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Diachronous key stratigraphic surfaces in low-accommodation settings fluvio-deltaic settings
Van Yperen, A.E. 1 , Holbrook, J.M. 2 , Poyatos-Moré, M. 1 , Midtkandal, I. 1
1 University of Oslo, Department of Geosciences, P.O. Box 1047 Blindern, 0316 Oslo, Norway
2 Texas Christian University, Department of Geological Sciences, TCU Box 298830, Fort Worth, Texas 76129

The adequate documentation and interpretation of regional-scale stratigraphic surfaces is paramount to establish correlations between continental and shallow marine strata. In active depositional systems however, these surfaces are often composite and hence diachronous, which contradicts their original appreciation. This is particularly true in low-accommodation settings due to their amalgamated nature. This study utilizes the ~400 km transect of the Cenomanian Mesa Rica Sandstone (Dakota Group, USA); an example of an exhumed depositional profile across a river-to-delta system. The near-continuous outcrop exposure 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 distributary channel deposits and delta-front strata. Additionally, multi-storey channel deposits bound by erosional composite scours incise into underlying deltaic deposits. These represent incised-valley fill deposits. The erosional composite surface below fluvial strata in the continental realm represents a sequence boundary/regional composite scour (RCS) and can be mapped for >300 km. Basal distributary composite scours, composite surfaces bounding incised valleys, and basal surfaces below dispersed
trunk channels incising into deltaic deposits occur at sub-regional scale. 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.