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Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, sub sidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, infl uence paleogeography and sediment-dispersal patterns, they should also yield similar patterns, or facies, of detrital zircon age spectra . Such age-distribution patterns should be documented on the craton in order to make meaningful comparisons to sedimentary rocks from suspect terranes along continental margins. The Rocky Mountains of western North America provide excellent outcrops of sedimentary rocks that record >500 m.y. of tectonostratigraphic evolution. One such Phanerozoic section is exposed along the margins of the Bighorn Basin in northwest Wyoming, from which we report over 4000 U/Th/Pb detrital zircon ages from 48 samples that span a stratigraphic interval from the Middle Cambrian Flathead Sandstone through the Eocene Willwood Formation. These data provide one of the most complete records of detrital zircon age patterns from this part of cratonic North America.The stratigraphic record of the Bighorn Basin is subdivided into four tectonostratigraphic assemblages (TSA1-TSA4). These assemblages record an initial passive margin, followed by a transition to a convergent margin, followed by a marine-dominated retroarc foreland basin, followed by a retroarc foreland segmented by local basement uplifts. This tectonostratigraphic architecture is expressed as four, fi rst-order patterns within the detrital zircon age distributions.TSA1 represents a Paleozoic-Triassic proximal continental margin assemblage dominated by Proterozoic zircons with abundant grains in the 1600-1950 Ma range, a Grenville population at ca. 1100 Ma, and a Phanerozoic population at ca. 420 Ma. TSA2 is a transitional assemblage associated with the Jurassic-Early Cretaceous organization of a west-facing convergent margin and Cordilleran orogen. The TSA2 detrital zircon age distribution is characterized by the appearance of Mesozoic grains, age peaks at ca. 420 and 600 Ma, and a dominant population of Grenville (1.0-1.1 Ga) grains with a suite of Proterozoic grains diminishing in abundance as age increases to 1.9 Ga. TSA3 sedimentary rocks were deposited in the Cretaceous Interior Seaway in a retroarc foreland basin and are dominated by zircons for which ages are close to the depositional age of the strata, refl ecting input from the active Idaho Batholith and Sierran segments of the Cordilleran magmatic arc. The older zircon fractions from TSA3 sedimentary rocks are characterized by a dominant detrital zircon age peak at 1.7-1.8 Ga, which probably refl ects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 refl ects the phase of basin fi ll associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this a...

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A high-flux, Late Cretaceous magmatic event in the western United States has been tested as a zircon source for high-resolution chronostratigraphic correlation in coeval sedimentary rocks in northwest Wyoming. Thirteen samples of Cenomanian-Coniacian sandstone in the Bighorn Basin yielded more than 1200 U/Th/Pb detrital zircon ages from the Mowry Shale, the Frontier Formation, and the Cody Shale. In addition, two individual clast ages were obtained from a conglomerate located near the top of the Frontier Formation. These formations are dominated by detrital zircon grains that yield paleontologically constrained depositional or near-depositional ages. Each sample has a minimum of 22 grains comprising the youngest age peak. Individual youngest peak ages range from 99.4 to 87.7 Ma, spanning Cenomanian through Middle Coniacian time (Gradstein et al., 2012). Three of four stratigraphic sections yield samples with minimum age peaks that young upward, are consistent with available paleontological control, and suggest an age resolution of one-two million years despite an estimated analytical error of 2 percent (+/-2 Ma for 100 Ma samples). An age reversal at the top of the fourth section demonstrates that recycling of older sediments into younger beds can be an important control on the age of zircon populations, even during intervals of sediment accumulation dominated by first-cycle zircons from an active magmatic arc. The presence of nearly depositional age volcanic cobbles at the top of the Frontier Formation implies rapid erosion and transport of coarse material from a volcanic source eastward into the foreland basin. The new detrital zircon data, in conjunction with available paleontological constraints, provide a framework for detailed stratigraphic correlation.

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