DETRITAL-ZIRCON AGES OF UPPER MISSISSIPPIAN–LOWER PENNSYLVANIAN DEPOSITS INDICATE MORROWAN INCEPTION OF THE ANCESTRAL ROCKY MOUNTAINS OROGEN

Detrital-zircon (DZ) U–Pb age distributions in upper Paleozoic strata throughout the Ancestral Rocky Mountains (ARM) region from Oklahoma westward change abruptly upward in the Early Pennsylvanian, marking a striking provenance shift in western Laurentia. Widespread red-weathering Upper Mississippian to Lower Pennsylvanian successions of the Rocky Mountain region overlie a Middle Mississippian carbonate platform and are overlain in turn by Morrowan–upper Atokan deltaic and marine strata of the Wyoming and Arizona shelves, Paradox basin, and Central Colorado trough. Generally interpreted as loessites and reworked loessites, the redbeds have a consistent, distinctive DZ age spectrum suggestive of sources in the late Paleozoic orogens of northern and eastern Laurentia. Because the redbeds are difficult to date, the stratigraphic brackets indicate they may have accumulated during a time interval of as much as 20 m.y. The DZ age distributions of the redbeds are largely indistinguishable from those of Upper Mississippian strata ranging from the Midcontinent to the Marathon region of West Texas. The wide distribution of similar DZ attributes demonstrates that the redbeds have cosmopolitan DZ age characteristics shared by numerous Mississippian depositional systems of Laurentia.

The Chesterian–Morrowan eolian redbeds pass upward through a thin transition interval into clastic deposits that have DZ ages derived almost exclusively from local sources in the basement rocks of fault-bounded uplifts of the ARM. Some of the overlying Morrowan and Atokan marine and deltaic strata contain a mix of DZ ages from the subjacent strata and from local basement sources in the Uncompahgre and Ancestral Frontrange uplifts. Most overlying Atokan to Permian fluvial and alluvial deposits have DZ ages from exclusively local sources in the ARM uplifts; however, some specific units in that succession include cosmopolitan DZ age distributions, indicating continued eolian detrital contributions from distant sources mixed with the proximal detritus. The consistent Early Pennsylvanian provenance change along the ARM uplifts, recognized by several research groups, records basement exposure in nearby uplifts and indicates nearly synchronous initiation of those uplifts in middle to late Morrowan time.