A 3-D EARTH MODEL OF STACKED THRUSTS IN THE SEVIER THRUST BELT, BIG HOLE MOUNTAINS, EASTERN IDAHO

We constructed a three-dimensional earth model of the Pine Creek area in the Big Hole Mountains of eastern Idaho, where stacked Sevier thrust sheets are exposed at the surface. This area is the object of ongoing geologic mapping by BYU-Idaho field camp students, whose maps provided primary inputs to the model. The objectives of this research were to 1) interpret complex subsurface thrust belt geology, and 2) to develop methods to integrate geologic map data with well data in a structurally complex area. We used EarthVision software for modeling. The Big Hole mountains are bounded on the southwest by the Grand Valley normal fault and on the northeast by the Teton Valley normal fault, both of which are Basin and Range faults. At least two major Sevier thrust faults are under the range to the northeast of the model area. Strata range in age from Cambrian to mid-Cretaceous, all of which are duplicated vertically in five stacked thrust sheets with displacements on the order of kilometers to tens of kilometers. Two exploratory wells were drilled to depths of 13,000 feet in the model area in the 1980’s, and provided critical information. Our initial intent was to use geologic cross-sections as model input, but we quickly discovered that cross-sections are only useful when they pass through the wells, because the wells encountered duplexes at depth that are not evident at the surface. Modeling did show, however, that the duplexes caused a significant flattening of stratal dips at the surface, the cause of which was previously unknown. We constructed the model using the method of reference horizons to constrain geometries, and created additional strata by adding or subtracting their true stratigraphic thicknesses from the reference horizons. The model shows steep thrusts in the northeast, suggesting younger faulting and possible duplexes below the modeled volume. The model includes two duplexes encountered by the exploratory wells. The overall shape of the duplexes was interpreted by the extent of flattened dips at the surface, and the internal structure and strata were interpreted using the well data. We conclude that this area has potential for structural hydrocarbon traps. Future work will incorporate seismic profile data in the model.