HR: 17:45h
AN: H54B-08    [Abstracts]
TI: Distinguishing Long-Term Controls on Fluvial Architecture in the Lance Formation, Bighorn Basin, Wyoming
AU: * McHarge, J L
EM: jmcharge@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, 1000 University Ave., Laramie, WY 82071, United States
AU: Hajek, E A
EM: ehajek@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, 1000 University Ave., Laramie, WY 82071, United States
AU: Heller, P L
EM: heller@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, 1000 University Ave., Laramie, WY 82071, United States
AB: Allogenic processes are considered a prime control on the stratigraphic distribution of channel bodies, however, recent studies have indicated that autogenic stratigraphic organization may occur within fluvial systems on basin- filling time scales (10⁵-10⁶ years). Groupings or clusters of closely-spaced channel bodies can be produced by several different mechanisms, including both allogenic and autogenic processes. Commonly, sand- dominated intervals in stratigraphic successions are interpreted as incised-valley fills produced by base-level changes. In contrast, long-timescale organization of river avulsion can generate similar stratigraphic patterns. For example, sand-dominated intervals in the fluvial Lance Formation (Maastrichtian; Bighorn Basin, WY) have been interpreted as incised-valley fills formed during sea-level lowstand. However, closely-spaced sand bodies in the Ferris Formation (Lance equivalent; Hanna Basin, WY) are interpreted as aggradational in origin, and have been compared to autogenic avulsion stratigraphy produced in experimental basins. We evaluate the Lance Formation in the southern Bighorn Basin in an effort to determine whether these sand-dominated intervals are truly incised- valley fills resulting from sea-level changes, or if they were generated by autogenic processes. The Lance Formation crops out in the western and southern margins of the basin, exposing relatively proximal and distal portions of the system. By comparing alluvial architecture between exposures, we evaluate similarities and differences from upstream to downstream and look for evidence of intrinsic and extrinsic controls on deposition. In both localities, the Lance Formation comprises multi-story sheet sandstones and smaller, single-story sandstones. Observed changes from upstream to downstream in the system include: 1) increasing paleoflow depths (from ~30-60 cm to ~70-120 cm); 2) decreasing preservation of fine-grained material within channel bodies; 3) increasing proportion of amalgamated, multi-story sand bodies; and 4) increasing lateral continuity of multi-story sand bodies. These results indicate that upstream, channel-body spacing is dominantly controlled by aggradational processes and may be the result of autogenic avulsion clustering, whereas downstream, evidence of incision and amalgamation indicate that base-level may have limited and controlled sand-body architecture.
DE: 1861 Sedimentation (4863)
SC: Hydrology [H]
MN: 2007 Fall Meeting