CHARACTERISTICS AND IMPLICATIONS OF EARTHQUAKE-INDUCED SEDIMENTARY DEFORMATION FEATURES (SEISMITES) IN THE GREEN RIVER FORMATION (EOCENE), WYOMING, COLORADO, AND UTAH, USA
Lacustrine sediments of the Green River Formation were deposited during Eocene (53-45 Ma) in interconnected foreland basins along the Front Range of the Rocky Mountains across an area of > 62 150 km2 in present-day Wyoming, Colorado and Utah, USA. These deposits are up to 2 km thick, dominated by finely laminated carbonate mudstones. Deposition was taken place in an active tectonic setting during the late-stage activity of the Laramide Orogeny and the Sevier Fold and Thrust Belt. Sedimentary deformation structures are widespread in these deposits and despite the large number of geological studies devoted to the stratigraphy, they are poorly understood.
The principal aim of this thesis was to explore and document previously overlooked or unrecognized deformation structures through field observations of exceptional outcrops in Wyoming, Colorado and Utah. This work represents the first detailed study on such features in the Green River Formation, the best-documented ancient lake systems worldwide.
The investigation of outcrops revealed an unparalleled suite of sedimentary deformation structures in all the sub-basins, in a variety of depositional settings. These include convolute lamination, load structures, folds, hybrid brittle–ductile features, sedimentary dikes, microfaults, fluid-escape features, oil shale breccias, and mass-transport deposits. They occur in numerous laterally extensive deformed horizons, mainly in laminated carbonates, that could be traced and correlated laterally for >25 km. Their morphological attributes, lateral extent, and recurrence at different stratigraphic levels, along with the low-energy and low-gradient sedimentary environment and the active tectonic setting, indicate that they were induced by ancient syndepositional earthquakes and can be interpreted as ‘seismites’. This study has demonstrated that these structures can be used to infer tectonically active periods and to refine the paleotectonic history of the area. The reinterpretation of features previously described as the result of desiccation helped to better understand the depositional setting of the Green River Formation. Moreover, the occurrence of seismites was related to synchronous changes in basin evolution for the first time, implying that many abrupt changes in the history of the Green River Formation were tectonically, rather than climatically, driven. Clearly, seismites should be more widely utilized to recognize the contribution of tectonics on basin history
