Charles E. Savrda1
(1) Auburn University, Auburn, AL
ABSTRACT: Ichnofabrics and Substrate-Controlled Ichnofacies in Tertiary Deep-Water Stratigraphic Sequences, New Jersey Slope (ODP Site 1073)
Although ichnofacies and ichnofabrics are commonly employed to delineate sequence stratigraphic surfaces within shallow marine deposits, ichnologic signatures of deep-water sequences are virtually unstudied. This provided incentive for studies of a 144-m-thick package of Tertiary (Eocene-Pliocene) slope deposits recovered at ODP Site 1073 on the New Jersey margin.
These deposits comprise 15 depositional sequences characterized by basal glauconite sands, which record transgressive condensation, overlain by clay, silty clay, or biogenic pelagic muds, which represent highstand through lowstand stages. Sequence-bounding surfaces are sharp; most correspond to hiatuses defined by Sr-isotopic or biostratigraphic data and can be linked to previously dated seismic unconformities recognized beneath the shelf. They represent transgressive surfaces and, thus, genetic stratigraphic sequence boundaries.
The entire package is completely bioturbated and dominated by a softground ichnofossil assemblage (Chondrites, Teichichnus, Zoophycos, Phycosiphon, Planolites, Palaeophycus, Taenidium, Thalassinoides). Although there is little change in ichnofauna across individual sequence boundaries, overall burrow densities and abundance of softground Thalassinoides are highest in glauconitic intervals, reflecting diminished sedimentation rates with transgressive phases. In the broader view, burrow densities and importance of "deeper water" ichnotaxa (e.g., Zoophycos) generally decrease upwards through the whole package, reflecting increasing sedimentation rates as a consequence of successive sea-level cycles and associated episodes of clinoform progradation.
Most sequence boundaries are clearly manifested by conspicuous firmground Thalassinoides (Glossifungites ichnofacies). Unlike those that develop at sequence boundaries in shelf sequences in response to subaerial exposure and ravinement, these deep-water firmgrounds formed during transgression as a result of sediment starvation and bottom-current winnowing, facilitated by bioerosion, at or near the bases of slope clinoforms.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado