Abstract

Pennsylvanian rocks of the northern U.S. Rocky Mountains are mature quartzose sandstones. This paper uses detrital zircon geochronology on seven samples from the Wood River Formation, Tensleep Sandstone, and Weber Sandstone to determine if these sandstones have a common provenance, representing eastern Laurentian and Appalachian sand reworked within shallow-marine and eolian environments from the Wyoming craton westward to the Pioneer thrust plate of south-central Idaho. Our work suggests that this continental sand blanket was mixed with local sources on the south in the Yavapai-Mazatzal provinces of the Ancestral Rocky Mountains and in samples from the western Cordilleran thrust belt in south-central Idaho. In total, these Pennsylvanian sandstones contain a broad spectrum of detrital zircon U-Pb ages including, from old to young: A) minor Archean-age (3300–2550 Ma) populations; B) Paleoproterozoic (2000–1600 Ma), Mesoproterozoic (1470–1350 Ma), and major “Grenvillian” (1250–950 Ma) populations; and C) Cryogenian- to Ediacaran-age (665–565 Ma) and minor Paleozoic (495–410 Ma) populations. We interpret these detrital zircon ages to represent provenance mainly from the Appalachian mountain belt of eastern North America; however, central Appalachian versus northern Appalachian derivation is not clearly distinguished. The Weber Sandstone from the north flank of the Uinta Mountains in northeast Utah contains a strong 1700–1640 Ma age population derived from the Yavapai-Mazatzal provinces in the adjacent Ancestral Rocky Mountains. The shallow-marine Hailey Member of the Wood River Formation in south-central Idaho yields a population of >1800 Ma detrital zircons reworked from the uplifted Mississippian Copper Basin Formation. Both the Hailey and Wilson Creek Members of the Wood River Formation contain unique 640–490 Ma grains that may represent provenance from the Big Creek-Beaverhead plutonic belt of east-central Idaho and/or eastern Klamath terrane in the Klamath Mountains of northwest California and southwest Oregon. These new data support published models for Pennsylvanian–Permian transport of siliciclastic sediment with sources mainly from the North American craton, north of the Ancestral Rocky Mountains, into continental margin basins.

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