TRANS-HUDSON OROGEN AND WILLISTON BASIN IN MONTANA AND NORTH-DAKOTA -NEW COCORP DEEP-PROFILING RESULTS

COCORP (Consortium for Continental Reflection Profiling) deep reflecti on profiling across the Williston basin and underlying Trans-Hudson or ogen shows the following: (1) There is no evidence for a precursor rif t basin beneath the axis of the Williston basin. With the exception of small-scale structures (e.g., Nesson and Cedar Creek anticlines), the basement surface beneath the Williston basin is smooth and has a dish -shaped geometry. (2) Crustal penetrating reflections within the easte rn Trans-Hudson orogen dip eastward, in apparent contradiction to the ''conventional view'' that the Superior province underthrusts the east ern side of the orogen. (3) Basement reflections beneath the center of the orogen define a broad crustal-scale antiform, the axis of which c oincides approximately with the depocenter of the overlying Williston basin. Midcrustal reflections defining the crest and western flank of this antiform also coincide generally with the location and shape of t he source of the North American Central Plains conductivity anomaly. ( 4) Crustal penetrating reflections beneath the western side of the oro gen dip westward, supporting the current view that subduction polarity was to the west beneath the Wyoming province. (5) The eastern edge of the Wyoming province is marked by east-dipping upper-crustal reflecti ons and subhorizontal lower-crustal reflections that are truncated by the west-dipping reflectivity of the Trans-Hudson orogen.