The origin of the Neihart Quartzite, a basal deposit of the Mid-Proterozoic Belt Supergroup, Montana, U.S.A.

Abstract

AbstractThe Neihart Quartzite is the basal quartz arenite unit (≍270m thick) of the Mid-Proterozoic Belt Supergroup of western North America. Petrographic studies indicate a source area with plutonic granitic, metamorphic and felsic volcanic rocks. Extreme textural maturity and bimodality indicate an episode of aeolian transport for the detrital quartz grains. The lower 80% of the Neihart Quartzite were probably deposited by braided streams, whereas the upper 20% were deposited in shoreline environments. Residual material that was ‘stored up’ on the pre-Beltian cratonic surface and underwent aeolian reworking was the likely source material for most of the Neihart Quartzite. Less mature sediments in the top portion of the Neihart Quartzite indicate uplift and erosion of new source material during Neihart deposition. Other known cratonic quartz arenites, such as the St. Peter Sandstone (Ordovician), Lamotte Sandstone (Cambrian) and Flathead Quartzite (Cambrian), are thin (tens of metres thickness) and exhibit sheet-like geometry. In contrast, the Neihart Quartzite and its probable lateral equivalents are considerably thicker and increase in thickness towards the central portions of the basin. It thus appears that Belt sedimentation began with accumulation of a basal quartz arenite unit, and that sand for that unit was transported by braided streams from the surrounding craton to a gradually subsiding Belt basin.