Micro facies analysis and depositional environments of the carbonate unit in the Blue Nile Basin, Central Western Ethiopia

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Fifty-five (55) samples were selected for lithostragraihical and petrographic analysis from the carbonate unit. Lithostratigraphically, the unit is characterized by marls, calcareous shales, micritic, oolitic, fossiliferous and bioturbated micritic limestone. On the basis of petrographic investigations, seven (7) microfacies types are identified: Bioclastic-Intraclastic packstone/grainstone, Peloidal grainstone, Oolitic packstone / grainstone. Dolomitized packstone, Coral Framestone, Bioclastic packstone / grainstone and Bioclastic mudstone. In order to reflect dispositional energy conditions, the microfacies were grouped into three microfacies associations: (i) High-energy shallow marine environment (MFT1, MFT2 and MFT 3) (ii) Shallow marine to Reef environment associations (MFT5 and MFT6), and (iii) Low-energy, restricted marine environment associations (MFT4 and MFT7). Blue Nile Basin microfacies carbonates paleoenvironment Ethiopia Jurassic Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction The sedimentary history of the Mesozoic succession in Ethiopia is closely linked to the formation of rift basins along the periphery of the Gondwanan supercontinent, a process that began in the Upper Paleozoic and continued through the Tertiary Period (Mohr, 1962). A significant phase in this history was the Middle to Late Jurassic (Callovian–Kimmeridgian) marine transgression that affected the northeastern Horn of Africa and led to widespread carbonate deposition (Abate et al., 2015). This event, likely associated with regional subsidence and a major sea-level high stand, resulted in the drowning of the East African craton and the establishment of carbonate platforms across various basins (Bosellini, 1989 ; Russo et al., 1994). Consequently, the Upper Jurassic (Oxfordian-Kimmeridgian) carbonate successions of the East African margin and the Arabian platform are characterized by distinct carbonate and evaporitic units, with their boundaries reflecting regional shifts in tectonic regimes. Among these, the Ethiopian carbonate deposits provide a key record of these environmental changes (Bosellini, 1989 ). The Mesozoic sedimentary formations of Ethiopia are primarily found in the Blue Nile, Mekele, and Ogaden Basins (Assefa, 1991 ; Russo et al., 1994; Bosellini et al., 1997 ; Wolela Ahmed, 2007, 2009; Jain and Singh, 2019 a,b; Radwańska and Jain, 2020 ; Santos et al., 2021; Jain, and Schmerold, 2021 ; Jain et al., 2022 ; Jain and Schweigert, 2022 ; Singh et al., 2022 ; Salamon et al., 2022 , 2024 ). The Ogaden Basin in southeastern Ethiopia represents the largest and deepest of these basins, preserving thick Jurassic sedimentary sequences that record Ethiopia’s first major marine transgression, making it an important reference section for regional stratigraphy (Jain & Singh, 2019 a). In the Blue Nile Basin, the Antalo Limestone Formation, which conformably overlies the Gohatsion Formation, provides critical insights into Jurassic carbonate deposition in Ethiopia (Russo et al., 1994; Jain and Singh, 2019 b; Singh et al., 2022 ). The Antalo Limestone is a regionally extensive carbonate sequence that exhibits distinct lithological and faunal variations indicative of changing depositional environments. The lower portion of the formation consists of burrowed mudstones transitioning into oolitic and coquinoid limestones, rich in corals, stromatoporoids, bivalves, gastropods, and benthic foraminifera. These features suggest deposition in a well-oxygenated, shallow marine platform with active biogenic and chemical sedimentation. The middle section is characterized by marly limestone and marl, hosting a diverse fossil assemblage that includes ammonites and brachiopods, suggestive of deposition in a shelf to open marine environment with moderate energy conditions. The uppermost part of the Antalo Limestone records a return to shallow water conditions, as evidenced by planar laminated oolitic and reefal limestones, coral patches, and shoal facies. These stratigraphic divisions and facies transitions within the Antalo Limestone are argued to be comparable to the carbonate units observed in the Mertule Mariam section, our study area, located in the Northern Ethiopia (Jain et al., 2022 ; Jain and Schweigert, 2022 ). The shallow marine deposits in the lower part of the Mertule Mariam section seemingly align with the burrowed mudstones and coral-rich limestone of the Antalo Limestone lower section (Russo et al., 1994). Similarly, the fossil-rich marl and limestone facies of the middle part of the Mertule Mariam section are consistent with the middle section of the Antalo Limestone, characterized by ammonite and brachiopod-rich marls (Jain & Schmerold, 2021 ). Finally, the high-energy oolitic and reefal facies in the upper part of the Mertule Mariam section presumably correspond to the laminated oolitic limestone and reefal deposits of the Antalo Formation's upper section. Overall, the Mesozoic succession of Ethiopia provides a critical archive of the paleogeographic and paleoenvironmental evolution of the region, recording the interplay of marine transgressions, tectonic subsidence, and carbonate platform development during the Jurassic period (Jain & Schmerold, 2021 ). Numerous geological studies have been conducted on the Blue Nile Basin; however, research on the paleontological and environmental aspects of the carbonate units in the region remains limited (e.g., Russo et al., 1994; Dawit, 2010; Jain and Singh, 2019 a,b; Radwańska and Jain, 2020 ; Santos et al., 2021; Jain, and Schmerold, 2021 ; Jain et al., 2022 ; Jain and Schweigert, 2022 ; Singh et al., 2022 ; Salamon et al., 2022 , 2024 ). Notably, due to the inaccessibility between Gunde Weyn and Mekane Selam, the facies, paleontology, and depositional environment of the carbonate units in the Mertule Mariam section have not been explored. Furthermore, there has been no investigation into the correlation of this section with other geological formations across Ethiopia. This study builds on existing stratigraphic frameworks, particularly regarding the Mertule Mariam section, by presenting new findings that contribute to a more comprehensive regional stratigraphic synthesis. Through the integration of micropaleontological, petrographic, and lithostratigraphic analyses, we aim to refine depositional environment interpretations, establish detailed microfacies relationships, and enhance understanding of the lateral and vertical continuity of carbonate successions. Ultimately, this research seeks to provide new insights into the paleoenvironmental and paleogeographic evolution of the Mertule Mariam section and its role within Ethiopia’s Mesozoic depositional history. 2. Geological Setting The Blue Nile Basin, located on the Northwestern Ethiopian Plateau, is characterized by a Mesozoic succession that is 1200–1400 meters thick, underlain by Neoproterozoic basement rocks and capped by Early-Late Oligocene and Quaternary volcanic rocks (Gani et al., 2009). The basin also contains rare Paleozoic sediments (Getaneh Assefa, 1991 ; Merla, 1997). It is composed of five primary stratigraphic units: the Adigrat Sandstone, Gohatsion Formation, Antalo Limestone, Mugher Mudstone, and Debre Libanos Sandstone (Getaneh Assefa, 1991 ; Russo et al., 1994; Chernet et al., 2019 ; Jain and Singh, 2019 a). The thickness of the limestone ranges from 200 to 600 meters (Russo et al., 1994; Assefa, 1991 ) (Fig. 1.1 ). The sedimentary history of the Mesozoic succession in the Blue Nile Basin is closely associated with the formation of rift basins along the edges of the supercontinent Gondwana, beginning in the Upper Paleozoic and continuing into the Tertiary Period (Mohr, 1962). The Jurassic transgression in Ethiopia is primarily represented by carbonates, including the Antalo Limestone, Hamanlei, and Urandab Formations, which conformably overlie the Adigrat Sandstone. These limestones, dating to the Pliensbachian/Aalenian, are first found in the Ogaden Basin (Beyth, 1971; Kazmin, 1973) (Fig. 8.1 ). Tectonic evolutions in northeastern Africa, alongside fluctuations in sea level throughout geologic time, have played significant roles in the formation of these basins and the accumulation of thick Mesozoic sediments. In terms of structural evolution, most of these sedimentary basins are linked to extensional tectonic events that have occurred intermittently from the Late Paleozoic to the Tertiary Period. The Ogaden, Blue Nile, and Mekelle Outlier are considered intracontinental rift-related basins formed due to extensional stresses associated with the break-up of Gondwanaland, spanning from the Upper Paleozoic through to the Tertiary Period (Mohr, 1962; Blandford, 1970). The facies and depositional patterns observed in the Mertule Mariam section are comparable to carbonate units in other regions of Ethiopia and East Africa, providing opportunities for regional correlations with Mekele basin and Ogaden basin (see also Jain et al., 2022 ) (Fig. 7.1 ). The Mekelle Basin is one of the major Mesozoic sedimentary basins in Ethiopia, located in northern Ethiopia. It extends from the Amba-Alage Mountain in the south up to Wukro town in the north, Abi-Adi in the west, and reaches the western escarpment of the Ethiopian Rift Valley to the east (Alemu et al., 2018). The basin forms a nearly circular outlier covering approximately 8,000 km² around Mekelle (Beyth, 1972a ; Bosellini et al., 1997 ). Structurally, the Mekelle Outlier is interpreted as an intramontane basin that formed due to the uplift of two east-west trending structural highs located approximately between 13°N and 14°N latitudes, between the Wukro fault belt and the Precambrian basement rocks (Beyth, 1972b) and began in the Upper Paleozoic and continued through the Tertiary Period (Mohr, 1962; Blandford, 1970). Recent studies by Tadesse et al. (2018) have further refined the structural evolution of the Mekelle Outlier, describing it as a sag basin based on geological field data, remote sensing, and geophysical gravity analysis. The facies and depositional patterns observed in the Mertule Mariam section are comparable to carbonate units in other regions of Ethiopia and East Africa, providing opportunities for regional correlations (Fig. 7.1 ). The carbonate unit in the Blue Nile Basin, referred to as the 'Antalo Limestone Formation,' comprises up to 420 meters of shallow marine origin carbonates (Russo et al., 1994). It conformably overlies the Gohatsion Formation and can be subdivided into three sections: lower, middle, and upper limestone. The lower 180 m-thick section consists of burrowed mudstones, transitioning upwards into oolitic and coquinoid limestone, rich in corals, stromatoporoids, bivalves, gastropods, and benthic foraminifera, with occasional marl intercalations. This section represents a shallow water environment (Russo et al., 1994). The 200 m-thick middle section is characterized by highly fossiliferous, interbedded marly limestone and marl. The ammonites with brachiopods and bivalves suggest a shelf to open marine environment (Russo et al., 1994). The upper 50 m-thick section consists of planar laminated oolitic and reefal limestone, interpreted to indicate a return to shallow water conditions. The presence of oolitic bars, coral patches, and offshore to inshore facies suggests a similar shallow water environment to the lower section. In addition to tectonic controls on the formation of Ethiopian sedimentary basins, fluctuations in sea level driven by tectonic activity and climate variations have influenced the deposition of various sedimentary successions throughout geologic time. In fact, large-scale down warping of the East African continent during the Upper Triassic to Lower Jurassic resulted in widespread deposition of fluvio-deltaic Adigrat Sandstone, extending into western and northern Ethiopia (Dawit, 2010). Continued rifting and subsidence of the region, including adjacent areas of Saudi Arabia, Somalia and Yemen, led to significant marine transgression from the east and southeast (Jain, 2019 b). This event flooded extensive regions and facilitated the deposition of the Jurassic marine carbonate units, which are the primary focus of this study (Dainelli, 1943 ). With the eventual arching and doming of the Arabian-Somalian massif in the Late Jurassic, marine regression commenced, leading to the deposition of varied facies, including restricted marine, lagoonal, and supratidal to intertidal deposits within structurally controlled settings. The sediments deposited following the retreat of the sea include the Agula Shale in the Mekelle Outlier, the Gebredare Formation in the Ogaden Basin, and the Muger Mudstone in the Blue Nile Basin (Ethiopian Ministry of Mines, 2011). 