Paleocurrents in the Triassic of Northeastern British Columbia
Triassic strata of the Rocky Mountain Foothills in northeastern British Columbia consist predominately of fine–grained clastics in the lower part, medium– to coarse–grained in the middle and lower upper parts and fine–grained in the uppermost part. Some bioclastic carbonates and evaporites also occur in the middle and upper parts. Triassic rocks rest disconformably upon upper Palezoic chert, and are overlain disconformably by Jurassic shales in the southern part of the area and lower Cretaceous shales in the north. This, together with an unconformity bevelling progressively older beds northeasterly indicates uplift in the northeast. Seventy sections were measured to establish the stratigraphic facies relations and history of sedimentation. This work was augmented by textural studies on hand specimens from certain sections and by observations on bedding thicknesses. Generally, lower Triassic beds are thinner and finer–grained than middle and upper parts except for the youngest beds. Also, as sediments are coarser in the east than west, the following history of easterly transgression, followed by westerly regression and again followed by easterly transgression, was established.
Paleocurrents based on the observations of 2,500 current structures such as planar cross–bedding, festoon bedding, current and wave ripple marks, and flow markings indicated that the probable direction of sedimentary transport was toward the west and southwest. Triassic sediments deposited during a regressive phase underwent textural changes expressed as decrease in grain size and gross lithologic aspect from coarse in the east to fine in the west; at a given section younger beds are generally coarser than older beds, and axes of maximum thickness for successively younger formations occur successively westerly from those of older formations. These observations implied an apparent migration of depositional sites in the direction of sedimentary transport during the regressive phase of sedimentation. Such apparent migratory behavior of depositional sites in a direction away from the source area, and coinciding with the direction of sedimentary transport, was a response to increased erosion and consequent sedimentation arising from uplift in the northeast. It appears that paleoslope attitude was the dominant regional control of the resulting sedimentary trends.