Sequence stratigraphy in strike variable fluvio-lacustrine systems : evidence from Eocene Lake Uinta and modern analogs

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Gearon, James Hooker

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This thesis presents an outcrop-based, near depositional-strike cross section of fluvio-lacustrine interactions along the well-studied Nine Mile Canyon near Helper, Utah, highlighting the strike variability of facies and depositional environments in the Sunnyside Interval fluvio-lacustrine deposits. We utilize a new, high-resolution correlation panel to demonstrate the existence of a new lacustrine sequence stratigraphic concept: the Supply-Generated Sequence (SGS), which allows subdivision of the cyclically deposited alluvial, fluvial, and lacustrine facies in the Sunnyside Interval based on identifiable stratigraphic horizons, indicating periods of marked clastic input followed by subaqueous lake deposition. This novel conceptual framework helps address the inadequacy of previous sequence stratigraphic interpretations of the Sunnyside Interval. Conventional interpretations rely on premises regarding base level and sediment discharge that were developed for marine settings and do not apply directly to lacustrine systems because the creation and preservation of sedimentary deposits must logically occur during base-level rise in lake basins. In marine settings, base-level draw-downs are interpreted as the common causal mechanism for deltaic progradation, leaving sediment supply as a secondary consideration. In lacustrine systems, accommodation and sediment-supply are generated by water and sediment discharge routed through tributary rivers, making pulses of deposition inherently transgressive in nature. Our findings corroborate recent studies that consider the large, channelized sandstone deposits (shown to be accompanied by δ13C negative excursions) of the Sunnyside Interval to be discharge macroforms deposited during orbitally forced high-discharge early Eocene Hyperthermal events that lasted ~40 kyr. We also document the lateral complexity and discontinuity of hyperthermal sandstone event deposits and show the approximate durations of Sunnyside Interval SGSs to be 40-50 kyr, adding further evidence for the hyperthermal forcing mechanism model. We examine four previously proposed analogs of the Sunnyside Interval System, comparing basin type, morphology, lake level change, and depositional slope. We submit that the Neales River Delta in Lake Eyre, Australia is the best modern analog for the Sunnyside Interval System is due to its morphological and stratigraphic similarities. Ultimately, we characterize the Sunnyside Interval System as a “Supply-Controlled Splay-Delta” with high sediment and water supply during transitory hyperthermal events and low sediment and water supply during inter-hyperthermal periods. The depositional system acted as both a terminal splay or a fluvially dominated delta guided by the interplay of supply and accommodation in Eocene Lake Uinta.


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