SEDIMENT DEFORMATION IN THE JORDAN SANDSTONE AND INFLUENCES ON MODERN HYDROGEOLOGY

Soft to firm sediment deformation features occur over a ~10m stratigraphic interval that straddles a contact between hummocky cross-stratified and overlying trough cross-stratified sandstone lithofacies of the Upper Cambrian (Furongian) Jordan Formation, southeast Minnesota, U.S.A. Features include ball and pillow structures, sandstone dikes and sills, diastasis cracks, and possible fluid escape structures. A conspicuous deformation feature occurs in a coarse- to very coarse- grained trough cross-stratified lithofacies, such that decimeter-thick beds appear to have been boudined. Boudins range from elongate, rounded blocks to angular, brecciated blocks, and are present across hundreds of meters of strike exposure. The variability of boudins is interpreted to reflect a range of competence of the sandstone during deformation Shaking caused by seismic events, pore water overpressure, or resonant vibrations from passing storm waves are potential drivers of the deformation. The primary goal of this study is to document and describe these deformation structures to better understand their genesis and influence on the physical properties of the Jordan sandstone. One hypothesis being tested is that bedding parallel macropore groundwater conduits, common in the Jordan Sandstone today, were incipiently developed early in the burial history of the formation, via the same processes responsible for the deformation features.