Benthic foraminifera at the Cretaceous-Tertiary boundary around the Gulf of Mexico

Cretaceous-Tertiary (K-T) boundary sections in northeastern Mexico contain marly formations separated by a controversial elastic unit. Benthic foramin ifera in seven sections indicate middle and lower bathyal depths of deposit ion for the marls, with the exception of the upper bathyal northernmost sec tion. Mixed neritic-bathyal faunas were present in the clastic unit, indica ting redeposition in the deep basin by mass-wasting processes resulting fro m the K-T bolide impact in the Gulf of Mexico. Benthic foraminifera in the Mexican sections, and at other deep-sea locations, were not subject to majo r extinction at the time of impact, but there were temporary changes in ass emblage composition. Benthic faunas indicate well-oxygenated bottom waters and mesotrophic conditions during the late Maastrichtian and increased food supply during the latest Maastrichtian. The food supply decreased drastica lly just after the K-T boundary, possibly because of the collapse of surfac e productivity. Cretaceous and early Paleogene benthic foraminifera, howeve r, did not exhibit the benthic-pelagic coupling of present-day faunas, as d ocumented by the lack of significant extinction at the K-T collapse of surf ace productivity. Much of the food supplied to the benthic faunas along thi s continental margin might have been refractory material transported from l and or shallow coastal regions. The decrease in food supply at the K-T boun dary might be associated with the processes of mass wasting, which removed surface, food-rich sediment. Benthic faunas show a staggered pattern of fau nal recovery in the lowermost Paleogene, consistent with a staged recovery of the vertical organic flux but also with a gradual buildup of organic mat ter in the sediment.