Syntectonic sedimentation in the Proterozoic upper Belt Supergroup, northwestern Montana

Abstract

Deepening environments in the Proterozoic Libby Formation record a tectonically induced style of sedimentation distinctly different from that of older Belt rocks. Facies associations and sedimentary structures indicate that deposition in the lower Libby Formation occurred above fair-weather wave base. Thick, widespread hummocky cross-stratified quartzite in the upper Libby Formation lacks the association of shallow-water features present in the lower Libby Formation, suggesting that upper Libby deposition occurred below fair-weather wave base and above storm wave base. Independent evidence for tectonism during deposition of the Belt Supergroup exists but is plagued by poor age control. The angular unconformity that occurs between Libby-equivalent rocks and the overlying Windermere Supergroup indicates tectonic activity between deposition of the Belt-Purcell and Windermere Supergroups. The interpreted subsidence or rise in basin water level combined with newly uplifted source areas as recorded in the upper Libby Formation and Garnet Range Formation could have been an early manifestation of movements that produced this unconformity.