Greenhorn Limestone


The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States. It preserves fossils dating back to the Cenomanian and Turonian of the Late Cretaceous period.
The formation was named for the Greenhorn Station on Greenhorn Creek in Colorado in 1896 by Grove Karl Gilbert; and it is the eponym of the Greenhorn Marine Cycle of the Cretaceous Western Interior Seaway. With the underlying Graneros Shale, it records the progressive stage of Greenhorn Marine Cycle while the overlying Carlile Shale records the regressive stage.
The unit under that name is recognized in the Great Plains Region from Minnesota and Iowa to New Mexico to Montana and the Dakotas.
In much of Alberta and Saskatchewan, the "Second White-Specked Shale" contains limy equivalents of the Greenhorn.
In Kansas, the Greenhorn Formation is divided into the Lincoln Limestone, Hartland Shale, Jetmore Chalk, and Pfeifer shale members, each noted by changes in chalkiness and limestone rhythmite patterns. In Colorado and western Kansas Hydrocarbon exploration, the divisions are Lincoln Limestone, Hartford Shale, and Bridge Creek Limestone. In other states, where the formation is less developed, the unit is not subdivided and is named the Greenhorn Limestone, as a formation or as a member of another formation, e.g., Cody Shale.

Lithologic character

The Greenhorn Formation is characterized as shale to chalky shale, light bluish-gray in color, with rhythmic beds of chalk or limestone that become marly closer to the Rocky Mountains. The shale can weather to buff under hilltops.
Exposures show many thin, rust-colored bentonite beds, several of which are consistent and widespread marker beds. These orange seams in the weathered shale and the yellow/orange stainings of some of the weathered limestones in the Greenhorn are associated with volcanic events in the Sevier orogeny. The oceanic iron and volcanic sulfur that precipitated with the volcanic ash into the calcareous mud formed pyrite, which later altered to selenite, siderite, and limonite, leading to the yellow to orange staining.

Paleofauna

The formation is recognized for its sequence of index fossils, including Ostrea, Ammonoidea, Belemnitida, and Inoceramidae.
Dinosaur remains have been recovered from the formation.