Abstract

U-Pb detrital zircon (DZ) ages (n = 807) from Lower Cretaceous and lowermost Upper Cretaceous synorogenic conglomerate and interbedded sandstone deposited in and near the foredeep of the Cordilleran foreland basin in central Utah indicate stratigraphic compositional variation among the deposits. Eight DZ age populations, ranging from Archean through Mesozoic, are present in the synorogenic foredeep deposits in varying proportions and permit definition of three compositional suites, termed here chronofacies. Chronofacies A, present in uppermost Neocomian–lowermost Aptian foredeep deposits, contains Archean through early Paleozoic DZ grains derived from Jurassic–Pennsylvanian strata of the thrust belt. Chronofacies B, in Aptian–Albian foredeep deposits, contains Archean and Paleoproterozoic grains with an age peak near 1850 Ma, a population distribution similar to that of lower Paleozoic quartzites of the thrust belt. Chronofacies C, in Albian–lower Cenomanian foredeep deposits, contains a trimodal population distribution of Paleoproterozoic and Mesoproterozoic grains similar to that of Cambrian–Neoproterozoic quartzite strata of the thrust belt. The chronofacies of the foredeep deposits thus record systematic erosional unroofing of the thrust belt during Early Cretaceous time.

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