The base of the Gaptank Formation is redefined as the base of the conglomerate member, and the underlying Chaetetes-bearing limestone member is transferred to the Haymond Formation. This change gives the Gaptank Formation a more distinctive lithologic base having greater areal continuity. The Gaptank Formation is subdivided into a conglomerate member (bottom) and a limestone member (top) that intertongue, in part, with a sandstone and shale member (middle).
Gaptank strata display cyclical repetition and illustrate differences in deposition arising from differences in subaqueous topography. Three submarine land forms persisted as distinctive depositional sites: a shallow shelf (undaform), a sloping surface (clinoform), and a basin (fondoform). Progressive basinward extension of the shallow-shelf areas resulted in more widespread shallow-water environments during deposition of the Gaptank Formation. In outcrop, many individual Gaptank limestone beds can be traced laterally from a shallow-shelf facies into a shelf-edge facies, then into an upper-slope facies. Other modifications in the sedimentary pattern resulted from gentle warping in different areas and at different times during deposition. Comparison of lithologic and depositional features in the Gaptank Formation with those of the late Pennsylvanian strata along the eastern shelf of the Midland basin in central and north-central Texas shows striking similarities.