A STRUCTURAL AND PETROGRAPHIC STUDY OF A PALEOPROTEROZOIC D1 FOLD IN METAMORPHOSED SEDIMENTARY ROCKS, SOUTH DAKOTA

Along Sheridan Lake Road in the Black Hills uplift is an exposure of metamorphosed sedimentary rocks that displays structures from the first two Paleoproterozoic deformation events in the Black Hills. This study is focused on the folded and metamorphosed sediments exposed in a 60 m long roadcut. The exposed rocks are interpreted to be part of a turbidite sequence composed of two rock types including an impure quartzite and a biotite-chlorite phyllite. Relict graded bedding indicates that bedding is overturned. The fold is steeply plunging to the SE and has been interpreted by previous workers to represent a classic example of a fold associated with earliest Paleoproterozoic deformation event (D1). No fabrics associated with this first event have yet to be identified in this outcrop, however; these rocks do display a strong foliation which strikes approximately S30^oE and dips approximately 80^oSW. This foliation is interpreted to be related to the second deformation event (D2) that may have overprinted evidence of original fabrics related to D1. The NW limb of the fold is over turned and dips to the NE, and the SE limb has a dip to the SW. Analysis of microstructures in orientated samples taken at the site will be tested for the presence of D1 fabrics. This analysis will be used to further characterize the structural features related to the D1 event. This will improve correlation to the regional deformation events.