3. Methods A detailed field description of the limestone unit was conducted in the Mertule Mariam Section which is found in the Blue Nile Basin. The thickness of the lithology in the studied section were measured and logged by taking consideration of each bed. Sedimentary structures, visible fossils and grains, bedding and overall stratification were described. Based on lithologic changes, 34 representative rock and marl samples were taken and snapped from bottom to top for further investigation and analysis. These 34 samples were prepared under thin section and petrographically examined to analyze the microfacies. The grain type (fossils, intraclasts, ooids, peloids and shell fragments) and proportion, fabrics grain shape, matrix and diagenetic feature under thin section were distinguished based on Dunham's (1962) classification scheme. The microfacies analysis was performed using Flügel (2010) microfacies classification principle. The identified microfacies in the studied limestone section were related with the Wilson (1975) and Flügel (2010) standard microfacies (SMF) approach. The depositional environments and facies belts were defined and recognized based on the studies of Wilson (1975) facies belts. 4. Results 4.1. Stratigraphy The Mertule Mariam carbonate section is conformably overlain by a clastic unit of sandstone and mudrocks and underlain by gypsum layers, marking the base of the Gohatsion Formation. Field investigations identified nine distinct lithofacies within the Mertule Mariam carbonate section (Fig. 3.1 ). 4.1.1. Gypsum The gypsum layer exposed at the base of the carbonate in the Mertule Mariam section, indicates the start of the underling 2-meter thick Gohatsion Formation (Fig. 3.1 ), characterized by whitish to grayish color (Fig. 5.1 ) 4.1.2. Marl This unit has variable thickness, extending from 2 to 127 meters (Fig. 3.1 ) intercalated with cliff forming micritic limestones. It shows a yellow, gray and light color in the field. The unit is mainly described by fine grained and loose sediments containing micro- and macro invertebrates like Pholadomya ( Bucardiomya ) somaliness Sowerby and Nanogyra nana Sowrbey. This rock unit varies from place to place in terms of fossil content, consisting of high numbers of foraminifera and ostracods in some locality to a complete absence of fossils in other places (Fig. 5.2 ) 4.1.3. Micritic limestone This unit is also variable in thickness, from 127 to 192 meters (Fig. 3.1 ). It is dominated by well-compacted fine grained carbonate cement (90%). This micritic limestone unit occurs as a cliff intercalated with marls (Fig. 5.3 ). 4.1.4. Oolitic limestone This unit is horizontally bedded with variable thickness, extending 192 to 217 meters (Fig. 3.1 ). It forms cliff faces and ranges in colour from yellow to gray (Fig. 5.4 ). Well-cemented ooid grains (85%) (allochems) are the main constituents. 4.1.5. Calcareous Shale This is a 237 to 251 m-thick unit (Fig. 3.1 ). It also intercalates with micritic limestone and marl unit in its upper part. The unit shows fissility and has a grayish color. The 4 m-thick shale is exposed in a road cut. It superimposes the 20- m-thick unexposed area at the middle part of the studied section (Fig. 5.5 ). 4.1.6. Fossiliferous limestone This is a 251 to 260 m-thick unit (Fig. 3.1 ) exposed in a road cut characterized by yellow to brown color. It is mainly layered intercalating with marls with shell fragments of brachiopods, crinoids and bivalves (80%) (Fig. 5.6 ). 4.1.7. Bioturbated micritic limestone The is a 260 to 320 m-thick cliff-forming, micrite-rich (75%) cliff forming horizontally bedded unit (Fig. 3.1 ). Thin fragments (10%) of brachiopod, bivalve and crinoid shells are highly cemented within the micrite. The unit is exposed in a road cut and is white to gray in color. At the upper part of the section the unit is intercalated with thin beds of shale, mud and marl. This intercalated bed thickness varies between 20 to 50 cm. Bioturbation is commonly observed, especially at the base of the unit (Fig. 5.7 ). 4.2. Microfacies Analysis In carbonate rocks, texture is primarily influenced by the hydrodynamic conditions of the depositional environment, while the presence and abundance of bioclasts determine grain types and proportions (Flügel, 2010). Based on these factors, the biogenic composition, textural properties, and lithological characteristics of the Mertule Mariam section (34 thin sections) has yielded eleven microfacies. In the present study, the classification schemes of Wilson (1975), Flügel (1982, 2004), and Flügel's ramp microfacies types (MFT; Flügel, 2004) is followed. The characteristics of each microfacies type are discussed below. 4.2.1. Bioclastic- Intraclastic packstone/grainstone (MFT 1) This microfacies is mainly composed of intraclasts (75%) and bioclasts (23%) including fragments of echinoid plate, bivalve and foraminifera (miliolids) (Fig. 6.1 .B). The grains show various bioturbated, poorly sorted, rounded and irregular features. Each grain is cemented with a sparite cement (2%) while the surface of each grain is dominantly filled by micrite. Some intraclasts and bioclasts micritization (Fig. 6.1 .A&B). Interpretation Intraclastic grainstones are interpreted as having been deposited by storm-wave erosion and reworking of various sediment types in shallow-marine environments (Flügel, 2004). The presence of intraclasts and the grain-supported fabric indicates high energy condition. Thus, this facies was deposited in high energy shoal environment with open circulation in the presence of benthic foraminifera and other bivalve shell fragments. The Bioclastic-Intraclastic packstone/grainstone can be compared with SMF Type 10 of Wilson (1975), corresponding to FZ 7. 4.2.2. Peloidal grainstone (MFT 2) This microfacies is characterized by a high abundance of micritized small sized peloids (90%), and minor ooids (5%) (Table.1.1). Few bioclasts of miliolid group foraminifera and intraclasts are also observed (Fig. 6.1 .C). The peloid allochems are medium to well-rounded and well-sorted. Most of the grains show a micrite cement characteristics. Interpretation The high degree of sorting and rounding suggests constant, high-energy conditions above fair-weather wave base, within the upper shoreface environment. This microfacies is compared with SMF Type 15 of Wilson (1975), and correspond to FZ 6, i.e., deposited in moderate circulation conditions with winnowing by wave currents. 4.2.3. Oolitic packstone/grainstone (MFT 3) The allochems are well-rounded and moderately to well sorted concentric ooid grains (95%) and bioclast (3%) of bivalve, echinoid and foraminifera (dominantly miliolids) (Fig. 6.1 .D&E) (Table.1.1). Most ooids typically consists of a nucleus (often a small grain or fragment) surrounded by concentric layers of calcium carbonate or other minerals. The ooid and bioclastic grains are cemented by coarse sparite cement. The outer surfaces of some ooids are micritized (Fig. 6.1 .D) and their nuclei consists of miliolids (Fig. 6.1 .E). Interpretation The presence of fossils within ooids indicates short-term stable conditions during ooids growth (Flugel 2004). The bioclasts and thin-rim ooids with sparry calcite indicate comparatively high-energy conditions in a platform margin shoal (Flügel, 2010). Ooids often form through the accumulation of carbonate minerals during wave action, indicating dynamic sedimentary processes. This microfacies is similar with SMF 15 and FZ 6 of Flügel (2010). 4.2.4. Dolomitized packstone (MFT 4) The microfacies is largely dominated by light gray micrite supported dolomite crystals (65%) (Table.1.1). The scattered dolomite crystal cemented by micrite cement consists of brownish to yellowish rhombic shape (Fig. 6.1 .F). Some parts of the feature show black to deep black color. Interpretation According to Amthor and Friedman ( 1991 ) the very fine dolomite crystal occurs restricted to the peritidal settings. The fine crystal size represents a result of early dolomitization precursor of peritidal lime mudstone or pencontemporaneous neomorphism or an early diagenetic dolomite (Zenger, 1983; Amthor & Friedman, 1991 ). However, dolomite does not indicate a depositional facies, but a diagenetic facies. Due to presence of the fine grain size, quartz grains, fenestral fabric, lack of fauna, and vertical changes suggest that deposition occurred in a low-energy, restricted intertidal and supratidal environment (Wilson and Evans, 2002). The brown coloration may be due to the presence of iron oxides (such as hematite or limonite) that yield reddish or brownish hues to the rock. Therefore, this microfacies is deposited in shallow subtidal to lower intertidal environments, corresponding to SMF-23 of Wilson (1975) and Flugel (2010) of facies belt 8. 4.2.5. Coral Framestone Microfacies (MFT5) The microfacies dominantly contains a matrix-bounded coralline foraminifera and bivalves (40%); it also contains algae (Fig. 6.1 .G). The microfacies is dominated by corals and micrite (60%) (Table.1.1). Interpretation The coral framestone microfacies is predominantly formed from coral fragments and various biogenic materials typically in a warm, shallow marine environment where coral reefs thrive along the platform margin. Often associated with high-energy reef settings, coral framestone indicates the dynamic conditions favorable for coral growth. This facies is comparable to SMF 7 in FZ 5 and RMF 12 (Wilson 1975; Flugel 2004). 4.2.6. Bioclastic packstone/grainstone (MFT 6) This microfacies is composed of large bivalve shells (80%) and echinoid plate allochems cemented by micrite cement (20%) (Table.1.1). Most of them have been micritized marginally to completely (Fig. 6.1 .H). The outer surface of shells are slightly affected by fine-grained to moderately coarse-grained calcite (sparite) cement (Fig. 6.1 .H). Interpretation These deposits show evidence of intense reworking with a predominantly open marine fauna suggesting a marine environment with high hydrodynamic energy. The microfacies represent bioclastic shoals on the platform margin or on highs in the platform interior corresponding to SMF 12/FZ 6 of Flugel (2004). The coarse bioclasts indicate comparatively high-energy conditions of a shoal at the platform margin (Flügel, 2010). 4.2.7. Bioclastic mudstone (MFT 7) The main distinctive feature of this facies is the abundance of sponge spicules (20%) within a micrite-dominated matrix (80%) with some gastropod shell, brachiopod, echinoid plate, bivalve, and miliolid bioclasts (Table.1.1). Diagenetic stylolite structures and fracture are also observed. In addition to them, most bioclasts are affected by fine crystalline calcite cementation (Fig. 6.1 .I). Interpretation The presence of abundant sponge spicules and miliolid foraminifera within a homogenous micrite and overlain by skeletal sandy limestone and underlain by coral framestone suggest that the bioclastic spiculite wackstone was deposited in the proximal open marine environment below fair-weather wave base with normal circulation. This microfacies corresponds with FZ3 facies zone of Flugel (1982) and Wilson (1975), suggesting proximal open marine settings with open circulation. This microfacies is comparable to SMF 8 of Flügel (2010). 