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Neoichnological Characterization of a Storm-Dominated Muddy Shelf

Abstract

Sediment box cores acquired from the Gulf of Mexico enable ichnological characterization of a modern storm-dominated muddy shelf, and can be used as an analog for recognizing shelf deposits in the rock record. Below 15 m water depth, trace diversities and burrow densities are low. Between 15 and 35 m, box cores exhibit high diversities and densities of traces. Below 70 m, sediments are devoid of burrows, and infaunal populations from grab samples are equally scant. No data are available between 35 and 70 m. The ichnological trends derived from the Gulf of Mexico box cores indicate that the shallow shelf residing below fair-weather wave base (15 to 40 (?) m) is the most favorable regime for infaunal colonization. Consequently, trace diversities and burrow densities are highest in this zone, and decrease in both the landward and seaward directions. The implications of this neoichnological study are that deeper water deposits, particularly in muddy shelves like the modern central Texas shelf, are unlikely to be pervasively bioturbated. This trend is certainly true for many Cretaceous-aged black shales of the Western Interior Seaway of North America. Using the Gulf of Mexico as an analog, the absence of bioturbation in muddy marine sediments likely reflects deposition on the outer shelf (below approximately 70 m water depth), and does not necessarily reflect dysaerobic conditions or anoxia at the sea floor.