IN THE LAND OF UNCONFORMITIES: TIME AND SPACE RELATIONS OF COMPLEX 763-740 MA VOLCANOGENIC-GLACIOGENIC ROCKS IN THE GREATER MOUNT ROGERS TRISTATE AREA OF VA, NC, AND TN

A complex of late Neoproterozoic bimodal volcanic, volcaniclastic, fluvial, and glaciogenic rocks deposited in distinct basins in the western Blue Ridge (VA-NC-TN), non-conformably overlie 1.32-1.05 Ga basement rocks and are non-conformably overlain by fluvial conglomerates that contain 570 Ma basalt flows. Field relations combined with zircon U-Pb ages from rhyolites (n=35) and samples of volcanogenic rocks for detrital zircons (n=28) demonstrate that multiple unique basin deposits span ca. 30 m.y. and locally constrain the transition to glaciogenic rocks at ca. 750 Ma, ca. 33 m.y. older than deposits associated with Sturtian Snowball Earth.

Volcanic rocks, herein called rhyolites, include porphyritic and banded flows, welded tuffs, and breccia that were deposited from 763-740 Ma on an irregular paleotopography of basement rocks and a lower complex (LC) of conglomerate, basalt, arkose, siltstone, rhyolite, and volcanoclastic rocks. Rhyolite was erupted onto basement rocks and LC at 761, 759, 755, and 754 Ma. Rhyolite intruded 763 Ma rhyolite at 749 Ma. An isolated area of 745-740 Ma rhyolite occurs within a broader area of rhyolite that are dated at 755–750 Ma. Conglomerates contain 780-760 Ma rhyolite clasts and detrital zircons, and rare rhyolite dikes and masses in basement support distal flows from volcanic centers that are no longer preserved. Numerous ca.757 Ma gabbro dikes in basement could have fed the LC basalt flows.

Volcanogenic rocks locally transition upward to glaciogenic deposits. Glaciogenic rocks include map- and outcrop-scale lacustrine deposits in restricted basins. Maroon rhythmite contain drop stones (ice rafts) of basement, rhyolite, and other rocks, and also interbedded sandstones (turbidites) that very laterally from east to west, and overlain by diamictite (till) containing clasts of basement and rhyolite within a silty matrix. Distal rhyolite flows at 750 Ma within rhythmite, a block of 750 Ma rhyolite within rhythmite, and 748 Ma rhyolite overlain by rhythmite, constrain the transition from volcanism to a pre-Sturtian glaciation. Volcanism lasted 40 m.y. and glaciation at least 15 m.y. but cycles of erosion and deposition of the rocks punctuated an intricate geologic framework.