5. Discussion 5.1. Facies association and depositional environment 5.1.1. Facies Association 1: High-energy shallow marine environment This facies association includes three major microfacies types; bioclastic-intraclastic packstone/grainstone (MFT 1), Peloidal grainstone (MFT 2) and oolitic packstone/grainstone (MFT 3). The bioclastic-intraclastic packstone/grainstone (MFT 1) is a high-energy shoal with storm-wave reworking containing intraclasts and bioclasts (echinoid plates, bivalves, and foraminifera). The Peloidal grainstone (MFT 2) suggests high-energy settings with high degree of sorting and containing rounding peloid grains suggesting constant conditions above fair-weather wave base, in the upper shoreface environment. The oolitic packstone/grainstone (MFT 3) is a shoal environment with ooid grains suggesting dynamic processes and stable conditions for ooid growth. The depositional environments for this facies association is characterized by high energy often influenced by storm events, wave action and strong hydrodynamic conditions. These environments are typically found in shoals, platform margins or reef areas. 5.1.2. Facies Association 2: Shallow Marine to Reef Environments The shallow marine to reef environment facies association consists of two microfacies: Coral Framestone (MFT 5) and Bioclastic Packstone/Grainstone (MFT 6). The first microfacies type is characterized by the high-energy reef environment dominated by coral fragments, marine organisms and biogenic materials, whereas the second type is marked by intense reworking of large bivalve shells and echinoid plates. The shallow marine to reef environments facies association includes the environments associated with reef growth, corals, and other biogenic materials in warm and shallow marine settings. 5.1.3. Facies Association 3: Low-Energy, Restricted Marine Environments The low-energy, restricted marine environment facies association includes Dolomitized Packstone (MFT 4) and Bioclastic Mudstone (MFT 7). The Dolomitized Packstone (MFT 4) is mainly a low-energy, restricted intertidal to supratidal environment microfacies influenced by early dolomitization. The Bioclastic Mudstone (MFT 7) is a low-energy, open marine environment microfacies found below the fair-weather wave base with a micrite-dominated matrix and abundant sponge spicules. This facies association includes environments characterized by lower energy, such as intertidal zones, restricted marine settings or areas where diagenetic processes such as dolomitization dominate. 5.2. Depositional environment of the Mertule Mariam section The studied section presents a diverse sequence of carbonate and siliciclastic deposits that reflect dynamic depositional environments ranging from tidal flats and lagoons to high-energy shoals and open marine systems. This diversity is evident in the lithological variations, sedimentary structures, and fossil assemblages observed throughout the section. The ~ 60 m-thick upper part of the section is dominated by micritic limestones with intercalations of marls and dolomites. Thin section analysis identifies this limestone as micrite or mudstone (Dunham, 1962 ), with bioturbation and marked by a gradual decrease in fossil content upward, indicating deposition in a low-energy, restricted marine environment. The presence of unfossiliferous mudstone facies with high micrite content (98–99%) suggests sedimentation in intertidal flats, where limited energy conditions restricted biogenic activity. The alternating marl and dolomite layers reflect episodic shifts in salinity and energy, possibly tied to tidal influences or short-term climatic variations. In contrast, the middle part of the section is characterized by marls interbedded with shales, fossiliferous limestones, dolomitized and variegated mudstones, reflecting deposition in a more dynamic, lagoonal to shallow marine environment. The peloidal-bioclastic wackestone/packstone facies in this interval, with peloids and bivalve bioclasts, supports deposition in a shallow lagoon setting, where restricted circulation and moderate energy prevailed. The presence of benthic foraminifera within marls further suggests low-energy lagoonal environment with occasional influx of higher-energy conditions, as evidenced by storm-induced winnowing and sediment reworking. The lower part of the section transitions into a higher-energy depositional setting, as indicated by the presence of oolitic and bioclastic grainstone facies. The 125 m-thick marl unit intercalated with a 2-m thick oolitic limestone layer reflects deposition in high-energy shoals near platform margin. The abundance of ooids, peloids, and bioclasts in these facies is consistent with wave-dominated environments above fair-weather wave base, where constant agitation promoted grain rounding and sorting. Coral framestone facies, dominated by reef-building corals, further supports the interpretation of reefal shoals within high-energy zones with open marine circulation. At the ba s e of the section, the 125 m-thick marl layer with abundant brachiopods and foraminifera transitions into a 65 m-thick micritic limestone unit. This interval reflects an open marine environment below fair-weather wave base, with low-energy conditions favoring the preservation of fine-grained micrite and diverse skeletal fragments. Bioclastic mudstone and wackestone facies in this interval indicate a well-circulated, low-energy setting, consistent with a proximal open marine environment. Sponge spicules and skeletal fragments in these facies provide additional evidence of normal marine conditions with moderate nutrient availability. The depositional sequence as a whole shows an upward-deepening trend, transitioning from high-energy shoals and lagoons to low-energy open marine environments. This reflects a transgressive-regressive cycle, where sea-level fluctuations influencing facies distribution and depositional patterns. 6. Conclusions This study provides the first detailed analysis of the carbonate units exposed at the Mertule Mariam section (Blue Nile Basin), identifying seven distinct microfacies types grouped into four facies associations: tidal flat, lagoon, high-energy shoals, and open marine environments. These facies reflect a diverse depositional system within a shallow marine environment, ranging from shallow inner ramp of near-coastal tidal settings to restricted lagoonal systems, high-energy shoals, and proximal mid- to outer-ramp open marine settings. Field observations combined with petrographic analysis suggests that the carbonate units were deposited under dynamic environmental conditions influenced by variations in energy, circulation, and sediment supply. The observed facies transitions capture the interplay between regressive and transgressive cycles, providing valuable insights into the stratigraphy and paleoenvironment of the Jurassic carbonate platform in the region. The carbonate units are lithostratigraphically and biostratigraphically correlative with previously studied units in the Blue Nile Basin, including the Antalo Limestone Formation. Regionally, these deposits are equivalent to carbonate units in the Ogaden Basin and Mekele Basin, indicating a widespread depositional system linked to the Jurassic marine transgression across East Africa. To enhance the understanding of the stratigraphy and paleoenvironment of the Mertule Mariam section, more detailed studies in micropaleontology and biostratigraphy are needed, allowing for more precise age constraints, paleoecological interpretations, and refined correlations within the basin and across the region. Such efforts will further contribute to reconstructing the paleogeographic and tectonic evolution of the East African margin during the Jurassic. Declarations Author Contribution all 4 authors contribute for the research and manuscript preparation References Adams, A.E., & Mackenzie, W.S. (1998). A Color Atlas of Carbonate Sediments and Rocks under the Microscope. Manson Publishing, London, 179 pp. Ahr, W.M. (1973). The carbonate ramp: An alternative to the shelf model. Transactions of the Gulf Coast Association of Geological Societies, 23, 221–225. Al-Aydrus, M.A.A. (2011). Microfacies analysis and depositional environments of tertiary carbonate sequences in Socotra Island, Yemen. Geological Bulletin of Turkey, 54(1). Alemu, T., Abdelsalam, M. G., Dawit, E. L., Atnafu, B., & Mickus, K. L. (2025). The Paleozoic and Mesozoic Mekele Sedimentary Basin in Ethiopia: Journal of African Earth Sciences 143 (2018) 40-58. Al-Husseini, M.I. (1997). Jurassic sequence stratigraphy of the western and southern Arabian Gulf. GeoArabia, 2(4), 361–382. Amthor, I.E., & Friedman, G.M. (1991). Dolomite-rock textures and secondary porosity development in Ellenburger Group carbonates (Lower Ordovician), west Texas and southern New Mexico. Sedimentology, 38, 343–362. Asrat, A. (2015). Geology, geomorphology, geodiversity, and geoconservation of the Sof Omar Cave System, southeastern Ethiopia. African Earth Sciences. Assefa, G. (1980). Stratigraphy and sedimentation of the type Gohatsion Formation (Lias-Malm) Abay River basin, Ethiopia. Ethiopian Journal of Science (Sinet), 3, 87–110. Assefa, G. (1981). Gohatsion Formation: A new Lias-Malm lithostratigraphic unit from the Abay River Basin, Ethiopia. Geoscience Journal, 2(1), 63–88. Assefa, G. (1991). Lithostratigraphy and environment of deposition of the Late Jurassic–Early Cretaceous sequences of the central part of the Northwestern Plateau, Ethiopia. Neue Jahrbuch für Geologie und Paläontologie, 182, 255–284. Aydin, A., & Rona, P. A. (1981). Upper Jurassic Carbonate Successions in East Africa and the Arabian Platform: Tectonic and Environmental Implications. Journal of Sedimentary Petrology, 51(1), 15-24. Bassoullet, J. (1997). Les grands foraminifères: Biostratigraphie du Jurassique ouest-européen et Méditerranéen: Zonations parallèles et distribution et microfossiles. Bulletin des Centres de Recherche Exploration-Production Elf-Aquitaine Mémoires, 17, 293–304. Batista dos Santos Filho, M.A., Piovesan, E.K., Fauth, G., & Srivastava, N.K. (2015). Paleoenvironmental interpretation through the analysis of ostracodes and carbonate microfacies: Study of the Jandaíra Formation, Upper Cretaceous, Potiguar Basin. Brazilian Journal of Geology, 45(1), 23–34. Benssaou, M., & Hamoum, N. (2001). The western Anti-Atlas of Morocco: Sedimentological and paleogeographical formation studies in the Early Cambrian. 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Memorie della Società Geologica Italiana, 49, 95–116. Bucur, I.I., & Sasaran, E. (2005). Micropaleontological assemblages from the Upper Jurassic-Lower Cretaceous deposits of Trascău Mountains and their biostratigraphic significance. Acta Paleontologica Romaniae, 5, 27–38. Cantrell, D.L., & Hagerty, R.M. (1999). Microporosity in Arab Formation carbonates, Saudi Arabia. GeoArabia, 4(2), 129–154. Chernet, S.G., Atnafu, B., & Asrat, A. (2019). Stratigraphic and lithofacies analysis of the Gohatsion Formation in the Blue Nile Basin, central Ethiopia: Implications for depositional setting. Journal of African Earth Sciences. Dainelli, G. (1943). Geologia dell'Africa Orientale. Reall Accad. Italia, Rome. Dawit, E.L., & Bussert, R. (2009). Latest Ordovician-Early Silurian postglacial tide-dominated shelf sediments in northern Ethiopia. 6th Annual Conference SEPM-CES Sediment 2009, Abstracts, Krakow, Poland, 148. Dunham, R.J. (1962). Classification of carbonate rocks according to their depositional texture. In W.D. Ham (Ed.), Proceedings of Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir, Tulsa, 1, 108–121. Embry, A.F. III, & Klovan, J.S. (1971). A Late Devonian reef tract on northeastern Banks Island, N.W.T. Bulletin of Canadian Petroleum Geology, 4, 730–781. Emig, C.C., Bitner, M.A., & Alvarez, F. (2013). Phylum Brachiopoda. Zootaxa, 3703(1), 75–78. Jain, S. (2019a). First Bathonian (Middle Jurassic) nautiloid Paracenoceras Spath from Ethiopia. Journal of African Earth Sciences, 149, 82–96. Jain, S. (2019b). Middle Bathonian Indonesian Macrocephalites cf. etheridgei (Spath) from SW Somalia. Journal of African Earth Sciences. Volume 151, 202–211. Jain, S., & Schmerold, R. (2021). Callovian and Kimmeridgian fossils and stratigraphy of the Blue Nile Basin (central western Ethiopia). Annales Societatis Geologorum Poloniae, 91(4), 413-440. Jain, S., Mullugetta, M., Benzaggagh, M., Salamon, M. A., & Schmerold, R. (2022). Discovery of chitinoidellids and calpionellids from the Blue Nile Basin and the Jurassic‐Cretaceous boundary in Ethiopia. Cretaceous Research, 132, 105112. https://doi.org/10.1016/j.cretres.2021.105112. Jain, S., & Schmerold, R. (2021). The Callovian‐Kimmeridgian Biostratigraphy of the Blue Nile Basin (Ethiopia). Annales Societatis Geologorum Poloniae, 91, 287–307. doi: https://doi.org/10.14241/asgp.2021.12 Jain, S., Schmerold, R., & Getachew, M. (2020). Discovery of the Middle Callovian ammonite Erymnoceras in the Blue Nile Basin (Ethiopia). Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen, 297/1, 27–35. Jain, S., & Schweigert, G. (2022). On the dimorphic occurrence of the upper Tithonian ammonite genus Djurjuriceras Roman from the Blue Nile Basin (Ethiopia). Paläontologische Zeitschrift, 96, 655–668. DOI.org/10.1007/s12542‐022‐00620‐y. Jain, S., & Singh, A. (2019). First calcareous nannofossil record from the Jurassic strata exposed in the Blue Nile Basin (Ethiopia). Journal of African Earth Sciences. 158, Article 103553, doi.org/10.1016/j.jafrearsci.2019.103553. Radwańska, U., & Jain, S. (2020). First Late Jurassic echinoid record of Pygurus meslei Gauthier from the Antalo Limestone Formation (Blue Nile Basin, Ethiopia). Journal of African Earth Sciences, 170, 103898. doi.org/10.1016/j.jafrearsci.2020.103898. Salamon, M. A., Benyoucef, M., Jain, S., Benzaggagh, M., Płachno, B. J., Abdelhamid, M.A.M., Ahmad, F., Azar, D., Bouchemla, I., Brachaniec, T., El Ouali, M., El Qot, G., Ferré, B., Gorzelak, P., Krajewski, M., Klompmaker, A. A., Mekki, F., Poatskievick‐Pierezan, B., & Slami, R. (2024). Jurassic and Cretaceous crinoids (Crinoidea, Echinodermata) from the southern shelf of Tethys (northern and eastern Africa, Middle East, Asia, and India). Palaeontographica, Abteilung A: Palaeozoology – Stratigraphy, 328(1–6), 1–991‐99. https://doi.org/10.1007/s12371‐024‐00973‐7 Salamon, M. A., Jain, S., Brachaniec, T., Duda, P., Płachno, B. J., & Gorzelak, P. (2022). Ausichicrinites zelenskyyi gen. et sp. nov., a first nearly complete feather star (Crinoidea) from the Upper Jurassic of Africa Royal Society Open Science 9(7), doi.org/10.1098/rsos.220345. Santos, A., Jain, S., & Diez, J. B. (2022). Upper Jurassic palynology from the Blue Nile Basin (Ethiopia). Review of Palaeobotany and Palynology. Review of Palaeobotany and Palynology 285, 104361. doi.org/10.1016/j.revpalbo.2020.104361. Singh, A., Jain, S., Benzaggagh, M., Schweigert, G., Salamon, M. A., & Mulugeta, M. (2022). Late Tithonian nannofossils from Dejen area, the Blue Nile Basin, central western Ethiopia. Paleoworld. doi.org/10.1016/j.palwor.2022.10.003. Table Table 1.1: Microfacies type codes, microfacies names, thin section photograph, main components and depositional environment of each microfacies type. Microfacies type code Microfacies name Thin section photograph Carbonate grains Cement Depositional environment Peloid % Ooid% Bioclast% intraclast Micrite% Sparit% MFT1 Bioclastic- Intraclastic packstone/grainstone Fig.6.1 A&B 0% 0% 23% 75% 0% 2% Shoal MFT2 Peloidal grainstone Fig.6.1C 90% 5% 0% 0% 5% 0% Lagoon MFT3 Oolitic packstone/grainstone Fig.6.1D&E 0% 95% 3% 0% 0% 2% Shoal MFT4 Dolomitized packstone Fig.6.1F 0% 0% 0% 0% 45% 0% Tidal MFT5 Coral Framestone Microfacies Fig.6.1G 0% 0% 40% 0% 60% 0% Shoal MFT6 Bioclastic packstone/grainstone Fig.6.1H 0% 0% 80% 0% 20% 0% Shoal MFT7 Bioclastic mudstone Fig.6.1I 0% 0% 20% 0% 80% 0% Open marine Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6963278","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":484216364,"identity":"61859f58-91d4-42df-8e28-42d334c3636a","order_by":0,"name":"Mahider Mulugeta","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABAElEQVRIiWNgGAWjYFACHoYDDAwWEPYHAxsgydh4gAgtEkAGMwPjjII0kJYGgloYYFqYeT4cBovh1aLbfvbgoZttEonz3fsPfuAxOG+3tv0w0JYam2hcWszO5CUczgVq2XjmMLOEhMHt5G1nEoFajqXlNuDSciDHAKJlRjKDhAFQi9kBoBbGhsO4tZx/A9Uy/zHzjwSDc8lm5x8S0HIDast8CWY2iQMGB+zMbhCy5QbQlpxzEsYbeJLNLBsMkhPMbgBtScDnl/M5xp9zymxk57cffHz7zx87e7Pz6Q8ffKixwakFDgwOQOhEsMoEQspBQB5qqD0xikfBKBgFo2BkAQAf72psyWuVrQAAAABJRU5ErkJggg==","orcid":"","institution":"Bahir Dar University","correspondingAuthor":true,"prefix":"","firstName":"Mahider","middleName":"","lastName":"Mulugeta","suffix":""},{"id":484216365,"identity":"7f77def6-9146-446b-980a-85c7d5658b85","order_by":1,"name":"Balemwal Atnafu","email":"","orcid":"","institution":"Addis Ababa University","correspondingAuthor":false,"prefix":"","firstName":"Balemwal","middleName":"","lastName":"Atnafu","suffix":""},{"id":484216366,"identity":"b650b6a7-5007-487f-a01c-d39023699d94","order_by":2,"name":"Sreepat Jain","email":"","orcid":"","institution":"DG-2","correspondingAuthor":false,"prefix":"","firstName":"Sreepat","middleName":"","lastName":"Jain","suffix":""},{"id":484216369,"identity":"5463edf4-753c-4473-93e6-c8c51f4a3612","order_by":3,"name":"Andualem Taye","email":"","orcid":"","institution":"Mekdela Amba University","correspondingAuthor":false,"prefix":"","firstName":"Andualem","middleName":"","lastName":"Taye","suffix":""}],"badges":[],"createdAt":"2025-06-24 08:23:11","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6963278/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6963278/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86675395,"identity":"64f9aedc-4eb8-45f6-b8fe-6b97524c62f7","added_by":"auto","created_at":"2025-07-14 11:58:09","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":335485,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 1.1\u003c/strong\u003e: General stratigraphy of the Blue Nile Basin. Age, lithology, thickness, and description of the formations exposed at the Dejen area are shown as a representative locality for the Blue Nile Basin (modified after Jain et al., 2020, 2022).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6963278/v1/078a9249c743122504d48b0e.png"},{"id":86675645,"identity":"4814cd18-8503-45bb-b5b8-4096b4ed39f8","added_by":"auto","created_at":"2025-07-14 12:06:09","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":250961,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 2.1:\u003c/strong\u003e Study area. (A) Position of Ethiopia in context of Africa; (B) Locations of the Mekele, Blue Nile and Ogaden mesozoic sedimentary basins of Ethiopia (modified after Jain et al., 2022). The orbitolinid bearing lower Aptian sediments at Graua (Bosellini et al., 1999) are also marked; (C) Locations of the East Gojjam section (this study).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6963278/v1/bbd6d89b1a69154a16acf7d9.png"},{"id":86675398,"identity":"32cdfdc7-9cad-414a-a604-1864860c21b8","added_by":"auto","created_at":"2025-07-14 11:58:09","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":231703,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 3.1:\u003c/strong\u003e The stratigraphic log of the Mertule Mariam section (this study).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6963278/v1/1e6d2d54e1a1c56b7885bdca.png"},{"id":86675400,"identity":"5a136853-88d3-4cf3-9d33-e022de023f13","added_by":"auto","created_at":"2025-07-14 11:58:09","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":121494,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 4.1: \u003c/strong\u003eThe distribution of samples within stratigraphic log.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6963278/v1/85d59acc6f33fadae627aa97.png"},{"id":86675649,"identity":"248d3206-06fa-4103-a1a8-a03be0af54a2","added_by":"auto","created_at":"2025-07-14 12:06:09","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1069788,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure\u003c/strong\u003e \u003cstrong\u003e5.1\u003c/strong\u003e Field photograph shows Gypsum\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u003c/strong\u003e \u003cstrong\u003e5.2\u003c/strong\u003e Field photograph shows marl\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u003c/strong\u003e \u003cstrong\u003e5.3\u003c/strong\u003e Field and thin section photograph shows bioclastic micritic limestone\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure 5.4\u003c/strong\u003e Field and thin section photograph shows oolitic limestone\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure 5.5\u003c/strong\u003e Field photograph shows calcareous shale\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure 5.6\u003c/strong\u003e Field and thin section photograph shows fossiliferous limestone\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u003c/strong\u003e \u003cstrong\u003e5.7\u003c/strong\u003e Field photograph shows bioturbated micritic limestone\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6963278/v1/04b42e3cf8eb8240c4d42848.png"},{"id":86675405,"identity":"14c03cc1-6e57-4be1-91cb-d1e71617b6d6","added_by":"auto","created_at":"2025-07-14 11:58:09","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":874183,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 6.1:\u003c/strong\u003e Microphotographs of the most important microfacies types of the Mertule Mariam section under ppl.\u003c/p\u003e\n\u003cp\u003e(A) and (B) Bioclastic- Intraclastic packstone/grainstone type microfacies;\u003c/p\u003e\n\u003cp\u003e(C) Peloidal grainstone type microfacies;\u003c/p\u003e\n\u003cp\u003e(D) Oolitic packstone/grainstone type microfacies;\u003c/p\u003e\n\u003cp\u003e(E) Oolitic packstone/grainstone type microfacies with miliolid foraminiferas in the nucleus\u003c/p\u003e\n\u003cp\u003e(F) Dolomitized packstone type microfacies;\u003c/p\u003e\n\u003cp\u003e(G) Coral Framestone type Microfacies\u003c/p\u003e\n\u003cp\u003e(H) Bioclastic packstone/grainstone type microfacies;\u003c/p\u003e\n\u003cp\u003e(I) Bioclastic mudstone type microfacies.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6963278/v1/e493c3e72bfc24ac6c68f0d4.png"},{"id":86675403,"identity":"aabbd944-74c6-4724-aade-e67a3b43c3db","added_by":"auto","created_at":"2025-07-14 11:58:09","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":376828,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 7.1\u003c/strong\u003e: Proposed regional correlation of the Carbonate unit throughout Ethiopian basins (Blue Nile, Mekele and Ogaden (Blue Nile Basin (the present study and Russo et al., 1994), Mekele Basin (Levitte, 1970 and Bosselini et al., 1997) and Ogaden Basin (Abate et al, 1974 and Hunegnaw, 1998).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6963278/v1/6cfa059c02dd3b9fbc9cdfc3.png"},{"id":86675412,"identity":"f1d284d4-ac30-4a50-8cb0-83e830d1e086","added_by":"auto","created_at":"2025-07-14 11:58:09","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":439644,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 8.1\u003c/strong\u003e: Reconstruction of Tithonian (Late Jurassic) paleoenvironment of the region (modified after Al-Husseini, 1997). (B) Location of the study area at Mertule Mariam within the Blue Nile Basin (C) Location of the study section. CCD = Calcite Compensation Depth.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6963278/v1/19076b65d31e3e9b67a971ae.png"},{"id":93506315,"identity":"4ec6c67f-206f-4e49-8a7c-3183f132db01","added_by":"auto","created_at":"2025-10-14 14:47:15","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4478760,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6963278/v1/c1f483e2-fd24-49be-9907-b79c88c7779b.