Stratigraphy of the Cretaceous/Tertiary (K/T) boundary transition in northeastern and east-central Mexico

The Cretaceous-Tertiary (K-T) transition and the nature and tempo of its associated mass extinction has become one of the controversial topics in ecology during the last fifteen years. Although it is generally agreed that nearly 60% of the fauna disappeared at this transition {dollar}sim{dollar}65 my ago. There is no consensus as to the actual cause. Some researchers have proposed a catastrophic event, the impact of a huge meteorite and various locations have been proposed as the impact site. The {dollar}sim{dollar}180 km-diameter Chicxulub-structure on the north of the Peninsula of Yucatan, Mexico is now commonly interpreted as the putative impact site based on magnetic anomalies that would suggest impact rings produced when an extraterrestrial body struck the Earth. Non-catastrophic arguments have also been proposed to explain the K-T mass extinction event, including climate cooling, sea-level changes and massive volcanism near the end of the Cretaceous.; Outcrops from 9 localities in northeastern Mexico and in east-central Mexico have been analyzed to evaluate the nature of the K-T transition in Mexico and its possible relationship to the Chicxulub structure.; Major objectives and results of this investigation were: (1) The age and stratigraphy of the siliciclastic deposits found at or just below the K/T boundary in NE Mexico sections. These deposits have been interpreted by some workers as Yucatan impact-generated megatsunami waves. High resolution stratigraphic analysis revealed that these deposits pre-date the K/T boundary event by a short time interval of less than 200 kyr. (2) Evaluate the nature and tempo of siliciclastic deposition, whether short-term tsunami induced, or long-term. Results revealed the presence of numerous disconformities, discrete intervals of bioturbation and intervals of normal hemipelapic sedimentation which suggest deposition occurred over an extended time interval, rather than within a few hours to days via megatsunami.