PRECAMBRIAN GEOLOGY OF WIND CAVE NATIONAL PARK, BLACK HILLS, SOUTH DAKOTA

As part of a collaboration with the National Park Service and the South Dakota Geological Survey to prepare a geologic map of Wind Cave National Park, we mapped the previously undivided Precambrian rocks exposed in a nearly 20 km² area in the northwest portion of the Park. Four distinct meta-sedimentary units were defined in the area. These are intruded by bodies of Harney Peak-equivalent granite. Meta-sedimentary Unit 1 (lowest) is dominantly indistinctly bedded coarse-grained quartz-biotite-microcline-sillimanite schist interbedded with and overlain by quartz-plagioclase-biotite-muscovite +/- microcline rocks, bedded on a 5 cm to 1m scale. Unit 2 is dominantly clean quartzite and interbedded siliceous schists. Quartzite beds are typically 20 cm to 2 m thick. Poorly exposed fine-grained quartz- muscovite schist, fine-grained quartz-biotite schist, and coarse-grained quartz-biotite-sillimanite schist make up roughly half the unit. Unit 3 is thin-bedded to laminated quartz-biotite +/-microcline +/- muscovite +/- sillimanite +/- plagioclase +/- garnet schist interbedded with 10 cm to 1 m biotite quartzite beds. This unit is heavily intruded by granite pegmatite sills and is poorly exposed in the area. Unit 4 is clean quartzite interbedded with quartz- biotite schist and quartz-biotite-garnet-sillimanite +/- muscovite schist. In sum these units likely represent metamorphosed coastal platform deposits. Several hundred individual bodies, primarily sills, of Harney Peak-equivalent granite pegmatite in aggregate occupy 4-5% of the area.

Layers in much of the area dip steeply to moderately WSW to W and are folded around south-plunging to horizontal axes into overturned tight to isoclinal folds. In the eastern part of the area, dips are more moderate in a late stage dome interpreted to overlie a body of granite. Another large granite body is exposed north of the park, and exerts structural influence on the rocks in the northern part of the park, deflecting fold axes and layering to the northeast. The dominant foliation in the area is penetrative and, as evidenced by outcrop scale folds, deformed by the tight folds. Sparse spaced foliation is axial planar to these folds, and a late low angle foliation, likely related to granite doming, is present in a very few places.