Abstract

The Cryogenian Perry Canyon Formation, formally named herein with a type section located in northern Utah, consists of a 0.3- to 1.5-km-thick succession of diamictite- and volcanic-bearing strata that record glacial events and early rifting along western Laurentia. The formation is divided into seven informal members, from bottom to top: (1) arkosic grit deposited as proximal turbidites during onset of rifting; (2) quartzite and grit deposited as medial turbidites; (3) pebbly slate with dropstones deposited during an older glacial episode; (4) slate with interbedded quartzite deposited mostly as distal turbidites; (5) mafic volcanic and intrusive rocks; (6) diamictite with basement and volcanic clasts deposited during younger glaciation; and (7) graywacke with rare carbonate layers. The formation has a basal nonconformity on Archean to Paleoproterozoic basement rocks of the Facer Formation and is overlain by arkosic quartzite of the Maple Canyon Formation and argillite of the Kelley Canyon Formation. Changes in member thicknesses reflect syndepositional faulting. Vertical and lateral changes in provenance and maximum depositional ages were evaluated from detrital zircon (DZ) U-Pb geochronology of five well-exposed stratigraphic sections across the sedimentary basin. DZ patterns for the arkosic grit record local sources during onset of rifting, patterns for the pebbly slate and slate members record distal Laurentian sources and possible sediment recycling, and patterns for diamictite and graywacke record basement sources along the rift margin and reworking of mafic to felsic volcanic rocks. Young zircon grains derived from volcanic rocks give maximum depositional ages of 703 ± 6 Ma and 667 ± 5 Ma for the lower part of the diamictite and graywacke members, respectively, suggesting a late Sturtian age for the younger glaciation. Associated cap carbonates, however, have Marinoan-style features, indicating that global correlations based only on lithology may not be valid. DZ data for the Kelley Canyon Formation record a change to distal Laurentian sources during final stages of early rifting. Mafic volcanic and intrusive rocks are subalkaline to alkaline and have trace-element ratios indicative of a rift setting. Volcanic clasts in diamictite include basalt, trachyte, and rhyolite. Volcanic zircon grains have εHf = –1 to –21, which record crustal contamination during igneous activity. Rare mafic to ultramafic dikes in the upper part of the section were intruded during waning stages of igneous activity. Early rifting spanned ca. 720 Ma to 660 Ma but was incomplete and followed by deposition of mature quartzite of the overlying lower Brigham Group across a broad basin, indicating that Rodinia rift models showing pre–720 Ma separation of western Laurentia may need revision. Final rifting and transition to drift occurred ca. 550 Ma during deposition of the upper Brigham Group.

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