CHUMP (CHUAR-UINTA MOUNTAIN-PAHRUMP) STRATA OF THE WESTERN U.S. RECORD CRETACEOUS – LIKE OCEAN ANOXIC EVENTS (OAES) BEFORE SNOWBALL EARTH

The correlative middle Neoproterozoic Chuar, Uinta Mountain, and middle Pahrump groups (ChUMP) of western North America record the nascent breakup of Rodinia, large perturbations in the carbon cycle, and biosphere change. In particular, the spectacularly preserved Chuar Group of Grand Canyon provides one of the best organic records of this time, which foreshadowed ~85 million years of extreme climate change including Snowball Earth conditions. Here we present new U-Pb detrital zircon, ⁴⁰Ar/³⁹Ar marcasite, C-isotope, ƐNd, and microfossil data from the Chuar Group.

A small population (n=11) of detrital zircon grains yield a maximum depositional U-Pb age of ca. 782 Ma for the base of the Chuar Group. Older detrital zircon populations indicate a combination of local and distant sources, and an overall unroofing sequence. ⁴⁰Ar/³⁹Ar on marcasite nodules from the base of the Awatubi Member yield a plateau age of 751+/-17 Ma constraining the base of the uppermost and best developed of three positive carbon-isotope excursions, which lasted until 742 +/- 6 Ma. ƐNd isotope values range from -8.68 to -5.73 and correspond to decreasing feldspar/kaolinite ratios and positive carbon-isotope values, showing a correlation between more juvenile crustal production, increased silicate weathering rates, and organic carbon burial. In-depth fossil counts show a decline of acritarch diversity and the appearance of vase-shaped microfossils associated with the Awatubi positive excursion.

Detrital zircon and carbon-isotope data sets correlate with the Uinta Mountain and Pahrump groups, as do microfossil trends. ChUMP strata also correlate with the Callison Lake dolostone in northwestern Canada, and are consistent with records in Svalbard, Scotland, Norway, and Namibia, confirming that significant global changes in the carbon, biologic, and weathering cycles occurred in the 740-780 Ma time period. Intriguingly, ChUMP excursions are reminiscent of ocean anoxia events of the Phanerozoic. For example, Middle Cretaceous strata record comparable variations, which are attributed to eustatic rise, silicate weathering, extinction events, and climate instability in response to greater oceanic crust production.