Maximum age of the basal Cretaceous Chinkeh Formation sandstones, Maxhamish Lake area, Liard Basin, British Columbia

Abstract

Sandstone, shale and conglomerate of the Chinkeh Formation represent shoreface and channelized environments forming the base of the Cretaceous section in Liard Basin of northeast British Columbia, southeast Yukon and southwest Northwest Territories. Biostratigraphic constraints on the formation indicated a Hauterivian to Early Albian age. New U-Pb detrital zircon ages obtained from a subsurface core in the Maxhamish Lake area indicate a maximum depositional age of 112.6 ± 2.0 Ma and an Early Albian to latest Aptian age. Grain roundness and the diversity of zircon crystal morphologies suggest that most of the Cretaceous-aged zircons were derived from plutonic sources. Intrusions of the Anvil Suite, located approximately 550 km westnorthwest of the Maxhamish Lake area in the mid-Cretaceous, form the likely source. Erosion of the few kilometres of sediment originally overlying the intrusions would take at least a million years and plutonic zircon would not enter the sediment supply until then. Because of this delay in plutonic zircon availability, the new detrital zircon results suggest that the Chinkeh Formation in the Maxhamish Lake area is more likely Early Albian than latest Aptian in age, and support the Early Albian age assigned on the basis of foraminifera. Detrital zircon geochronology has proven to be a very useful tool in differentiating the Chinkeh Formation from similar Triassic strata.