Epiclastic Versus Pyroclastic: Revised Age Constraints on Cryogenian Glacial Deposits in the Pocatello Formation, Idaho, U.S.A.

Mark David Schmitz, Boise State University, Department of Geoscience, Boise, United States, Vincent Isakson, Boise State University, Boise, ID, United States, Carol M Dehler, Utah State University, Logan, UT, United States, Adolph Yonkee, Weber State University, Ogden, UT, United States and Francis A Macdonald, University of California Santa Barbara, Earth Science, Santa Barbara, United States

Contact First Author: Mark David Schmitz; markschmitz@boisestate.edu

Previously Published Material: Cordilleran Tectonics Workshop, University of British Columbia-Okanagan

Abstract ID#: 35973

 

English Abstract:
Neoproterozoic glacial successions are recognized worldwide, and interpreted to record global glaciations as part of the ‘Snowball Earth’ model (Kirschvink, 1992; Hoffman et al, 1998). Although a fundamental tenet of the Snowball Earth hypothesis is a global synchrony of glaciogenic deposits, the number, timing, and correlation of these glacial episodes remain controversial, due to incomplete successions and limited geochronologic constraints.

The Neoproterozoic Pocatello Formation of southeastern Idaho exposes a thick (>1km), well-preserved stratigraphic succession containing glaciogenic strata and a cap dolostone with Marinoan characteristics (Dehler et al, 2011). Stacked diamictite-bearing intervals appear to record two glacial episodes, but absolute ages and relation to other Neoproterozoic glacial events are uncertain. Published ages from the lower Scout Mountain Member of the Pocatello Formation (Fanning and Link, 2004) are now known to be maximum depositional ages from epiclastic volcanic detritus (Keeley et al, 2013). We present new high-precision (CA-IDTIMS) U-Pb zircon ages of ~697 Ma from two felsic pyroclastic flows within the lower diamictite of the Pocatello Formation, which place the first robust constraints on the ‘Sturtian’ glaciation in this sector of the Laurentian margin. Exposures of the lower Pocatello Formation in Idaho thus appear to record only the latter half of the long Sturtian Snowball Earth glaciation (Rooney et al, 2014). By contrast, we will also introduce new data that falsify the earlier identification of a green argillite bed in the upper Scout Mountain Member as a ~667 Ma ”reworked fallout tuff bed” (Fanning and Link, 2004), and demote its significance to a maximum depositional age from epiclastic detritus. Without a firm minimum age bracket, the correlation of the upper diamictite and associated cap carbonate in the Pocatello Formation to the Marinoan deglaciation remains tenable.

Dehler et al (2011) GSA Field Guide 21:181-192. Fanning and Link (2004) Geology 32:881–884. Hoffman et al (1998) Science 281:1342-1346. Kirschvink, J.L. (1992) in Schopf and Klein, The Proterozoic biosphere: a multi-disciplinary study: Cambridge University Press, p. 51-52. Keeley et al (2012) Lithosphere doi:10.1130/L226.1. Rooney et al (2014) PNAS 111:51–56.

Back to: The Neoproterozoic-Paleozoic Earth System: Records From an Earth in Transition