STRUCTURE OF SNEDAKER BASIN QUADRANGLE-IMPLICATIONS FOR REGIONAL TECTONICS AND HYDROCARBON EXPLORATION ALONG NORTHERN HELENA SALIENT, MONTANA THRUST BELT

ABSTRACT Laramide structures dominate the Snedaker Basin area and consist of north-northeast-verging folds and thrust faults involving rocks ranging from the Newland Formation of the Precambrian Belt Supergroup to the Lower Cretaceous Blackleaf Formation(?). There are four thrust plates plus parautochthon. The thrust plates are: the previously unrecognized Rock Creek thrust plate which is an inverted limb of a nearly recumbent nappe; the Jim Ball Basin thrust plate; the Wagner Gulch thrust plate; and the Scout Camp thrust plate. The Jim Ball Basin plate is probably a klippe of the Scout Camp plate. Snedaker Basin is located along the northern margin of the Helena salient. This margin of the salient resembles a duplex that changes eastward along strike into an imbricate fan, with the Scout Camp thrust fault forming the roof of the duplex and a basal decollement between Belt strata and basement probably forming the floor. The direction of tectonic transport was east-northeasterly, with some thrusts having demonstrable left-lateral movement as well as dip slip. An estimated 18 mi (29 km) of north-south stratal shortening occurred along these faults. Folding preceded and accompanied thrusting with the result that thrust faults cut folds and other thrust faults. Hydrocarbon exploration should be directed toward fracture reservoirs, because indigenous intergranular and intercrystalline porosity is low in most pre-Morrison strata, the dominant rocks in the subsurface of this area. The Heath Formation (Mississippian) is the most abundant hydrocarbon source rock, but maturation proceeded beyond the preservation limit of oil and wet gas, and probably dry gas. Hydrocarbon generation occurred after thrusting in response to tectonic burial. Any other source rocks present are as mature as the Heath, and, therefore, dry gas is the only hydrocarbon that may be present in this part of the thrust belt.