Stratigraphy
Metasedimentary rocks exposed north of the Cheyenne belt are generally interpreted as having been deposited in an Atlantic-type cratonic margin. The Snowy Pass Supergroup and Phantom Lake metamorphic suite are preserved in both the Sierra Madre and Medicine Bow mountains. Consisting of quartzite, conglomerate, volcaniclastic and metavolcanics, the Phantom lake metamorphic suite is thought to record late Archean sedimentation in southern Wyoming followed by an orogenic episode marked by volcanics and volcaniclastic rocks in this package. Within the Snowy Pass Supergroup, the Deep Lake group records the transition from Archean tectonics to a stable craton capable of supporting thick sediments along its margin. The upper Deep Lake group and lower Libby Creek group reveal a rifting event evidenced by similar paleocurrent directions in both fluvial and marine sediments which would be expected in an elongated intracratonic basin but not in an open continental margin. The Deep Lake and Libby Creek groups are complicated by advancement of glacial ice. Retreat of this continental scale ice sheet re-established a major delta system recorded throughout the Libby Creek group. These rocks lend the most support to a rifting model as they represent deltaic sedimentation in a trough oriented parallel to the continental margins similar to Mesozoic rifting of South America and modern continental margins in the Gulf of California today. Deposition of the French slate open marine black shales and volcanics of the Towner greenstone represent the approach of an island arc from the south.
Metasedimentary rocks exposed north of the Cheyenne belt are generally interpreted as having been deposited in an Atlantic-type cratonic margin. The Snowy Pass Supergroup and Phantom Lake metamorphic suite are preserved in both the Sierra Madre and Medicine Bow mountains. Consisting of quartzite, conglomerate, volcaniclastic and metavolcanics, the Phantom lake metamorphic suite is thought to record late Archean sedimentation in southern Wyoming followed by an orogenic episode marked by volcanics and volcaniclastic rocks in this package. Within the Snowy Pass Supergroup, the Deep Lake group records the transition from Archean tectonics to a stable craton capable of supporting thick sediments along its margin. The upper Deep Lake group and lower Libby Creek group reveal a rifting event evidenced by similar paleocurrent directions in both fluvial and marine sediments which would be expected in an elongated intracratonic basin but not in an open continental margin. The Deep Lake and Libby Creek groups are complicated by advancement of glacial ice. Retreat of this continental scale ice sheet re-established a major delta system recorded throughout the Libby Creek group. These rocks lend the most support to a rifting model as they represent deltaic sedimentation in a trough oriented parallel to the continental margins similar to Mesozoic rifting of South America and modern continental margins in the Gulf of California today. Deposition of the French slate open marine black shales and volcanics of the Towner greenstone represent the approach of an island arc from the south.