STRATIGRAPHIC FRAMEWORK AND PROVENANCE OF THE LOWER BELT SUPERGROUP (NEWLAND FORMATION),HELENA EMBAYMENT, CENTRAL MONTANA

The Belt Supergroup is well exposed in western Montana where most of the unit is allochthonous due to Sevier thrusting. However, east of the Helena salient several autochonous 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.

Deposits of the lower Newland locally contain pebble-sized lithic clasts derived from the underlying Chamberlain Formation suggesting basinal reorganization at the onset of Newland deposition. Cores from wells along 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. This unit is 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 lower Newland 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.

It has recently been proposed that the Belt Supergroup is an allochthonous block that was translated up to 1300 km north (Hildibrand 2015). However, this study suggests the Lower Belt in the Helena Embayment has not undergone significant latitudinal translation relative to cratonic North America.