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

Abstract

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.