Cotter reported in 1975 that Deltaic deposits at the base of the Upper Cretaceous regressive elastic wedge known as the Ferron Sandstone Member of the Mancos Shale are exposed in depositional dip section for more than 45 miles (72 km) along the southern part of the Castle Valley in east-central Utah. These deposits were formed as the Late Cretaceous Last Chance Delta prograded northward as a high-constructive lobate complex. In the relatively stable depositional basin the facies were dispersed laterally to produce a deltaic sequence that typically comprises 40 to 80 feet (12 to 24 m) of siltstone, sandstone, and some coal. Thin prodelta deposits at the base are overlain by a thicker, laterally continuous sheet of delta front deposits, and these, in turn, are overlain by the coal-bearing delta plain facies. Delta front beds typically are horizontal or have slight primary inclination to the north, but several delta-front sequences are steeply inclined to the west or north-west. These inclined Gilbert-type deltaic sequences possibly developed as distributary channels issued into a less saline western embayment.
Cotter concluded that in the framework of this northerly deltaic depositional system, the laterally continuous basal sandstone zone is most likely the delta front facies of the Vernal Delta. Higher sandstone zones are distributary and fluvial channel deposits. Published reports of relationships in the Ferron Sandstone of the Wasatch Plateau gas fields present rather sketchy and, in some respects, conflicting information (Edson et al., 1954; Walton, 1954). It appears that a significant portion, though not all, of the gas is produced from relatively tight and very low permeability sandstone in the laterally continuous delta front facies at the base of the Vernal Delta sequence. Additional gas production is from the higher, less widespread, distributary and fluvial channel sandstones.
Coal/lignite sequences are known to be present NW of Hanksville near the Dirty Devil River. An attempt to mine this low-grade coal/lignite in the early 1980s. Lenardite is also known to be present in the underclays of the two intermittent, highly faulted thin coal seams present.
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