Biochronology of selected Bowser Basin strata; tectonic significance

Abstract

Recent evaluation and reinterpretation of fossil floral and faunal data more clearly define the ages of strata exposed in the Groundhog Coalfield and the surrounding Bowser Basin of north-central British Columbia. In the Groundhog Coalfield, Bowser Lake Group strata consist of an overall coarsening-upwards cycle divisible into four informal stratigraphic units, which are, from oldest to youngest, the Jackson, Currier, McEvoy, and Devils Claw units. The section has an unconformable relationship with underlying Bajocian Spatsizi marine shales, volcanics, and arenaceous sediments. Marine macrofossils indicate a Callovian to Oxfordian age for the Jackson unit. The fossil plant succession present in the overlying Currier unit indicates Late Jurassic affinities. Recent unpublished palynologic data from lower McEvoy rocks in the northern Groundhog Coalfield suggest a Barremian age. The palynoassemblage present in the lower Devils Claw unit in the central part of the Groundhog Coalfield suggests a late middle Albian age.Rocks of the Sustut Group have an angular unconformable relationship with underlying Bowser Lake Group strata near the eastern margin of the Bowser Basin. The palynoassemblage present in Sustut Group rocks from the southern Sustut Basin suggests a Campanian to Maastrichtian age range, in contrast to a probable late Albian to Campanian age range for the northern Sustut Basin and a middle to late Albian age from Sustut Group outliers present within the northern Bowser Basin. Within the Groundhog Coalfield, Devils Claw strata have a conformable or paraconformable relationship with underlying Bowser Lake Group strata.Regional discrepancy in the age and geometry of the sub-Sustut unconformity is attributed to a time-transgressive unconformity that resulted from cratonward advance of an isostatically induced peripheral bulge. Age and contact relationships suggest that deformation in the Bowser Basin and surrounding Sustut Basin must have spanned the time period that corresponds to a second uplift pulse of the Columbian orogen (Aptian to early Cenomanian) and the uplift pulse related to the Laramide orogen (Campanian to Maastrichtian).