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Indicators of Paleoseismicity in the Lacustrine Sediments of the Eocene Green River Formation, Wyoming, Colorado and Utah (USA)

Abstract

The carbonate-rich sediments of the Eocene Green River Formation (53–43 Ma) were deposited in a complex lake system in present-day Wyoming, Colorado and Utah during the Laramide orogeny. Although syndepositional tectonic activity has been inferred previously for this interval, sedimentary deformation features linked to ancient earthquakes have never previously been studied. Stratigraphic intervals with synsedimentary deformation features were studied at several locations in all the sub-basins to provide information on the paleotectonic history of the area. Laterally extensive deformed horizons are common in laminated deposits of Fossil Basin (WY), and include folds, water-escape structures, dikes, microfaults, and chaotically brecciated intervals that preserved flow structures. These indicate hydrofracturing, dewatering and the liquified state of sediments during deformation. Sediments deposited in sublittoral environments of the Bridger Basin (WY), at the boundary between the Tipton Shale and Wilkins Peak members, also show evidence of synsedimentary deformation. These features, some traceable laterally for tens of kilometers, imply tectonically influenced changes in sedimentary processes during that time. Pervasive deformation features are also present in the Mahogany Oil Shale Zone in the Piceance Creek and Uinta basins (CO, UT). Behavior ranges from brittle (fragmented laminites and faulted beds) to plastic (convolution, folds), to cm- to m-scale cracks filled with injected sediment. Deformation styles were controlled by the rheological properties of the host sediment. Grain size and morphology, abundance of organic matter (which influences ductility) and degree of diagenesis (compaction, cementation) all, in turn, influenced the deformation structures. All these structures resulted from increased pore pressures from cyclic loading, and vertical and horizontal stresses induced by ground motion. Mass-transport deposits, triggered by seismic shaking, contain chaotic deformation features. Typically, undeformed strata separate the deformed horizons, which indicates that brief (tectonically-driven) events affected only near-surface sediments that were rheologically susceptible to syndepositional deformation. In many examples, the deformed horizons are overlain by beds showing contrasting lithology and/or degrees of bioturbation. This implies that many abrupt changes in the Green River Formation were tectonically, rather than climatically driven.