REVISED AGE CONSTRAINTS FOR CRYOGENIAN GLACIAL DEPOSITS IN THE POCATELLO FORMATION, SOUTHEASTERN IDAHO

Neoproterozoic glacial successions are recognized worldwide and have been interpreted to record global glaciations as part of the snowball Earth model (Kirschnink, 1992; Hoffman et al., 1998). A fundamental tenet of the Snowball Earth hypothesis is the global synchrony of glaciogenic deposits. However, the number, timing and ultimate duration of glacial episodes remain controversial as successions are incomplete and geochronologic constraints are limited or have conflicting interpretations. Currently published data constrain two low latitude, snowball Earth glaciations: the “Sturtian” glaciation at ~716 – 662 Ma (Macdonald et al., 2010 – Rooney et al., 2014) and the “Marinoan” glaciation at ~655 – 635 Ma (Zhang et al., 2008 – Condon et al., 2005).

The Neoproterozoic Pocatello Formation of southeastern Idaho possesses thick, well-preserved stratigraphic succession divided into the basal Bannock Volcanic Member, the Scout Mountain Member and an informal upper member. Diamictite bearing intervals within the Scout Mountain Member may record multiple glacial episodes, but age determination for the Pocatello Formation has proved problematic. Published ‘Sturtian’ ages ranging between ~709-685 Ma for the lower Scout Mountain Member are now known to be maximum depositional ages of epiclastic detritus (Fanning and Link, 2004, 2008; Keeley et al., 2013). The Scout Mountain member also contains glaciogenic strata and a cap-carbonate with Marinoan characteristics, thus may be younger than previously thought (Dehler et al., 2011; Petterson et al., 2011).

We present new high precision U-Pb zircon ages of ~697 Ma from two felsic, pyroclastic beds within the lower diamictite of the lower Scout Mountain member. The ages for the event beds provide robust maximum age constraints for the overlying ‘Sturtian’ glaciogenic strata. Equivalent (~697 Ma) youngest dates determined for the lower diamictite indicate the deposit may have formed during or shortly after the event beds. We also present new data indicating the previously established minimum age constraint of ~667 Ma from a “reworked fallout tuff” (Fanning and Link, 2004) is a maximum depositional age from epiclastic detritus. The lack of a firm minimum age control supports the correlation of the upper diamictite and associated cap-carbonate as potentially Marinoan in age.