Biotic turnover in benthic foraminifera across the cretaceous/tertiary boundary at El Kef, Tunisia

The Cretaceous/Tertiary boundary section exposed near El Kef, Tunisia is the most complete boundary sequence known to date. It contains nearly 1 m of black and gray boundary clay containing planktic foraminiferal Zone POa, b, 2 m of clayey shales of P1a (Globigerina eugubina) Zone and 4 m of shales and marls of P1b (G. taurica) Zone. Quantitative analysis of the benthic foraminiferal fauna suggests that deposition during the latest Cretaceous occurred in an upper slope to outer shelf environment which shallowed at the K/T boundary to an outer to middle shelf depth and shallowed further by P1b time to mid-shelf depth. A major reduction in benthic diversity occurred near the K/T boundary with about 50% of the fauna disappearing. Diversity remained an average of 37% lower during deposition of the first 3 m of sediment above the boundary (POa, b-P1a) and productivity was very low. Surviving and thriving foraminifers during this interval were primarily low oxygen tolerant epifaunal and infaunal species. A sharp decrease in the low oxygen tolerant fauna and appearance of a shallow mid-shelf fauna at about 4 m above the boundary (P1b Zone) signals a second regression, return to higher oxygen levels and higher productivity. Although the environmental effects of the K/T boundary event can be inferred from benthic faunas, the ultimate cause remains elusive. Faunal changes prior to and the long recovery period after the K/T boundary are difficult to explain by a single impact hypothesis.