The terminology introduced by Rich (1951) can be applied to the subsurface, post-Canyon rocks of west-central Texas. Rich's term "unda" denotes an environment within the zone of wave action; "clino" denotes a slope environment, and "fondo" denotes an environment in the deepest part of a sea. West-east cross sections in the subject area show a cyclic development of the unda, clino, and fondo environments advancing generally westward as the Midland basin was filled. The filling of the Midland basin was accomplished by repeated, alternating phases of deposition; a clastic or sand-shale phase and a carbonate-black shale phase. The term "cyclo-phase" is used to describe such deposition and includes rocks formed in all three environments: unda, clino, and fondo. Each phase of d position was probably related to sea-level. Turbidity currents were possibly responsible for transporting material that was formerly of unda origin to a topographically lower environment.

Several oil fields of west-central Texas produce from cyclo-phase sandstones. These sandstones are locally called "Canyon sands" but it can be shown that they are actually Cisco or younger in age.

Stratigraphic, structural, and depositional conditions are variously responsible for hydrocarbon accumulation in certain cyclo-phase fields of west-central Texas. Anticlinal conditions are responsible for trapping in the undaform sandstone of the Group 4,000^prime pool. An updip extension of the sand body is responsible for accumulation in the clino-fondoform sandstones of the Lake Trammel and Sweetwater "Canyon Sand" fields. The configuration of the undaform edge encouraged migration of oil into the clinofondoform sandstone of the Flowers field. Local thickening of an undaform sandstone forms a trap in the North Hamlin field.

Because of the cyclic nature of the upper Pennsylvanian-lower Permian deposition, many more fields of the types discussed may be anticipated in the cyclo-phase sandstones of west and west-central Texas.