STRUCTURAL AND MAGMATIC EVOLUTION OF THE HELENA SALIENT: NEW GEOLOGIC MAPPING IN THE DEVILS FENCE ANTICLINORIUM

The Disturbed Belt of western Montana describes a series of east-vergent Late Cretaceous folds and thrust faults that have imbricated Precambrian strata of the Belt Supergroup with Paleozoic miogeoclinal strata and Mesozoic sedimentary rocks of the Rocky Mountain foreland. Deposition of the Elkhorn Mountain volcanics and emplacement of the coeval Boulder Batholith and associated satellite plutons were roughly synchronous with structural deformation. The genetic relationship between deformation and regionally extensive magmatism during the Late Cretaceous is a matter of debate.

The Devils Fence anticlinorium is a thin-skinned deformational system within the hanging wall of the Lombard thrust plate of the Helena salient within the Disturbed Belt. The anticlinorium was mapped in the late 1940's and early 1950's as part of a regional investigation into the geology and mineral deposits of the Boulder Batholith. New 1:24,000 scale geologic mapping of the region, undertaken under the auspices of the USGS EDMAP program, in association with Montana Bureau of Mines and Geology, will produce an updated series of 7.5 minute quadrangle maps, detailed cross sections, and a significant amount of new geochemical and geochronological data. Our aim is to document the genetic relationship between Late Cretaceous contractional deformation and magma emplacement. Detailed mapping in conjuction with geochemical and geochronological analysis will allow us to produce an accurate assessment of the timing of deformation and magma emplacement within the Devils Fence anticlinorium.

New mapping has confirmed the basic framework established in earlier studies and has identified a number of previously unrecognized subsidiary contractional structures on the flanks of the main anticlinorium. Deformed sedimentary strata are intruded by a compositionally diverse suite of stocks, dikes, and sills including pyroxene diorite, hornblende granodiorite, and andesite porphyry, presumably related to the emplacement of the Cretaceous Boulder Batholith. New geochronologic constraints on the timing of deformation and magmatism are provided by U/Pb ages from the Doherty Mt and Sagebrush Park stocks as well as the basal member of the Elkhorn Mountain volcanic package.