Evolution of the Western Canada Foreland in the Liard Basin, Northwest Canada

Abstract

Liard Basin, a subbasin of the Western Canada Sedimentary Basin, occupies an important re-entrant along the northwest, foreland edge of the Western Canada Foreland Basin (WCFB). It is characterized by anomalously thick Upper Paleozoic and mid-Cretaceous sections and bounded on the east and north by long-lived, deep-seated structures. Significant syn-depositional, west-side-down normal faulting occurred along the east margin during foreland basin evolution making the Liard a unique area in the WCFB. Albian to Campanian foreland basin strata preserved in the Liard Basin overlie a major regional unconformity that has completely removed the stratigraphic record of the Upper Jurassic - Lower Cretaceous foreland basin found to the south. The foreland basin succession in the Liard Basin comprises two unconformity bounded packages. The lower package includes a thick (1.0–1.6 km) succession of Early Albian-Lower Cenomanian marine shale with 3 shoreface sandstone units, and gradationally overlying Middle and Upper Cenomanian conglomerate-dominated alluvial – fluvial deposits (150–180 m). The disconformably overlying upper package includes up to 300 m of Santonian to Campanian marine shale and nonmarine sandstone that form the preserved basal part of a largely eroded 1.5–2.5 km thick Late Cretaceous-Paleocene clastic wedge. Thermochronological modelling indicates significantly less material was deposited near the northern edge of the basin. Two distinct detrital zircon age signatures occur in the lower wedge. Albian sandstones were largely derived from Paleozoic and Triassic sediments and are dominated by 370–450, 550–650, 950–1200 and 1600–1900 Ma zircon populations that originally came from Caledonian, Timanide, Grenvillian and Laurentian sources. These sandstones contain only 5–10% first cycle Cordilleran material, which was sourced from relatively close by. In contrast, Cenomanian coarse clastic strata are dominated by Cordilleran sourced zircons, 60% of which can be tied to well dated plutonic suites in southeast Yukon. Detrital zircon age profiles for Campanian sandstones of the upper wedge are eagerly anticipated. Rapid subsidence of the Liard Basin occurred in the Albian (post 108 Ma) after a period of explosive volcanism to the west. Reduced rates of subsidence in the Cenomanian combined with deformation and uplift of the foreland belt to the northwest resulted in coarse clastic facies flooding across the basin and a change in the detrital zircon age signature.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016