Abstract

Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, subsidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, influence 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, first-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, reflecting 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 reflects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 reflects the phase of basin fill associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this assemblage record erosion and redeposition of all previous sedimentary rocks from surrounding basement uplifts.

Patterns of detrital zircon ages reflect fundamental changes in paleogeography and sediment dispersal at the 10–100 m.y. time scale and are clearly related to major tectonic events or phases. Detrital zircon ages also provide evidence for linkages between convergent margin processes such as arc magmatism and sedimentation in the retroarc foreland. During these times of strong arc-retroarc linkage, detrital zircon geochronology provides a potentially useful tool for high-resolution chronostratigraphy.

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