ChUMP CONNECTION (CHUAR-UINTA MOUNTAIN- PAHRUMP): GEOCHRONOLOGIC CONSTRAINTS FOR CORRELATING CA. 750 MA NEOPROTEROZOIC SUCCESSIONS OF SOUTHWESTERN LAURENTIA

This study reports new geochronologic constraints on both authigenic and detrital grains from Neoproterozoic successions of the western U.S., and fortifies previous correlation. We report new direct dates using microprobe U-Pb zircon dates on authigenic monazite from the Chuar Group, Ar-Ar dates on authigenic marcasite nodules from the Chuar Group, and detrital zircon and muscovite ages from sandstones of the upper Crystal Springs Fm (Pahrump Group, Death Valley, California) and Chuar Group and underlying upper Nankoweap Fm (Grand Canyon Supergroup, Grand Canyon, Arizona). Chuar Group black shales contain monazites of two populations. Larger grains (10-30 µ) are rounded and exhibit zoning patterns (1.74-1.65 Ga) consistent with their derivation as detrital grains from the local basement. A smaller population (1-5 µ) shows delicate textures that we interpret as early diagenetic authigenic products. These have distinct geochemistry (high Nd, low Th, and low La) relative to detrital grains and yield a preliminary age of 804 ± 20 Ma from the lower Chuar Group. This date is compatible stratigraphically with a 751 ± 17 Ma Ar-Ar plateau age (770 ± 30 Ma total gas age) on marcasite nodules and the published 742 ±6 Ma zircon age from an ash bed in the upper Chuar Group. New detrital zircon studies show a small number of zircon grains in the upper Nankoweap Fm (n=2) and the upper Crystal Spring Formation that are ca. 750 Ma.

These new ages: 1) reinforce previous correlations of the Chuar Group and the middle Pahrump Group with the Uinta Mountain Group of northern Utah (ChUMP hypothesis); 2) calibrate regional C-isotope curves and provide more concise timing on the burial of large volumes of organic carbon within a cratonic seaway (ChUMP seaway) that was potentially of comparable scale to the Cretaceous Western Interior Seaway; and 3) support previous models for a major (300 Ma) unconformity between the lower and upper parts of the Crystal Spring Formation.