Fission‐track ages of detrital zircons from Cretaceous strata, southern British Columbia: Implications for the Baja BC hypothesis

Abstract

The Methow/Tyaughton basin contains a thick sequence of mid‐Cretaceous (Albian‐Cenomanian) marine clastic sediments, which have been interpreted as synorogenic fill deposited during collision and accretion of the Insular superterrane at the North American margin. Three distinct, regionally extensive petrofacies have been recognized in this sequence: (1) a west‐derived volcanic petrofacies, (2) a cherty petrofacies derived from a local intrabasinal high, and (3) an east‐derived arkosic petrofacies. Grain ages for detrital zircon are used here to identify provenance. The fission track (FT) method is used to date seven sandstone samples, with ∼45 grain ages per sample. Grain age distributions show marked differences between petrofacies and close similarities within a petrofacies. All petrofacies are dominated by a young population of grain ages (113 to 89 Ma), statistically indistinguishable from the depositional ages of the samples. This population is attributed to contemporaneous volcanism in the source region. The age of older populations varies among the petrofacies, indicating that each petrofacies was derived from different source terrains. Most important, all Arkosic samples have nearly identical grain age distributions, even though they come from widely separated parts of the Methow/Tyaughton basin. This evidence suggests that the Arkosic petrofacies is an overlap sequence that ties together disparate basement terranes, including the Bridge River, Cadwallader, and Methow terranes, by the Albian (∼100 Ma). The source of the Arkosic petrofacies provides an important constraint on the mid‐Cretaceous location of these outboard terranes. This petrofacies increases in thickness and coarseness to the east, suggesting a source inboard of the Methow/Tyaughton basin. The detrital composition of the sediment indicates that high‐grade metamorphic rocks and S‐type plutonic rocks were present in the source. Given the rapid onset of arkosic sedimentation in the Albian, we infer that this petrofacies was derived from newly uplifted metamorphic and plutonic rocks. We consider two interpretations for this arkosic source. An in situ interpretation would derive these sediments from metamorphic and plutonic rocks of the Omineca Crystalline belt, which presently lies to the east of the basin. The onset of arkosic sedimentation during the Albian is compatible with evidence from the Alberta foreland basin that indicates Albian uplift and denudation of the Omineca Crystalline belt. However, Nd/Sm data from the Arkosic petrofacies (Barfod and Nelson, 1992) indicate a fairly primitive source, which is not consistent with the presence of Precambrian rocks in the Omineca belt. The second interpretation accounts for paleomagnetic data that indicates the outboard terranes of western British Columbia have been transported 3000 km northward during the latest Cretaceous and early Tertiary. In this case, the arkosic sediments would most likely have been derived from an inboard continental setting near the present‐day latitude of central Mexico. A possible source might have been the western half of the Peninsular Ranges batholith, which is characterized by plutonic rocks with relatively primitive isotopic characteristics.