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Piegan Group: Helena and Wallace Formations (MTYpg;0)

Geologic Formation in MT, ID

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Lithology

Limestone

Dolostone

Argillite

Other (4,657 mi²)

Geologic Age

Mesoproterozoic (1600 Ma – 1000 Ma ago)

Geologic Unit Label

Ypg

Helena Formation (Yh): Northwestern Montana: (formerly “lower Wallace”): cycles of basal white quartzite or intraclast beds overlain by couplets of green siltite and argillite, capped by dolomite beds. Calcite pods and ribbons (molar tooth structure) common. West-central Montana: gray to dark gray limestone and dolomitic limestone with siltite partings. Thickness as much as 2,000 m (6,562). Wallace Formation: Tan-weathering, dolomitic quartzite and siltite, and black argillite with calcite ribbons (molar tooth structure) in graded pinch-and-swell couples and couplets. Thickness as much as 2,500 m (8,202 ft).

— USGS

Taxonomy

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Geologic Formation