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Marias River Formation (MTKmr;0)

Geologic Formation in MT

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Lithology

Phosphorite

Chert

Phosphorite

Other (4,678 mi²)

Geologic Age

Cenomanian – Santonian (100.5 Ma – 83.8 Ma ago)

Geologic Unit Label

Kmr

Marias River Formation (Kmr) - Kevin Member (Kmk): Dark gray shale with calcareous concretions and numerous bentonite beds. Upper part calcareous. MacGowan Concretionary Bed (middle part of member): grayish brown concretionary dolostone and limestone with phosphatic pellets, and gray to black chert pebbles. Marine. Thickness as much as 350 m (1,148 ft). Ferdig Member (Kmf): Gray and dark gray shale with thin siltstone or sandstone beds at the top. Marine. Thickness as much as 125m (410 ft). Cone Member (Kmc): Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft). Floweree Member (Kmfl): Dark gray, noncalcareous shale with interbeds of siltstone. Calcareous concretions and scattered granules of dark gray chert. Shallow marine. Thickness as much as 30 m (98 ft).

— USGS

Taxonomy

Classified As

Geologic Formation