RED-WING-CREEK STRUCTURE, NORTH-DAKOTA - PETROGRAPHIC AND GEOCHEMICALSTUDIES, AND CONFIRMATION OF IMPACT ORIGIN

The 9 km diameter Red Wing Creek structure, North Dakota, is located w ithin the oil-rich Williston Basin at 47 degrees 36'N and 103 degrees 33'W. Earlier geophysical studies indicated that this subsurface struc ture has a central uplift, surrounded by an annular crater moat, and a raised rim. Breccias were encountered during drilling between similar to 2000 and 2800 m depth in the central uplift area, and the presence of shatter cone fragments in drill core samples was suggested to indi cate an impact origin of the Red Wing Creek structure. We studied the petrographic and geochemical characteristics of samples of well cuttin gs from two boreholes at the center of the structure: the True Oil 22- 27 Burlington Northern and True Oil 11-27 Burlington Northern wells. W e found planar deformation features (PDFs) in quartz with up to three sets of different crystallographic orientations in sandstone- and silt stone-dominated samples from the True Oil 11-27 borehole. U-stage meas urements of the crystallographic orientations of the PDFs showed the o ccurrence of the shock-characteristic (0001), {10 (1) over bar 3}, {10 (1) over bar 2} {11 (2) over bar 2}, {10 (1) over bar 1}, and {51 (6) over bar 1} orientations, with a dominance of (0001) and {10 (1) over bar 3} orientations. The relative frequencies of the orientations ind icate a shock pressure of at least 12-20 GPa. These results provide un ambiguous evidence for shock metamorphism at Red Wing Creek and confir m that the structure was formed by impact.