Clay mineralogy and chemistry of fi ne-grained sediments.Environment analysis around the K/P boundary at Sekarna/Kasserine Island, Tunisia

摘要

The Cretaceous-Paleogene fully marine stratigraphic section in the El Kef region, Tunisia was offi cially designated as the global stratotype of the K/P boundary. This marine basin is terminated towards the south by the Kasserine Island, Tunisia, where the K/P boundary is located within a marly non-fossiliferous zone (NF zone) abundant in phosphorites between lower Eocene phosphate-bearing limestones and Maastrichtian to Campanian limestones. The fi ne-grained grain size fractions of these sediments (< 2μm, 2 – 63 μm) are indicative of air-borne component within the late Cretaceous through Paleogene sedimentary series and are used for mineralogical and chemical analyses (major, trace elements, Au, PGE). These studies led to a tripartite chemicalmineralogical subdivision of the Cretaceous-Paleogene series and can assist in pinpointing the processes conducive to the large mass extinction during this part of the Earth’s history. The late Cretaceous “alkaline world” lasted beyond the K/P boundary and overlaps with the “noble world”. Illite-smectite mixed-layers are the fi rst-order marker minerals resulting from the alteration of a volcanic ash precursor of basic (to intermediate) composition. Ferroan dolomite is a second-order marker mineral brought about by the peculiar conditions of this calcareous environment of deposition. The “acidic world” during the Paleogene is characterized by the phyllosilicate kaolinite and by goyazite attesting to low pH, probably caused by some kind of “acid rain”. Harsh chemical conditions during the “noble world” (non-fossiliferous zone) provoked inhospitable living conditions. There is a sharp onset marked by elevated base metal contents and mineralogically by marls enriched in pure zincian spinel of non-volcanic origin. The chemical variation of Au and PGE are at variance with an idea of a single event during this period of time and point to multistage processes and/ or chemical and mineralogical differentiation.