The last big marine transgression of the Western Interior Seaway: Almond Formation development from barrier spits across south Wyoming

The stratigraphic response to marine shoreline transgressions have been debated for the last six decades. A particularly interesting aspect is that transgressive ravinement often removes evidence of the shoreline itself, until there is turnaround to regression. To elucidate this topic, we present a high-resolution chronostratigraphic study of an overall transgressive coastline, documented with evidence from over 1450 wells covering an area of 6200 km2 to define the fundamental sandbodies and relevant stratigraphic surfaces. The mapping of 15 elongate sandstone units stacked into 6 reservoirs shows that there was a spectacular westward rising of back-stepping barrier spits, sourced by long-shore currents mobilizing sand from a delta to the north. Each fundamental barrier spit unit (BSU) is a 6–12 metres thick, regressive, southward elongate and narrowing sandbody generated by longshore drift of sandy sediment. Each barrier sand body and its related overlying transgression is estimated to represent some 20 Ky. Strong transgressive ravinement events periodically flooded the barrier spits and shifted the deposition westward in four to 35 km steps. Westward transgressive-stepping across and into a structurally-generated embayment progressively dampened wave energy with each backstep. Morphologically, there were two endmember sandbody types: 1) narrow, drum stick-like sandbodies built mostly by longshore accretion, i.e. true barrier bars, and 2) broader and more extensive regressive belts of sandstone that resemble strandplains. The former display typically longer backsteps across lagoonal areas whereas the latter show a more aggradational stacking arrangement which evidences a quasi-balance between accommodation and sediment supply. There is no evidence of a smooth and continuous transgression controlling sandstone deposition and preservation. A hybrid punctuated transgression model, that considers the alternation of long-term coastal retrogradation and short-lived barrier spit development by longshore accretion and seaward progradation, is the most appropriate model for creating and preserving Upper Almond sandbodies.