BIOSTRATIGRAPHIC OUTLINE OF THE LATE PALEOZOIC CHILLIWACK GROUP – REASSEMBLING A BROKEN FORMATION

The Devonian-Triassic Chilliwack Group is comprised of geographically isolated outcrops in northwest Washington State and adjacent southwest British Columbia. Heavy forest cover, tectonic disruption, Quarternary volcanism and erosion have made biostratigraphic reconstruction elusive. However, petrographic studies of late Paleozoic sedimentary rocks reveal a distinct sequence of four temporal horizons. (1) The Red Mountain limestone, defined by an Eostaffela/Millerella fauna, consists of 4 outcrops of early Pennsylvanian age. Described clastic sequences lying structurally below the Red Mountain limestone have proved to be either Mesozoic Cultus Formation or late Pennsylvanian rocks. (2) Upper clastic unit (Monger, 1966) includes widespread volcaniclastics of coarse sandstone to boulder congromerate texture. Endemic fauna and early fusulinids such as Profusulinella indicate an early to mid-Pennsylvanian age and a stratigraphic position above the Red Mountain limestone. (3) The Blue Creek unit consists of imbricated lenses of late Pennsylvanian to early Permian limestone interbedded with green/black chert and tuff. Each outcrop has a unique lithological and faunal succession and fusulinids are abundant, varied and occasionally well-preserved. Taxa include, but are not limited to, species of Ozwainella, early Parafusulina, Pseudofusulinella, Paraschwagerina, Schwagerina, and Schubertella. (4) The Black Mountain unit is comprised of 70 m of siltstone conformably overlain by 110 m of cherty limestone with tuffaceous and shaly interbeds, and the brachiopod fauna is the same throughout. Species of Schwagerina and Pseudofusulinella indicate a mid-Permian age. These species do not appear in the Blue Creek unit, even in reworked beds, so it is assumed that the Black Mountain limestone is the youngest of the Paleozoic Chilliwack outcrops