Description and origin of the lower part of the Mesaverde Group in Rifle Gap, Garfield County, Colorado.

Rifle Gap cuts through the central part of the Grand Hogback and the gap exposes the entire Upper Cretaceous Mesaverde Group. The best outcrops are on the roadcuts through the coal-rich lower part of the group. These roadcuts border the Rifle Gap dam and reservoir on the southwest, where the coal-rich section can be examined with little climbing away from the road. This coal-rich section consists of 1700 ft (518 m) of uppermost Mancos Shale, overlain by the Iles Formation and the lower part of the Williams Fork Formation of the Mesaverde Group. These formations are composed of members and informal units formed in a late Campanian coastal setting of deltas and intervening strandplains which supported vast peat swamps. The age and the coastal subenvironments of deposition of these units are indicated by fossils and sedimentary structures which can be studied on the roadcuts. The highest, most prominent roadcut exposes a thick, white sandstone called the Trout Creek Sandstone Member, Iles Formation, the correlations of which have been reassessed recently. This sandstone previously was correlated to a sandstone exposed at New Castle: the Rollins Sandstone Member, Mesaverde Formation, of the southern Piceance Creek basin. However, later field mapping showed that the two sandstones cannot be traced to each other but are separated by 310-450 ft (93.9-136.4 m) of strata along the central Grand Hogback. The recorrelations of the Trout Creek and adjacent sandstone members have influenced reconstruction of the local, late Campanian paleogeography of the oscillating shoreline. The sketches of paleogeographic reconstruction show an oscillating shoreline that trended northeast-southwest. This shoreline migrated inland to the area of Wolf Creek on the northern border of the Piceance Creek basin, and seaward only as far as the area of New Castle, until deposition of the Haas sandstone unit of the lower Williams Fork Formation. During the deposition of the Haas, the shoreline continued migrating seaward past New Castle. The general trend and seaward extent of this shoreline are based on regional correlations of ammonite zones and on local paleocurrent indicators from deposits of longshore drift and storm scour exposed in the central Grand Hogback. -Author