Intertongued Upper Cretaceous Volcanic and Nonvolcanic Rocks, Central-Western Montana

Abstract

A thick sequence of intertongued volcanic and nonvolcanic rocks, exposed on the northeast limb of a syncline immediately west of Garrison, central-western Montana, is a western facies of the Elkhorn Mountains Volcanics of Late Cretaceous age. This sequence, here named the Golden Spike Formation, ranges from 4000 to 8000 feet in thickness. In the northwestern sixth of the 9-mile-long outcrop belt, nonvolcanic rocks predominate. Nonvolcanic detritus which forms this relatively thin fluviatile sequence was derived from tectonic uplifts a short distance to the west or northwest of Garrison. In the southeastern area, a relatively thick sequence comprises mostly lavas and volcaniclastic rocks. Most volcaniclastic rocks were probably deposited by epigene processes ( e.g. , weathering, erosion, and redeposition by mudflows, landslides, or running water) and were derived from centers of Elkhorn volcanism to the east. This depositional pattern resulted in intimate interfingering of volcanic and nonvolcanic beds; the authors observed only limited mixing of these two components in individual beds. Throughout the formation many volcanic tongues wedge out within a short distance to the northwest. The virtual absence of volcanic debris in the fluviatile rocks deposited to the northwest indicates either that explosive volcanic activity was minimal in the Elkhorn volcanic field during deposition or that prevailing westerly winds kept pyroclastic material out of the Golden Spike area.