Carbon-isotope and microfaunal stratigraphy bounding the Lower Aptian Oceanic Anoxic Event 1a in northeastern Tunisia

Abstract

Using an integrated multidisciplinary approach the upper part of the Lower Cretaceous northeastern Tunisian Jebel Ammar sedimentary succession was examined in detail. The method applied included lithologic and microfacies analyses, micropalaeontology, sedimentology, variations in organic matter (OM) content and carbonate carbon stable isotope (δ13C) record. A major result was the identification in this sector of Tunisia of the Early Aptian (Bedoulian) OAE1a event in a biostratigraphically well-calibrated context, its location keyed to planktonic foraminiferal zones and isotopic stages.The most striking feature to the Jebel Ammar Aptian sequence is the presence of a 25 m interval of black limestones and marly limestones, of which the microfacies shows that these darker beds consist of wackestones with the presence of abundant radiolarians, a fair number of diversified planktonic and relatively rare benthic foraminifers, together an indication of a pelagic palaeoenvironment. The foraminiferal marker Schackoina (Leupoldina) cabri (still very rare at the beginning of its range) first occurs about six metres above the base of this interval, but becomes much more abundant in its uppermost part, together with the radially- elongate-chambered praehedbergellids. This “acme” of S. (L.) cabri is nearly contemporary with a radiolarian bloom. Rock Eval analyses show TOC values up to 4.59% and a Tmax ranging between 441 and 513 °C, which indicates an overmature OM. The δ13C isotope curve shows an evolution similar to those recognized worldwide. The lower part of the darker beds includes a marked shift in isotope values from −2.40 to +3.02 ‰/PDB. This increase is assumed to be equivalent to the isotopic C4 stage of Menegatti et al. (1998, Paleoceanography, 13, 530–545). The signature of the middle and upper part of the isotopic curve is tentatively interpreted as comprising the C5–C7 stages. The first occurrence of S. (L.) cabri is located in the upper part of C4 and its acme as well as the radiolarian bloom is situated within the span of the C7 stage. These results are fairly consistent, though with minor discrepancies, with what has been published from several parts of the North Tethyan margin and more particularly the Lower Aptian type area of southeastern France, where S. (L.) cabri first occurs at the C3/C4 stage boundary with an acme that begins in the lower part of C7.