La Salle Anticlinorium

La Salle Anticlinorium
Name	La Salle Anticlinorium
Type	Anticlinorium
Location	Idaho, Montana, Wyoming, Colorado
Era	Paleozoic, Mesozoic, Cenozoic
Lithology	limestone, shale, sandstone, dolomite, quartzite

La Salle Anticlinorium is a major north–south trending structural high in the northern Rocky Mountains region that spans parts of Idaho, Montana, Wyoming, and Colorado. The feature is an elongate anticlinorium that juxtaposes older Cambrian through Paleozoic strata against younger Mesozoic cover and is integral to understanding foreland deformation related to the Laramide Orogeny, the Sevier Orogeny, and intracontinental stress transmission from the Cordilleran orogen. Its exposures have been focal points for mapping by the United States Geological Survey, academic researchers from Stanford University, University of Wyoming, and Montana State University, as well as industry geologists from Anadarko Petroleum Corporation, ExxonMobil, and BP.

Geology

The anticlinorium exposes a composite architecture where Precambrian basement provinces such as the Yavapai Province, Arcadia Formation-aged terranes, and Grenville orogeny-related rocks underlie a cover of Cambrian Flathead Formation-type sandstones, Ordovician Bighorn Dolomite-equivalent limestones, and Mississippian carbonates related to the Madison Group. Surficial cover includes Pleistocene glacial deposits correlated with the Laurentide Ice Sheet and interbedded Pleistocene loess. The geomorphology links to drainage systems including the Missouri River, Yellowstone River, and tributaries that incised the anticlinorial crest during Neogene and Quaternary exhumation. Tectonostratigraphic correlations draw on analogs from the Laramie Range, Bighorn Mountains, Absaroka Range, and the Wind River Range.

Stratigraphy and Structure

Stratigraphic columns across the structure record a succession from Cambrian quartzites and Ordovician limestones through Devonian reef complexes similar to the Fossil Basin facies and into Pennsylvanian cyclothems comparable to those in the Ancestral Rocky Mountains domain. Structural elements include tight folds, regional-scale thrusts comparable to features in the Sevier fold-and-thrust belt, duplexes akin to those documented in the Wasatch Range, and reactivated basement-involved uplifts analogous to the Laramide uplifts of the Black Hills. Normal faults related to Basin and Range extension overprint the anticlinorium along its western flank near the Snake River Plain and intersect wrench faults reminiscent of the Teton Range structural grain. Cross-sections integrate work by the American Association of Petroleum Geologists and mapping compiled in USGS Professional Papers.

Tectonic Setting and Formation

Formation models invoke interactions among the Farallon Plate subduction, eastward propagation of the Cordilleran magmatic arc, and continental intraplate shortening during the Laramide Orogeny and contractional pulses in the Sevier Orogeny. The anticlinorium is often interpreted as a basement-cored uplift related to flat-slab subduction events similar to reconstructions of the Cenozoic western North American margin and the collision dynamics exemplified by the Shatsky Rise–Hawaiian-Emperor seamount chain tectonic analogs. Geochronologic constraints derive from U-Pb zircon dates obtained by laboratories at University of California, Berkeley, argon–argon thermochronology from Lamont–Doherty Earth Observatory, and low-temperature thermochronometers used in studies of the Great Plains–Rocky Mountains transition.

Paleontology and Fossil Record

Sedimentary units on the anticlinorium preserve fossils ranging from Cambrian Explosion trilobites and brachiopods through Devonian stromatoporoid reefs and Mississippian crinoid assemblages to Pennsylvanian plant-bearing cyclothems with lycopsids comparable to specimens from the Mazon Creek and Paleobotany assemblages. Mesozoic strata record marine invertebrates and microfossils useful for biostratigraphy, with ammonoids and bivalves correlated to stages defined in the Geological Society of America stratigraphic framework. Vertebrate discoveries include Cenozoic mammalian faunas comparable to the Bridger Formation and Harrison Formation records; paleontological curation has involved the Smithsonian Institution, American Museum of Natural History, and regional museums such as the Montana Historical Society.

Economic Resources and Mineralization

The anticlinorium hosts mineralization including Mississippi Valley-type lead–zinc deposits analogous to occurrences in the Missouri River Basin, barite occurrences of interest to mining firms like Teck Resources, and carbonate-hosted copper–silver mineralization similar to deposits in the Animas River district. Hydrocarbon prospectivity has been assessed for potential traps within Pennsylvanian carbonate and fractured Paleozoic reservoirs; seismic surveys by Schlumberger and well logs from operators including ConocoPhillips inform plays compared against analogs in the Williston Basin and Denver Basin. Groundwater resources and geothermal gradients have been evaluated in studies involving the Department of Energy and regional water management agencies analogous to assessments in the Great Basin.

Research History and Mapping

Mapping and synthesis began with regional surveys by the USGS in the late 19th and early 20th centuries, building on field campaigns by geologists associated with Yale University and the University of Chicago. Key contributions include structural interpretations by researchers affiliated with Princeton University and stratigraphic correlations advanced by teams at the Colorado School of Mines. Major publications appeared in journals such as Geology, the Journal of Geophysical Research, and the AAPG Bulletin. Ongoing research integrates remote sensing from Landsat and NASA airborne geophysical data, numerical modeling from groups at Caltech and MIT, and collaborative projects funded by the National Science Foundation and industry consortia.

Category:Geology of the Rocky Mountains Category:Structural geology