Superposition of Four Generations of Folds and Development of Horse Creek Dome, Black Hills, South Dakota

ABSTRACT

The study area is a part of the Precambrian basement of the Black Hills that was uplifted during Laramide Orogeny. Early Proterozoic meta-conglomerate and greywacke units occupy the study area and form a crescent fold which is unusual structural pattern, mapped in the Precambrian core of the Black Hills. This study aims to determine how the crescent shape was formed via fieldwork investigation and structural data analysis. Four Precambrian (F1, F2, F3 and F4) folding events affected the study area prior to the uplift. Minor folds indicated that F1 and F4 exhibited isoclinal folding, F2 reflected similar folding and F3 displayed kink band and chevron folding. F2 is associated with axial plane cleavage (S2), trends towards the west, and is associated with regional metamorphism. F3 trends NNW and is associated with spaced cleavage (S3). F4 trends WSW and is associated with spaced crenulation cleavage (S4). The area was structurally divided into five domains. Poles to bedding (S0) and axial plane cleavage (S2) equal area stereographic projections show steeply plunging maxima that have monoclinic symmetry, resulting from superposition of two oblique F2 and F3 folding events. Domain I contains the Horse Creek dome that resulted from superposition of F3 on F2. The meta-conglomerate unit formed a crescent fold, due to superposition of the first two generations of folds. The crescent fold follows the Horse Creek dome, trends and opens westerly, and it is parallel to F4 folds.

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