Stratigraphic base level and fluvial architecture: Ericson Sandstone (Campanian), Rock Springs Uplift, SW Wyoming, USA

The concept of stratigraphic base level, or the ratio between accommodation and sediment supply (A/S ratio), has been used to analyse the Rusty and Ca nyon Creek Members of the Campanian Ericson Sandstone in the Rock Springs U plift, SW Wyoming, USA. The Ericson Sandstone was deposited under fluvial t o estuarine conditions in a foreland basin setting influenced both by Sevie r-style (thrust belt) tectonism and by more local, Laramide-style, foreland uplifts. The depositional setting was situated several tens to a few hundr ed kilometres from the nearest shoreline. Therefore, sea level change at th e contemporaneous shoreline probably had little, if any, influence on the d evelopment of the sedimentary architecture. The Rusty Member shows an alternation between incised valleys filled by mul ti-storey estuarine channel sandstones showing palaeoflow to the south and delta plain sediments with single-storey channels with no evidence of tidal influence, which show palaeoflow to the east. This cyclicity is interprete d as recording repeated uplift of the Wind River Range to the north, causin g valley incision and reduction of the A/S ratio. During quiescent periods, the A/S ratio increased allowing the valleys to fill and delta plain condi tions to be subsequently re-established because of increased sediment suppl y ham the thrust belt in the west. A regional unconformity at the base of the Canyon Creek Member truncates th e Rusty Member, and represents a significant reduction of the A/S ratio cau sed by Laramide tectonia: uplift. The Canyon Creek Member is a multistorey, multi-lateral fluvial channel sandstone, where channel preservation and th ickness increase upwards, suggesting an increase of the A/S ratio. The Cany on Creek Member channels are interpreted to have been sinuous, meandering c hannels from the observed sedimentary structures and fill patterns, despite their sand-rich nature. It is argued that grain size is a poor indicator o f channel planform, and that there was very law preservation potential for fine material because of a relatively low A/S ratio. The top of the Canyon Creek Member is a regionally correlative surface mark ing an abrupt increase of the A/S ratio. This surface is termed an expansio n surface, denoting an abrupt increase in accommodation. The overlying Almo nd Formation shows a single-storey alluvial architecture with a very high p reservation of fine-grained material. An assumed correspondence in time of the Late Absaroka thrust phase in the Sevier belt to the west and the forma tion of the sharp top of the Canyon Creek Member suggests that the thrust p hase caused a basin-wide abrupt increase of subsidence that changed the all uvial architecture. As an alternative to sequence stratigraphic: nomenclature defined for strat a controlled by shoreline movements, a scheme relating systems tracts and s urfaces to changes in stratigraphic base level is proposed. Such a scheme i s useful where correlations to shoreline strata are ambiguous or cannot be made, or where tectonics and climate are important controls.