Depositional history of the Greenhorn Formation (Upper Cretaceous), Northwestern Black Hills
The Upper Cretaceous (Cenomanian - Turonian) Greenhorn Formation in northeast Wyoming is predominantly a calcareous to noncalcareous shale with abundant limestone concretions. Five lithologic units are recognized: thinly laminated shale, laminated and variably bioturbated mudstone, thoroughly bioturbated mudstone, limestone beds, and sandstone beds. The lower Greenhorn Formation is characterized by dark-colored thinly laminated shale with rare biogenic and current-formed structures. It was deposited by pelagic settling in relatively quiet waters in anaerobic conditions. Rare storms reworked the sediments producing thin bioturbated intervals and thin limestone beds. The upper Greenhorn Formation is characterized by laminated and variably bioturbated mudstone interstratified with thinly laminated shale, thoroughly bioturbated mudstone, and limestone beds. It was deposited by pelagic settling in relatively quiet water in dysaerobic conditions and was intermittently influenced by currents. Sandstones in the upper Greenhorn Formation were deposited by currents that transported sediment from a western source. The trend upward in the Greenhorn Formation of greater amounts of terrigenous sand, carbonate grains, current-formed structures, and bioturbation reflects an increase in current activity and oxygen levels interpreted to result from a relative local shoaling of the sea.
- Research Organization:
- College of William and Mary, Williamsburg, VA (USA)
- OSTI ID:
- 7036313
- Journal Information:
- Mt. Geol.; (United States), Journal Name: Mt. Geol.; (United States) Vol. 25:3; ISSN MOGEA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
580100* -- Geology & Hydrology-- (-1989)
ANAEROBIC CONDITIONS
CARBONATE ROCKS
CRETACEOUS PERIOD
DEPOSITION
FEDERAL REGION VIII
GEOLOGIC AGES
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
GEOLOGY
LIMESTONE
LITHOLOGY
MESOZOIC ERA
NORTH AMERICA
ROCKS
SANDSTONES
SEDIMENTARY BASINS
SEDIMENTARY ROCKS
SHALES
SILTSTONES
STRATIGRAPHY
SURFACE WATERS
USA
WYOMING