Graneros Shale

Graneros Shale
Name	Graneros Shale
Period	Cenomanian
Type	Geological formation
Region	Western Interior Seaway

Graneros Shale The Graneros Shale is a Cretaceous shale unit within the Western Interior region that records marine transgression across the North American craton during the Cenomanian stage, linking to studies in United States Geological Survey, Geological Society of America, American Museum of Natural History, Smithsonian Institution, and University of Kansas. It serves as a key stratigraphic marker in correlations among outcrops in Colorado, Kansas, New Mexico, Wyoming, and Montana, and features in regional syntheses by researchers associated with Harvard University, Yale University, Stanford University, University of Texas at Austin, and University of California, Berkeley.

Geology and Stratigraphy

The unit occupies a stratigraphic position above the mudstones and sandstones correlated with the Dakota Group and below carbonate-rich units that include equivalents of the Greenhorn Limestone and Benton Shale in syntheses published by United States Geological Survey, Kansas Geological Survey, Colorado Geological Survey, New Mexico Bureau of Geology and Mineral Resources, and authors from Paleontological Society. Regional cross-sections link the unit with transgressive systems tracts mapped in basin studies led by researchers at University of Chicago, Princeton University, Columbia University, and Iowa Geological Survey, and the unit's boundaries are used in chronostratigraphic frameworks endorsed by International Commission on Stratigraphy. Correlation panels show relationships to the Mancos Shale in exposures studied by teams from University of Wyoming, South Dakota School of Mines and Technology, and Montana Bureau of Mines and Geology.

Lithology and Sedimentology

The Graneros lithofacies consist predominantly of dark gray to black marine shale interbedded with thin siltstone and turbiditic sandstone laminae documented in fieldwork by University of Kansas, Colorado School of Mines, University of New Mexico, Brigham Young University, and Utah Geological Survey. Geochemical analyses performed at Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, Lawrence Livermore National Laboratory, Los Alamos National Laboratory, and Oak Ridge National Laboratory report elevated total organic carbon and variable sulfur content indicative of dysoxic to anoxic bottom waters, while petrographic investigations by British Geological Survey, École Normale Supérieure, Max Planck Institute for Chemistry, University of Leeds, and University of Edinburgh document clay mineral assemblages dominated by illite, smectite, and kaolinite. Sedimentological features described in publications from Society for Sedimentary Geology, American Association of Petroleum Geologists, Petroleum Exploration Society of Great Britain, Canadian Society of Petroleum Geologists, and Society of Economic Geologists include lamination, pyrite framboids, and loading structures associated with rapid transgressive deposition.

Paleontology

Fossil content includes marine invertebrates such as ammonites, inoceramid bivalves, and gastropods reported in monographs from American Museum of Natural History, Smithsonian Institution, Paleontological Research Institution, Natural History Museum, London, and Royal Tyrrell Museum, as well as microfossils including planktonic foraminifera and calcareous nannofossils studied by teams at Lamont–Doherty Earth Observatory, University of Miami, University of Barcelona, University of Tokyo, and Chinese Academy of Sciences. Vertebrate remains including isolated fish scales and occasional marine reptile fragments have been described in papers affiliated with University of Kansas Natural History Museum, Denver Museum of Nature & Science, New Mexico Museum of Natural History and Science, Field Museum, and Carnegie Museum of Natural History. Palynological assemblages used for biostratigraphy were analyzed by researchers at University of Leicester, Royal Belgian Institute of Natural Sciences, University of Oslo, Ludwig Maximilian University of Munich, and University of Copenhagen.

Depositional Environment and Paleogeography

Interpretations place deposition within the western arm of the Western Interior Seaway during a major Cenomanian transgression documented in paleogeographic reconstructions by Paleomap Project, NOAA, USGS Coastal and Marine Geology Program, International Ocean Discovery Program, and groups at University of Kansas. Facies models proposed by University of Texas at Austin, University of Oklahoma, Texas A&M University, Oklahoma Geological Survey, and Bureau of Economic Geology indicate deposition on an outer shelf to upper slope under dysoxic conditions influenced by sea-level rise, storm events, and sediment supply from ancestral river systems traced to orogenic sources such as sediment provenances studied by U.S. Geological Survey geochemists and isotope specialists at University of Arizona and Pennsylvania State University.

Economic Importance and Resource Potential

The unit's elevated organic content has drawn interest from energy and resource organizations including American Association of Petroleum Geologists, U.S. Department of Energy, Energy Information Administration, Schlumberger, and Halliburton for shale gas and shale oil potential, while regional assessments by Colorado School of Mines, Kansas Geological Survey, New Mexico Bureau of Geology, Wyoming Oil and Gas Conservation Commission, and Montana Bureau of Mines and Geology evaluate conventional reservoir seals and source rock relationships. The shale also hosts mineralogical concentrations relevant to geotechnical studies conducted by Federal Highway Administration, Army Corps of Engineers, Bureau of Land Management, Environmental Protection Agency, and state departments of transportation, and has been considered in carbon sequestration and underground storage investigations performed by National Renewable Energy Laboratory, Lawrence Berkeley National Laboratory, NETL, and International Energy Agency research groups.

History of Investigation and Naming

Early descriptions and naming of the unit involved survey work by personnel affiliated with United States Geological Survey, Kansas Geological Survey, Colorado School of Mines, New Mexico Bureau of Geology, and 19th–20th century geologists associated with Geological Society of America, American Philosophical Society, Royal Society, Smithsonian Institution, and universities such as Harvard University and Yale University. Subsequent mapping, stratigraphic revision, and regional synthesis were advanced through collaborative projects linked to USGS Professional Papers, monographs from Kansas Geological Survey Bulletin, field guides from Rocky Mountain section of the Geological Society of America, and doctoral research produced at University of Kansas, Colorado College, and University of New Mexico.

Category:Shale formations