Abstract:
Sixteen sharp erosional surfaces are recognized in an 144-m-thick condensed package of Tertiary (Eocene-Pliocene) clinoform-toe sediments recovered at Ocean Drilling Program (ODP) Site 1073 on the New Jersey slope. Most of these surfaces correspond to significant hiatuses and/or extremely condensed intervals defined by Sr isotopes and/or biostratigraphic data, and many can be linked, at least tentatively, 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 muddy sands above. The entire Tertiary package is thoroughly bioturbated and dominated by ichnotaxa representing softground conditions (Chondrites, Teichichnus, Zoophycos, Phycosiphon, Planolites, Palaeophycus?, Taenidium, and Thalassinoides type 1). Burrow densities, burrow preservation, and the relative importance of select ichnotaxa vary upward through the Tertiary package, reflecting changes in the relative degree of condensation and associated glauconite authigenesis with 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 clearly marked by Thalassinoides type 2, burrow systems that penetrate deeply (up to 2 m) into subjacent clays and are characterized by extremely sharp walls and coarser glauconitic fills. Thalassinoides type 2 records firmground conditions and, hence, represents the Glossifungites ichnofacies. Unlike firmground ichnofabrics that develop at sequence boundaries in shallower shelf sequences in response to subaerial exposure and transgressive ravinement, the Tertiary firmgrounds on the New Jersey margin formed in deep water in response to phases of rapid transgression and negative sediment budget; overconsolidated mud substrates were exhumed as a result of sediment starvation and bottom-current winnowing at or near the bases of slope clinoforms. These observations extend the previously established sequence stratigraphic utility of substrate-controlled ichnofacies to deeper water facies.