Abstract:
Ocean Drilling Program Leg 171B recovered excellent records of the extinction and its aftermath. ODP Site 1049 uniquely recovered the boundary beds in three holes less than 30 m apart. All three holes contain a graded bed of green spherules that rest sharply on slumped uppermost Maastrichtian calcareous ooze and contain abundant Cretaceous microfossils as well as centimeter-scale clasts of Cretaceous chalk. The thickness of the spherule bed ranges from 9 to 17 cm. Primary depositional mechanisms or syndepositional slumping has apparently significantly affected the thickness of the spherule bed within a small area. Hence, maps of thickness variation of ejecta fall out, and impact models based on them, should be viewed with caution.The graded bed is overlain by a limonitic layer and a 3-6 cm thick dark gray calcareous ooze that contains a pronounced iridium anomaly and minute planktic foraminiferal assemblages indicative of Early Danian Foraminiferal Zone P-alpha. We believe that at least part of this clay layer represents material suspended by the impact. The concentration of iridium reaches low levels within 3 cm of the limonitic layer suggesting that most of the bolide remains are concentrated in the upper part of the spherule bed and the lower part of the overlying gray ooze. A similar lithologic succession is present at sites 1050 and 1052 except that no spherule or limonitic layers were recovered at either site. The K-T sequence drilled by Leg 171B demonstrates that the impact event that produced the spherule bed was associated with a sudden, dramatic, extinction of marine protists. In turn, the floral and faunal recovery was, at least in part, remarkably fast as suggested by a succession of nannofossil blooms in the earliest Paleocene and the appearance of the earliest Cenozoic foraminifer species, Parvulorugoglobigerina eugubina, immediately above the limonitic horizon. The rare occurrence of P. eugubina at El Kef and other shallow marine sites suggests that this species appeared in both open ocean and shallow marine regions immediately after the extinction.