Molas Formation

Molas Formation
Name	Molas Formation
Type	Geological formation
Period	Coniacian–Campanian
Primary lithology	Sandstone, shale, conglomerate
Otherlithology	Coal, siltstone
Namedfor	Molas Pass
Namedby	Charles R. Keyes
Region	Colorado, New Mexico, Utah
Country	United States

Molas Formation The Molas Formation is a Late Cretaceous to Paleogene sedimentary sequence exposed in the San Juan Mountains and adjacent basins of the southwestern United States. It is notable for coarse clastic deposits, coal-bearing strata, and an assemblage of fossil plants and vertebrate trace evidence that inform reconstructions of Western Interior Seaway margin dynamics, Laramide orogeny foreland basin sedimentation, and Paleogene landscape evolution. Studies of the unit have linked it to regional tectonics affecting the Rocky Mountains, Animas River, and intermontane basins.

Geology and Stratigraphy

The Molas Formation sits stratigraphically above units correlated with the Dakota Sandstone and below Paleogene synorogenic sequences tied to the Navajo Sandstone–age successions. Regional correlations tie the formation to the Coniacian through Campanian chronostratigraphy used in mapping by the U.S. Geological Survey, the Colorado Geological Survey, and university geoscience departments at University of Colorado Boulder and New Mexico Institute of Mining and Technology. Stratigraphic frameworks integrate biostratigraphy, detrital zircon geochronology from laboratories at Arizona State University and University of Arizona, and palynostratigraphic ties developed with researchers at Smithsonian Institution paleobotany collections. Interfingering with contemporaneous units in the San Juan Basin and the Paradox Basin reflects syndepositional tectonism linked to the Sevier orogeny and later reworking during the Laramide orogeny.

Age and Paleoenvironments

Radiometric constraints from detrital zircons dated at facilities affiliated with Massachusetts Institute of Technology and Geological Society of America meetings place portions of the Molas in the Coniacian to Campanian; local coal seams yield paleofloral assemblages correlated to Maastrichtian–Paleocene transitions studied by teams from University of Kansas and Yale University. Paleoenvironments ranged from coastal plain and deltaic swamps to fluvial braidplain systems draining rising highlands associated with San Juan Mountains uplift. Climate reconstructions use comparisons with contemporaneous floras from Hell Creek Formation and Fruitland Formation and faunal turnover events discussed at conferences hosted by Society of Vertebrate Paleontology.

Lithology and Sedimentology

The Molas comprises conglomerates, lithic sandstones, siltstones, shales, and coal reflecting variable energy conditions documented in sedimentologic work by groups at Colorado School of Mines and University of New Mexico. Facies analyses reference depositional models developed for the Cretaceous coastal systems of the Western Interior Seaway and utilize sedimentary logs archived at the American Association of Petroleum Geologists core repositories. Sediment provenance studies tie detritus to sources in the Uncompahgre Uplift and Wasatch Range and involve mineralogical analyses performed at Stanford University electron microprobe labs.

Fossil Content

Fossils include plant megafossils, palynomorphs, freshwater mollusks, and vertebrate trace fossils recorded in collections at the Denver Museum of Nature & Science and the New Mexico Museum of Natural History and Science. Paleobotanical material parallels assemblages cataloged in the Instituto de Geología (UNAM) comparative collections and has been discussed in monographs associated with the Paleontological Society. Ichnofauna includes dinosaur tracks and crocodilian traces analogized to tracksites from the Morrison Formation and Cretaceous track assemblages reported from Utah Geological Survey surveys. Coal-associated plant floras show affinities with taxa described by researchers at Royal Botanic Gardens, Kew and Brown University paleobotany groups.

Distribution and Type Localities

Exposures are widespread across southwestern Colorado, northwestern New Mexico, and parts of eastern Utah, with key localities near Molas Pass in the San Juan Mountains and along tributaries of the Animas River. Type sections and measured sections have been logged by personnel from the U.S. Geological Survey and academic teams from Colorado State University and deposited in institutional archives at the New Mexico Bureau of Geology and Mineral Resources. Mapping projects coordinated with Bureau of Land Management resource inventories document lateral facies changes toward the Paradox Basin margin.

Economic Importance and Resources

The Molas Formation hosts thin coal seams that were evaluated during surveys by the U.S. Bureau of Mines and regional energy assessments by the Department of Energy and state agencies. Coarse conglomerates have been quarried locally for road aggregate under permits issued by San Juan County authorities, and its petroleum-system potential has been appraised in basin studies conducted by industry groups including American Petroleum Institute-affiliated consultancies. Groundwater aquifers within Molas sandstones have been examined in hydrologic studies led by Environmental Protection Agency-funded teams and state water resources divisions.

History of Research and Naming

The formation was named for Molas Pass by early 20th-century geologists working in the San Juan region, with formal descriptions appearing in reports circulated through the U.S. Geological Survey and regional geological societies such as the American Geophysical Union. Subsequent work refining its age, facies, and paleontology has been carried out by researchers affiliated with University of Colorado Boulder, Colorado School of Mines, New Mexico Institute of Mining and Technology, and international collaborators presenting at meetings of the Geological Society of America and the Society for Sedimentary Geology.

Category:Cretaceous geology of North America