Influence of Regional Tectonics and Local Structure on Deposition of Morrow Formation in Western Anadarko Basin

Abstract

In the Panhandles of Texas and Oklahoma, the Morrow Formation is the basal transgressive, predominantly clastic unit of the Pennsylvanian System unconformably overlying the Mississippian. The upper boundary is delineated by the base of the Thirteen-Finger limestone, an electric-log marker, and the lower boundary by the Pennsylvanian-Mississippian contact. The Panhandle area is bordered on the west by the Sierra Grande uplift and on the south by the Amarillo uplift. The major tectonic features within the area are the western Anadarko basin, southern Hugoton embayment, Cimarron arch-Keyes dome, Dalhart basin, and several smaller structures in the unstable belt marginal to the deeper Anadarko basin. For the purpose of this study, the Morrow Formation was subdivided into three lithologically distinct units that are recognizable on electrical and sample logs. The lower unit, a wedge of predominantly terrigenous clastic material, is overlapped by the relatively calcareous middle unit. In the predominantly shaly upper Morrow, marine and lagoonal shale grades upward into non-marine shale and coal near the top of the unit. This suggests filling of the basin which caused a temporary retreat of marine environments prior to renewed transgression in the post-Morrow units. Regionally, Mississippian rocks are progressively truncated away from the basin axis. The absence of Chesterian beds over several structures suggests local pre-Pennsylvanian movement. Lower Morrow sandstone accumulations correlate closely with the configuration of the pre-Pennsylvanian surface. Apparently, significant structure existed before, or developed during, deposition of the lower unit. Isopachous and structural data indicate continued activity of these structures throughout Morrow deposition. Distribution of coarse terrigenous clastics within the predominantly fine terrigenous clastic Morrow Formation suggests one source of sand west of the Panhandle area and a second source on the northeast. The Wichita-Amarillo uplift south of the depositional basin began to supply coarse clastic material during deposition of the upper Morrow unit. In the lower unit, widespread sheet sandstone and sandstone beds that parallel trends of early Morrow shorelines are interpreted to be nearshore marine sediments deposited as the early Morrow sea overlapped onto the shelf areas. The thin, lenticular upper Morrow sandstone bodies are interpreted as having been deposited by a complex stream system flowing southeast toward the basin axis.