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Micro facies analysis and depositional environments of the carbonate unit in the Blue Nile Basin, Central Western Ethiopia","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe sedimentary history of the Mesozoic succession in Ethiopia is closely linked to the formation of rift basins along the periphery of the Gondwanan supercontinent, a process that began in the Upper Paleozoic and continued through the Tertiary Period (Mohr, 1962). A significant phase in this history was the Middle to Late Jurassic (Callovian\u0026ndash;Kimmeridgian) marine transgression that affected the northeastern Horn of Africa and led to widespread carbonate deposition (Abate et al., 2015). This event, likely associated with regional subsidence and a major sea-level high stand, resulted in the drowning of the East African craton and the establishment of carbonate platforms across various basins (Bosellini, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Russo et al., 1994). Consequently, the Upper Jurassic (Oxfordian-Kimmeridgian) carbonate successions of the East African margin and the Arabian platform are characterized by distinct carbonate and evaporitic units, with their boundaries reflecting regional shifts in tectonic regimes. Among these, the Ethiopian carbonate deposits provide a key record of these environmental changes (Bosellini, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1989\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe Mesozoic sedimentary formations of Ethiopia are primarily found in the Blue Nile, Mekele, and Ogaden Basins (Assefa, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Russo et al., 1994; Bosellini et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Wolela Ahmed, 2007, 2009; Jain and Singh, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2019\u003c/span\u003ea,b; Radwańska and Jain, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Santos et al., 2021; Jain, and Schmerold, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Jain et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Jain and Schweigert, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Singh et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Salamon et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2022\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The Ogaden Basin in southeastern Ethiopia represents the largest and deepest of these basins, preserving thick Jurassic sedimentary sequences that record Ethiopia\u0026rsquo;s first major marine transgression, making it an important reference section for regional stratigraphy (Jain \u0026amp; Singh, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2019\u003c/span\u003ea). In the Blue Nile Basin, the Antalo Limestone Formation, which conformably overlies the Gohatsion Formation, provides critical insights into Jurassic carbonate deposition in Ethiopia (Russo et al., 1994; Jain and Singh, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2019\u003c/span\u003eb; Singh et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe Antalo Limestone is a regionally extensive carbonate sequence that exhibits distinct lithological and faunal variations indicative of changing depositional environments. The lower portion of the formation consists of burrowed mudstones transitioning into oolitic and coquinoid limestones, rich in corals, stromatoporoids, bivalves, gastropods, and benthic foraminifera. These features suggest deposition in a well-oxygenated, shallow marine platform with active biogenic and chemical sedimentation. The middle section is characterized by marly limestone and marl, hosting a diverse fossil assemblage that includes ammonites and brachiopods, suggestive of deposition in a shelf to open marine environment with moderate energy conditions. The uppermost part of the Antalo Limestone records a return to shallow water conditions, as evidenced by planar laminated oolitic and reefal limestones, coral patches, and shoal facies.\u003c/p\u003e\u003cp\u003eThese stratigraphic divisions and facies transitions within the Antalo Limestone are argued to be comparable to the carbonate units observed in the Mertule Mariam section, our study area, located in the Northern Ethiopia (Jain et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Jain and Schweigert, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The shallow marine deposits in the lower part of the Mertule Mariam section seemingly align with the burrowed mudstones and coral-rich limestone of the Antalo Limestone lower section (Russo et al., 1994).\u003c/p\u003e\u003cp\u003eSimilarly, the fossil-rich marl and limestone facies of the middle part of the Mertule Mariam section are consistent with the middle section of the Antalo Limestone, characterized by ammonite and brachiopod-rich marls (Jain \u0026amp; Schmerold, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Finally, the high-energy oolitic and reefal facies in the upper part of the Mertule Mariam section presumably correspond to the laminated oolitic limestone and reefal deposits of the Antalo Formation's upper section.\u003c/p\u003e\u003cp\u003eOverall, the Mesozoic succession of Ethiopia provides a critical archive of the paleogeographic and paleoenvironmental evolution of the region, recording the interplay of marine transgressions, tectonic subsidence, and carbonate platform development during the Jurassic period (Jain \u0026amp; Schmerold, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eNumerous geological studies have been conducted on the Blue Nile Basin; however, research on the paleontological and environmental aspects of the carbonate units in the region remains limited (e.g., Russo et al., 1994; Dawit, 2010; Jain and Singh, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2019\u003c/span\u003ea,b; Radwańska and Jain, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Santos et al., 2021; Jain, and Schmerold, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Jain et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Jain and Schweigert, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Singh et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Salamon et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2022\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Notably, due to the inaccessibility between Gunde Weyn and Mekane Selam, the facies, paleontology, and depositional environment of the carbonate units in the Mertule Mariam section have not been explored. Furthermore, there has been no investigation into the correlation of this section with other geological formations across Ethiopia.\u003c/p\u003e\u003cp\u003eThis study builds on existing stratigraphic frameworks, particularly regarding the Mertule Mariam section, by presenting new findings that contribute to a more comprehensive regional stratigraphic synthesis. Through the integration of micropaleontological, petrographic, and lithostratigraphic analyses, we aim to refine depositional environment interpretations, establish detailed microfacies relationships, and enhance understanding of the lateral and vertical continuity of carbonate successions. Ultimately, this research seeks to provide new insights into the paleoenvironmental and paleogeographic evolution of the Mertule Mariam section and its role within Ethiopia\u0026rsquo;s Mesozoic depositional history.\u003c/p\u003e"},{"header":"2. Geological Setting","content":"\u003cp\u003eThe Blue Nile Basin, located on the Northwestern Ethiopian Plateau, is characterized by a Mesozoic succession that is 1200\u0026ndash;1400 meters thick, underlain by Neoproterozoic basement rocks and capped by Early-Late Oligocene and Quaternary volcanic rocks (Gani et al., 2009). The basin also contains rare Paleozoic sediments (Getaneh Assefa, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Merla, 1997). It is composed of five primary stratigraphic units: the Adigrat Sandstone, Gohatsion Formation, Antalo Limestone, Mugher Mudstone, and Debre Libanos Sandstone (Getaneh Assefa, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Russo et al., 1994; Chernet et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Jain and Singh, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2019\u003c/span\u003ea). The thickness of the limestone ranges from 200 to 600 meters (Russo et al., 1994; Assefa, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1991\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e1.1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe sedimentary history of the Mesozoic succession in the Blue Nile Basin is closely associated with the formation of rift basins along the edges of the supercontinent Gondwana, beginning in the Upper Paleozoic and continuing into the Tertiary Period (Mohr, 1962). The Jurassic transgression in Ethiopia is primarily represented by carbonates, including the Antalo Limestone, Hamanlei, and Urandab Formations, which conformably overlie the Adigrat Sandstone. These limestones, dating to the Pliensbachian/Aalenian, are first found in the Ogaden Basin (Beyth, 1971; Kazmin, 1973) (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e8.1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eTectonic evolutions in northeastern Africa, alongside fluctuations in sea level throughout geologic time, have played significant roles in the formation of these basins and the accumulation of thick Mesozoic sediments. In terms of structural evolution, most of these sedimentary basins are linked to extensional tectonic events that have occurred intermittently from the Late Paleozoic to the Tertiary Period. The Ogaden, Blue Nile, and Mekelle Outlier are considered intracontinental rift-related basins formed due to extensional stresses associated with the break-up of Gondwanaland, spanning from the Upper Paleozoic through to the Tertiary Period (Mohr, 1962; Blandford, 1970).\u003c/p\u003e\u003cp\u003eThe facies and depositional patterns observed in the Mertule Mariam section are comparable to carbonate units in other regions of Ethiopia and East Africa, providing opportunities for regional correlations with Mekele basin and Ogaden basin (see also Jain et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e7.1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe Mekelle Basin is one of the major Mesozoic sedimentary basins in Ethiopia, located in northern Ethiopia. It extends from the Amba-Alage Mountain in the south up to Wukro town in the north, Abi-Adi in the west, and reaches the western escarpment of the Ethiopian Rift Valley to the east (Alemu et al., 2018). The basin forms a nearly circular outlier covering approximately 8,000 km\u0026sup2; around Mekelle (Beyth, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1972a\u003c/span\u003e; Bosellini et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). Structurally, the Mekelle Outlier is interpreted as an intramontane basin that formed due to the uplift of two east-west trending structural highs located approximately between 13\u0026deg;N and 14\u0026deg;N latitudes, between the Wukro fault belt and the Precambrian basement rocks (Beyth, 1972b) and began in the Upper Paleozoic and continued through the Tertiary Period (Mohr, 1962; Blandford, 1970). Recent studies by Tadesse et al. (2018) have further refined the structural evolution of the Mekelle Outlier, describing it as a sag basin based on geological field data, remote sensing, and geophysical gravity analysis. The facies and depositional patterns observed in the Mertule Mariam section are comparable to carbonate units in other regions of Ethiopia and East Africa, providing opportunities for regional correlations (Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e7.1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe carbonate unit in the Blue Nile Basin, referred to as the 'Antalo Limestone Formation,' comprises up to 420 meters of shallow marine origin carbonates (Russo et al., 1994). It conformably overlies the Gohatsion Formation and can be subdivided into three sections: lower, middle, and upper limestone.\u003c/p\u003e\u003cp\u003eThe lower 180 m-thick section consists of burrowed mudstones, transitioning upwards into oolitic and coquinoid limestone, rich in corals, stromatoporoids, bivalves, gastropods, and benthic foraminifera, with occasional marl intercalations. This section represents a shallow water environment (Russo et al., 1994).\u003c/p\u003e\u003cp\u003eThe 200 m-thick middle section is characterized by highly fossiliferous, interbedded marly limestone and marl. The ammonites with brachiopods and bivalves suggest a shelf to open marine environment (Russo et al., 1994).\u003c/p\u003e\u003cp\u003eThe upper 50 m-thick section consists of planar laminated oolitic and reefal limestone, interpreted to indicate a return to shallow water conditions. The presence of oolitic bars, coral patches, and offshore to inshore facies suggests a similar shallow water environment to the lower section.