Depositional morphotypes and characterization of silica diagenesis in Ypresian-Priabonian gypsiferous carbonates, Gafsa, Southern Tunisia

Silica diagenetic features have been recognized in the Ypresian – Priabonian gypsiferous carbonates of Kef Edour Member in Gafsa basin, southern Tunisia. These rocks host a broad range of nodular features including anhydrite / gypsum, nodules and silica geodes. Three types of silica geodes have been recognized based on colour, form, chemical and mineralogical composition. A zoned texture characterizes most of geodes, with (1) a rim, 3­4 cm thick, composed of mixed microquartz and (2) length-slow chalcedony and (3) euhedral Megaquartz lining the inner cavity of the nodule. XRD analysis reveals that the quartz is the only silica phase present in most samples. The other silica forms (opaline group = opal-A, opal-C, opal-CT) are undetectable with XRD. FTIR and 29 Si MAS NMR analyses confirm the presence of diagenetic hydroxylated silica forms (silanol sites with two bridging oxygen sites Q 2 (–(O) 2 Si(OH) 2 ) interpreted as amorphous opaline precursors of the quartz. Other non-siliceous groups have been detected, such as SO 4 (sulphate evaporites) CO 3 (calcium and magnesium carbonates), and confirmed by petrographic observations. In Mg-rich and gypsiferous host sediments, silica diagenesis tends to proceed discontinuously. During early silicification, pore fluid chemistry (pH < 9) tolerates silica replacements and the development of the microcrystalline and fibrous form of quartz, whereas this silica replacement is slowed down or curtailed later in the innermost parts of anhydrite nodules due pore fluid chemistry (pH values > 9) allowing the development of euhedral silica form by force of crystallization supported by its crystallinity degree and its low specific surface area and ultramicroporosity that make it the most mineralogically competent and the most chemically resistant in such environments. Silica supply could be derived ultimately from (1) minor volcanogenic/hydrothermal venting contributions during early diagenetic stages, and (2) diagenesis of the extraformational and overlying detrital quartz and clay-rich sediments.