The Castlegate Formation, part of the Mesaverde Group in east-central Utah, is a dominantly fluvial succession showing variable channel-belt stacking and some significant brackish water influence. The formation consists of 1) a lower succession of amalgamated fluvial sandbodies (lower Castlegate), 2) a middle interval characterized by isolated, tidally-influenced sandbodies and well preserved floodplain deposits (middle Castlegate) and 3) an upper interval of amalgamated sandbodies (Bluecastle Tongue) similar to the lower Castlegate member. It is within the tidally and brackish water influenced (characterized by Teredolites, Cylindrichnus, double mud drapes, tidal bedforms and multiple reactivation surfaces) middle Castlegate interval that an upward fining succession of tidal flat deposits has been observed. In outcrop, a typical succession is mud-prone and composed of flaser and wavy bedding overlain by lenticular bedding, followed by coal. Thick, ripple-laminated sandstones often punctuate the succession, some of which are interpreted as tidal sandwaves, oriented in an ebb-current direction. This overall fining upward trend is interpreted in terms of the progradation of tidal flats, possibly in the uppermost reaches of an estuary. The flaser and wavy bedding represents deposition in the mixed flats, the lenticular bedding would indicate mud flats and the coal that caps the succession was deposited in a salt marsh. Both tidally influenced distributaries and complex prograding sandwaves occur within the tidal flat succession.

This tidal flat succession has been traced along the Book Cliffs, 60 km in a basinward direction, from Willow Canyon, where it is 10 m thick, to Horse Canyon, where it is approximately 30 m thick. At Horse Canyon there is a greater abundance of flaser and wavy bedding, although it interfingers with fluvial sandbodies and coastal plain carbonaceous shale. The prograding tidal flat model is consistent with the overlying succession of laterally accreting sandbodies and floodplain deposits indicative of meandering streams, suggesting a final regression for the Castlegate sequence. The lower, backstepping portions of the identified tidal flat succession is believed to be time-equivalent with the Anchor Mine Tongue transgression at the time-equivalent shoreline around the Green River area.