Geologic map of Lake Francis Case region showing study localities (marked as stars). (1) south of South Dakota Highway 44 bridge; (2) north and immediately south of SD 44 bridge; (3) Buryanek Recreation Area; (4) west of the town of Academy (unnamed game production area); (5) Boyer Game Production Area; (6) Dude Ranch Lakeside Use Area; (7) cliffs north of Chamberlain; (8) Kiowa Game Production Area; (9) cliffs downstream from Fort Randall Dam. Reconnaissance observations were made at numerous other localities. BPVA observations made at sites 1–6.
Photographs of the BPVA in stratigraphic context. Labels as follows: (1) marls in upper part of the Niobrara Chalk; (2) contact between the Niobrara Chalk and Pierre Shale; (3) Ardmore ash layers in Burning Brule Member; (4) BPVA with top and bottom marked by arrows; and (5) modern soil profile. Bar scales approximate. A, Site south of Highway 40 bridge (site 1 of Fig. 1), coordinates = 43.366297° −99.120225°. B, Site along Lake Francis Case between sites 5 and 6 of Fig. 1, coordinates = 43.688377° −99.446582°. C, Site immediately south of Highway 40 bridge (site 2 of Fig. 1), coordinates = 43.382770° −99.141760°. The yellow jarosite that replaced the original gypsum vein fill is particularly evident here.
Photographs of the BPVA. Bar scales approximate. A, View of veins in sub-vertical cliff face between sites 1 and 2 in Figure 1. B, Vertical view of typical vein network geometry (same outcrop as in A). Note 55 mm camera lens for scale. C, Close up of vein at site 5 in Fig. 1, with core of selenite, and thin margin of yellow jarosite. D, Same outcrop as C, but near more highly weathered margin, with vein composed of all jarosite and with a more irregular margin.
Circular histogram plots with sliding sectors of observed strikes from the study sites overlain on a model distribution of preferred orientations (see text for further explanation). A, Readings from the BPVA from sites 1–3. B, Readings from BPVA at site 5. C, Readings from the underlying Niobrara Chalk at sites 1–9.
A, Photograph of weathering profile in the Niobrara Chalk marl (from near site 7, Fig. 1). Bar scales approximate. Labels as follows: (1) darker and still organic-rich, relatively unweathered marl; (2) weathered rind of white-colored marl; (3) Quaternary or younger colluvium and alluvium. B, Fine-grained and fibrous gypsum veins along an older joint with a well-defined margin of weathered host rock within fresher marl of the Niobrara Chalk. Note coin for scale.