DRAINAGE EVOLUTION IN AN INCIPIENT FORELAND BASIN FROM U/PB DETRITAL ZIRCON GEOCHRONOLOGY, JURASSIC-CRETACEOUS WESTERN INTERIOR BASIN

The Late Jurassic to Early Cretaceous Cordilleran foreland basin fill of North America consists of two tectonostratigraphic wedges separated by a major sub-Cretaceous unconformity (ca. 20 Ma). New U/Pb detrital zircon dates from these two clastic wedges at Great Falls in west-central Montana and Grande Cache in west-central Alberta suggest a shift from broad continental provenance to more local sources in both foreland clastic wedges.

It has been demonstrated in Alberta that the Jurassic drainage system of the first tectonostratigraphic wedge contains evidence for two depositional phases: 1) sediment distributed along basin axial channel systems containing grains reworked from Triassic-Jurassic eolian deposits of the southwestern U.S. (Paleozoic and Neoproterozoic zircons); 2) an abundance of sediment locally sourced from uplifted strata of the western margin of North America (Paleoproterozoic and Archean zircons). Both the Great Falls and Grande Cache data sets span the second clastic wedge of the Early Cretaceous. In the oldest lithostratigraphic units, Mesozoic zircons from the Cordillera comprise 4-12% of the samples. Paleozoic and early Mesoproterozoic (1040 Ma) zircon ages combined with paleoflow measurements, lead to an interpretation that axial rivers transported recycled sediment from the southwestern U.S. An abrupt increase in Mesozoic zircons (i.e. 112 and 165 Ma ages) composing 72-95% of the data is interpreted to record a shift to local, Cordilleran sources of sediment likely delivered to the basin by transverse rivers. The American and Canadian data, as well as published detrital zircon ages from western Montana and northeastern Alberta all show the shift from axial drainages that sourced a broad continental catchment to Cordilleran dominated age spectra. This shift is diachronous being recorded earlier in strata at Great Falls. This suggests that uplift and denudation of the orogen may have occurred first in the northern U.S.

Data from both clastic wedges indicate that the accommodation space in the basin was initially under-filled and dominated by axial drainage adjacent to the orogen. Accommodation space was filled as a result of increased sediment supply from the emergent orogen, shifting axial drainage networks further eastward into the basin.