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Dolomite Decollements--Exception or Rule?

J. L. Coleman, Jr., J. A. Lopez

Classical thrust-belt papers emphasize that major decollement development is restricted to structurally incompetent units, i.e., shales or evaporites. Detailed study of thrusts in the southern Appalachians of Alabama, Georgia, and Tennessee suggests that in the middle third of the fold and thrust belt, a zone exists where the main structural translation is along a decollement within the 4,000-ft thick, Cambrian-Ordovician Knox Group dolomites. This decollement apparently varies stratigraphically within the Knox, suggesting that no one stratigraphic interval controls its position. Seismic stratigraphy tied to well and outcrop data shows that dolomite decollements appear to be tectonically placed adjacent to shale-basin footwalls.

Apparently, upper level decollement folding may contort and compress the shale beds, so that bedding-plane slip becomes ineffective and the shale "pile" acts as a cohesive, ductile mass, resistant to faulting and energy release. With continued horizontal compression, the rock pile must give. The shear strength of the shale pile is greater than the underlying dolomite, which either detaches along the first available porosity zone or along a zone that develops shear brittle fracture due to an inherent structural weakness. Three possible features may promote decollement development within a dolomite section: (1) the dolomite section is relatively more competent than adjacent, less competent units and, therefore, may concentrate stress within it as opposed to the incompetent section; (2) orosity zones within a basically nonporous dolomite section may be inherent planes of weakness, which may be overpressured if in fluid communication with overpressured shales; and (3) evaporitic units associated with shallow marine units may have been present within the dolomite section and acted as glide surfaces. We believe features 1 and 2 combined may be more critical than any single factor in controlling the detachment level.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.