\u003c/p\u003e\u003cp\u003eIn addition to tectonic controls on the formation of Ethiopian sedimentary basins, fluctuations in sea level driven by tectonic activity and climate variations have influenced the deposition of various sedimentary successions throughout geologic time. In fact, large-scale down warping of the East African continent during the Upper Triassic to Lower Jurassic resulted in widespread deposition of fluvio-deltaic Adigrat Sandstone, extending into western and northern Ethiopia (Dawit, 2010). Continued rifting and subsidence of the region, including adjacent areas of Saudi Arabia, Somalia and Yemen, led to significant marine transgression from the east and southeast (Jain, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2019\u003c/span\u003eb). This event flooded extensive regions and facilitated the deposition of the Jurassic marine carbonate units, which are the primary focus of this study (Dainelli, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1943\u003c/span\u003e). With the eventual arching and doming of the Arabian-Somalian massif in the Late Jurassic, marine regression commenced, leading to the deposition of varied facies, including restricted marine, lagoonal, and supratidal to intertidal deposits within structurally controlled settings. The sediments deposited following the retreat of the sea include the Agula Shale in the Mekelle Outlier, the Gebredare Formation in the Ogaden Basin, and the Muger Mudstone in the Blue Nile Basin (Ethiopian Ministry of Mines, 2011).\u003c/p\u003e"},{"header":"3. Methods","content":"\u003cp\u003eA detailed field description of the limestone unit was conducted in the Mertule Mariam Section which is found in the Blue Nile Basin. The thickness of the lithology in the studied section were measured and logged by taking consideration of each bed. Sedimentary structures, visible fossils and grains, bedding and overall stratification were described. Based on lithologic changes, 34 representative rock and marl samples were taken and snapped from bottom to top for further investigation and analysis. These 34 samples were prepared under thin section and petrographically examined to analyze the microfacies. The grain type (fossils, intraclasts, ooids, peloids and shell fragments) and proportion, fabrics grain shape, matrix and diagenetic feature under thin section were distinguished based on Dunham's (1962) classification scheme. The microfacies analysis was performed using Fl\u0026uuml;gel (2010) microfacies classification principle. The identified microfacies in the studied limestone section were related with the Wilson (1975) and Fl\u0026uuml;gel (2010) standard microfacies (SMF) approach. The depositional environments and facies belts were defined and recognized based on the studies of Wilson (1975) facies belts.\u003c/p\u003e"},{"header":"4. Results","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e4.1. Stratigraphy\u003c/h2\u003e\u003cp\u003eThe Mertule Mariam carbonate section is conformably overlain by a clastic unit of sandstone and mudrocks and underlain by gypsum layers, marking the base of the Gohatsion Formation. Field investigations identified nine distinct lithofacies within the Mertule Mariam carbonate section (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e).\u003c/p\u003e\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\u003ch2\u003e4.1.1. Gypsum\u003c/h2\u003e\u003cp\u003eThe gypsum layer exposed at the base of the carbonate in the Mertule Mariam section, indicates the start of the underling 2-meter thick Gohatsion Formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e), characterized by whitish to grayish color (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e5.1\u003c/span\u003e)\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section3\"\u003e\u003ch2\u003e4.1.2. Marl\u003c/h2\u003e\u003cp\u003eThis unit has variable thickness, extending from 2 to 127 meters (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e) intercalated with cliff forming micritic limestones. It shows a yellow, gray and light color in the field. The unit is mainly described by fine grained and loose sediments containing micro- and macro invertebrates like \u003cem\u003ePholadomya\u003c/em\u003e (\u003cem\u003eBucardiomya\u003c/em\u003e) \u003cem\u003esomaliness\u003c/em\u003e Sowerby and \u003cem\u003eNanogyra nana\u003c/em\u003e Sowrbey. This rock unit varies from place to place in terms of fossil content, consisting of high numbers of foraminifera and ostracods in some locality to a complete absence of fossils in other places (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5.2\u003c/span\u003e)\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\u003ch2\u003e4.1.3. Micritic limestone\u003c/h2\u003e\u003cp\u003eThis unit is also variable in thickness, from 127 to 192 meters (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e). It is dominated by well-compacted fine grained carbonate cement (90%). This micritic limestone unit occurs as a cliff intercalated with marls (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5.3\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\u003ch2\u003e4.1.4. Oolitic limestone\u003c/h2\u003e\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eThis unit is horizontally bedded with variable thickness, extending 192 to 217 meters (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e). It forms cliff faces and ranges in colour from yellow to gray (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5.4\u003c/span\u003e). Well-cemented ooid grains (85%) (allochems) are the main constituents.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003ch2\u003e4.1.5. Calcareous Shale\u003c/h2\u003e\u003cp\u003eThis is a 237 to 251 m-thick unit (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e). It also intercalates with micritic limestone and marl unit in its upper part. The unit shows fissility and has a grayish color. The 4 m-thick shale is exposed in a road cut. It superimposes the 20- m-thick unexposed area at the middle part of the studied section (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e5.5\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e4.1.6. Fossiliferous limestone\u003c/h2\u003e\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eThis is a 251 to 260 m-thick unit (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e) exposed in a road cut characterized by yellow to brown color. It is mainly layered intercalating with marls with shell fragments of brachiopods, crinoids and bivalves (80%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e5.6\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003e4.1.7. Bioturbated micritic limestone\u003c/h2\u003e\u003cp\u003eThe is a 260 to 320 m-thick cliff-forming, micrite-rich (75%) cliff forming horizontally bedded unit (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e). Thin fragments (10%) of brachiopod, bivalve and crinoid shells are highly cemented within the micrite. The unit is exposed in a road cut and is white to gray in color. At the upper part of the section the unit is intercalated with thin beds of shale, mud and marl. This intercalated bed thickness varies between 20 to 50 cm. Bioturbation is commonly observed, especially at the base of the unit (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e5.7\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e4.2. Microfacies Analysis\u003c/h2\u003e\u003cp\u003eIn carbonate rocks, texture is primarily influenced by the hydrodynamic conditions of the depositional environment, while the presence and abundance of bioclasts determine grain types and proportions (Fl\u0026uuml;gel, 2010). Based on these factors, the biogenic composition, textural properties, and lithological characteristics of the Mertule Mariam section (34 thin sections) has yielded eleven microfacies. In the present study, the classification schemes of Wilson (1975), Fl\u0026uuml;gel (1982, 2004), and Fl\u0026uuml;gel's ramp microfacies types (MFT; Fl\u0026uuml;gel, 2004) is followed. The characteristics of each microfacies type are discussed below.\u003c/p\u003e\u003cdiv id=\"Sec14\" class=\"Section3\"\u003e\u003ch2\u003e4.2.1. Bioclastic- Intraclastic packstone/grainstone (MFT 1)\u003c/h2\u003e\u003cp\u003eThis microfacies is mainly composed of intraclasts (75%) and bioclasts (23%) including fragments of echinoid plate, bivalve and foraminifera (miliolids) (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.B). The grains show various bioturbated, poorly sorted, rounded and irregular features. Each grain is cemented with a sparite cement (2%) while the surface of each grain is dominantly filled by micrite. Some intraclasts and bioclasts micritization (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.A\u0026amp;B).\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eInterpretation\u003c/strong\u003e\u003cp\u003eIntraclastic grainstones are interpreted as having been deposited by storm-wave erosion and reworking of various sediment types in shallow-marine environments (Fl\u0026uuml;gel, 2004). The presence of intraclasts and the grain-supported fabric indicates high energy condition. Thus, this facies was deposited in high energy shoal environment with open circulation in the presence of benthic foraminifera and other bivalve shell fragments. The Bioclastic-Intraclastic packstone/grainstone can be compared with SMF Type 10 of Wilson (1975), corresponding to FZ 7.\u003c/p\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\u003ch2\u003e4.2.2. Peloidal grainstone (MFT 2)\u003c/h2\u003e\u003cp\u003eThis microfacies is characterized by a high abundance of micritized small sized peloids (90%), and minor ooids (5%) (Table.1.1). Few bioclasts of miliolid group foraminifera and intraclasts are also observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.C). The peloid allochems are medium to well-rounded and well-sorted. Most of the grains show a micrite cement characteristics.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eInterpretation\u003c/strong\u003e\u003cp\u003eThe high degree of sorting and rounding suggests constant, high-energy conditions above fair-weather wave base, within the upper shoreface environment. This microfacies is compared with SMF Type 15 of Wilson (1975), and correspond to FZ 6, i.e., deposited in moderate circulation conditions with winnowing by wave currents.\u003c/p\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section3\"\u003e\u003ch2\u003e4.2.3. Oolitic packstone/grainstone (MFT 3)\u003c/h2\u003e\u003cp\u003eThe allochems are well-rounded and moderately to well sorted concentric ooid grains (95%) and bioclast (3%) of bivalve, echinoid and foraminifera (dominantly miliolids) (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.D\u0026amp;E) (Table.1.1). Most ooids typically consists of a nucleus (often a small grain or fragment) surrounded by concentric layers of calcium carbonate or other minerals. The ooid and bioclastic grains are cemented by coarse sparite cement. The outer surfaces of some ooids are micritized (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.D) and their nuclei consists of miliolids (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.E).\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eInterpretation\u003c/strong\u003e\u003cp\u003eThe presence of fossils within ooids indicates short-term stable conditions during ooids growth (Flugel 2004). The bioclasts and thin-rim ooids with sparry calcite indicate comparatively high-energy conditions in a platform margin shoal (Fl\u0026uuml;gel, 2010). Ooids often form through the accumulation of carbonate minerals during wave action, indicating dynamic sedimentary processes. This microfacies is similar with SMF 15 and FZ 6 of Fl\u0026uuml;gel (2010).\u003c/p\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section3\"\u003e\u003ch2\u003e4.2.4. Dolomitized packstone (MFT 4)\u003c/h2\u003e\u003cp\u003eThe microfacies is largely dominated by light gray micrite supported dolomite crystals (65%) (Table.1.1). The scattered dolomite crystal cemented by micrite cement consists of brownish to yellowish rhombic shape (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.F). Some parts of the feature show black to deep black color.