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