Ichnofabrics of a Pleistocene slope succession, New Jersey margin: relations to climate and sea-level dynamics
Abstract Trace fossil and sedimentologic data were collected at 10 cm intervals throughout a virtually continuous, ∼520 m-thick Upper Pleistocene siliciclastic succession recovered at ODP Site 1073 (New Jersey margin). These data were examined in the context of isotopic, seismic, and palynologic proxies to document depositional and ichnologic responses to Quaternary climate and sea-level dynamics in an upper slope setting. Two broad, texturally defined sedimentary facies assemblages are recognized. The clay-rich assemblage reflects relatively rapid deposition, in part from turbidity currents and suspended plumes, and appears to be linked to cooler, sea-level fall and lowstand phases. The sand-rich assemblage reflects overall slower sedimentation by offshelf spillover and periodic winnowing and erosion, under the influence of contour and other bottom currents, during warmer transgressive/highstand phases. Facies of the sand-rich assemblage (muds, sandy muds, muddy sands, and sands) are completely bioturbated and are characterized by high densities of biogenic structures, including distinct burrow forms. In contrast, the clay-rich assemblage includes facies that exhibit limited biogenic disruption (laminated clay/silty clay and graded silt- or fine sand-to-clay couplets) and, on the whole, is characterized by lower densities of biogenic structures, most of which are diffuse burrow mottles. These ichnofabric trends reflect changes in ecological and taphonomic parameters (e.g. substrate stability and consistency, residence time of sediments in the benthic boundary layer, and degree of inter- and intrabed textural variability), many of which were governed by variations in sedimentation rate as mediated by glacio-eustacy. Recurring, identifiable ichnofossils are most common in facies of the sand-rich assemblage in the upper half of the Pleistocene succession. Finer-grained components of this assemblage (e.g. muds and sandy muds), representing quieter phases of highstand deposition, are characterized by a low-diversity distal Cruziana ichnofacies assemblage dominated by Schaubcylindrichnus and Phycosiphon (=Anconichnus), both of which represent the work of deposit-feeding worms. Narrow facies transitions that correspond to phases of rapid transgression are marked by occurrences of Thalassinoides. These crustacean burrow systems were excavated in coarser-grained, higher-energy facies (sands and muddy sands) and in closely associated firmground substrates produced by exhumation of consolidated muds by bottom currents and/or mass-wasting processes. The association of firmground ichnofabrics with marine flooding events in an upper slope setting demonstrates that the sequence stratigraphic utility of the Glossifungites ichnofacies extends beyond shelf depositional sequences to deeper-water deposits. The distribution of texturally defined facies and general ichnofabric parameters (e.g. degree of bioturbation) appears to respond to glacio-eustatic cycles through the entire Pleistocene record. However, the Schaubcylindrichnus–Phycosiphon assemblage and Thalassinoides are rare or absent altogether in the lower half of the succession, indicating that biological responses to, and the ichnologic record of, climate and sea-level dynamics may be strongly dependent on margin physiographic and bathymetric evolution.