RIFTING OF SOUTHWEST LAURENTIA DURING THE STURTIAN-MARINOAN INTERGLACIAL AND ITS CONSTRAINTS ON INITIATION OF THE PASSIVE MARGIN

The Kingston Peak Formation in the Panamint Range represents the stratigraphically most complete section of Cryogenian strata along the SW margin of Laurentia. Two glacigenic diamictites and their associated cap carbonates, the older Surprise Member and Sourdough Member and the younger Wildrose Member and Noonday Formation (Sentinel Peak Member), provide timing constraints to bracket the inter-glacial succession to between ca. 713 Ma and 635 Ma, the ages of inferred correlative glacial cap carbonate rocks dated elsewhere. This timing constraint is further strengthened by the presence of a sharp decline in C isotopes in the Thorndike Member, which occurs immediately beneath the Wildrose Member; this decline is readily correlated with the global Trezona anomaly.

Within the inter-glacial succession, new mapping in the northern Panamints has documented the presence of a previously unrecognised suite of coarse sedimentary rocks herein defined as the Argenta Member of the Kingston Peak Formation. The Argenta consists largely of poorly-sorted breccias and conglomerates containing an assemblage of gravel-sized clasts dominated by granitic gneiss, schist, feldspar augens, vein quartz and quartzite fragments, and locally carbonate rocks. These compositions indicate derivation from a basement provenance and record deposition in alluvial-fan to coarse-braided fluvial settings; their textural and compositional immaturity implies relatively short distances of transport. Mapping shows that the Argenta defines wedge-shaped packages as much as 200 m thick and that the base of the Argenta is a significant angular unconformity. Combined, these features are evidence that deposition occurred during a phase of extensional tectonism interpreted as recording the initial dismemberment of the Rodinia supercontinent. Best estimates place the timing of this tectonism at ca. 650 –