CENOZOIC TECTONICS OF THE BLACK HILLS, SOUTH DAKOTA: DEFORMATION OF A UNIQUE WHITE RIVER GROUP (ORELLAN) LAKE DEPOSIT AND OTHER REGIONAL EVIDENCE

Deposition and deformation of Tertiary deposits provide clues of Cenozoic tectonic history in the Black Hills region of South Dakota. The Black Hills uplift resulted from Laramide basement deformation and is represented by a crescentric, asymmetric, doubly plunging anticline forming a positive topographic feature in the western Central Plains region. Deformation includes basement thrust faulting, west and east vergent monoclines, anticline and syncline pairs, and conjugate faulting. Subsequent exhumation of the Black Hills resulted in deposition of Cenozoic surficial deposits including the White River and Arikaree Groups. Recent studies within the 7.5 minute Rockerville Quadrangle, east-central Black Hills, have revealed evidence of a post-Laramide phase of uplift and deformation. Paleozoic source water springs fed a fresh water lake during the Late Eocene-Early Oligocene. Limestone deposited in this Orellan lake lies unconformity on the underlying Paleozoic and Mesozoic strata. A gentle tilt (approximately 5 degrees) to the northeast and folding of the lake deposit parallel to the long axis of the lake (NW-SE) is sub-parallel to older, low amplitude Laramide folds in the underlying strata. This suggests post-Oligocene deformation and rotation of the deposit.

Other indications of post-Oligocene uplift of the Black Hills region include normal faulting in the Badlands area to the southeast, and in the Short Pine Hills and Slim Buttes of northwestern SD. Offsets in these regions are generally less than a few hundred feet within the White River and Arikaree Groups. Documented timing constraints based on offsets in Tertiary strata indicate episodes of deformation during late Oligocene (31ma-25ma) and early Miocene (20ma) in Nebraska. Changes in base level are represented in the mid-Tertiary sedimentary record in areas surrounding the Black Hills. The Eocene Chadron Formation (White River Group) and the Oligocene Gering Formation and Miocene Upper Harrison Beds (Arikaree Group) contain coarse clastics, providing evidence of erosion of westward upland source areas. Uplift and erosion on a regional scale is likely related to a continuum of Laramide deformation of this northern Rocky Mountain region, creating an overall flexure with broad-beam style upwarping of the crust.