The Morrow Formation is the basal, transgressive, predominantly clastic unit of the Pennsylvanian System and unconformably overlies Mississippian. The formation is subdivided into three distinct units. The lower is predominantly terrigenous and is overlapped by the relatively calcareous middle unit. The upper is a predominantly marine and lagoonal shale that grades upward into non-marine shale and coal. Underlying Chester rocks are locally absent and distribution of sand bodies in the Morrow suggest movement prior or during deposition of the initial Pennsylvanian sediments and subsequent filling of the basin. A clastic source area is suggested west and northwest of the Panhandle area and another probably was northeast.

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