--> Controls on Fluvial and Lacustrine Facies and Stacking Patterns of the Paleocene-Eocene Hanna Formation, Hanna Basin, Wyoming

AAPG ACE 2018

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Controls on Fluvial and Lacustrine Facies and Stacking Patterns of the Paleocene-Eocene Hanna Formation, Hanna Basin, Wyoming

Abstract

Upstream fluvio-lacustrine deposits of sediment routing systems can record external controls on deposition, (tectonism, climatic change), because they have not experienced downstream signal attenuation (transient sediment storage in floodplains). However, autogenic and tectonic processes contribute noise, making isolating external controls difficult. Recent work in Rocky Mountain Laramide basins revealed changes in processes-architectures that coincide with climatic warming (hyperthermals) during the Paleogene (e.g. Paleocene-Eocene boundary: PETM). A basin-wide shift toward multistory sandstone sheets with upper flow regime sedimentary structures is attributed to vegetation change and increased precipitation during the PETM

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We propose external and autogenic controls on fluvio-lacustrine stratigraphy and stacking in the Paleocene-Eocene Hanna Formation (Fm), Hanna Basin, WY. Stratigraphy along Hanna Draw Road records a transition from fine-grained deposits of small (1-3 m deep), southward-flowing, single story rivers, shallow lakes, and lake margins, to an ~45 m thick coarser-grained, multistory channel belt deposit with northwest directed flow, then a return to southward-flowing fine-grained alluvial-lacustrine deposits. Similar alluvial-lacustrine deposits are attributed to a high, steady subsidence rate relative to sediment flux, resulting in a shallow, emergent water table. We hypothesize that the aforementioned amalgamated fluvial deposit may result from intense weathering, erosion, and sedimentation related to the PETM climate/vegetation change. To test this we collected bulk organic matter and examined stable carbon isotopes across this transition. We compared the ~150 m succession to older Paleocene deposits of the Hanna Fm. Similar amalgamated channel deposits are found stratigraphically lower, and have been interpreted as tectonically driven. Although our data reveal negative-positive isotopic excursions across the ~150 m section, they do not clearly correlate with changing environments, organic matter type, or global climate change. The Hanna Basin received sediment from uplifts to the north, south, and west, while an eastern uplift periodically blocked eastward transport, significantly influencing basin hydrology. Thus, whereas some upstream deposits may reflect global climate forcing, the Hanna Basin stratigraphy may be overprinted by autogenic processes and Laramide tectonics that influenced basin hydrology, masking the climate signal.