Characteristics and implications of sedimentary deformation features in the Green River Formation (Eocene) in Utah and Colorado

The Eocene Green River Formation and associated fluvial strata host a large variety of widely distributed and laterally extensive synsedimentary to early post-depositional deformation features. Although mostly overlooked in past stud-ies, these structures provide insight into the rheology of the sediments and hydrodynamic conditions at the time of deformation. They also have implications about basin configuration and bottom topography, as well as the structural history of the region.In the Uinta and Piceance Creek Basins of Utah and Colorado, deformed intervals are present in littoral (sand-dom-inated) to profundal (oil shale) lacustrine deposits, as well as fluvial facies. Deformation structures exhibit a wide range of morphology, size, and style, ranging from ductile (convolution, folding, load and pinch-and-swell structures) to brittle (microfaults, breccias), to sedimentary injection into fissures and cracks forming dikes, to various mass transport deposits. In most cases, these layers are bound above and below by undisturbed beds of similar facies with horizontal bedding planes, which implies short-lived but recurring events that affected only sediments with a susceptible rheological state. Deformation typically occurred intrastratally, i.e. under shallow burial. The variations in deformation style, size, morphology, and areal extent are related to differences in grain size and type, water content, cementation, organic content, thickness of susceptible material, lateral facies variation, and the nature of the driving force. Some of the sedimentary injection features overprint each other, indicating multiple events affecting a single interval.We interpret most of these features as seismically induced (‘seismites’) based on: (1) the tectonic setting of the basin; (2) the sedimentary environment and sedimentological characteristics of the host succession; (3) the lateral extent of deformation structures; (4) their recurrence at different stratigraphic levels; and (5) their similarity to those ascribed to seismically induced deformation elsewhere and reproduced experimentally. They developed as a result of increased pore pressure and a complex combination of extensional, compressive, and shear stresses acting in many directions caused by shaking during strong earthquakes. Mass transport deposits indicate in situ disruption and failure of the imperceptibly dipping lake floor. These ‘seismites’ represent the direct sedimentary record of syndepo-sitional tectonic activity in the Uinta Basin.