A LARAMIDE CONNECTION BETWEEN WYOMING AND WISCONSIN: DETRITAL ZIRCON GEOCHRONOLOGY OF UPLAND CONGLOMERATE, UPPER MISSISSIPPI RIVER VALLEY

In the U.S. upper mid-continent region, chert-clast conglomerate of pre-glacial (i.e., pre-Quaternary; pre-Illinoian) age is preserved as far-traveled, but discontinuous, upland deposits along the modern Mississippi and Missouri River valleys. Detrital zircon ages in a sample of upland conglomerate from the Driftless Area of southwestern Wisconsin are consistent with recycled derivation from the western U.S. Cordillera. Precambrian and Paleozoic ages in our sample are a match with sand of the northern North American passive margin which was uplifted and recycled into the Cordilleran retroarc foreland basin during the Sevier and Laramide orogenies. Mesozoic and Cenozoic ages record well-known Cordilleran magmatic pulses with almost peak-for-peak matches to our sample. The youngest zircon ages and regional geologic constraints suggest a depositional interval ca. 55–45 Ma. We propose that the subducted Shatsky conjugate oceanic plateau provided a mechanism for extreme long-distance progradation of gravels from the Northern Rocky Mountains to Wisconsin during the Laramide orogeny. Lithospheric rebound following passage of the subducted plateau resulted in a migrating wave of anomalous relief that carried gravel from the Northern Rocky Mountains to the mid-continent region. This transient landscape effect was terminated by Late Eocene–early Miocene base-level capture by the north-flowing Bell River. Deposition of Wyoming-derived clastic sediment in Wisconsin provides an important record of early Paleogene transient landscapes in the mid-continent region.