We interpret the near K/T boundary clastic deposits of northeastern Mexico as deposited over an extended time period, during the last 170 to 200 k.y. of the Maas-trichtian and by normal sedimentary processes that include gravity flows and turbidity currents, rather than impact-generated tsunami waves. This deposition scenario is indicated by multiple horizons of bioturbation within the top unit 3 and near the base of the middle unit 2; the presence of thin layers enriched in fine clay-minerals and planktic foraminifera, suggesting hemipelagic sedimentation within unit 3; the presence of a marl layer of Maastrichtian age above the clastic deposit; the occurrence of distinct layers enriched in zeolites in unit 3; and the presence of lithologically, sedimentologically, and mineralogically distinct units and subunits that are correlatable over more than 300 km. Such correlations do not support a chaotic deposition as predicted for an impact-generated tsunami event. We interpret the clastic beds of northeastern Mexico as having accumulated during the major eustatic sealevel lowstand near the end of the Maastrichtian. In this scenario, the unconformity at the base of the clastic deposit represents a type 1 sequence boundary, where deltaic sediments were eroded and transported into deeper waters, depositing the spherulerich layer of unit 1. Continued sea-level lowering resulted in erosion and bypass of shelf sediments and the deposition of the sandstone of unit 2. Subsequently, stabilization of the sea-level lowstand resulted in episodes of decreased erosion and sediment transport alternating with normal hemipelagic sedimentation, thus depositing the sand and the silt and shale layers of unit 3. The sea-level rise during the last 50 to 100 k.y. of the Maastrichtian resulted in the normal hemipelagic sedimentation observed in the pre-K/T boundary marl layer above the clastic deposit.
|Number of pages
|Special Paper of the Geological Society of America
|Published - Jan 1 1996
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