THE CRETACEOUS-TERTIARY TRANSITION ON THE SHALLOW SAHARAN PLATFORM OFSOUTHERN TUNISIA

A multidisciplinary approach to the study of a K/T boundary section on the Saharan Platform based on planktic and benthic foraminifera, calc areous nannofossils, lithology, stable isotopes, mineralogy and geoche mistry reveals a biota stressed by fluctuating hyposaline, hypoxic lit toral and nearshore environments, productivity changes, and a paleocli mate altering between seasonal warm to temperate and warm/humid condit ions. Benthic foraminifera indicate that during the last 300 kyr of th e Maastrichtian (CF1, Micula prinsii) deposition occurred in a inner n eritic (littoral) environment that shallowed to a near-shore hyposalin e and hypoxic environment during the last 100-200 kyr of the Maastrich tian. These conditions were accompanied by a seasonal warm to temperat e climate that changed to warm/humid conditions with high rainfall, by decreasing surface productivity, and significantly decreasing plankti c and benthic foraminiferal species richness. The K/T boundary is mark ed by an undulating erosional contact overlain by a 10 cm thick sandst one layer which is devoid of any exotic minerals or spherules. Their a bsence may be due to a short hiatus and the fact that the characterist ic clay and red layer (zone PO) are missing. During the earliest Dania n (Pla), low sea-levels prevailed with continued low oxygen, low salin ity, high rainfall, high erosion and terrigenous sediment influx, acco mpanied by low diversity, low oxygen and low salinity tolerant species . These environmental conditions abruptly ended with erosion followed by deposition of a phosphatic siltstone layer that represents condense d sedimentation in an open (transgressive) marine environment. Above t his layer, low sea-levels and a return to near-shore, hyposaline and h ypoxic conditions prevailed for a short interval [(base of Plc(2))] an d are followed by the re-establishment of normal open marine condition s (inner neritic) comparable to the late Maastrichtian. This marine tr ansgression is accompanied by increased productivity, and the first di versified Danian foraminiferal assemblages after the K/T boundary even t and represents the return to normal biotic marine conditions. Though the WT Seldja section represents one of the most shallow marginal sea environments studied to date for this interval, it does not represent isolated or atypical conditions. This is suggested by the similar glo bal trends observed in sea-level fluctuations, hiatuses, as well as fa unal assemblages. We conclude that on the Saharan platform of southern Tunisia, longterm environmental stresses beginning 100-200 kyr before the K/T boundary and related to climate, sea-level, nutrient, oxygen and salinity fluctuations, were the primary causes for the eventual de mise of the Cretaceous fauna in the early Danian. The K/T boundary bol ide impact appears to have had a relatively incidental short-term effe ct on this marine biota.