Chapter 38 Neoproterozoic strata of southeastern Idaho and Utah: record of Cryogenian rifting and glaciation

Abstract

AbstractNeoproterozoic strata in southeastern Idaho and Utah include the <766 Ma Uinta Mountain Group and Big Cottonwood Formation (Fm.) deposited in an east-trending rift basin and, to the west, the lower part of a westward-thickening rift to passive-margin succession that initiatedc.720 Ma. The latter contains a lower diamictite and volcanic succession, with a complex stratigraphic interval of Cryogenian marine glacial deposits (Pocatello and Mineral Fork formations and correlatives). This is overlain by a mostly terrigenous succession of <667 Ma strata assigned to the upper member of the Pocatello Fm. and Brigham Group in southeastern Idaho, to the Kelley Canyon Fm. and Brigham Group in northern and western Utah, and to the McCoy Creek Group and Prospect Mountain Quartzite in adjacent Nevada. Although the Brigham Group and correlative deposits contain no direct evidence for glaciation, widely developed, though stratigraphically restricted, incised valleys, with erosional relief from a few metres to as much as 160 m, are inferred to represent subsequent times of Cryogenian glacially lowered sea level. Overall interpretations of the stratigraphy and sedimentology of these rocks have changed little in the past 10–15 years. The most important recent advances relate to U–Pb geochronology. In strata that lie unconformably below demonstrable glacial deposits, the lower Uinta Mountain Group (formerly thought to bec.900 Ma) contains populations of detrital zircons as young as 766±5 Ma. Cryogenian magmatism north of the Snake River Plain in central Idaho is recognized near House Mountain, east of Boise atc.725±5 Ma, in the Pioneer Mountains Core Complex at about 695 Ma, and in central and east-central Idaho at 685–650 Ma. Clasts interpreted to be from the rift-related Bannock Volcanic Member of the Pocatello Fm. are dated at 717±4 Ma and 701±4 Ma. The overlying diamictite-bearing Scout Mountain Member contains a mafic lapilli tuff near the base (686±4 Ma) and a reworked fallout tuff near the top (667±5 Ma). Strongly negative C-isotope data have been obtained from some of the carbonate rocks, although the latter constitute only a small fraction of the succession. Palaeomagnetic data are available only for the Uinta Mountain Group, and suggest an equatorial palaeolatitude.