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Mary Kraus1, A.J. Pulham1
(1) Univ of Colorado, Boulder, CO

Abstract: Genesis of different sandstone body stacking patterns in the Fluvial Willwood formation, Bighorn Basin, Wyoming

Sandstone stacking patterns and avulsion styles differ in two time-equivalent stratigraphic sections of the Eocene Willwood Formation. Differences are attributed to sediment accumulation area, which was two times faster in the McCullough Peaks (MP) study area than in the elk Creek (EC) area.

Both areas have major sheet sandstones, up to 1.5 km wide, which are interpreted as deposits of trunk rivers. Those in MP are up to 20 m thick with 6 vertically stacked stories, showing that vertical amalgamation was important to sandbody genesis. Those in the more slowly aggrading EC area are only 10 m thick with 2-3 stories. Vertical amalgamation was less important, and lateral amalgamation was responsible for the extensive sandbodies. The paleosol stratigraphy shows that the sheets had an early incisional phase; the depth of scour is approximately half of total sandbody thickness in each area. Following the incisional phase, the channel systems built above their floodplains. The aggradational phase produced ~10 m of sediment in the Mp area and only ~r in the EC area.

Avulsion styles also differed between the two study areas. Avulsion deposits are easily recognized in EC based on their paleosol attributes. Avulsion complexes are less apparent in MP; however, some ribbon sandbodies are present as nested groups that underlie the major sheet sandstones. They are interpreted as feeder channels to avulsion complexes and, thus, highlight the presence of avulsion complexes. Ribbon sandstones are less common and smaller in EC, and nesting has not been observed.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana