16 Sedimentology, Geochemistry, Chemostratigraphy, and Geomechanics of Mudrock Systems

Monday, 9 March 2020
1:30 PM - 5:30 PM
Meeting Room 202CD

VOLCANOGENIC ZIRCONS FROM MUDROCKS: A NEW TOOL FOR CHRONOSTRATIGRAPHY

Paper number: 16-2
Program: Theme Sessions
Time: 1:35 PM – 1:55 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 1

doi: 10.1130/abs/2020SC-343823

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