INVESTIGATING LITTORAL SEDIMENT TRANSPORT AND MIXING IN THE NORTHERN GULF OF MEXICO USING DETRITAL ZIRCON U-PB GEOCHRONOLOGY

Coastal sediment in the northeastern Gulf of Mexico is primarily delivered by the Mississippi, Mobile, and Apalachicola river systems. While the drainage area and source rock within these river systems are known, an understanding of subsequent littoral sediment transport and mixing remain largely qualitative at a regional scale. We present detrital zircon U-Pb geochronology from the Mississippi, Mobile, and Apalachicola rivers (N=3, n=714), as well as beach and barrier island deposits (N=4, n=813) along the northeastern Gulf Coast to determine sediment source(s), littoral transport, and mixing dynamics. Results show the Mississippi River has a distinct age spectra, defined by dominant Cenozoic populations, relative to the Mobile and Apalachicola rivers, which exhibit U-Pb age spectra with dominant Paleozoic and Proterozoic populations. Cross Correlation Coefficient, Likeness, Kolmogorov-Smirnov, and Kuiper tests using DZstats show that subsequent littoral beach and barrier island deposits are similar to the Apalachicola and Mobile rivers, and dissimilar to the Mississippi River. Monte Carlo unmixing models using DZmix show that coastal deposits east of Mobile Bay are composed of 73.4-61.4% Apalachicola drainage, 36.0-25.0% Mobile drainage, and 2.0-1.5% Mississippi drainage; while west of Mobile Bay coastal deposits are composed of 42.7-31.1% Apalachicola drainage, 67.3-53.8% Mobile drainage, and 3.3-1.5% Mississippi drainage. All coastal deposits contain 17.8-20.9% grain abundance of zircons with low (< 0.1) Th/U ratios, suggesting a metamorphic sediment source terrane. Our results provide a detrital zircon chemical-age evaluation that suggests that the Apalachicola drainage system represents the major sediment source for beach and barrier island deposits along the northeastern Gulf Coast. This study illustrates how detrital zircon U-Pb geochronology can be used in coastal studies and establishes a modern analog for detrital zircon studies for the Mesozoic-Cenozoic Gulf Coastal Plain in the southeastern United States.