Paleocene-Eocene Sedimentary Record - a Deep-Time Window into the Future?
Understanding the character and consequences of ancient climate variability is the key to planning for modern and future climate change and its impact on the society. Our atmosphere is heading towards a state far beyond the boundaries of all modern observations and Quaternary climate proxy calibrations. As an effort towards understanding the potential fundamental differences between icehouse and greenhouse worlds, we have studied the sedimentary and geochemical record of Paleocene-Eocene alluvial strata (Colton and Green River Fm) from the southwestern Uinta Basin, Utah. This greenhouse alluvial record indicates stratigraphic changes in the character of channel fill facies and architecture that correlate to changes in organic carbon isotope values, interpreted to reflect a terrestrial record of the PETM as well as the post-PETM hyperthermals and intermittent cooler periods. The key stratigraphic change is an occurrence of two fundamentally different types of channel fills. Type 1 channel fills consist of dominantly trough-cross stratified sandstones that occur in different types of bars as well as in channel thalweg. These channels are highly lenticular in shape, have highly irregular lower boundaries, and commonly display multiple internal erosion surfaces, in places draped by muddy intervals. Thus, the Type 1 channels have the characteristics of “normal” braided river deposition, with episodes of dune migration, erosion and bypass during high discharge periods, and mud drape formation during low discharge periods. Type 2 channel fills consist of dominantly plane-parallel- and climbing-ripple-laminated sandstones that are organized into very simple downstream or obliquely accreting highly aggradational bars. These channels have erosional bases that are less irregular. The Type 2 channels locally lack the highly lenticular internal architecture and internal erosion surfaces. These sedimentary structures as well as the lack of multiple internal erosion surfaces suggest that the Type 2 channels were locally filled very rapidly, probably causing consequent instant channel avulsions. Such characteristics suggest extremely episodic water and sediment discharge and occurrence of short-lived, high-energy discharge events that moved most of the seasonal sediment budget. Thus, our dataset suggests high-frequency Early Eocene fluctuations in precipitation seasonality, with occurrence of catastrophic water and sediment discharge events during hyperthermals.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009