PALEOSEISMIC INDICATORS IN THE LACUSTRINE GREEN RIVER FORMATION (EOCENE, USA) - CHARACTERISTICS AND IMPLICATIONS

Their typically heterolithic nature and deposition in an overall quiet-water environment make lacustrine deposits particularly suitable for the analysis of soft-sedimentary deformation features induced by shaking during ancient earthquakes. Lacustrine sediments of the Eocene Green River Formation (53 – 43 Ma) accumulated in an interconnected lake system of ponded basins in Wyoming, Utah and Colorado during the Laramide orogeny. Despite the known syndepositional tectonic activity, horizons of soft-sediment deformation in the Green River Formation have not previously been studied and linked to tectonic movements.

Synsedimentary deformation features were classified and interpreted according to their geometric characteristics at several locations in the various sub-basins. Laterally extensive deformed horizons are present in the laminated lacustrine deposits of Fossil Basin. Deformation features include folds, water escape structures, sedimentary dikes, microfaults, and chaotically brecciated intervals with flow structures indicating a fluidized state of sediment during the deformation event. The style of deformation was governed by the varying rheological properties of the sediment and the various driving forces.

Numerous deformation features were also found in the Mahogany Oil Shale Zone, an extensive stratigraphic marker present in the Piceance Creek and Uinta basins, which contains several petroliferous shale beds. Deformation style ranges from brittle (fragmented laminites and faulted beds) to plastic (convolution, folding), to sedimentary injection into cm- to m-scale cracks in the form of dikes and dikelets. The degree of ductility in these profundal deposits was controlled by the amount of organic matter in the sediment.

Based on the tectonic setting of the basin, the detailed characteristics of the deformed features, and the presence of undeformed layers adjacent to deformed horizons, these features are interpreted as having been developed as a result of increased pore pressure and vertical and horizontal stresses induced by seismic activity. As soft-sediment deformation features induced by earthquakes are indicative of synsedimentary tectonism, they provide information about the location and timing of the tectonic movements in or at the margins of the lake sub-basins.