Evolution of a Permo-Triassic sedimentary mélange, Grindstone terrane, east-central Oregon

Abstract

The Grindstone terrane in east-central Oregon is one of the few areas in western North America where large blocks of unmetamorphosed Devonian, Mississippian, and Permian limestones are inter mixed with Permian and Lower Triassic radiolarian chert and Pennsylvanian?, Permian, and Triassic volcaniclastic rocks. Although originally described as parts of a coherent succession, we interpret the Grindstone rocks to be a sedimentary melange composed of Paleozoic limestone slide and slump blocks that became detached from a carbonate shelf fringing a volcanic knoll or edifice in Late Permian to Middle Triassic time and were intermixed with Permian and Triassic slope to basinal clastic and volcaniclastic rocks in a forearc basin setting. Paleogeographic affinities of the Grindstone limestone faunas and volcaniclastic debris in the limestone and clastic rocks all indicate deposition in proximity to an island-arc system near the North American craton. The Grindstone terrane deposits are unconformably overlain by Upper Triassic to Middle Jurassic sequences of the Izee terrane. Although lithologic and faunal differences indicate that the Grindstone and Izee terranes together represent a tectonic block separate from the adjacent Baker terrane, all three terranes were juxtaposed by Late Triassic or Early Jurassic time. We recommend reduction to informal status for the Coffee Creek (Mississippian), Spotted Ridge (Pennsylvanian?), and Coyote Butte (Permian) formations because (1) they cannot be mapped or traced beyond limited areas, and (2) these older rocks are chaotically inter-mixed with younger chert and volcaniclastic rocks. New biostratigraphic data indicate that the bulk of the Spotted Ridge volcaniclastic rocks represent a part of the Triassic Vester Formation of the Izee terrane.