Abstract: Fluvial Sheet Sandstone Development Above a Tectonically-Controlled Sequence Boundary: Canyon Creek Member, Erickson Sandstone (Campanian), Rock Springs Uplift, Wyoming

Ole J. Martinsen, Brit Sauar

The Erickson Sandstone of the Rock Springs Uplift, southwest Wyoming, consists of three predominantly sandy members, all deposited in fluvio-deltaic conditions. The uppermost member, the Canyon Creek, varies in thickness from 30 m to approximately 150 m from northwest to southeast in the uplift. Its lower boundary is erosional but regionally flat, and can be followed for at least 6000 sq km in southwest Wyoming. The erosion surface is considered to relate to local, basement-controlled (Laramide) uplift, because a widespread tectonic event can be demonstrated across the Wyoming foreland at this time with local truncation, among other places in the Moxa Arch, to the west. The Canyon Creek Member is made up of a sheet of multi-lateral, multi-storey, sinuous, meandering channel sandstones The combination of stacked sinuous channel sandstones in a sheet above a lowstand erosion surface is unusual, and perhaps reflects a very low-gradient alluvial plain.

Each channel body fines upwards from gravelly sandstone to fine sandstone. Internally, low-angle, lateral accretion surfaces and trough cross-stratification dominate. Paleocurrents show a multi-directional flow mode, but opposed paleoflow directions can be documented in adjacent sets. Bioturbation by Planolites is common, but also Thalassinoides, Asterosoma and Ophiomorpha are observed, suggesting, at least periodically, brackish to saline water conditions. This suggests a tidal influence during deposition of the Canyon Creek Member.

The upper boundary is flat, and marks the passage from slow rates of increase of accommodation in the Canyon Creek Member (multi-storey channel sandstones), to more rapid rates of increase in accommodation space to allow for deposition of single-storey, widely separated channel sandstones and abundant floodplain fines in the overlying Almond Formation. This vertical association probably reflects rising base-level, post-dating the basement-cored Laramide uplift, induced either by crustal loading of the uplift itself, by increased loading in the thrust belt to the west as a result of late Absaroka thrusting, or by a eustatic sea-level rise.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994