THE SOUTHERN PART OF THE SOUTHWEST MONTANA THRUST BELT: A PRELIMINARY RE-EVALUATION OF STRUCTURE, THERMAL MATURATION AND PETROLEUM POTENTIAL

ABSTRACT Two major thrust sheets are exposed in the Lima area of extreme southwestern Montana: the Tendoy and Medicine Lodge allochthons Drilling in front of the Tendoy thrust has revealed blind thrusts and additional thrust imbrication of the Lima thrust system in the subsurface beneath the Lima anticline. The frontal Tendoy sheet involves Mississippian through Cretaceous rocks of the southwest-plunging nose of the Mesozoic Blacktail-Snowcrest uplift that were thrust onto the uplift probably during Paleocene time. Rocks of the exposed Tendoy sheet have never been deeply buried, based on a uniform conodont CAI (color alteration index) of 1 and organic geochemistry of Paleozoic rocks from the Tendoy thrust sheet. Directly above and west of the Tendoy sheet lie formerly more deeply buried rocks of the Medicine Lodge thrust sheet. Greater burial depth is suggested by more elevated conodont CAI values The rocks in the Medicine Lodge allochthon have been transported eastward from the miogeocline a much greater distance than those in the Tendoy sheet, which originated near the Cordilleran hingeline. The Mesozoic Blacktail-Snowcrest uplift east of the Tendoy sheet is asymmetric, with its southeastern vertical to overturned limb exposed along the Snowcrest Range. Northwest-dipping, basement-involved thrusts on the southeast limb may merge with depth into a major sub-Snowcrest Range thrust which flanks and is chiefly responsible for this asymmetric uplift. The uplift was an area of more rapid subsidence with respect to platform areas in the vicinity of the Ruby syncline and the Centennial Basin to the southest during Mississippian through Permian time The inferred sub-Snowcrest Range fault may thus represent a reactivated earlier zone of basement weakness. The petroleum potential of the area has not been fully evaluated However, the intersection of thrust-belt and cratonic trends, similar to the Uinta uplift farther south, probably formed a number of structural traps which have not yet been tested. Both potential petroleum source-beds and reservoir rocks are present in this area that has remained cool between batholiths and mantling volcanic rocks to the north and younger volcanic rocks to the south in the Snake River Plain.