TECTONIC IMPLICATIONS OF A CHEMOSTRATIGRAPHIC STUDY OF EARLY PALEOPROTEROZOIC CARBONATES FROM THE SOUTHEASTERN MARGIN OF THE WYOMING CRATON

The Paleoproterozoic southern margin of the Wyoming Craton (WC), while well exposed in the Sierra Madre and Medicine Bow Mountains is poorly exposed further east. Deposited on this margin are the Slaughterhouse and Nash Fork (NFF) formations of the Snowy Pass Supergroup. The lower part of the NFF, Medicine Bow Mountains contains carbonates with ¹³C-enrichment up to +28‰, whereas carbonates higher in the formation have d¹³C values between 0 and 2.5‰. This dramatic change in the isotopic composition of the Paleoproterozoic ocean is constrained to ~2.1 Ga (Karhu, 1993). Carbonates of the Slaughterhouse Formation, Sierra Madre have similar levels of ¹³C-enrichment, supporting correlation with the NFF. The age of the Whalen Group in the Hartville Uplift (the easternmost exposure of the WC) is poorly constrained. Although Hofmann & Snyder (1985) found that some stromatolites in this group are similar to the ones in the NFF (Hadrophycus immanis), this unit has still been considered to be Archaean in age. Carbonates of the Rawhide Canyon section that contain these stromatolites have d¹³C values up to +8.2‰ suggesting correlation with the lower part of the NFF and their deposition along the continuous southern margin of the WC. The rest of the Whalen Group - including carbonaceous schists with BIFs, carbonates with d¹³C values between 0 and 2 ‰, and mafic volcanics - are likely correlative with the upper NFF and the overlying Towner Greenstone. Carbonates of the Estes and Roberts Draw formations, in the Black Hills were deposited on the eastern margin of the WC and have d¹³C values similar to the upper NFF. Along with the geochronologic constraints these data suggest that the rifting and breakup on the eastern margin occurred between 2.1 and 2.0 Ga. These chemostratigraphic constraints suggest that the Snowy Pass Supergroup is significantly older than and unrelated to the ~1.78 Ga Medicine Bow Orogeny.