Indicators of paleoseismicity in the lacustrine sediments of the Green River Formation (Eocene), Wyoming, Colorado and Utah

Balázs Töró, Brian R. Pratt, and Robin W. Renaut
University of Saskatchewan, Department of Geological Sciences
Saskatoon, Saskatchewan, Canada
[email protected]

Lacustrine sediments of the Eocene Green River Formation (53–43 Ma) were laid down in an interconnected lake system in Wyoming, Colorado and Utah during the Laramide orogeny. Despite the known syndepositional tectonic activity, hitherto deformation features related to ancient earthquakes had yet to be explored.

Laterally extensive horizons of soft-sediment deformation are common in littoral to profundal deposits in all the subbasins. Deformation style ranges from plastic (convolution, folding) to more brittle (e.g. fragmented intervals, breccias, microfaults). The style of deformation was governed by the rheological properties of the sediment, which in turn were influenced by grain size and morphology, richness in organic matter, and its diagenetic state. Sediment injection features indicate hydrofracturing, dewatering and fluidization which formed cm- to m-scale cracks and dikes. Injections in littoral to sublittoral carbonates are generally filled with silt- or sand-sized sediment, with incorporated fragments of the host material. In profundal sediments the injections are commonly filled with carbonate mud, which in many cases is silicified.

All these structures are interpreted as having developed as a result of increased pore pressure and vertical or horizontal stresses induced by seismic activity. Mass transport deposits, triggered by seismic shaking, contain chaotic deformation features. In general, deformed horizons are separated by undeformed strata, which implies short-lived events that affected only near-surface sediments possessing a susceptible rheological state at the time. In many cases deformed horizons are overlain sharply by beds of contrasting lithology and/or degree of bioturbation, suggesting abrupt tectonically driven changes in lake sedimentation.

AAPG Search and Discovery Article #90183©2013 AAPG Foundation 2013 Grants-in-Aid Projects