COEVAL, IN- AND OUT-OF-SEQUENCE DEFORMATION WITHIN THE FRONTAL THRUST SHEETS OF THE TENDOY MOUNTAINS, SW MONTANA

Structural and synkinematic stratigraphic relationships within the Tendoy Mountains, Montana recess, record broadly coeval, in-, and out-of-sequence deformation within the Sevier foreland. New mapping of the Dixon Mountain 7.5 minute quadrangle (DOI-USGS EDMAP) provides evidence of the emergence and emplacement of several thrust sheets. Exposures of the Beaverhead Conglomerate, an extensive Cretaceous alluvial deposit, record details of synsedimentary deformation along the thrust front. During emplacement, the Tendoy thrust sheet overrode proximal Beaverhead detritus and developed kilometer scale transverse folding in response to complex oblique ramp geometry. The Deadwood Gulch thrust, a leading-edge imbricate of the Tendoy, was later emplaced over the Tendoy thrust and sourced olistostromes within younger Beaverhead strata. While the majority of displacement on the Four Eyes Canyon thrust, the next hinterward major thrust sheet was in sequence, it also experienced late emergent out-of-sequence activity as recorded by overthrusting of synthetically derived synorogenic conglomerates within the Tendoy sheet wedge-top basin. Hinterward dips within these conglomerates suggest that there was some late stage in-sequence movement along the underlying Tendoy thrust or on a more frontal décollement associated with Beaverhead deformation within the Red Rock Valley.