Abstract

Pennsylvanian sedimentation in the Palo Duro basin was strongly influenced by tectonic events. Highlands surrounding the basin were uplifted early in the Pennsylvanian Period and were major sources of clastic sediment. Arkosic sandstones ("granite wash") contain abundant feldspar and granite rock fragments derived from exposed Precambrian basement rocks.

Granite wash deposits are thickest in the northeastern and northwestern portions of the basin, and lobes of sandstone extend to the southern basin margin. Individual beds, which are 10 to 40 ft (3 to 15 m) thick and are laterally discontinuous, cannot be correlated more than a few tens of miles. Granite wash deposits vary from medium-grained sandstone to conglomerate, and upward-fining sequences are common. The sandstones contain scours and large-scale trough or foreset cross-beds. Limestone commonly is interbedded with granite wash sandstone and shale.

Thick wedges of Pennsylvanian granite wash adjacent to mountain fronts are characteristic fan delta deposits. Coarse clastic material was carried off the uplifts to adjacent fans by high-gradient braided streams. Proximal to the source areas, sediments are entirely terrigenous clastics, which were probably deposited in subaerial delta plain environments. Coal and thin limestone beds are interbedded with delta plain sandstones. Carbonates become more abundant downdip where they were deposited periodically in subaqueous, distal fan delta environments. Periodic movement along faults may have been responsible for the initiation of major clastic cycles.