Sediment dispersal in an evolving foreland: Detrital zircon geochronology from Upper Jurassic and lowermost Cretaceous strata, Alberta Basin, Canada

Abstract

The Alberta foreland basin is a classic example of a retro-arc foreland basin, yet the early stages of its development remain poorly understood. Several contrasting hypotheses have been proposed to explain the source areas and dispersal patterns of sediment in western Canada during the Late Jurassic initiation of the foreland basin. Here, we use detrital zircon uranium-lead (U-Pb) geochronology, sandstone petrography, paleocurrent measurements, and regional correlations to reconstruct the early basin evolution, including sediment provenance and depositional history. These data indicate sediment in the early foreland basin was delivered via two principal sedimentary systems: a south-to-north axial river system, and transverse fluvial systems that emanated from the adjacent Cordillera. Accordingly, sandstones of the Jurassic foreland, associated with the Minnes Group and equivalent Kootenay and Nikanassin formations, are divided into two informal groups, type 1 and type 2. Type 1 sandstones are mature quartz arenites, present along the entire north-south length of the Alberta Basin, and generally at the base of the succession. Type 1 sandstones have zircons with age populations between 980 and 2000 Ma, similar to sediments of Jurassic and Lower Cretaceous strata in the western United States. These deposits are interpreted to have been derived from southern sources and transported axially to the north along the earliest foredeep of the Cordilleran foreland basin. Type 2 sandstones by contrast, are less mature, containing higher quantities of chert and lithic fragments, and are dominated by 1765–2100 Ma zircons with a smaller population at 2500–2800 Ma. The zircon age populations of type 2 sandstones are similar to populations recorded in the Neoproterozoic to Triassic miogeocline strata of the adjacent fold-and-thrust belt. Type 2 sandstones are common in the western, orogenic side of the basin, but they extend eastward across the basin in fluvial sediments in the upper portion of the succession. Changes in provenance and sediment composition are associated with the evolution of paleodrainages and the increasing importance of Cordilleran erosion to the sediment budget. The progressively greater influx of orogen-derived material relative to subsidence displaced the axial fluvial system toward a more cratonward-position. The collected data support the hypothesis that much of the sediment was initially transported northward by an axial drainage network, followed by Cordilleran-sourced sediments fed by transverse river systems. The present study attempts to unravel predictable patterns of sediment dispersal in evolving foreland basins, while testing whether clear changes in sediment composition of the first clastic pulse of sediment are related to hinterland exhumation, changing drainage divides, weathering processes, or varied provenance.