ABSTRACT: The conodont biostratigraphy of the lower "Wolfcamp Shale" and the fusulinid biostratigraphy of the underlying Strawn Group from Post West Field subsurface cores, Garza County, Texas, constrain the time of the subsidence that formed the northern Midland Basin. The cored wells are paleogeographically located on the northern part of the Horseshoe Platform in the northern Midland Basin. All fusulinid data are from the Strawn limestone and all conodont data are from the overlying "Wolfcamp Shale". Based on fusulinid biostratigraphy, the cyclic carbonate sequences of the Strawn Group in the Post West Field cores range in age from middle early Desmoinesian to latest Desmoinesian (Zones Df2, Df3, and Df4; Moscovian). There are unconformities at the base and top of the Strawn limestone. Based on conodont biostratigraphy, basinal mudstone deposition of the "Wolfcamp Shale" began during the middle Missourian on the Strawn erosional paleotopographic high at the PostWest Field. The lower "Wolfcamp Shale" (Wolfcamp D log unit = Penn Shale = Cline Shale) is middle Missourian to early Virgilian (middle Kasimovianâ€“early Gzhelian) in age, and the overlying Wolfcamp C2 log unit is early to late Virgilian (Gzhelian) in age.