2019 AAPG Annual Convention and Exhibition:

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Exploiting Autogenic Sedimentary Processes to Synchronize Geologic and Modern Timescales of Environmental Change

Abstract

Sedimentary rocks host a suite of geochemical and biologic substrates useful for reconstructing paleoenvironmental conditions. These proxy records provide the opportunity to independently constrain and quantify the magnitudes of change in various boundary conditions that may drive major shifts in basin stratigraphic patterns. As such they are now important in the development of basin evolution models and prediction of reservoir characteristics. However, the robustness and utility of these predictions rests, in part, on the accuracy of the paleoenvironmental proxy records. Similarly, any use of paleo-records as an analog for future environmental change also rests on the supposition that proxy records accurately capture the magnitude and rate of change. This supposition is in conflict with the known incompleteness of the stratigraphic record and the inability of most age models to resolve the timing of bed-scale depositional patterns (i.e., those from which proxy records are derived). The de facto approach is to linearly interpolate between dated horizons, yet it is unclear if this approach is justified given the stochastic nature of sediment deposition. Using a physical experiment wherein a fluviodeltaic system evolved under purely autogenic processes we assessed the aliasing of a known, synthetic climate changes of varying magnitude and frequency preserved by the resultant stratigraphy. We find substantial modification occurs when the timescale of climate variability is shorter than the timescale on which compensational depositional patterns occur, but is faithfully preserved when the timescale of climate change is longer than this compensational timescale. Preliminary field studies spanning the Paleocene-Eocene Thermal Maximum in the Bighorn Basin (Wyoming, USA) are consistent with this conclusion. Thus, we advocate framing proxy-derived paleoenvironmental records within this compensational stratigraphic framework. It appears to dictate not only the timescale at which the record will be complete (i.e., well-sampled in the time domain), but also when the assumption of linear sedimentation models can be used to accurately estimates the rates of change. Ultimately, this may be a fruitful approach to more explicitly synchronize records spanning modern systems, the Recent, and the Ancient as it resolves the underlying assumptions regarding the distribution of time in the stratigraphic record.