Paper No. 1
Presentation Time: 9:00 AM-6:00 PM
SIGNFICANCE OF AN ESE FRACTURE DIRECTION IN TERTIARY STRATA OF SOUTH DAKOTA AND NEBRASKA?
A rich array of fracture types in the Tertiary strata of Nebraska and South Dakota includes faults (predominantly normal), clastic dikes, veins (chalcedony, calcite and gypsum), joints, and occasional deformation bands. These fractures show a diverse array of morphologies orientation distributions and complex age relationships. Generally north trending features can be associated with reactivation of Laramide features (a ‘Black Hills’ trend). More NE to ENE trending structures have been associated with reactivation of basement features such as those along the Colorado Lineament. The Toadstool fault in northwest Nebraska falls in this category. However, a significant ESE to SE trending and regionally distributed set of features is largely unrecognized and unexplained. These features include normal faults and conjugate joints at Slim Buttes and Pine Hills in NW South Dakota, faults and clastic dikes in Badlands National Park, the White Clay fault in NW Nebraska, one of several fault and joint and chalcedony vein directions at Toadstool Geologic Park in NW Nebraska, and a joint set in Brule Formation strata in the North Platte River valley area of Nebraska. The faults and joints at Slim Buttes clearly formed before the unconformably overlying Arikaree Group strata were deposited. The White Clay fault is argued to have been active during and influenced the deposition of the basal Ogallala Group strata. The ESE trending faults at Toadstool postdate diagenetically produced silica mobilization and chalcedony vein formation, which is possibly coeval with Arikaree Group deposition and loading. Explanations for the regional ESE direction of fracturing include: a) reactivation over time of a consistently oriented, but unidentified, structural element; b) loading by a regionally consistent stress field that persisted from pre- to post- Arikaree Group deposition times; or c) loading by a regionally consistent stress field during an earlier and shorter time frame, followed by subsequent reactivation. The third possibility is favored at present with a pre-Arikaree inception of fracturing.