EVIDENCE FOR LATE-STAGE LARAMIDE N-S SHORTENING ALONG THE NORTHERN CLOSURE OF HUDSON DOME, SOUTHWESTERN WIND RIVER BASIN, WYOMING

Geologic mapping and structural analysis of Hudson Dome and its extension to the north as Plunkett Anticline provide support for previously recognized late-stage N-S Laramide shortening along the southern and southwestern margin of the Wind River Basin. The previous evidence comes primarily from the interchange zones between adjacent en echelon periclinal basin-margin folds to the south of Hudson Dome, which trend sub-parallel to the Wind River Mountains. From its southern closure, Hudson Dome maintains a moderate NNW trend, roughly consistent with regional ~NE-SW Laramide shortening. However, the overall fold geometry and axial trace of Hudson Dome are complex along the northern part of the dome and its continuation as Plunkett Anticline. The dome and anticline axial trace is folded about a series of younger WNW-striking fold axial traces, consistent with the late-stage N-S shortening recognized in the fold interchange zones to the south. Three major WNW-trending axial traces within the Cretaceous Cody Shale, McGowan Anticline and two adjacent synclines, occur immediately to the east of the refolded Hudson-Plunkett axial trace and are also consistent with late-stage ~N-S shortening.

Geological mapping and analysis of bedding and fracture orientations and the mode of fracture formation indicate that buckle folding and very localized forced folding mechanisms are responsible for the formation of Hudson Dome and Plunkett Anticline. However, early stage fractures produced during the regional NE-SW shortening were reactivated and/or reoriented along the refolded segment of the Hudson-Plunkett axial trace. Reactivation is locally indicated by stepped offsets of the early fractures and by the orientation of shear bands consistent with both the regional NE-SW and local N-S shortening directions.

Fracture measurements recorded in the field were subdivided and stereographically plotted based on their respective domains of folding along the extent of the Hudson-Plunkett axial trace. The folds contain six major fracture sets that are reoriented about the late-stage fold axial traces. These data indicate a shift in Laramide paleostress orientation from the regional ~NE-SW compression to a local N-S shortening component.