STRUCTURE , STRATIGRAPHY AND GEOCHRONOLOGY OF THE BANNOCK VOLCANIC AND SCOUT MOUNTAIN MEMBERS, POCATELLO FM, SE IDAHO: GRADATIONAL CONTACT CONSTRAINS TIMING OF RODINIAN RIFTING AND STURTIAN GLACIATION

Rift-related volcanic and glaciogenic strata of the Neoproterozoic Pocatello Fm at the base of the Cordilleran miogeocline in southeast Idaho record the Early Cryogenian Sturtian glaciation and Rodinian rifting. Basal metabasalts of the Bannock Volcanic Mb (BVM) underlie and are in gradational contact with the lower diamictite of the Scout Mountain Mb (SMM). Two new ICP-MS and four SHRIMP U-Pb zircon ages in conjunction with previously dated samples constrains the BMV to be < 705 Ma and > 686 Ma and suggests that the overlying Scout Mountain Member is < 686 Ma.

On Oxford Mountain a pyroclastic flow a few tens of meters above greenschist metabasalt of the BVM has a SHRIMP concordia age of 702 ±5 Ma for a population of euhedral zoned igneous zircons. Different samples from the same stratigraphic interval have zircon populations with LA ICP-MS ages at 673 ± 7 Ma and 696 ±9Ma. These volcaniclastic beds grade upward to sandy Scout Mountain diamictite that contain a population of euhedral zircons with a LA ICP-MS age of 688 ± 7 Ma. Although it is now clear that several zircon populations appear to be present in the Oxford Mountain tuff, data from different laboratories using two methods are consistent with reports by Fanning and Link (2004; 2008) of zircon populations with SHRIMP ages of 709 ± 4 and 686 ± 5 Ma from this stratigraphic interval. More precise dating is in progress.

In the basal SMM south of Portneuf Narrows in the N Bannock Range, detrital zircon populations in reworked volcanic diamictite have SHRIMP ages of 705 ± 5 Ma and 704 ± 5 Ma. A rhyolite clast from a cobble conglomerate above this lower diamictite yields 689 ± 4 Ma. A previously reported age from a tuff above the upper diamictite is 667 ± 2 Ma.

The presence of the reworked ~700 Ma population south of Portneuf Narrows allows a stratigraphic correlation along 75 km of strike to Oxford Mt and suggests regional felsic Cryogenian magmatism or ash dispersal. The favored interpretation is that the pyroclastic flow on Oxford Mt represents a fixed and punctuated long-lived (20 m.y.) felsic volcanic center that post-dates BVM basalt deposition (<705 Ma) and erupted in two pulses at ~700 and ~680 Ma, the younger of which incorporated the older population and was not subsequently reworked.