SYNSEDIMENTARY LOW ANGLE NORMAL DETACHMENT IN WHITE RIVER GROUP STRATA OF NW SOUTH DAKOTA
The half grabens are associated with a low-angle normal detachment in the basal orange and white sandstones of the White River Group. High-angle normal faults within the Chadron and the overlying Brule Fm. connect in a listric geometry to the detachment which has moved toward the north-northeast. Tilted Fort Union strata and detachment structures suggest a deeper detachment also exists. This transport direction is based on the geometry of outcrop scale structures and stereonet plots and fault block rotation directions. Overlying the sandstone is a unit of smectitic brown mudstone. We propose that the mud may have acted as a sealing layer, leading to increased pore pressures within the underlying sandstones. A significantly thinner or missing brown mudstone to the south where deformation is also absent would be consistent with this proposal.
We observed fill geometries and sedimentary truncation of faults within the Brule Formation that indicates faulting was partly synsedimentary. Deposition possibly contributed to faulting by increasing the overburden, compacting the mudstones and further increasing the pore pressure. Measurements of paleocurrents in the sandstone of the lower Brule Fm. indicate a NW to N transport direction, different from the expected regional slope to the NE or E. This could be due to a local anomaly in fluvial meander direction or to fluvial diversion in response to the faulting.
Based on our data, we have concluded that the deformation in Reva Gap is not due to mass wasting as suggested in earlier literature. It is somewhat reminiscent of the Heart Mountain detachment in Wyoming as some form of gravity sliding. We consider the possibility that the deformation was related to a laterally migrating compaction front triggered by a deeper-seated fault to the north-northeast of Reva Gap. This idea requires further field research.