The Pennsylvanian–Early Permian Ancestral Rocky Mountains, Proximal Source of Sediment and Barrier to Regional Dispersal—The Story from Detrital Zircons with a Few Surprises along the Way

March 1, 2022 2:00 PM


William A. Thomas, Ph.D.
Hudnall Professor Emeritus of Geology, University of Kentucky
Visiting Scientist, Geological Survey of Alabama


Interpretations of sedimentary provenance from detrital-zircon data depend on two primary considerations—matching of detrital-zircon age distributions with crystallization ages of the inferred provenance, and tracking a dispersal pathway from source to sink. Prior to tectonic uplift of the Ancestral Rocky Mountains (ARM), a craton-wide erosion surface on Mississippian limestones was covered by the initial late Chesterian–Morrowan sedimentary deposits of the Absaroka sequence; the cover strata have cosmopolitan distributions of detrital-zircon age groups from various sources around the margins of the Laurentian craton. The late Morrowan rise of the ARM basement uplifts provided a proximal source for coarse clastic deposits, which have detrital-zircon age groups corresponding to the age of ARM basement rocks (herein termed “intraARM detritus”). The proximal sediment in alluvial-fan to marine-delta-front systems, however, did not spread widely from the source ARM uplifts, and “extraARM detritus” from distant sources around the margins of the craton mingled with the proximal intraARM detritus adjacent to the uplifts. Not far from the uplifts, extraARM detritus completely overwhelmed most of the intraARM detritus. Active uplift of the ARM basement structures ceased in the Late Pennsylvanian; erosional denudation and subsequent Permian onlap onto the truncated basement rocks covered the ARM uplifts. The Permian deposits, ranging from shallow marine to eolian, have cosmopolitan detrital-zircon distributions that represent sources from around the craton. Identification of provenances and tracking of dispersal pathways are based on a compilation of new and previously published U-Pb age data from a concentration in the proximal intraARM deposits, from regions near the ARM uplifts, from more distant intracratonic basins, and from Paleozoic orogens around the margins of the Laurentian craton.

William A. Thomas

University of Texas at Austin

University of Texas

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