Sorting and wave abrasion; controls on composition and diagenesis in Lower Frontier sandstones, southwestern Wyoming

Lower Frontier sandstones on the Moxa arch were deposited in a wave-dominated, multi-river delta plain which prograded eastward into the Cretaceous Interior seaway; headwaters were in the Sevier thrust belt. Sands were deposited as fluvial units, as marine shoreline sequences, and as offshore sand ridges. Fluvial sandstones, some pebbly, are dominated by trough crossstratification and by climbing ripple lamination. These sandstones are associated with flood-basin mudstones that contain abundant root traces and lack burrows. Marine shoreline sediments show a transition from burrowed lower shoreface parallel- and hummocky-stratified and ripple-laminated sandstones with mudstones, to surfgenerated cross-stratification, to foreshore parallel stratification, and finally to structureless backshore sandstones. Offshore sandstones are mostly burrowed, but storm-generated parallel and hummocky stratification is present. Depositional hydrodynamics largely controlled diagenesis. Fluvial sandstones contain a greater percentage of rock fragments and have less quartz than equivalent marine sandstones owing to the combined effects of sorting and wave abrasion. The coarsest sands and gravels, which were trapped on the deltaic plain, were richer initially in rock fragments than finer grained sands that were passed to the shoreline. These latter sands had a significant portion of unstable grains, including chert, destroyed by wave abrasion. Composition significantly influenced diagenesis. Quartz-rich, permeable marine sandstones were cemented early by silica (because quartz overgrowths preferentially form on monocrystalline quartz), whereas primary porosity in quartz-poor fluvial sandstones was largely preserved through this stage. Fluvial sandstones were affected more by grain dissolution, calcite replacement, calcite precipitation in open pores, and by subsequent carbonate dissolution. Fluvial sandstones generally have the best porosity and permeability mostly due to creation of secondary porosity. Compaction in finer grained sandstones destroyed permeability by squeezing clays into open spaces. Other major diagenetic processes are late-stage growth of kaolinite and continued precipitation of silica.