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Book Chapter

Middle Cryogenian (“Sturtian”) Pocatello Formation: Field relations on Oxford Mountain and the Portneuf area, southeast Idaho

By
Joshua A. Keeley
Joshua A. Keeley
Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA
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Paul K. Link
Paul K. Link
Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA
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Published:
January 01, 2011

Abstract

Detailed mapping along the east face of Oxford Ridge in the southern Bannock Range, southeast Idaho determines the stratigraphic placement and lateral extent of strata in the Scout Mountain Member of the Neoproterozoic Pocatello Formation. The lower “transitional unit” overlies the Bannock Volcanic Member and consists of 70 m of massive diamictite with argillitic and vesicular basaltic clasts up to cobble size intercalated with thin metabasalt and hyaloclastite units. Overlying the transitional unit is a 150–190-m-thick, massive, brown-green to purple sandy diamictite with dominantly quartzose cobble clasts. Interbedded with this middle unit is a 60-m-thick epiclastic volcanic interval informally named the Oxford Mountain tuffite. An upper sandstone unit up to 100 m thick lies above the diamictite at the head of Fivemile Creek in the southern portion of the map area.

The volcanic interval contains plagioclase-phyric volcanic lithic sandstone, porphyritic volcanic lithic fragments and rounded cobbles in tuffaceous diamictite and a reworked stratified lapilli-tuff. It is interstratified with quartzose and volcanogenic diamictite and can be traced along 5.5 km of strike. On Oxford Mountain, laser ablation–inductively coupled plasma mass spectrometry U-Pb zircon ages presented here and additional sensitive high-resolution ion microprobe ages constrain the underlying Bannock Volcanic Member to be 717–686 Ma and require that the overlying Scout Mountain Member is younger than 685 Ma.

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Contents

GSA Field Guide

Geologic Field Trips to the Basin and Range, Rocky Mountains, Snake River Plain, and Terranes of the U.S. Cordillera

Jeffrey Lee
Jeffrey Lee
Department of Geological Sciences 400 E. University Way Central Washington University Ellensburg, Washington 98926 USA
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James P. Evans
James P. Evans
Department of Geology Utah State University 4505 Old Main Hill Logan, Utah 84322-4505 USA
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Geological Society of America
Volume
21
ISBN electronic:
9780813756219
Publication date:
January 01, 2011

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