Anna van Yperen, Miquel Poyatos-Moré , John Holbrook, Ivar Midtkandal

Mouth bars are the fundamental architectural elements of proximal deltaic successions. Understanding their internal architecture and deciphering the relative impact and complex interaction of coastal processes (fluvial-, tide- and wave-dominated) is paramount to the interpretation of ancient deltaic successions. This is particularly challenging in low-accommodation systems, because they are commonly characterized by thin, condensed and top-truncated sections. In this study, we analyze the exhumed Cenomanian Mesa Rica Sandstone (Dakota Group, Western Interior Seaway, USA), which encompasses a fluvio-deltaic system covering a ~450 km profile oriented parallel to depositional-dip direction. The study targets the proximal deltaic expression of the system, with 22 sedimentary logs (total of 390 m) spatially correlated within a ~25 km2 study area at the rim of the Tucumcari Basin. Analysis of facies distributions, depositional architecture and spatial extent of stratigraphic surfaces reveals a 6–10-m-thick, sharp-based and sand-prone deltaic package, comprising several laterally-extensive (>800 m width) mouth bars. Composite erosional surfaces infilled with multi-story fluvial and marine-influenced channel deposits (12–20 m thick, 100–250 m wide) scour locally into the deltaic package. Based on differences in sedimentary structures, bed thicknesses, occurrence of interflood beds and bioturbation indexes, we distinguish four different subenvironments within single mouth bars. These range from mouth bar axis, off-axis, fringe to distal fringe deposits, and each reflect differences in hydraulic conditions as moving away from the main active feeding channel. The interpreted mouth bar components also show intra-mouth-bar variability in dominant process regime, with overall river dominance but local preservation of tide influence in the fringe and distal fringe components. Mouth bar deposits amalgamate to form an extensive sand-rich sheet body throughout the study area, in which interflood mudstone to very-fine grained sandstone beds are nearly absent. These features are interpreted to reflect successive coalescence of mouth bars in a low accommodation / supply (A/S) setting. These conditions promoted channel avulsion/bifurcation and thus the potential reworking of previously deposited mouth bar fringe and distal fringe sediments, where tide influence tends to be better recorded. Results of this study evidence a common mixed nature and internal process-regime variability within mouth bar components. They also caution against the possible loss of preservation of subordinate processes (e.g. tidal indicators), and consequent underestimation of the true mixed influence in low-accommodation deltaic settings.