Nature and significance of the diachronous contact between the Hazelton and Bowser Lake groups (Jurassic), north-central British Columbia

摘要: Abstract In north-central British Columbia, the contact between Lower to Middle Jurassic Hazelton Group and Cretaceous Bowser Lake is an upward gradational change from fine-grained, commonly tuffaceous sedimentary lithofacies coarser more heterogeneous assemblages that include abundant chert clasts generally lack distinct tuff beds. Paleontologic dating reveals lowest older (Bajocian) in north-most exposures younger (Early Oxfordian) southwest toward centre of Basin. Interpretation stratigraphic changes relies upon age data ammonites, a lesser extent marine bivalves; new bivalve species with biostratigraphic significance, Myophorella richardsi n. sp., named herein. many places groups than previously understood. Earlier treatments this boundary over-emphasized paleontologic content time correlations rather lithologic criteria. The diachroneity base resulted from: southwestward migration initial front coarse, chert-bearing, clastic detritus derived Cache Creek terrane, which started Bajocian; subsequent northward coarse facies containing volcanic shed highland near south margin Although activity immediate vicinity Basin diminished significantly Bajocian, volcanism continued basin into latest Jurassic. This southern was likely source for beds uppermost Continued input ash recorded by U-Pb ages detrital zircons are coeval depositional inferred fossils. large volume systems, however, diluted influx prevented accumulation within proper.