Abstract

Depositional hydrodynamics were a major control on the diagenesis of the lower Frontier sandstones of the Moxa arch. The unit was deposited in a delta which prograded into the western margin of the interior Cretaceous seaway during Cenomanian time; the headwaters were in the Sevier thrust belt. The delta is thought to have been wave-dominated. Sandstones were deposited in three units: as fluvial point bars and related avulsion bodies; as marine shoreline sandstones; and as offshore, storm-generated bars. The latter consist of more-or-less burrowed sandstones and siltstones in horizontal stratification and in hummocky cross-bedding; the sandstones and siltstones alternate with mudstone.

The fluvial sandstones are richer in rock fragments and have less quartz (approximately a 15 to 20 per cent difference) than equivalent marine sandstones. The coarsest and more quartz-poor sandstones and gravels tended to be trapped on the delta plain, and a large portion of the rock fragments delivered to the sea was subsequently destroyed by abrasion along the beach. This had a significant effect on diagenesis, resulting in the fluvial sandstones, especially the well-developed point bars, having the best porosity and permeability. The quartz-rich, predominantly marine sandstones were preferentially cemented by silica, while primary porosity in the quartz-poor, fluvial sandstones was largely preserved through this stage. The fluvial sandstones then were preferentially cemented with calcite which later dissolved. Permeability in the finest grained sandstones was substantially destroyed by compaction, by the authigenic growth of clays, and by quartz overgrowths.