Rocky Mountain (53rd) and South-Central (35th) Sections, GSA, Joint Annual Meeting (April 29–May 2, 2001)

Paper No. 0
Presentation Time: 10:15 AM

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


DEHLER, Carol M.1, PRAVE, Anthony R.2, CROSSEY, Laura J.3, KARLSTROM, Karl E.3, ATUDOREI, Viorel3 and PORTER, Susannah M.4, (1)Department of Geology, Utah State Univ, 4505 Old Main Hill, Logan, UT 84322, (2)Univ St Andrews, St Andrews, Fife, KY16 9AL, United Kingdom, (3)Earth & Planetary Sciences, Univ of New Mexico, Northrop Hall, Albuquerque, NM 87131, (4)Department of Organismic and Evolutionary Biology, Harvard Univ, 26 Oxford Street, Cambridge, MA 02138, chuaria@cc.usu.edu

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 d13Corg and d13Ccarb £ ~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 d13Corg 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 d13Ccarb 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.