Cores recovered in the Caribbean Sea on JOIDES Leg 15 exhibit a superb assemblage of biogenic sedimentary structures. They are identified as distinctive trace fossils (ichnogenera) similar to those well-known from land-based stratigraphic sections. They represent the burrowing behavior of benthic animals living contemporaneously with sedimentation.

Some of the best examples are at sites 146 and 149 in the Venezuelan basin. These adjoining sites were drilled in abyssal depths of 3,949 and 3,472 m, respectively, and provide a complete section from Coniacian to Pleistocene. Faunal and sedimentologic evidence from the cores suggests that deposition was abyssal.

The most distinctive biogenic structures are Zoophycus (a spiral web), Teichichnus (a laminated trough), and Chondrites (a regularly branched system). These represent the deep-water trace fossil assemblage of Seilacher, which has been documented in rocks of various ages deposited in bathyal and abyssal frameworks from widespread geographic areas.

Because similar trace fossils are present throughout the Phanerozoic, it is disappointing that they are not recognized in modern deep-sea sediments, and the animals responsible for them have not been identified. One reason is that the traces are accentuated only with time. Our material shows a progressive enhancement of biogenic structures, from vague outlines in the softer Tertiary sediments to increasingly marked contrast and detail downward in the cores. Furthermore, little knowledge exists about modern abyssal burrowers and their burrows, and few cores have been taken for the identification of potential fossil traces.

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