LATE TONIAN PALEOGEOGRAPHY OF SOUTHWESTERN NORTH AMERICA

In light of new tandem U-Pb zircon geochronology using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-IDTIMS), derived age models for late Tonian strata in Grand Canyon (AZ), Death Valley (CA), and the Uinta Mountains (UT) now allow stronger correlation and the ability to construct high resolution paleogeographic maps.

In this paper, we present U-Pb CA-IDTIMS ages for detrital zircons extracted from basal sandstones within the Nankoweap Fm, Chuar Group (Grand Canyon), the Horse Thief Springs Fm, middle Pahrump Group (Death Valley), and the Jesses Ewing Canyon Fm, Uinta Mountain Group (UMG, Uinta Mountains), collectively referred to as the ChUMP units (Chuar-Uinta-Pahrump). All CA-IDTIMS analyses were performed on detrital zircon grains that were previously identified using LA-ICPMS analyses.

The basal ChUMP units yield maximum depositional ages between 775 and 766 Ma. The Nankoweap Fm sample yields a 775.6 Ma ± 0.3 age (n=14 grains) and has a single grain at 770.1 ± 0.5 Ma. The Jesse Ewing Canyon Fm yields two young populations of 775.1 ±0.7 Ma (n=3 grains) and 766.3 ±0.5 Ma (n=3 grains). The Horsethief Springs Formation sample yields grains averaging 775.4 ± 0.7 Ma. The maximum depositional age (MDA) for the initiation of the Chuar basin is <775 Ma, or possibly <770 Ma. The MDA for the middle Pahrump basin is <775 Ma, and the MDA for the Uinta Mountain Group is <766 Ma. It is possible that all three basins initiated as late as 766 Ma, or, the UMG basin initiated some millions of years after the Chuar and middle Pahrump basins started forming.

The new TIMS ages are integrated with existing stratigraphic, paleontologic, paleomagnetic, and geochronologic constraints on the ChUMP units. Patterns emerge such as: a) the middle Pahrump and Chuar Group basins received 775 Ma grains from a felsic source; b) the UMG basin received input from two felsic sources that are 775 and 766 Ma; c) all ChUMP units have vase-shaped microfossils at their succession tops with the Chuar top constrained between 751 and 729 Ma; and d) the ChUMP units are in near-equatorial latitudes at ca. 760 Ma.

This richness of multiple ChUMP data sets allows the construction of relatively high-resolution paleogeographic maps for the late Tonian of southwestern North America, including at least two time slices at ca. 770 and 740 Ma.