Ichnofabrics in Eocene to Maestrichtian Sediments from Deep Sea Drilling Project Site 605, off the New Jersey Coast

Abstract

Ichnofabrics in the sediments drilled at Site 605 reflect environmental changes. Long-term changes allow the section to be differentiated into four units, and short-term fluctuations define cycles in the range of several decimeters. The composition of the ichnofabrics is controlled by the availability of nutrients within the sediment (reflecting productivity in the surface waters and the sedimentation rate) and by oxygen content within the respiration water (related to deepwater circulation patterns as well as to the organic carbon content of the sediment). INTRODUCTION Many studies of deep-sea environments relate biogenic structures to ecologic conditions (e.g., Ekdale, 1977; Ekdale et al., 1984; Wetzel, 1981, 1983a, b). In deep-sea sediments, trace fossils can provide valuable information on the paleoecology and geologic history of the benthic macro fauna. This is important in that most macrorganisms living in the deep-sea environment are not preserved in the body fossil record (Ekdale, 1977). This study is based on observations of biogenic sedimentary structures in sediments drilled at Deep Sea Drilling Project Site 605, which is situated on the upper continental rise off New Jersey at about 2,200 m water depth (Fig. 1). The bottom of the hole was 816.7 m below the seafloor; the upper 154 m were washed, but rotary drilling recovered the lower 662 m of sediment. The sediments drilled at Site 605 are heavily bioturbated. The deposits were normally several times reworked by organisms; thus, the continuing response of the burrowing macro fauna to environmental changes is well preserved, allowing a reconstruction of past environments. Five lithologic units were distinguished at Site 605: Unit I, 198 m of Pleistocene gray, silt-rich clay; Unit II, 153 m of lower to middle Eocene biosiliceous nannofossil chalk rich in radiolarians and diatoms; Unit HI, 214 m of lower to middle Eocene greenish gray nannofossil limestone with varying amounts of foraminifers and calcified radiolarians; Unit IV, 176 m of Paleocene dark greenish gray clayey nannofossil marls and limestones; and Unit V, 77 m of lower Paleocene to Maestrichtian olive gray, clayey limestone. An unconformity occurs between the upper Eocene and Pliocene. Throughout the drilled section, carbonate content is usually greater than 60%. The organic carbon content varies between 0.1 and 0.6% (with a mean of 0.22%). The (compacted) sedimentation rates vary from 2 to 4 cm/10 yr. for Units III, IV, and V, whereas Unit II has van Hinte, J. E., Wise, S. W., Jr., et al., Init. Repts. DSDP, 93: Washington (U.S. Govt. Printing Office). 2 Address: Geologisch-Palaontologisches Institut der Universitat, Sigwartstrase 10, D 7400 Tubingen, Federal Republic of Germany. a markedly higher rate with an average of more than 10 cm/10 yr. The Paleogene section shows evidence of cyclic sedimentation, especially in Units III and IV. An unusually high biosiliceous productivity began during the Eocene Epoch (Units II and III), reaching a constant high level in Unit II. METHODS Biogenic structures were identified by means of typical cross-sections. Because such identifications may be subjective, the trace fossils were defined only at the ichnogenus level (Hantzschel, 1965, 1975). Sometimes a subdivision of one ichnogenus was possible when distinct maxima in diameter distribution and differences in sediment infill were found (Wetzel, 1981). In general, two types of biogenic sedimentary structures can be distinguished: (1) trace fossils with a well-defined shape and sharp and distinct outlines and (2) biodeformational structures, which have indistinct outlines and features and which destroy preexisting structures. This study is based on visual observations of wet cores. This is important to note, because other methods of observation, for example, X-ray radiography (Wetzel, 1981), wetting with oil (Bromley, 1981), or staining (Risk and Szczuczko, 1977), allow the recognition of more types of trace fossils but fewer biodeformational structures. As an additional complication, diagenesis may enhance certain burrow types, whereas others are more or less masked. In this description only trace fossils are discussed. In Hole 605 sediments, trace fossils at and near the sediment surface had no chance of being preserved in the fossil record; hence, only more deeply burrowed traces are considered. RESULTS Description of Trace Fossils Chondrites Chondrites are three-dimensional burrow systems which normally branch downward into the sediment at angles of 30-60°. They are simple tunnels or wall-lined tubes (Wetzel, 1981) that consist of (1) a connection from the seafloor to (2) a typically branched lower part. In general, tunnels or tubes become more horizontal with depth below the surface of the seafloor (Fig. 2). Different types were identified in the sediments drilled at Site 605 by their differing diameter and sediment fill, as suggested by Wetzel (1979, 1981). Chondrites in Hole 605 also had reworked other burrows (= composite burrows; Fig. 3).