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Firmground Ichnofabrics in Deep-water Sequence Stratigraphy, Tertiary Clinoform-toe Deposits, New Jersey Slope

¹ Department of Geology and Geography, 210 Petrie Hall, Auburn University, AL 36849-5305
² Department of Geological Sciences, Rutgers University, Piscataway, NJ 08855-1179
³ Institut für Geologie und Paläontologie, Universität Stuttgart, Herdweg 51, D-70174 Stuttgart, Germany
⁴ Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK

Sixteen erosional surfaces are recognized in a 144-m-thick condensed package of Tertiary (Eocene-Pliocene) clinoform-toe sediments recovered at ODP Site 1073 on the New Jersey slope. Most of these surfaces are associated with significant hiatuses or extremely condensed intervals defined by Sr isotopes or biostratigraphic data, and many can be linked to sequence boundaries defined in onshore and shelf seismic studies. All surfaces define the bases of fining upward sequences; they separate clay or biogenic muds below from authigenic glauconitic sandy muds or sands above. The entire Tertiary package is thoroughly bioturbated and dominated by ichnotaxa representing softground conditions. Burrow densities, burrow preservation, and the relative importance of certain ichnotaxa vary through the Tertiary package, reflecting changes in water depth, relative degree of condensation, and associated glaucony authigenesis, all related to margin progradation. Nonetheless, when individual sequences are considered, little or no change in softground ichnofossil assemblages is recognized across bounding surfaces. However, most surfaces are marked clearly by firmground Thalassinoides, burrow systems that penetrate deeply (up to 2 m) into subjacent clays and are characterized by extremely sharp walls and coarser glauconitic fills. In shallower shelf sequences, firmground ichnofabrics develop at sequence boundaries in response to subaerial exposure and transgressive ravinement. In contrast, the Tertiary firmgrounds on the New Jersey margin formed in deep water in response to phases of rapid transgression and net erosion; consolidated mud substrates were exhumed as a result of sediment starvation and bottom-current winnowing, facilitated by bioerosion, at or near the bases of slope clinoforms. These observations extend the previously established sequence stratigraphic utility of the substrate-controlled Glossifungites ichnofacies to deeper water facies.