Metasedimentary rocks in the Medicine Bow Mountains and Sierra Madre are divided into four groups. The > 3-km-thick Phantom Lake Metamorphic Suite contains strongly deformed metavolcanic and metasedimentary rocks that are crosscut by late Archean granites. The > 2.5-km-thick Deep Lake Group unconformably overlies the Phantom Lake Suite and late Archean granites and contains fluvial sediments, including radioactive quartz-pebble conglomerates, and glaciomarine deposits. Both successions are intruded by large sills of tholeiitic gabbro. The 4.5-km-thick lower Libby Creek Group is inferred to be in thrust-fault contact with older units and contains sediments recording transgressions and regressions across a macrotidal delta. This succession is intruded by the 2,000-m.y.-old Gaps Intrusion and comagmatic tholeiitic to weakly alkalic dikes. The 3-km-thick upper Libby Creek Group is bounded by a thrust fault below and by the Cheyenne Belt above and contains carbonates and marine slates. The early Proterozoic Deep Lake, lower Libby Creek, and upper Libby Creek Groups collectively are named the Snowy Pass Supergroup.

Lithologies and stratification sequence in the well-preserved Medicine Bow Mountain section suggest transgressive, miogeoclinal sedimentation during the early Proterozoic. Paleocurrent data indicate that fluvial, then deltaic, sedimentation of the Deep Lake and lower Libby Creek Groups took place on a southwest-dipping paleoslope, parallel to the inferred south cratonic boundary of the Wyoming Province. This and a few west-directed paleocurrents suggest a continental or microcontinental block to the south, bounding sedimentation in a northeast-elongated basin. A rift setting for deposition of these units explains the transgressional character of the sediments, the deltaic sedimentation with paleocurrents parallel to the cratonic boundary, and the 120° bend in the Cheyenne Belt between the Medicine Bow Mountains and the Sierra Madre. The upper Libby Creek Group is interpreted to represent open marine conditions following separation of the two continental blocks. Tholeiitic sills in the Deep Lake Group and tholeiitic to weakly alkalic dikes in the Libby Creek Group are thought to be related to basaltic igneous activity associated with compound early Proterozoic rifting between 2,300 and 2,000 m.y. ago.

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