Stratigraphic framework and provenance of the lower belt (Newland Formation), Belt Supergroup, Helena Embayment, central Montana

Abstract

The Belt Supergroup is exposed in western Montana where most of the unit is allochthonous due to Sevier thrusting. However, east of the Helena salient several autochthonous sections of Newland Formation pin it to central Montana. This study focuses on the stratigraphy, provenance, and structural framework of the Newland across this boundary. Measured sections in the Big Belt, Little Belt, and Big Snowy Mountains, along with previously published sections (Zieg 1981) were used to correlate the Newland and interpret its origin. Detrital zircon geochronology (U-Pb by LA-ICP-MS) was used to establish provenance. Clay mineralogy was used to determine burial depth across the embayment. Deposits of the lower Newland locally contain intraformational conglomerates suggesting basinal reorganization at the onset of Newland deposition. Core from a well south of the Volcano Valley fault demonstrate thickening of the lower Newland in the footwall and indicate down-to-the-south faulting (buttress fault). The Volcano Valley fault was reactivated as a thrust fault during Sevier shortening. Above these local units, the lower Newland is a regionally extensive parallel laminated tan calcareous shale containing thin beds of microspar, interpreted as a deep-water depositional system across the entire Helena embayment, indicating the Belt Basin extended further east than most current depictions. Detrital zircon geochronology of the Newland Formation from the Big Belt Mountains suggests diverse sources. The primary source of ~2.45-2.61 Ga, may correspond to the Medicine Hat Block and/or crystalline basement in the western part of the basin. A younger Paleoproterozoic source (~1.79-1.98 Ga) matches the Great Falls Tectonic Zone, and two less prominent Archean sources of ~2.61-2.8 GA and ~3.0-3.2 Ga may reflect contributions from intra-basinal crystalline basement. Elemental analyses complement the detrital zircon geochronology, but do not distinguish distal Archean contributions (e.g., Canadian Shield) from a more proximal Wyoming Province. Xrd analysis of clays reveals the Helena embayment experienced a minimum burial depth of 3,300 m. It has been proposed that the Belt Supergroup is an allochthonous block that translated 1300 km north (Hildebrand, 2013). However, this study suggests the Lower Belt in the Helena embayment has not undergone significant latitudinal translation relative to cratonic North America.