Magmatic unconformities and stratigraphic relations in the Peridotite zone, Stillwater Complex, Montana

Abstract

New stratigraphic relations between cumulate layers and units are documented in the Peridotite zone of the Stillwater Complex, a late Archean mafic to ultramafic layered intrusion exposed along the northern edge of the Beartooth uplift, Montana. First, low-angle unconformities occur between all types of successive stratigraphic units and at all scales within the Peridotite zone. Second, new stratigraphic units and divisions, based on the unconformities and type of cumulate layering, within the Peridotite zone are proposed and are correlated with published stratigraphic columns. The stratigraphic units, identified on the basis of rock type(s) and contacts with adjacent units, from smallest to largest a cyclic units, multicyclic units, megacyclic groups, and subzones. The three larger units are usually bounded by low-angle magmatic unconformities. The three subzones and five of the six megacyclic groups identified in the Peridotite zone can be traced over a distance of 17.7 mi (28.3 km). The stratigraphic relations in the Peridotite zone indicate that low-angle crosscutting magmatic unconformities occur at all scales and that lateral variations within and between the identified stratigraphic units are common. The relations suggest an extremely dynamic and rapidly changing environment during formation of the Peridotite zone. Overprinted on the stratigraphic relations is a larger evolution of cumulus rock types in the Peridotite zone from more primitive stratigraphic units dominated by olivine and olivine–bronzite cumulates to more evolved stratigraphic units dominated by bronzite and bronzite–olivine cumulates. Several major magmatic, or possibly structural, events appear to have affected the Peridotite zone during its formation, as indicated by thickness-variation patterns in megacyclic groups and subzones. The thickest massive chromite layers in the Peridotite zone are associated with two of the major magmatic events.