Correlation of the members of the Jurassic Snowshoe Formation in the Izee basin of east-central Oregon

Abstract

The lithostratigraphic terminology of the type Snowshoe Formation of the John Day inlier is revised. The lower member is correlated with and renamed the Warm Springs Member. The middle and upper members are renamed the Schoolhouse and South Fork Members respectively. A local zonal scheme based on ammonites and related to the standard schemes of North America and northwest Europe is applied to the interdigitating units deposited in the Izee basin. Marked diachronism is evident so that the depositional model of the Basey and Silvies Members synchronously providing sediment to the Izee basin must be rejected. It is replaced by a sequential model whereby an early Bajocian Basey-to-Schoolhouse-to-Warm Springs Member lateral transition eastward is followed by a late Bajocian Silvies-to-Schoolhouse Member lateral transition westward. This latter event together with the deposition of the succeeding South Fork Member of Bathonian age is represented south of the South Fork of the John Day River by an unconformity and as such denotes a major event in the Jurassic history of eastern Oregon.