EVALUATING CYCLES IN DISTAL FLOODPLAIN DEPOSITION WITHIN THE EARLY PALEOGENE HANNA BASIN, WYOMING, USA

This study analyzes overbank depositional cycles in the Early Paleogene Hanna Basin (south-central Wyoming, U.S.A.) and compares them to similarly-aged floodplain depositional cycles in the Bighorn Basin (northwest Wyoming, U.S.A.) to identify any common sedimentologic patterns, and evaluate local, regional, or even global climatic/tectonic controls on basin deposition. Within the Hanna Basin we identified characteristic cycles of organic-rich, finer-grained lacustrine, paludal, and fluvial depositional environments, generally stacked between ledge-forming sandstone units, deposited in more channelized environments. Similar cycles occur in succession over 18 times within the study area.

We conducted a detailed analysis of two cycles in the upper Hanna Formation, including stratigraphic thickness, lithologic description, X-ray fluorescence (n=40), δ¹³C bulk organic carbon (n=30), and total organic carbon (TOC) (n=30). The two cycles are ~14 and ~25 m thick, and are comprised of coal layers, fissile organic-rich shales, slightly mottled gray siltstones, and bioturbated yellow sandstones. These are all interpreted as overbank facies that represent deposition along a vegetated lake margin and more open lake/wetland environment. Up-section transitions in these lithofacies record changes in the relative inputs of water, sediment, and organic matter to the floodplain. Throughout the section, lithology and % TOC covary. The δ¹³C values range between about -24 and -28‰ (VPDB), and show cyclical fluctuations up-section that are unrelated to the lithology and % TOC in the sample, suggesting non-local controls on δ¹³C composition.

These cycles of overbank deposition in the wet Hanna Basin appear complementary to fluvial avulsion cycles in the well-studied and well-drained Willwood Formation of the Bighorn Basin, which have been identified as red paleosols, deposited during times of river stability, alternating with sandy crevasse splay units. We hypothesize the lithologic differences between the cycles document the different wet versus dry local climates of the two basins, and that the repetitive nature of their deposition is related to either autogenic cycles in overbank deposition or perhaps Milankovitch precessional cycles, as have been previously proposed in the Bighorn Basin.