2D AND 3D MODELING OF THE LARAMIDE FOLD GEOMETRY OF DERBY DOME AND ITS EN ECHELON INTERCHANGE WITH DALLAS DOME, SOUTHERN WIND RIVER BASIN, WYOMING

A series of left stepping, en echelon, periclinal folds along the SW margin of the Wind River Basin are associated with the Laramide, basement-cored uplift of the Wind River Mountains of central Wyoming. The folds, which include Dallas Dome and Derby Dome, trend NW-SE and verge SW, consistent with regional NE-SW shortening during Laramide time. However, the origin of the en echelon fold pattern has been variably attributed to the reactivation of basement faults, late-stage N-S Laramide shortening, or a combination of both mechanisms. Analysis of the 3D geometry of Derby Dome (and associated faulting), its fold mechanism, and its en echelon offset with Dallas Dome are consistent with regional NE-SW Laramide shortening followed by a local phase of late-stage N-S shortening strain concentrated in the interchange zone, which is also recognized in the basin-margin folds to the SE. Data used for the analysis include the recently completed geologic mapping, well logs, fracture orientation data, and a seismic reflection line. 2D cross section balancing and restoration were completed using the Move® software suite to validate previous and new cross sections that were used to construct a 3D model. Move® algorithms were used to identify strain concentrations on selected stratigraphic horizons within the model.

Fracture patterns away from the interchange zone are consistent with a syn- and pre-folding regional NE-SW Laramide shortening and buckle folding rather than forced folding. Shear and dilatant fracture tendency models, with principal shortening directions parallel to bedding up-dip directions, account for >80% of the measured fractures. On the backlimb and forelimb of the fold they are consistent with the regional NE-SW Laramide shortening. Along the periclinal fold closures, fractures correlate with bedding orientation on the fold, consistent with a buckling mechanism. Fold restoration models for Derby Dome are consistent with strain concentrations on the forelimb and hinge-parallel principal elongation.

Fracture patterns in the interchange between Derby and Dallas domes are consistent with late-stage folding and the rotation of previously formed Laramide fractures and formation of new fractures formed during the interchange process.