Hot shale in an ice world : paleoceanographic evolution of the northern Godwana margin during the Early paleozoic (Tanezzuft Formation, Tunisia)

The Tanezzuft Formation deposited in marine periglacial conditions on the northern Gondwana margin during the end of the "ice-house" climate that characterized the lowermost Silurian. The basal part of this sedimentary sequence is characterized by organic-rich facies with locally very high measured Total Organic Carbon (TOC) content up to values greater than 20%. While deposition of organic-rich sediments during greenhouse time interval is well known, deposition of black shales during ice-house conditions is poorly documented. The extraordinary paleoceanographic conditions that led to the accumulation and preservation of enormous amounts of organic matter in periglacial settings, makes this formation an atypical example of black shales deposition.The study area is situated on the North African Platform in southern Tunisia, on the northern flank of the Ghadames Basin. Petrophysical logs, biostratigraphic, organic- and inorganic-geochemical data from nine wells, are here integrated with the aim of reconstructing the depositional history of the Tanezzuft Formation and the role played by organic matter production, preservation and dilution through time. Data indicate that dilution - that is, depositional style and framework - had an important control on lateral and vertical distribution of the C organic-rich facies. TOC and Hydrogen Index (HI) maxima are found in correspondence of the main transgressive phases, with repetitive stacking patterns strongly associated with source rock properties. Organic matter production, as observed by detailed palynofacies analysis, was mainly marine in nature, with important contribution by Amorphous Organic Matter, Leiosphaeridia/Tasmanaceae and minor amounts of graptolites fragments. Geochemical data indicate that the organic-richest interval ("Hot Shale") of the Tanezzuft Formation deposited under severe anoxic conditions that resulted in enhanced organic matter preservation at the sea-floor. OF-Mod 3D, an organic facies modelling software tool (by SINTEF), is used in order to reconstruct and quantify the peculiar processes that controlled the exceptional accumulation of organic matter at the time of deposition. Modelling results indicate that water stratification was the most plausible process that drove organic-matter sedimentation under mesotrophic conditions. Density stratification was probably controlled by the deglaciation phase after the Hirnantian glaciation and/or by the flooded complex basin physiography that induced an inefficient circulation of the water masses. The complete recovery from these extreme paleoceanographic conditions implied progressive processes that took several hundred thousand years.