The Abiod at Ellès (Tunisia): stratigraphies, Campanian-Maastrichtian boundary, correlation

Abstract

In central Tunisia near the village of Elles is Wadi Ed Dam. There the Abiod Formation, underlain by the Kef marls and capped by the El Haria marls, crops out in an almost uninterrupted exposure about 286 m thick. In view of its possible use as a regional litho- and bio- stratigraphic reference section for the southern border of the Tunisian trough it is examined in detail here. Fossils studied include foraminifera (115 samples representing 6 micropaleontologic zones, see chart) and ammonites. The biohorizons established are based on the first (FO) or last occurrence (LO) of certain species of planktonic foraminifers in addition to the FO of already established foraminiferal zonal markers. Ammonite markers found include both the FO and LO of some forms, or just one or the other. The location of the Campanian-Maastrichtian (Cp-Ma) boundary time line at Elles is established by comparison with that determined at Kalaat-Senan and with that of the international stratotype of Tercis (France), ratified by the International Union of Geological Sciences. At Kalaat-Senan the Cp-Ma boundary is set at the lower third of the upper indurated limestone bar of the Abiod Formation - the Ncham Member. Its location at Kalaat-Senan is based on the upper limit of occurrence of Nostoceras (Nostoceras) hyatti and Pseudokossmaticeras brandti together with the FO of Nostoceras magdadiae, a Maastrichtian ammonite. At Elles several of these same markers are present, along with the respective planktonic foraminiferal zones. The length in years of some lithologic units and paleontologic zones in the Upper Cretaceous succession of wadi Ed Dam was calculated using cyclostratigraphy. The Abiod Formation represents 13.2 Ma, the Radotruncana calcarata zone 0.79 Ma, and the Nostoceras (Bostrychoceras) polyplocum zone 0.97 Ma. A proposed subdivision of the Upper Campanian is recommended at the base of the Bostrychoceras zone. Li et alii's 1999 proposal for fixing the location of the Cp-Ma boundary at the top of a limestone bed in the El Haria marls is reviewed.