PETROLOGIC APPROACHES TO CORRELATE LATE MESOPROTEROZOIC TO NEOPROTEROZOIC STRATIGRAPHIC SUCCESSIONS IN WESTERN NORTH AMERICA

Researchers continue to refine correlations among Mesoproterozoic and Neoproterozoic strata in the western U.S., relying upon combined lithostratigraphic, biostratigraphic, paleomagnetic, and chemostratigraphic analysis. We hypothesize that these strata record four punctuated depositional/tectonic episodes yielding different petrologic fingerprints. Our approach is to apply classical petrologic techniques in a class laboratory to test this model. Episode I (1250-1050 Ma) represents regional deposition prior to complete assembly of Rodinia. Episode II (1050-800 Ma) is characterized by regional erosion following the Grenville orogeny leaving little in the way of a sedimentary record. Episode III (800-740 Ma) reflects regional deposition associated with initiation of rifting of Rodinia, and closes with the climatic transition leading to global, low-latitude glaciation. Episode IV (740-540 Ma) consists of Sturtian to Cambrian deposition during continued rifting and thermal subsidence and extreme climate fluctuations associated by some with a 'snowball Earth'.

Late Mesoproterozoic successions include the Unkar, lower Pahrump and Apache Groups (AZ/CA), the Debaca 'terrane' (NM), and possibly the lower Uinta Mountain Group (UT). These sections are associated with thick red-bed siliciclastic deposits and ca. 1.1 Ga mafic intrusives. The 1250-1070 Ma Unkar Group records Grenville-age NW-directed shortening and orthogonal extension.

Mid-Neoproterozoic deposits representing Episode III consist of organic-rich mudrocks and carbonates that unconformably overlie Episode I strata. The ca. 800-742 Ma Chuar Group (AZ) is interpreted to record intracratonic extension during fragmentation of Rodinia. Other Mid-Neoproterozoic successions (the upper Pahrump Group (CA) and western Uinta Mtn. Group (UT)) share lithologic, paleontologic, and chemostratigraphic characteristics and potentially record rifting of the Cordilleran margin. Late Neoproterozoic deposits of Episode IV include the uppermost Pahrump Group, overlying Death Valley strata, and possibly the Sixtymile Fm. (AZ/CA), which range from fine- to very coarse-grained siliciclastic and carbonate rocks, and rest unconformably above Episode III strata. Lithic fragment composition, detrital quartz characteristics, and heavy mineral assemblages will be used to test for systematic differences among the three temporally distinct depositional suites.