Detrital Zircon Geochronology of Upper Cretaceous to Paleocene Sandstones From South‐Central Wyoming: Evidence for Middle Campanian Laramide Deformation.

Laramide deformation during the Late Cretaceous through early Eocene interrupted the east‐flowing drainage systems from the Sevier hinterland and segmented the Western Interior Foreland Basin in much of western North America into a series of intermontane basins and Precambrian basement‐cored uplifts. In Wyoming, the timing of Laramide deformation and its impacts on drainage patterns during the overlap in the Sevier and Laramide orogenies remain under debate. New detrital zircon U‐Pb geochronologic data from 11 outcrop samples (n = 3,085) in south‐central Wyoming suggest at least two provenance‐distribution phases, distinguished by a substantial increase in zircons of late Paleoproterozoic age (1800–1600 Ma) recorded at approximately 79 Ma. The most likely source of late Paleoproterozoic age zircons is Precambrian basement‐cored uplifts, specifically the Sierra Madre Mountains, signaling the possible middle Campanian initiation of Laramide uplift, earlier than previously recorded. When combined with complementary data sets from surrounding basins, the first provenance‐distribution phase is age discordant, with the earliest evidence of 1800–1600 Ma zircons in the Piceance Basin of Colorado (late Santonian to early Campanian). The second provenance phase shows repeating age distribution patterns that begin with significant contributions from 1800 to 1600 Ma zircon grains and end with much broader and evenly distributed population signatures, confirming previous interpretations of significant Laramide deformation during the Maastrichtian (approximately 70 Ma) as well as uplift and recycling of Upper Cretaceous formations at about 67 Ma. Plain Language Summary: During the Late Cretaceous, Wyoming was part of an epicontinental seaway that extended from Canada to Mexico. This Western Interior Seaway was a low spot in the North American continent formed from mountain building and crustal loading of the Wyoming fold‐and‐thrust belt to the west, an event known as the Sevier orogeny. At some point in the Late Cretaceous, an event known as the Laramide orogeny segmented the Western Interior Foreland Basin into a series of intermontane basins by basement‐cored uplifts that are today's mountain ranges. The timing of the Laramide orogeny, and its impacts on the east‐flowing drainage systems from the Sevier hinterland, has been the subject of research and argument for more than a century. New age data obtained from zircons preserved in outcrop samples collected in south‐central Wyoming—showing unique zircon populations that were most likely derived from the basement core of nearby mountain ranges—suggest at least two episodes of Laramide uplift in the Upper Cretaceous through Paleocene strata. The first event occurred about 79 million years ago, earlier than previously recorded in Wyoming. The second event recurred approximately 67 million years ago, coincident with the main phase of Laramide deformation recorded throughout much of Wyoming. Key Points: Geochronologic data from Upper Cretaceous and Paleocene strata in Wyoming record a substantial increase in late Paleoproterozoic age zirconsThese zircons suggest exhumation of the Sierra Madre Mountains, implying Laramide uplift at about 79 Ma, earlier than previously documentedRepeated pulses of Paleoproterozoic age zircons indicate renewed uplift at 67 Ma, coincident with the primary phase of Laramide deformation [ABSTRACT FROM AUTHOR]