LINKING MID-NEOPROTEROZOIC SUCCESSIONS IN THE WESTERN U.S.: THE CHUAR GROUP-UINTA MOUNTAIN GROUP-PAHRUMP GROUP CONNECTION (CHUMP)

Detailed study of well-preserved Neoproterozoic strata in the western U.S. permits regional correlation based on lithologic, paleontologic, chemostratigraphic and physical stratigraphic characteristics. Correlation of these data offers a firm foundation for understanding regional paleogeography and related geochemical and biological dynamics, and for testing global chemostratigraphic models for the Neoproterozoic.

The Chuar Group (1600 m thick), Grand Canyon, consists of organic-rich mudrocks with m-scale dolomite and sandstone beds and represents nearshore marine deposition. Chuaria is present throughout the succession, Melanocyrillium appears in the upper ~300 m, and variability in d¹³C_org and d¹³C_carb £ ~10‰. Unconformably below the Chuar Group is the ~1.1 Ga Cardenas Basalt and within the top few cms is a 742 Ma ash. The unconformably (?) overlying Sixtymile Formation contains an incised valley (15 m deep) inferred to represent Sturtian-age (ca. 750-700 Ma) drawdown.

The Red Pine Shale (£ ~1500 m thick), Uinta Mountain Group, Uinta Mountains, consists of organic-rich mudrock with m-scale sandstone beds and represents marine deposition. The Red Pine Shale contains Chuaria and Melanocyrillium and shows variability in d¹³C_org of £ 12‰.

The upper Crystal Spring Formation and the Beck Spring Dolomite (£ 900 m thick), Pahrump Group, Death Valley, consist of marine to locally non-marine carbonate and fine- to coarse-grained siliciclastic rocks. These units display variability in d¹³C_carb of 8-10‰ and Melanocyrillium is present in the upper Beck Spring beds. A 1.08 Ga diabase dike is truncated by an unconformity at the base of this succession and the Beck Spring Dolomite is unconformably overlain by inferred Sturtian-age diamictite of the Kingston Peak Formation.

These correlations imply deposition in a communal ocean basin that formed just prior to the Sturtian glacial interval and indicate that these strata capture the transition to possible global-scale glacial conditions. This study further elucidates the connection between the profound biological, geochemical, climatic, and tectonic changes (eukaryotic radiation, large magnitude fluctuations in C-isotopic composition of seawater, equatorial glaciations, supercontinent rifting) that mark this eventful time in Earth history.