Paper No. 16-1
Presentation Time: 1:35 PM
VOLCANOGENIC ZIRCONS FROM MUDROCKS: A NEW TOOL FOR CHRONOSTRATIGRAPHY
LEE, Jacquelin Diane, Geology Department, The University of Kansas, 1414 Naismith Dr Room 254, Lawrence, KS 66047, MÖLLER, Andreas, Department of Geology, University of Kansas, 1475 Jayhawk Blvd, Lawrence, KS 66045, LUDVIGSON, Greg A., Kansas Geological Survey, University of Kansas, 1930 Constant Avenue, Lawrence, KS 66047, SUAREZ, Marina B., Department of Geology, University of Kansas, 1414 Naismith Drive, Lawrence, KS 66045, MCLEAN, Noah M., Department of Geology, The University of Kansas, Lawrence, KS 66045 and JOECKEL, R.M., Conservation and Survey Division, SNR, Department of Earth and Atmospheric Sciences, and State Museum, University of Nebraska-Lincoln, Hardin Hall, 3310 Holdrege St, Lincoln, NE 68583-0996
Mudrocks have traditionally not been targets for the extraction of heavy minerals (e.g. zircon) for U-Pb dating because their hydraulic equivalence size compared to quartz would be too small to be useful. However, based on the hypothesis that exposed land surfaces may capture volcanic ash and its zircons during times of explosive volcanism, hydraulic equivalence is not relevant. This study uses mature paleosols from the Ruby Ranch (RRM) and Yellow Cat Member (YCM) of the Lower Cretaceous Cedar Mountain Formation in Utah and compares U-Pb results to interspersed fluvial sandstones. We propose that fluvial sandstone deposits are less likely to capture deposition-age appropriate zircon grains because the fluvial channel is constantly flushed by detrital material, likely dominated by a reworked zircon load.
Samples were collected from two sections, at the type locality Ruby Ranch Road, and at Dinosaur National Monument. These sites that have been previously studied for lithostratigraphy and chemostratigraphy (identifying stable isotope excursions C9 and C10) (e.g. Ludvigson et al. 2010). Those results will be supplemented by our detailed U-Pb geochronology to provide constraints on the position of the Aptian/Albian boundary in the sections and the age of the isotope excursions.
Initial results were obtained from two paleosols, from the RRM and the underlying YCM, with more than 300 zircons analyzed for each sample. The upper paleosol (RRM) yields a main mode centered at ca. 124 Ma (>250 dates) and a group of 3 youngest grains at about 105 Ma, whereas the paleosol from the YCM has a youngest group of 6 analyses at ca. 130 Ma. The marked difference in the number of Lower Cretaceous grains is interpreted to reflect either the volcanic activity at these times, the "magmatic lull" between ca. 125-140 Ma (Laskowski et al. 2013), and/or exposure of these land surfaces at times of high and low volcanic productivity, respectively.
Laskowski, A.K., et al., 2013. Tectonics 32, 1027-1048.
Ludvigson, G.A., et al., 2010, J. Sed. Res. 80, 955-974. doi: 10.2110/jsr2010.086.
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