A robust age model for the Cryogenian Pocatello Formation of southeastern Idaho (northwestern USA) from tandem in situ and isotope dilution U-Pb dating of volcanic tuffs and epiclastic detrital zircons

Abstract

Tandem in situ and isotope dilution U-Pb analysis of zircons from pyroclastic volcanic rocks and both glacial and non-glacial sedimentary strata of the Pocatello Formation (Idaho, northwestern USA) provides new age constraints on Cryogenian glaciation in the North American Cordillera. Two dacitic tuffs sampled within glacigenic strata of the lower diamictite interval of the Scout Mountain Member yield high-precision chemical abrasion isotope dilution U-Pb zircon eruption and depositional ages of 696.43 ± 0.21 and 695.17 ± 0.20 Ma. When supplemented by a new high-precision detrital zircon maximum depositional age of ≤670 Ma for shoreface and offshore sandstones unconformably overlying the lower diamictite, these data are consistent with correlation of the lower diamictite to the early Cryogenian (ca. 717–660 Ma) Sturtian glaciation. These 670–675 Ma zircons persist in beds above the upper diamictite and cap dolostone units, up to and including a purported “reworked fallout tuff,” which we instead conclude provides only a maximum depositional age of ≤673 Ma from epiclastic volcanic detritus. Rare detrital zircons as young as 658 Ma provide a maximum depositional age for the upper diamictite and overlying cap dolostone units. This new geochronological framework supports litho- and chemostratigraphic correlations of the lower and upper diamictite intervals of the Scout Mountain Member of the Pocatello Formation with the Sturtian (716–660 Ma) and Marinoan (≤650–635 Ma) low-latitude glaciations, respectively. The Pocatello Formation thus contains a more complete record of Cryogenian glaciations than previously postulated.

Supplemental Material: Neoproterozoic to early Paleozoic tectono-stratigraphic framework for central Idaho: Windermere Supergroup in the northern sector of the U.S. Cordillera

“…667 Ma tuff of Daugherty Gulch (Lund et al, 2010;Isakson, 2017) in central Idaho may represent a maximum depositional age rather than a true depositional age (as we show in Fig. 8), similar to what Isakson et al (2022) advocated for the similar-age tuff above the upper diamictite in the Scout Mountain Member of the Pocatello Formation.…”

“…Farther down in the Cryogenian-early Ediacaran portion of the section, correlations are also debated. Recent geochronology results (Ellison, 2021;Isakson et al, 2022) suggest that deposition of the Pocatello Formation spanned both the Sturtian and Marinoan glacial intervals (as we show in Fig. 8).…”

“…720 Ma to <650 Ma; they record Sturtian (Fanning and Link, 2004) and probably Marinoan (Isakson et al, 2022) phases of glaciation. The Pocatello Formation contains rare interbedded Cryogenian tuffs that approximate their depositional ages (Fanning and Link, 2004;Keeley et al, 2013;Isakson et al, 2022). Lateral variations in lithology and thickness and the presence of local unconformities suggest localized, syndepositional faulting during this interval.…”

“…These rocks are overlain by diamictite-bearing, volcanic, and generally finer-grained strata (Pocatello Formation and associated units) that were deposited from ca. 720 Ma to <650 Ma; they record Sturtian (Fanning and Link, 2004) and probably Marinoan (Isakson et al, 2022) phases of glaciation. The Pocatello Formation contains rare interbedded Cryogenian tuffs that approximate their depositional ages (Fanning and Link, 2004;Keeley et al, 2013;Isakson et al, 2022).…”

“…650-635 Ma Marinoan intervals, is supported by stratigraphic and geochronologic studies in Australia (Ken dall et al, 2006;Cox et al, 2018), North America (Macdonald et al, 2010(Macdonald et al, , 2017Rooney et al, 2015;Nelson et al, 2020), China (Song et al, 2017;Lan et al, 2014), and Namibia (Hoffman et al, 2021), the occurrence, stratigraphic location, and timing of Cryogenian glacial events are still debated within the northern sector of the U.S. Cordillera (cf. Lund et al, 2003;Fanning and Link, 2004;Isakson et al, 2022).…”