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eInterpretation\u003c/strong\u003e\u003cp\u003eAccording to Amthor and Friedman (\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1991\u003c/span\u003e) the very fine dolomite crystal occurs restricted to the peritidal settings. The fine crystal size represents a result of early dolomitization precursor of peritidal lime mudstone or pencontemporaneous neomorphism or an early diagenetic dolomite (Zenger, 1983; Amthor \u0026amp; Friedman, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1991\u003c/span\u003e). However, dolomite does not indicate a depositional facies, but a diagenetic facies. Due to presence of the fine grain size, quartz grains, fenestral fabric, lack of fauna, and vertical changes suggest that deposition occurred in a low-energy, restricted intertidal and supratidal environment (Wilson and Evans, 2002). The brown coloration may be due to the presence of iron oxides (such as hematite or limonite) that yield reddish or brownish hues to the rock. Therefore, this microfacies is deposited in shallow subtidal to lower intertidal environments, corresponding to SMF-23 of Wilson (1975) and Flugel (2010) of facies belt 8.\u003c/p\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section3\"\u003e\u003ch2\u003e4.2.5. Coral Framestone Microfacies (MFT5)\u003c/h2\u003e\u003cp\u003eThe microfacies dominantly contains a matrix-bounded coralline foraminifera and bivalves (40%); it also contains algae (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.G). The microfacies is dominated by corals and micrite (60%) (Table.1.1).\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eInterpretation\u003c/strong\u003e\u003cp\u003eThe coral framestone microfacies is predominantly formed from coral fragments and various biogenic materials typically in a warm, shallow marine environment where coral reefs thrive along the platform margin. Often associated with high-energy reef settings, coral framestone indicates the dynamic conditions favorable for coral growth. This facies is comparable to SMF 7 in FZ 5 and RMF 12 (Wilson 1975; Flugel 2004).\u003c/p\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec19\" class=\"Section3\"\u003e\u003ch2\u003e4.2.6. Bioclastic packstone/grainstone (MFT 6)\u003c/h2\u003e\u003cp\u003eThis microfacies is composed of large bivalve shells (80%) and echinoid plate allochems cemented by micrite cement (20%) (Table.1.1). Most of them have been micritized marginally to completely (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.H). The outer surface of shells are slightly affected by fine-grained to moderately coarse-grained calcite (sparite) cement (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.H).\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eInterpretation\u003c/strong\u003e\u003cp\u003eThese deposits show evidence of intense reworking with a predominantly open marine fauna suggesting a marine environment with high hydrodynamic energy. The microfacies represent bioclastic shoals on the platform margin or on highs in the platform interior corresponding to SMF 12/FZ 6 of Flugel (2004). The coarse bioclasts indicate comparatively high-energy conditions of a shoal at the platform margin (Fl\u0026uuml;gel, 2010).\u003c/p\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec20\" class=\"Section3\"\u003e\u003ch2\u003e4.2.7. Bioclastic mudstone (MFT 7)\u003c/h2\u003e\u003cp\u003eThe main distinctive feature of this facies is the abundance of sponge spicules (20%) within a micrite-dominated matrix (80%) with some gastropod shell, brachiopod, echinoid plate, bivalve, and miliolid bioclasts (Table.1.1). Diagenetic stylolite structures and fracture are also observed. In addition to them, most bioclasts are affected by fine crystalline calcite cementation (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6.1\u003c/span\u003e.I).\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eInterpretation\u003c/strong\u003e\u003cp\u003eThe presence of abundant sponge spicules and miliolid foraminifera within a homogenous micrite and overlain by skeletal sandy limestone and underlain by coral framestone suggest that the bioclastic spiculite wackstone was deposited in the proximal open marine environment below fair-weather wave base with normal circulation. This microfacies corresponds with FZ3 facies zone of Flugel (1982) and Wilson (1975), suggesting proximal open marine settings with open circulation. This microfacies is comparable to SMF 8 of Fl\u0026uuml;gel (2010).\u003c/p\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"5. Discussion","content":"\u003cdiv id=\"Sec22\" class=\"Section2\"\u003e\u003ch2\u003e5.1. Facies association and depositional environment\u003c/h2\u003e\u003cdiv id=\"Sec23\" class=\"Section3\"\u003e\u003ch2\u003e5.1.1. Facies Association 1: High-energy shallow marine environment\u003c/h2\u003e\u003cp\u003eThis facies association includes three major microfacies types; bioclastic-intraclastic packstone/grainstone (MFT 1), Peloidal grainstone (MFT 2) and oolitic packstone/grainstone (MFT 3). The bioclastic-intraclastic packstone/grainstone (MFT 1) is a high-energy shoal with storm-wave reworking containing intraclasts and bioclasts (echinoid plates, bivalves, and foraminifera). The Peloidal grainstone (MFT 2) suggests high-energy settings with high degree of sorting and containing rounding peloid grains suggesting constant conditions above fair-weather wave base, in the upper shoreface environment. The oolitic packstone/grainstone (MFT 3) is a shoal environment with ooid grains suggesting dynamic processes and stable conditions for ooid growth. The depositional environments for this facies association is characterized by high energy often influenced by storm events, wave action and strong hydrodynamic conditions. These environments are typically found in shoals, platform margins or reef areas.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec24\" class=\"Section3\"\u003e\u003ch2\u003e5.1.2. Facies Association 2: Shallow Marine to Reef Environments\u003c/h2\u003e\u003cp\u003eThe shallow marine to reef environment facies association consists of two microfacies: Coral Framestone (MFT 5) and Bioclastic Packstone/Grainstone (MFT 6). The first microfacies type is characterized by the high-energy reef environment dominated by coral fragments, marine organisms and biogenic materials, whereas the second type is marked by intense reworking of large bivalve shells and echinoid plates. The shallow marine to reef environments facies association includes the environments associated with reef growth, corals, and other biogenic materials in warm and shallow marine settings.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec25\" class=\"Section3\"\u003e\u003ch2\u003e5.1.3. Facies Association 3: Low-Energy, Restricted Marine Environments\u003c/h2\u003e\u003cp\u003eThe low-energy, restricted marine environment facies association includes Dolomitized Packstone (MFT 4) and Bioclastic Mudstone (MFT 7). The Dolomitized Packstone (MFT 4) is mainly a low-energy, restricted intertidal to supratidal environment microfacies influenced by early dolomitization. The Bioclastic Mudstone (MFT 7) is a low-energy, open marine environment microfacies found below the fair-weather wave base with a micrite-dominated matrix and abundant sponge spicules. This facies association includes environments characterized by lower energy, such as intertidal zones, restricted marine settings or areas where diagenetic processes such as dolomitization dominate.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec26\" class=\"Section2\"\u003e\u003ch2\u003e5.2. Depositional environment of the Mertule Mariam section\u003c/h2\u003e\u003cp\u003eThe studied section presents a diverse sequence of carbonate and siliciclastic deposits that reflect dynamic depositional environments ranging from tidal flats and lagoons to high-energy shoals and open marine systems. This diversity is evident in the lithological variations, sedimentary structures, and fossil assemblages observed throughout the section.\u003c/p\u003e\u003cp\u003eThe ~\u0026thinsp;60 m-thick upper part of the section is dominated by micritic limestones with intercalations of marls and dolomites. Thin section analysis identifies this limestone as micrite or mudstone (Dunham, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1962\u003c/span\u003e), with bioturbation and marked by a gradual decrease in fossil content upward, indicating deposition in a low-energy, restricted marine environment. The presence of unfossiliferous mudstone facies with high micrite content (98\u0026ndash;99%) suggests sedimentation in intertidal flats, where limited energy conditions restricted biogenic activity. The alternating marl and dolomite layers reflect episodic shifts in salinity and energy, possibly tied to tidal influences or short-term climatic variations.\u003c/p\u003e\u003cp\u003eIn contrast, the middle part of the section is characterized by marls interbedded with shales, fossiliferous limestones, dolomitized and variegated mudstones, reflecting deposition in a more dynamic, lagoonal to shallow marine environment. The peloidal-bioclastic wackestone/packstone facies in this interval, with peloids and bivalve bioclasts, supports deposition in a shallow lagoon setting, where restricted circulation and moderate energy prevailed. The presence of benthic foraminifera within marls further suggests low-energy lagoonal environment with occasional influx of higher-energy conditions, as evidenced by storm-induced winnowing and sediment reworking.\u003c/p\u003e\u003cp\u003eThe lower part of the section transitions into a higher-energy depositional setting, as indicated by the presence of oolitic and bioclastic grainstone facies. The 125 m-thick marl unit intercalated with a 2-m thick oolitic limestone layer reflects deposition in high-energy shoals near platform margin. The abundance of ooids, peloids, and bioclasts in these facies is consistent with wave-dominated environments above fair-weather wave base, where constant agitation promoted grain rounding and sorting. Coral framestone facies, dominated by reef-building corals, further supports the interpretation of reefal shoals within high-energy zones with open marine circulation.\u003c/p\u003e\u003cp\u003eAt the ba\u003cb\u003es\u003c/b\u003ee of the section, the 125 m-thick marl layer with abundant brachiopods and foraminifera transitions into a 65 m-thick micritic limestone unit. This interval reflects an open marine environment below fair-weather wave base, with low-energy conditions favoring the preservation of fine-grained micrite and diverse skeletal fragments. Bioclastic mudstone and wackestone facies in this interval indicate a well-circulated, low-energy setting, consistent with a proximal open marine environment. Sponge spicules and skeletal fragments in these facies provide additional evidence of normal marine conditions with moderate nutrient availability.\u003c/p\u003e\u003cp\u003eThe depositional sequence as a whole shows an upward-deepening trend, transitioning from high-energy shoals and lagoons to low-energy open marine environments. This reflects a transgressive-regressive cycle, where sea-level fluctuations influencing facies distribution and depositional patterns.\u003c/p\u003e\u003c/div\u003e"},{"header":"6. Conclusions","content":"\u003cp\u003eThis study provides the first detailed analysis of the carbonate units exposed at the Mertule Mariam section (Blue Nile Basin), identifying seven distinct microfacies types grouped into four facies associations: tidal flat, lagoon, high-energy shoals, and open marine environments. These facies reflect a diverse depositional system within a shallow marine environment, ranging from shallow inner ramp of near-coastal tidal settings to restricted lagoonal systems, high-energy shoals, and proximal mid- to outer-ramp open marine settings.