Ichnofabrics of a Pleistocene slope succession, New Jersey margin: relations to climate and sea-level dynamics

Trace fossil and sedimentologic data were collected at 10 cm intervals thro ughout a virtually continuous, similar to 520 m-thick Upper Pleistocene sil iciclastic succession recovered at ODP Site 1073 (New Jersey margin). These data were examined in the context of isotopic, seismic, and palynologic pr oxies to document depositional and ichnologic responses to Quaternary clima te and sea-level dynamics in an upper slope setting. Two broad, texturally defined sedimentary facies assemblages are recognized, The clay-rich assemb lage reflects relatively rapid deposition, in part from turbidity currents and suspended plumes, and appears to be linked to cooler, sea-level fall an d lowstand phases. The sand-rich assemblage reflects overall slower sedimen tation 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, mu ddy sands, and sands) are completely bioturbated and are characterized by h igh densities of biogenic structures, including distinct burrow forms, In c ontrast, the clay-rich assemblage includes facies that exhibit limited biog enic disruption (laminated clay/silty clay and graded silt- or fine sand-to -clay couplets) and, on the whole, is characterized by lower densities of b iogenic structures, most of which are diffuse burrow mottles. These ichnofa bric trends reflect changes in ecological and taphonomic parameters (e,g, s ubstrate stability and consistency, residence time of sediments in the bent hic boundary layer, and degree of inter- and intrabed textural variability) , many of which were governed by variations in sedimentation rate as mediat ed 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 sand y muds), representing quieter phases of highstand deposition, are character ized by a low-diversity distal Cruziana ichnofacies assemblage dominated by Schaubcylindrichnus and Phycosiphon ( = Anconichnus), both of which repres ent the work of deposit-feeding worms. Narrow facies transitions that corre spond to phases of rapid transgression are marked by occurrences of Thalass inoides. These crustacean burrow systems were excavated in coarser-grained, higher-energy facies (sands and muddy sands) and in closely associated fir mground substrates produced by exhumation of consolidated muds by bottom cu rrents and/or mass-wasting processes. The association of firmground ichno-f abrics with marine flooding events in an upper slope setting demonstrates t hat the sequence stratigraphic utility of the Glossifungites ichnofacies ex tends beyond shelf depositional sequences to deeper-water deposits. The dis tribution of texturally defined facies and general ichnofabric parameters ( e.g. degree of bioturbation) appears to respond to glacio-eustatic cycles t hrough the entire Pleistocene record. However, the Schaubcylindrichnus-Phyc osiphon 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 de pendent on margin physiographic and bathymetric evolution. (C) 2001 Elsevie r Science B.V. All rights reserved.