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

Abstract

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 Canyon Creek Members of the Campanian Ericson Sandstone in the Rock Springs Uplift, SW Wyoming, USA. The Ericson Sandstone was deposited under fluvial to estuarine conditions in a foreland basin setting influenced both by Sevier‐style (thrust belt) tectonism and by more local, Laramide‐style, foreland uplifts. The depositional setting was situated several tens to a few hundred kilometres from the nearest shoreline. Therefore, sea level change at the contemporaneous shoreline probably had little, if any, influence on the development of the sedimentary architecture. The Rusty Member shows an alternation between incised valleys filled by multi‐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 interpreted as recording repeated uplift of the Wind River Range to the north, causing 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 conditions to be subsequently re‐established because of increased sediment supply from the thrust belt in the west. A regional unconformity at the base of the Canyon Creek Member truncates the Rusty Member, and represents a significant reduction of the A/S ratio caused by Laramide tectonic uplift. The Canyon Creek Member is a multi‐storey, multi‐lateral fluvial channel sandstone, where channel preservation and thickness increase upwards, suggesting an increase of the A/S ratio. The Canyon Creek Member channels are interpreted to have been sinuous, meandering channels from the observed sedimentary structures and fill patterns, despite their sand‐rich nature. It is argued that grain size is a poor indicator of channel planform, and that there was very low 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 marking an abrupt increase of the A/S ratio. This surface is termed an expansion surface, denoting an abrupt increase in accommodation. The overlying Almond Formation shows a single‐storey alluvial architecture with a very high preservation of fine‐grained material. An assumed correspondence in time of the Late Absaroka thrust phase in the Sevier belt to the west and the formation of the sharp top of the Canyon Creek Member suggests that the thrust phase caused a basin‐wide abrupt increase of subsidence that changed the alluvial architecture. As an alternative to sequence stratigraphic nomenclature defined for strata controlled by shoreline movements, a scheme relating systems tracts and surfaces to changes in stratigraphic base level is proposed. Such a scheme is useful where correlations to shoreline strata are ambiguous or cannot be made, or where tectonics and climate are important controls.

Towards the standardization of sequence stratigraphy

“…Concepts such as transgression and regression do not apply, unless correlation with a coeval coastline can be established (e.g., Kerr et al, 1999;Figure 27), but unconventional systems tracts such as "low-accommodation" versus "high-accommodation" may be useful for regional correlation (e.g., Olsen et al, 1995;Martinsen et al, 1999;Boyd et al, 2000;Arnott et al, 2002;Zaitlin et al, 2002;Leckie and Boyd, 2003;Ramaekers and Catuneanu, 2004;Leckie et al, 2004;Figure 28). Such systems tracts are defined by the ratio between fluvial architectural elements.…”

High resolution stratigraphy of initial stages of rifting, Sergipe-Alagoas Basin, Brazil

“…The predominance of multi-storey amalgamated sandstone bodies without deposits from outside channel along this unit indicates low accommodation creation rate and is related to a uniform A/S ratio with positive-close to 0 values (Martinsen et al 1999, Scherer et al2014.…”

“…Amalgamated sandstone bodies are replaced by isolated sandstone bodies allowing preservation of floodplain fine deposits. The character aggradational of fluvial channel sandstones and the substantial thickness of mudstones indicate a progressive increase of the accommodation creation rate and A/S ratio less than 1 (Martinsen et al 1999, Scherer et al 2014. This unit still belongs to the Rift Initiation Tectonic System Tract suggesting the shallow character of the basin, interpreted from the oxidation of mudstones, but the increase of A/S ratio indicates that the tectonic activity becomes more and more intense, however without individualization of half-grabens.…”

“…The unit III is characterized by a thick deltaic succession indicative of a deeper basin and high A/S ratio above 1 (Martinsen et al 1999, Scherer et al 2014. Besides, the scattering in paleocurrent direction and the fact that mudstones of unit III are superimposed on fluvial channel sandstones of unit II are consistent with the fragmentation of the wide basin in several smaller and deeper basin: the half-grabens (Prosser 1993, Bosence 1998, Gawthorpe & Leeder 2000.…”

Lithofacies, palynofacies, and sequence stratigraphy of Palaeogene strata in Southeastern Nigeria

“…These are interpreted to be the non-marine/coastal aggradational portion of the transgressive systems tract (Posamentier and Vail, 1988;Ye and Kerr, 2000). The fluvial architecture was probably also affected by changes in base level (Schumm, 1993;Martinsen et al, 1999).…”