ABSTRACT: Tectonics, Eustasy, and Sequence Stratigraphy -- The Middle Pennsylvanian-Wolfcampian of the Permian basin

SARG, J. F. "RICK," Independent Consultant, Midland, TX

The depositional patterns of sedimentary rocks are controlled by the interaction of tectonics, eustasy, and sediment supply. Tectonics and eustasy combine to cause relative changes of sea level that control the accommodation space for sediments. Sediment supply controls how much of the accommodation space is filled.

Tectonics has the greatest effect on accommodation. Long-term basin fill histories are interpreted as first-order tectonic events. Second-order tectonic events are initiated by increase in the rate of subsidence that progressively decay and may culminate in a period of uplift or structural growth. Three second-order tectonic events characterize the middle-late Paleozoic history of the Permian basin. These events occur over tens of millions of years and are (1) Givetian-Meramecian, (2) Chesterian-Desmoinesian, and (3) Missourian-Guadalupian. Sediment response to these tectonic events include initial backstepping carbonate platform deposition, followed by deepening and starvation of the basin areas resulting in black shale deposition. Prograding carbonate/deltaic sequences then progress vely fill the basin. Repeated cycles of Marathon-Ouachita thrust loading, compressional anticlinal growth, and relaxation are interpreted to have caused these tectonic events.

Eustasy controls the rate of relative sea level change and is the major controlling factor on the timing of stratigraphic discontinuities. The discontinuities bound sequences and subdivide them into systems tracts. The Middle Pennsylvanian-Wolfcampian of the Permian basin can be subdivided into 19-21 third-order sequences (1-5-m.y. duration) and include six Desmoinesian, four Missourian, five to six Virgilian, and four to five Wolfcampian cycles. The cyclothems of the mid-continent represent higher order depositional sequences that stack in an orderly fashion to comprise the systems tracts of the third-order sequences.

AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)