Low-accommodation and backwater effects on sequence stratigraphic surfaces and depositional architecture of fluvio-deltaic settings (Cretaceous Mesa Rica Sandstone, Dakota Group, USA)

Abstract

The adequate documentation and interpretation of regional-scale stratigraphic surfaces is paramount to establish correlations between continental and shallow marine strata. However, this is often challenged by the amalgamated nature of low-accommodation settings and control of backwater hydraulics on fluvio-deltaic stratigraphy. Exhumed examples of full-transect depositional profiles across river-to-delta systems are key to improve our understanding about interacting controlling factors and resultant stratigraphy. This study utilizes the ~400 km transect of the Cenomanian Mesa Rica Sandstone (Dakota Group, USA), which allows mapping of down-dip changes in facies, thickness distribution, fluvial architecture and spatial extent of stratigraphic surfaces. The two sandstone units of the Mesa Rica Sandstone represent contemporaneous fluvio-deltaic deposition in the Tucumcari sub-basin (Western Interior Basin) during two regressive phases. Multivalley deposits pass down-dip into single-story channel sandstones and eventually into contemporaneous distributary channels and delta-front strata. Down-dip changes reflect accommodation decrease towards the paleoshoreline at the Tucumcari basin rim, and subsequent expansion into the basin. Additionally, multi-storey channel deposits bound by erosional composite scours incise into underlying deltaic deposits. These represent incised-valley fill deposits, based on their regional occurrence, estimated channel tops below the surrounding topographic surface and coeval downstepping delta-front geometries. This opposes criteria offered to differentiate incised valleys from flood-induced backwater scours. As the incised valleys evidence relative sea-level fall and flood-induced backwater scours do not, the interpretation of incised valleys impacts sequence stratigraphic interpretations. The erosional composite surface below fluvial strata in the continental realm represents a sequence boundary/regional composite scour (RCS). The RCS’ diachronous nature demonstrates that its down-dip equivalent disperses into several surfaces in the marine part of the depositional system, which challenges the idea of a single, correlatable surface. Formation of a regional composite scour in the fluvial realm throughout a relative sea-level cycle highlights that erosion and deposition occur virtually contemporaneously at any point along the depositional profile. This contradicts stratigraphic models that interpret low-accommodation settings to dominantly promote bypass, especially during forced regressions. Source-to-sink analyses should account for this in order to adequately resolve timing and volume of sediment storage in the system throughout a complete relative sea-level cycle.