BENTHIC FORAMINIFERAL FAUNAL TURNOVER ACROSS THE CRETACEOUS/TERTIARY BOUNDARY IN NORTHEASTERN MEXICO

The environmental and ecological effects of the impact of an asteroid at the end of the Cretaceous (K/P boundary, 65 Ma) have been actively investigated but are not yet clear. Deep-sea benthic foraminifera are among the least affected groups, and we investigated the effects of the impact in on this group in Upper Cretaceous and lower Tertiary marly sediments in northeastern Mexico (Mendez and Velasco Formations), close to the site of impact on Yucatan Peninsula. A siliciclastic unit intercalated between the Mendez and Velasco Formations indicates the boundary. We quantitatively analyzed benthic foraminiferal faunas in 7 sections, obtaining information on paleobathymetry and paleoenvironmental conditions. Benthic foraminiferal assemblages contain calcareous and agglutinated taxa, mostly belonging to the Velasco-type fauna. Assemblages indicate middle to lower bathyal paleodepths, except for the two northernmost sections, which were upper bathyal. Only the clastic unit contains typically neritic taxa, mixed with bathyal forms, indicating that this unit is allochthonous, and slumped from the platform into the deeper basins, destabilized by the asteroid impact. Uppermost Cretaceous benthic foraminifera indicate mesotrophic conditions; those from the lowermost Tertiary are strongly impoverished in infaunal species, indicating nutrient-depleted conditions. These nutrient-poor conditions probably were the result of the collapse of surface water productivity at the K/P transition. In addition, mass wasting of sand-rich material in the Gulf of Mexico may have removed organic-rich sediments and left organic-impoverished substrates, starving benthic foraminifers. The disturbance in the deep-sea environment caused a temporary faunal turnover and no significant extinction of benthic species, probably because they largely used sedimentary organic material rather than fresh phytoplankton. Recovery of faunas took more than ~ 0.5 myr.