RE-EVALUATION OF THE PROTEROZOIC DEVILS ISLAND SANDSTONE, KEWEENAWAN RIFT, NORTHERN WISCONSIN

The Proterozoic Devils Island Sandstone of northern Wisconsin has been interpreted as a nearshore lacustrine deposit formed in the Keweenawan rift. This same interpretation had been given for the correlable Hinckley Sandstone in eastern Minnesota until recent study showed that the depositional environment more closely represents a floodplain in which braided streams interact with eolian dunes. The purpose of this project is to determine whether the Devils Island Sandstone also needs re-interpretation. Over the past three years, exposures along Lake Superior within Apostle Islands National Lakeshore and the Brule and Siskiwit Rivers have been measured and described. Four sandstone facies are similar to those in the Hinckley Sandstone. Beds and lenses of trough cross-strata, in some cases filling troughs scoured into underlying beds, are interpreted as predominantly fluvial in origin. Low-angle tangential cross-strata interpreted to be eolian dune strata. Sandy planar beds display subaqueous ripples, adhesion structures and wind-ripple laminations, indicating transitions among wet, damp and dry environments. Convolute bedding associated with all facies indicates rapid sand deposition on a saturated sand substrate. The Devils Island Sandstone contains an additional facies not present in the Hinckley Sandstone. This planar-bedded facies is composed of centimeter- to decimeter-scale sandstone beds interspersed with silty laminae, commonly in fining-upward packages. Sedimentary structures include mudcracks, subaqueous ripples, and small scale beds and lenses of cross-strata. Structures are also present within the silty planar facies that appear to be bedding-parallel, sinuous burrows; further study is needed to determine whether they are biogenic.