SCALE DEPENDENCE OF CHALCEDONY VEIN STRIKE UNIFORMITY WITHIN THE CHADRON FORMATION OF BADLANDS NATIONAL PARK

Fairly dense arrays of chalcedony veins occur locally within the Eocene Chadron Fm. (White River Group) of South Dakota and Nebraska. The subvertical veins are restricted to stratiform intervals that vary from 8 to > 20 m in thickness. Host rocks are volcanic ash rich siltstones. Veins are typically cms thick, zoned, display tip curls, and have structures indicating vertical shortening. Their stratiform distribution is consistent with formation by diagenesis related syneresis. Vein strike orientation patterns are the focus of this study. Over 1700 strike readings and GPS (hand-held) positions were recorded from three study sites in Badlands National Park. In the areas surveyed strikes were taken from the middle of every well exposed vein (avoiding tip curls). Chi-square tests on strikes from the sites indicate two have uniform/random strike distributions at scales of 1000s m^2 and larger, while the third is strongly non-uniform over an area of 10,000s m^2, with maximum values in bins from 0-10 degrees and 80-90 degrees. Previous work at Toadstool Geologic Park in NW Nebraska documented highly non-random vein patches with maximum values in bins of 50-60 and 95-100 degrees. A random/uniform distribution is consistent with a synersis generated polygonal pattern in the absence of tectonic stresses. However, within the two, overall uniform/random, Badland sites are commonly areas of several 100 m^2 whose veins (n>5) are all or mostly parallel, an extremely improbable geometry given a larger random/uniform distribution. Thus, vein strikes can be locally organized, but in aggregate and at a larger scale unorganized. The orientation pattern is scale dependent. Associated questions are: what are the scales and geometries of these organized patches, what produces local organization, is there an overall regional pattern not yet detected, and what does this indicate about local stress fields?