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Small Rivers and Big Fans: New Geochonologic Constraints From the Miocene-Pliocene Deep-Water Mexican Continental Margin


Empirical relationships of Quaternary source-to-sink sediment-dispersal systems have been used to predict the morphology and architecture of shelf, slope, and basin-floor depositional systems. For example, these relationships predict that during the Paleogene, large North American rivers delivered large volumes of sediment to Wilcox submarine fans in the Gulf of Mexico. However, relatively short rivers and small catchments can also deliver large volumes of sediment to the ocean in regions of wet, humid climate and rapid tectonic uplift, such as high-standing islands of the west Pacific Ocean. Here we present new detrital zircon U-Pb ages (1078 grains from nine samples) from Miocene-Pleistocene strata in deep-sea drilling cores in the Western Gulf of Mexico (Ocean Drilling Program sites 3, 87, 90, and 91) to interpret the evolution of sediment dispersal in the Mexican continental margin. The Miocene-Pleistocene Mexican continental margin is characterized by variable climate (the Miocene is interpreted to be relatively hot and humid) and short rivers tapping the nearby high-relief, tectonically active North American Cordillera. Miocene strata in the Western Gulf of Mexico include grain age populations from Tran-Mexican Arc (<20 Ma), Yucatan (500-400 Ma), Chiapas (1200-1000, 650-500 Ma), and Grenville (1300-1000) source areas of central and southern Mexico. This sediment includes up to coarse-grained sand and pebbles that were deposited ~500 km from the Mexican coastline. There is a dramatic shift in the grain age populations of Pleistocene strata, which show affinity to Mississippi River provenance. The biostratigraphic age of Pleistocene samples (191-130 ka) is coeval with an avulsion of the ancestral Mississippi Delta to form the Bryant Canyon-Fan in the central Gulf of Mexico. Detrital-zircon geochronology reveals variability in Western Gulf of Mexico submarine-fan provenance, from small drainages of south-central Mexico to the Mississippi River, thus informing the character and quantity of sediment supply to the deep sea. Furthermore, small rivers draining high-relief areas in hot, humid climates can deliver coarse-grained sediment to large, long run-out submarine fans. These settings promote liberation and rapid transport of large volumes of sediment per unit area of river catchment compared to cooler, drier, and larger catchments. These caveats play an important role in scaling relationships of Quaternary source-to-sink sediment-dispersal systems.