Pliensbachian [lower jurassic] biostratigraphy and ammonite fauna of the Spatsizi area, north-central British Columbia

Abstract

In the northern Spatsizi area where lateral variations in facies and structural complexity are common, the age relationships between rock units can be accurately established using ammonite faunas. The Pliensbachian ammonites described in this bulletin were collected as part of a biostratigraphic study of the Spatsizi Group and Cold Fish volcanics, which collectively range in age from Pliensbachian to Bajocian. The study area includes most of the Eaglenest Range in the Spatsizi Plateau Wilderness Park, an area situated on the Stikine Terrane - one of several geologically distinct crustal blocks that constitute the Western Cordillera. The Spatsizi area is bounded to the north by the Stikine Arch, a major east-west trending tectonic feature that was a fundamental influence on the pattern of deposition in north-central British Columbia during the Jurassic. The ammonite fauna was collected from sedimentary and volcanic rocks distributed along the southern flank of the Stikine Arch and exposed from beneath the Bowser Lake Group in small inliers. The Pliensbachian part of this sequence is represented by sedimentary strata interbedded with the Cold Fish volcanics; conglomerate and siltstone of the Joan Formation, and dark shale of the lower Wolf Den Formation. The ammonite fauna is dominated by Tethyan and east Pacific forms. Nearly 500 specimens are assigned to more than 27 species representing 17 genera. Metaderoceras talkeetnaense is described as new. The biostratigraphy is referred to the North American zonal scheme demonstrating that the Cold Fish volcanics west of Cold Fish Lake are Early Pliensbachian (Imlayi and Whiteavesi zones); the Joan Formation, Early Pliensbachian (Whiteavesi and Freboldi zones), and the lower Wolf Den Formation Late Pliensbachian (Kunae and Carlottense zones) in age.