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Mechanical Deformation Controlled by Amount of Organic Matter, Green River Formation, Colorado

George J. Grabowski, Jr., David R. Pevear

Sedimentary structures in profundal lacustrine sediments of the Mahogany zone of the Green River Formation, examined in a core from the center of the Piceance Creek basin, record (1) early compressive ductile, (2) later compressive brittle, and (3) latest tensile brittle deformation after deposition. The degree of ductility and timing of deformation were controlled by the amount of organic matter (OM) in the sediments.

Dolomites (mean 3% OM) were relatively nonductile during early burial and contain rare microfaults, loop structures (a variant of microfaults), and dewatering structures. Stylolites and hairline vertical fractures crosscut these early compressive ductile features and record compressive brittle deformation caused by continued burial. Sediments with more organic matter (mean 9-14%) were more ductile during early burial. These kerogenites are characterized by loop structures, tuffaceous and dolomitic laminar boudinages, microfaults, and rare horizontal shear folds. Organic-rich kerogenites (mean 20-25%, maximum 46% OM) were extremely ductile during burial, and contain abundant loop structures an shear folds. Where overlain by denser sediments, they have been thoroughly deformed by boudin ge processes to form autobreccias, previously termed "blebby and streaked" oil shale. Kerogenites do not have either stylolites or hairline vertical fractures.

Following maximum burial, the Mahogany zone was unburied to its present depth of 300 m. This unloading caused tensile brittle fracturing of all rocks, with accompanying nonferroan calcite cementation and evaporite mineral dissolution.

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