BARABOO INTERVAL QUARTZITES IN IOWA, USA: DETRITAL ZIRCON PROVENANCE OF THE WASHINGTON COUNTY QUARTZITE

The Washington County Quartzite (WCQ) in southeastern Iowa is the southernmost occurrence of the Baraboo interval quartzites in the Laurentian midcontinent. The WCQ is structurally, texturally, and compositionally similar to Baraboo Interval quartzites; interpreted to have been deposited in fluvial, deltaic, or shallow marine environments near the southern Laurentian continental margin on the Columbia supercontinent before ~1650 Ma. In the early 1960’s, the presence of a stratigraphic high near the town of Keota, Iowa was indicated by subsurface geophysical anomalies and stratigraphic test drilling confirmed the occurrence of a structural dome. Two wells drilled in Washington County, Iowa in 1972 penetrate Proterozoic metasedimentary rocks at depths greater than 2318 ft (707 m). These wells are located less than 1.5 mi (2.4 km) apart and reveal at least 500 ft (150 m) of relief along the Precambrian surface. About 15 ft (5 m) of poorly sorted dark pink quartzite and gray-green to maroon phyllite were recovered from the center of the paleotopographic high in the W.F. Flynn M-1 well, and the drill core are available at the Iowa Geological Survey Oakdale Rock Library. Quartzite was sampled from a depth of 2333 ft (711 m). Zircons were separated using traditional methods and analyzed by LA-ICPMS at the University of Arizona Laserchron Center. One hundred new zircon ages from the WCQ reveal a prominent ~1.78 Ga age peak; these zircons were derived from the local Yavapai basement rocks. A secondary peak at ~1.85 Ga indicates that the Penokean terrane, which occurs ~200 km to the north, also supplied sediment to the WCQ. Surprisingly, few grains from the Archean Superior province are present. The WCQ age spectrum is statistically indistinguishable from those of the of the lowermost members of the Baraboo Formation in southern Wisconsin, which also are dominated by proximally sourced sediment from local Yavapai and Penokean basement rocks. The maximum depositional age is 1733 Ma. Our detrital zircon age data supports the hypothesis that the WCQ is indeed correlative to other Baraboo interval quartzites in the region.