CALCAREOUS NANNOFOSSILS AND ELASTIC SEDIMENTS AT THE CRETACEOUS-TERTIARY BOUNDARY, NORTHEASTERN MEXICO

摘要

A quantitative analysis of calcareous nannofossil assemblages on the Cretaceous-Tertiary (K-T) boundary of the Mimbral and Mulato outcrops of northeast Mexico indicates that the sections are biostratigraphically complete across the boundary and that there is a prominent spherule-bearing elastic unit located precisely at the K-T boundary. The sections consist of uppermost Maastrichtian (Micula prinsii Zone) marry limestones of the Mendez Formation and marlstones of the lower Paleocene (Zone NP1) Velasco Formation separated by a distinct, 1-3-m-thick sandstone unit that has a basal spherule-bearing layer. The origin of this clastic unit and the time of deposition relative to the K-T mass extinctions have been the subjects of much controversy. Some workers attribute this unit to rapid tsunami-induced deposition triggered by the nearby Chicxulub impact, and others consider it a turbidite deposited at some time prior to the K-T mass extinctions. Cretaceous nannofossils abruptly decrease in abundance at the base of the spherule bed and only rare to few reworked specimens are present in the elastic unit and in the basal Velasco Formation. Survivors and Tertiary species are common above the elastic unit. Cretaceous nannoplankton show no evidence of recovery after deposition of the elastic unit, which indicates that extinctions probably occurred correlative with the deposition of the elastic unit and in association with the Chicxulub impact. [References: 27]