EARLY MIDDLE CAMBRIAN (GLOSSOPLEURA BIOZONE) TRILOBITES IN A STEPTOE SURROUNDED BY COLUMBIA RIVER BASALT NEAR SPOKANE, WASHINGTON
One small steptoe on the east side of Clear Lake, which is 10 km west-northwest of the city of Cheney, Washington and 25 km southwest of Spokane, is an exposure of relatively unmetamorphosed rock, beginning with a calcareous quartz arenite (quartzite), overlain by shale containing minor limestones beds and then a reddish mudrock facies. From the reddish mudrock, which had been exposed during construction activity completed several years ago, we discovered an early Middle Cambrian trilobite fauna consisting of inarticulate brachiopods, hyolithids, and trilobite genera including Amecephalus, Glossopleura, and Zacanthoides. This is the first reported occurrence of a Glossopleura Biozone fauna in Washington and the only report of fossiliferous strata of Paleozoic age within this region of the Columbia plateau. The nearest known occurrence of this age fauna is from the upper Rennie Shale and basal Lakeview Limestone, located some 100 km to the northeast on the southern flank of Packsaddle Mountain, near Lakeview, Idaho. The rocks and fauna of this small steptoe have a closer affinity to this Idaho location than to the more extensive upper Proterozoic and Cambrian facies exposed in northeastern Washington.