Jurassic stratigraphy of Central Oregon

Abstract

Ten formations are recognized in the Jurassic marine sequence of central Oregon. Angular unconformities separate these formations from the Paleozoic and Upper Triassic formations below and the Upper Cretaceous above. No marked diastrophism is indicated within the Jurassic sequence, though long intervals of nondeposition are present. A chronologic arrangement of the formations shows four formations in and near the Middle Lias, four in the early Middle Jurassic, one in the early Upper Jurassic, and one in the Upper Jurassic or Cretaceous. A marked threefold lithologic grouping of the sequence is evident but does not correspond to the chronologic arrangement. The oldest lithologic division consists of late Lower Lias shales and sandstones, over 2000 feet thick, at the isolated Donovan Ranch locality in the Silvies River Canyon near Burns. The second lithologic division contains three Lias formations and two early Middle Jurassic formations at the base of the section in the main Jurassic area. It is characterized by thin calcareous formations abounding in well-preserved fossils. The third division includes the remaining part of the Jurassic sequence consisting of two formations of the early Middle Jurassic and one of the early Upper Jurassic, as well as the uppermost formation of uncertain age. It is characterized by thick, noncalcareous formations of sandstone and shale in which fossils are less abundant and not well preserved.