Signature of Climate Control in Early Eocene Fluvial Channel Systems
Here we document climate control signature in Early Eocene fluvial-lacustrine succession of the Uinta Basin, Utah, US. A distinct feature of the Early Eocene river deposits is that in many stratigraphic intervals, the river channels are filled with sandstones with dominantly gradational plane-parallel or climbing-ripple lamination, convex-up low angle bedforms, with only a minor (5-10%) cross stratification that in most cases is the most common feature of sandy river deposits. These sandstones are organized into simple, thick, dominantly downstream accreting barforms, based by significant erosion surfaces. All these features indicate very high deposition rates, generally not expected in rivers, but rather in environments of rapid accumulation like deltaic mouth bars or deepwater turbidite fans. Moreover, these channels are large (up to tens of meters deep), and vertically as well as laterally amalgamated. The lateral and vertical channel amalgamation demonstrates high frequency of avulsions. Stratigraphic intervals with such river channels coincide with peaks of negative carbon 13 excursions, interpreted as peaks of Early Eocene transient global warming events or hyperthermals. Other stratigraphic intervals, associated with initial negative and final positive shits of the overall negative carbon 13 excursions, interpreted as onset and offset of the hyperthermals coincide with stratigraphic intervals of thick floodplain mudstones encasing small (up to a few meters deep) laterally partially amalgamated channels with infills that indicate high deposition rates, similar to the channels described above. However, these channels are small and indicate low peak discharge. Moreover, the thick floodplain mudstones indicate high fine-grained sediment production and storage. Yet other stratigraphic intervals exhibit more “normal” river deposition with migration of dunes, organised into complex downstream, laterally and upstream migrating barforms. Such rivers indicate significantly lower deposition rates, as well as notably more stable water supply. We show that the combination of high erosion and high deposition rates is characteristic for climate-controlled river systems, and suggest that this Greenhouse world fluvial succession displays climate controls that produce a distinct stratigraphic record and a distinct type of fluvial reservoirs.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California