Abstract

A crucial constraint on the evolution of the ca. 1100 Ma Midcontinent Rift (MCR) in North America comes from the Jacobsville Sandstone, Bayfield Group, and other equivalent sedimentary rocks (JBE) that overlie the volcanics and sediments deposited in the MCR basin near Lake Superior. The MCR began extending ca. 1120 Ma and failed—ceased extending—at ca. 1096 Ma, although volcanism continued for ∼10 m.y. The JBE’s age is poorly constrained, with proposed ages ranging from ca. 1100 to ca. 542 Ma (i.e., youngest Precambrian). It has been proposed that the JBE were deposited shortly prior to or during the time when the MCR failed due to regional compression occurring ca. 1060 Ma as part of the Grenville orogeny (1300–980 Ma). However, the JBE are not conformable with the youngest rift-filling strata and differ compositionally from them. We present an analysis of 2050 new detrital-zircon ages showing that the JBE are younger than 959 ± 19 Ma. Thus, the JBE and the compression recorded in them that inverted the basin postdate the Grenville orogeny and are unrelated to the rift’s failure. The JBE may be significantly, perhaps ∼200–300 m.y., younger than the maximum age from zircons and may have been deposited shortly after a Snowball Earth event.

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