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Per Kent Pedersen1, Ron Steel2
(1) Carleton University, Ottawa, ON
(2) Wyoming University, Laramie, WY

Abstract: Sequence stratigraphy and alluvial architecture of the Ericson Sandstone, southwestern Wyoming

In outcrops of the late Campanian Ericson Sandstone (Glades-Clay Basin area, southwestern Wyoming) two distinct sequence stratigraphic surfaces occur. Sequence-forming unconformities form at the base of thick sandsheets of amalgamated fluvial channel-belts, whereas the sharp tops of sandsheets, marking an abrupt change from amalgamated fluvial channels below to more loosely connected fluvial channels embedded in floodbasin fines above, form non-marine flooding surfaces. The unconformities subdivide the Ericson Sandstone into four low-order sequences. In individual sequences, vertical changes in alluvial architecture reflect changes in available accommodation in relation to sediment flux (A/S ratio changes). A high degree of channel amalgamation within the basal sandsheet, together with a high but upward-decreasing frequency of internal erosion surfaces, strongly suggests an early period of significant sediment by-pass and erosion, followed by sediment accumulation with shorter and shorter periods of by-pass. Thus this unconformity cluster, a series of high-order erosion surfaces together with the low-order sequence boundary, is an important component of the lower and middle levels of the amalgamated sheetsand. The more loosely connected fluvial channel-belts in the mid-to-upper portion of the low-order sequences imply a more complete storage of the sediment budget and a higher accommodation setting. This is also the interval likelely to show most brackish-water or tidal influence. A slight change to more amalgamated fluvial channels in the uppermost portion of the sequence, suggests renewed increasing sediment by-pass and reduced accommodation as the new sequence boundary conditions are approached. Vertical changes in fluvial style accompany the observed changes in alluvial architecture.

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