Tectonically induced change in lake evolution recorded by seismites in the Eocene Green River Formation, Wyoming

Seismogenic sedimentary structures – seismites – provide an opportunity to understand the effects of syndepositional tectonics on lacustrine sedimentation. The lowermost Wilkins Peak Member (Eocene Green River Formation, Wyoming, USA) contains laterally extensive intervals displaying folds, load structures, mixed brittle-ductile deformation, microfaults, breccias and sedimentary dykes, which formed intrastratally in a low-energy, sublittoral lacustrine environment. They developed in situ by liquefaction and plastic or brittle behaviour of semi-lithified sediment, or by remobilization and injection of granular material. Their morphological attributes and lateral extent, zonation in deformation intensity and the depositional setting all imply deformation triggered by earthquake-induced stresses in shallow-buried sediment. Their stratigraphic distribution coincides with a shift in hydrological conditions from a balanced-filled to an underfilled lake type, independent of climate change. This study is the first to ascribe seismites to a pulse of tectonic activity that caused a synchronous change in lacustrine evolution.