COMPARISON OF FLOODPLAIN AND AVULSION-ASSOCIATED DEPOSITS IN ANCIENT FLUVIAL SUCCESSIONS: IMPLICATIONS FOR CREVASSE-SPLAY DEPOSITION AND AVULSION STYLE

Extensive heterolithic avulsion deposits rapidly accumulate overbank during progradational avulsions. In contrast, incisional avulsions erode directly into floodplains and lack avulsion deposits. The nature of floodplain and avulsion deposits may be used to infer paleo-avulsion style and evaluate how fine sediment is distributed during basin filling. However, differentiating avulsion-associated and floodplain sediments in ancient successions is difficult because both may contain crevasse-splays and other coarse overbank deposits. In an effort to discriminate between floodplain and avulsion deposits, we investigate overbank material below and lateral to avulsive channel-belt deposits in three ancient formations. Grain-size, paleosol development, and stratigraphic architecture of overbank deposits were characterized in the Ferris (Cretaceous/Paleocene; Hanna Basin), Fort Union (Paleocene; Bighorn Basin), and Willwood (Paleocene/Eocene; Bighorn Basin) formations in Wyoming.

Ferris Fm floodplain deposits are dominantly dark, carbonaceous claystones containing rare, thin (< 10 cm-thick) sandy lenses and little evidence of paleosol development. Where present, crevasse-splay deposits adjacent to channels pinch out in a few tens of meters. No evidence of heterolithic avulsion sediments was found below channel deposits.

Fort Union and Willwood floodplain deposits contain extensively mottled paleosol horizons. Channel-margin deposits are commonly tan dm-to-m-scale horizons with weak paleosol development and extend laterally for kilometers. Heterolithic avulsion deposits comprising meter-scale coarsening-upward successions (clay to sand) showing no or little paleosol development commonly underlie channels in these formations.

These observations indicate that Fort Union and Willwood floodplains contain significant avulsion-associated overbank accumulations, while Ferris Fm floodplains contain virtually none. This suggests that Ferris channels may have avulsed by incision, and/or that crevasse-splay and other coarse-overbank deposition was suppressed in Ferris river systems. By comparing field data to modern systems and modeling results, we consider how floodplain drainage, channel scale and sediment load may have affected sedimentation in each system.