Oxygen-deficient waters at the bathyal depths of the Santa Cruz Basin in the California Continental Borderland create harsh conditions for marine life. A feeding strategy approach is used for descriptions of the life habits (i.e., the organism's motility and its living position with respect to the substrate) of benthic fauna and is intended to provide insight into how these organisms disrupt slope and basin floor sediment. Washes of sediment recovered by box corers show that the basin supports a surprisingly high density and diversity of benthic macrofauna. In contrast, however, biogenic sedimentary structures preserved in the sediment are low in density and diversity. Furthermore, bottom photographs taken at 117 stations reveal a biogenically produced, microhummocky topography with few resolvable biogenic traces.
Recognizable biogenic traces are of three main classes: tracks and trails, depressions, and fecal matter. Echinoderms, the most abundant epifauna on the slope and adjacent basin floor, produce most of the large tracks and trails. Significantly, because of the soft, soupy nature of the surficial bathyal sediment, many tracks and trails there are less distinct than similar markings found at abyssal depths. Depressions made by asteroids, regular echinoids, and bottom dwelling fish are most common at moderate depths (<800 m). A characteristic circular depression made by a feeding, tubulous polychaete is restricted to the lowermost slope and the adjacent basin floor. Holothurian fecal strands dominate the feces types that can be seen in bottom photographs. These holothurian feces take the clothesline form common to the abyss.
Open burrows are common in bottom photographs but are scarce in box core slabs. Photography and X-ray radiography of box-core sediment slabs reveal relatively homogeneous, burrow-mottled sediment. The paucity of distinct biogenic structures results from a lack of sediment density contrast for radiography and from the thixotropic response of the sediment to biogenic disturbance. Traces at the sediment-water boundary and within the sediment have poor preservation potential. Sediment from silled bathyal environments similar to the Santa Cruz Basin will probably come to appear in the rock record as homogeneously mottled and bioturbated mudstones with few preserved biogenic structures.
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Organisms of one sort or another today inhabit virtually every sediment environment on Earth, and the rock record tells us that this has been the case through the greater part of our planet’s history. Furthermore, organisms leave their mark in most sedimentary settings, either directly in the form of body fossils or indirectly as biogenic structures. In addition to their often profound modifying effects on substrates, ancient biogenic structures preserve a record of organism behavioral activity in response to substrate and other paleoenvironmental controls. Thus, biogenic structures can be highly useful as facies indicators and can provide valuable clues to the interpretation of paleodepositional environments. The purpose of this volume is to present a broad spectrum of case-book examples of the use of biogenic structures in the interpretation of depositional environments.