\u003c/p\u003e\u003cp\u003eField observations combined with petrographic analysis suggests that the carbonate units were deposited under dynamic environmental conditions influenced by variations in energy, circulation, and sediment supply. The observed facies transitions capture the interplay between regressive and transgressive cycles, providing valuable insights into the stratigraphy and paleoenvironment of the Jurassic carbonate platform in the region.\u003c/p\u003e\u003cp\u003eThe carbonate units are lithostratigraphically and biostratigraphically correlative with previously studied units in the Blue Nile Basin, including the Antalo Limestone Formation. Regionally, these deposits are equivalent to carbonate units in the Ogaden Basin and Mekele Basin, indicating a widespread depositional system linked to the Jurassic marine transgression across East Africa.\u003c/p\u003e\u003cp\u003eTo enhance the understanding of the stratigraphy and paleoenvironment of the Mertule Mariam section, more detailed studies in micropaleontology and biostratigraphy are needed, allowing for more precise age constraints, paleoecological interpretations, and refined correlations within the basin and across the region. Such efforts will further contribute to reconstructing the paleogeographic and tectonic evolution of the East African margin during the Jurassic.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eall 4 authors contribute for the research and manuscript preparation\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAdams, A.E., \u0026amp; Mackenzie, W.S. 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Classification of carbonate rocks according to their depositional texture. In W.D. Ham (Ed.), Proceedings of Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir, Tulsa, 1, 108\u0026ndash;121.\u003c/li\u003e\n \u003cli\u003eEmbry, A.F. III, \u0026amp; Klovan, J.S. (1971). A Late Devonian reef tract on northeastern Banks Island, N.W.T. Bulletin of Canadian Petroleum Geology, 4, 730\u0026ndash;781.\u003c/li\u003e\n \u003cli\u003eEmig, C.C., Bitner, M.A., \u0026amp; Alvarez, F. (2013). Phylum Brachiopoda. Zootaxa, 3703(1), 75\u0026ndash;78.\u003c/li\u003e\n \u003cli\u003eJain, S. (2019a). First Bathonian (Middle Jurassic) nautiloid \u003cem\u003eParacenoceras\u003c/em\u003e Spath from Ethiopia. Journal of African Earth Sciences, 149, 82\u0026ndash;96.\u003c/li\u003e\n \u003cli\u003eJain, S. (2019b). Middle Bathonian Indonesian \u003cem\u003eMacrocephalites\u003c/em\u003e cf. \u003cem\u003eetheridgei\u003c/em\u003e (Spath) from SW Somalia. 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Discovery of the Middle Callovian ammonite \u003cem\u003eErymnoceras\u003c/em\u003e in the Blue Nile Basin (Ethiopia). Neues Jahrbuch f\u0026uuml;r Geologie und Pal\u0026auml;ontologie \u0026ndash; Abhandlungen, 297/1, 27\u0026ndash;35.\u003c/li\u003e\n \u003cli\u003eJain, S., \u0026amp; Schweigert, G. (2022). On the dimorphic occurrence of the upper Tithonian ammonite genus \u003cem\u003eDjurjuriceras\u003c/em\u003e Roman from the Blue Nile Basin (Ethiopia). Pal\u0026auml;ontologische Zeitschrift, 96, 655\u0026ndash;668. DOI.org/10.1007/s12542‐022‐00620‐y.\u003c/li\u003e\n \u003cli\u003eJain, S., \u0026amp; Singh, A. (2019). First calcareous nannofossil record from the Jurassic strata exposed in the Blue Nile Basin (Ethiopia). Journal of African Earth Sciences. 158, Article 103553, doi.org/10.1016/j.jafrearsci.2019.103553.\u003c/li\u003e\n \u003cli\u003eRadwańska, U., \u0026amp; Jain, S. (2020). First Late Jurassic echinoid record of Pygurus meslei Gauthier from the Antalo Limestone Formation (Blue Nile Basin, Ethiopia). Journal of African Earth Sciences, 170, 103898. doi.org/10.1016/j.jafrearsci.2020.103898.\u003c/li\u003e\n \u003cli\u003eSalamon, M. A., Benyoucef, M., Jain, S., Benzaggagh, M., Płachno, B. J., Abdelhamid, M.A.M., Ahmad, F., Azar, D., Bouchemla, I., Brachaniec, T., El Ouali, M., El Qot, G., Ferr\u0026eacute;, B., Gorzelak, P., Krajewski, M., Klompmaker, A. A., Mekki, F., Poatskievick‐Pierezan, B., \u0026amp; Slami, R. (2024). Jurassic and Cretaceous crinoids (Crinoidea, Echinodermata) from the southern shelf of Tethys (northern and eastern Africa, Middle East, Asia, and India). Palaeontographica, Abteilung A: Palaeozoology \u0026ndash; Stratigraphy, 328(1\u0026ndash;6), 1\u0026ndash;991‐99. https://doi.org/10.1007/s12371‐024‐00973‐7\u003c/li\u003e\n \u003cli\u003eSalamon, M. A., Jain, S., Brachaniec, T., Duda, P., Płachno, B. J., \u0026amp; Gorzelak, P. (2022). \u003cem\u003eAusichicrinites zelenskyyi\u0026nbsp;\u003c/em\u003egen. et sp. nov., a first nearly complete feather star (Crinoidea) from the Upper Jurassic of Africa Royal Society Open Science 9(7), doi.org/10.1098/rsos.220345.\u003c/li\u003e\n \u003cli\u003eSantos, A., Jain, S., \u0026amp; Diez, J. B. (2022). Upper Jurassic palynology from the Blue Nile Basin (Ethiopia). Review of Palaeobotany and Palynology. Review of Palaeobotany and Palynology 285, 104361. doi.org/10.1016/j.revpalbo.2020.104361.\u003c/li\u003e\n \u003cli\u003eSingh, A., Jain, S., Benzaggagh, M., Schweigert, G., Salamon, M. A., \u0026amp; Mulugeta, M. (2022). Late Tithonian nannofossils from Dejen area, the Blue Nile Basin, central western Ethiopia. Paleoworld. doi.org/10.1016/j.palwor.2022.10.003.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Table","content":"\u003cp\u003e\u003cstrong\u003eTable 1.1:\u003c/strong\u003e Microfacies type codes, microfacies names, thin section photograph, main components and depositional environment of each microfacies type.\u003c/p\u003e\n\u003ctable style=\"float: ;border-collapse: collapse;border: none;margin-right: 6.75pt;width: 697px;\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" style=\"width:54.95pt;border:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eMicrofacies type code\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" style=\"width:64.5pt;border:solid windowtext 1.0pt;border-left:none;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eMicrofacies name \u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" style=\"width:57.95pt;border:solid windowtext 1.0pt;border-left:none;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eThin section photograph\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"4\" style=\"width:197.9pt;border:solid windowtext 1.0pt;border-left:none;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Carbonate grains\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" style=\"width:102.0pt;border:solid windowtext 1.0pt;border-left:none;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eCement\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" style=\"width:45.6pt;border:solid windowtext 1.0pt;border-left:none;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eDepositional environment\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width:45.65pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003ePeloid %\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:35.35pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eOoid%\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:58.2pt;border-top:none;border-left:none;border-bottom: solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eBioclast%\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 58.7pt;border-top: none;border-left: none;border-bottom: 1pt solid windowtext;border-right: 1pt solid windowtext;padding: 0in 5.4pt;vertical-align: top;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eintraclast\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:58.7pt;border-top:none;border-left:none;border-bottom: solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eMicrite%\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:43.3pt;border-top:none;border-left:none;border-bottom: solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eSparit%\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width:54.95pt;border:solid windowtext 1.0pt;border-top: none;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eMFT1\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:64.5pt;border-top:none;border-left:none;border-bottom: solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eBioclastic- Intraclastic packstone/grainstone\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:57.95pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;color:#00B0F0;'\u003eFig.6.1 A\u0026amp;B\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:45.65pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003e0%\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:35.35pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003e0%\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:58.2pt;border-top:none;border-left:none;border-bottom: solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003e23%\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 58.7pt;border-top: none;border-left: none;border-bottom: 1pt solid windowtext;border-right: 1pt solid windowtext;padding: 0in 5.4pt;vertical-align: top;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003e75%\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:58.7pt;border-top:none;border-left:none;border-bottom: solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp 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style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eShoal\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width:54.95pt;border:solid windowtext 1.0pt;border-top: none;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eMFT2\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:64.5pt;border-top:none;border-left:none;border-bottom: solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003ePeloidal grainstone\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:57.95pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;color:#00B0F0;'\u003eFig.6.1C\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:45.65pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n 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style=\"width:45.6pt;border-top:none;border-left:none;border-bottom: solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:12px;line-height:115%;font-family:\"Times New Roman\",serif;'\u003eOpen marine\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Blue Nile Basin, microfacies, carbonates, paleoenvironment, Ethiopia, Jurassic","lastPublishedDoi":"10.21203/rs.3.rs-6963278/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6963278/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study presents a detailed analysis of the facies and paleoenvironment of the 330 m-thick carbonate unit in the Mertule Mariam section, Ethiopia. Fifty-five (55) samples were selected for lithostragraihical and petrographic analysis from the carbonate unit. Lithostratigraphically, the unit is characterized by marls, calcareous shales, micritic, oolitic, fossiliferous and bioturbated micritic limestone. On the basis of petrographic investigations, seven (7) microfacies types are identified: Bioclastic-Intraclastic packstone/grainstone, Peloidal grainstone, Oolitic packstone / grainstone. Dolomitized packstone, Coral Framestone, Bioclastic packstone / grainstone and Bioclastic mudstone. In order to reflect dispositional energy conditions, the microfacies were grouped into three microfacies associations: (i) High-energy shallow marine environment (MFT1, MFT2 and MFT 3) (ii) Shallow marine to Reef environment associations (MFT5 and MFT6), and (iii) Low-energy, restricted marine environment associations (MFT4 and MFT7).\u003c/p\u003e","manuscriptTitle":"Micro facies analysis and depositional environments of the carbonate unit in the Blue Nile Basin, Central Western Ethiopia","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-14 11:58:04","doi":"10.21203/rs.3.rs-6963278/v1","editorialEvents":[{"type":"communityComments","content":1}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f82aaab4-7517-43e3-8d9f-77bfba90f0e3","owner":[],"postedDate":"July 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-10-14T14:39:06+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-14 11:58:04","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6963278","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6963278","identity":"rs-6963278","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}