Abstract

The Española Basin is a west-tilted half-graben formed from extensional stresses related to the formation of the Rio Grande Rift. Intrabasinal normal faulting created a series of sub-grabens within the Española Basin. Importantly, this stratigraphic record provides one of North America’s best Miocene vertebrate fossil records. Yet, the composite stratigraphic section necessary to place those fossil occurrences into a biostratigraphic framework is unavailable largely because the tectonic framework creates a complex mosaic of facies, making the correlation of individual stratigraphic sections difficult. However, numerous widespread and distinguishable ash beds occur within the strata that help correlation across complex structural and facies relationships. This paper provides a composite stratigraphic section targeting fossil-rich regions within the Bureau of Land Management’s Sombrillo Area of Critical Environmental Concern, documenting periods of progradation and retrogradation of the alluvial fan facies.

Integrating previously published climatic and tectonic data with new paleontological data into a stratigraphic and lithofacies framework bears on the environmental evolution during and after the middle Miocene Climatic Transition (MCT). From approximately 17 Ma to 14 Ma, the Española Basin experienced a period of warmer and wetter conditions during the middle Miocene Climatic Optimum (MCO) reflected by alluvial fan facies. Towards the end of the MCO, alluvial fan facies began to retrograde. We infer these results to indicate that climate provides more influence on sedimentation because alluvial fan retrogradation corresponded with less vegetation and, thus, less sediment stability associated with cooler conditions, as demonstrated by previous stable isotopic studies. As the regional climate cooled and dried, the Española Basin remained wet, although cooler conditions forced the local extinction of palm trees. This is also supported by the increase in antilocaprid abundance and the appearance of gomphotheres attracted to the riparian zones during this time of regional aridity.