COMPLEMENTARY RELATIONS BETWEEN ICHNOLOGY AND SEQUENCE STRATIGRAPHY: EXAMPLES FROM A CENOZOIC SLOPE SUCCESSION, NEW JERSEY MARGIN

Ichnologic studies can aid in recognition and interpretation of sequence stratigraphic surfaces. Conversely, studies in a sequence stratigraphic framework can provide an improved understanding of factors that control ichnofossil and ichnofabric distribution. This mutually beneficial relationship between ichnology and sequence stratigraphy is exemplified in the Eocene-Pleistocene marine slope succession recovered at ODP Site 1073 on the New Jersey margin.

The succession at Site 1073 includes (1) a thin (~144 m), stratigraphically condensed package of Eocene-Pliocene muds and glauconitic muddy sands that accumulated at the distal toes of prograding clinoforms, and (2) a thicker (~520 m) package of rapidly deposited Pleistocene middle to upper slope muds. Approximately 20 surfaces are recognized in the succession, mostly in the Tertiary package, based on seismic reflection surveys, sedimentologic and biostratigraphic studies, and/or strontium isotope stratigraphy. In core, nearly all of these surfaces can be recognized readily by their distinctive Thalassinoides-dominated firmground ichnofabrics. Unlike firmgrounds at sequence boundaries in shallower shelf sequences, which form as a result of subaerial exposure and transgressive ravinement, firmgrounds at Site 1073 formed in deep water in response to phases of rapid transgression and consequent sediment starvation, bioerosion, and bottom-current winnowing (i.e., they define transgressive surfaces).

Sedimentation rates undoubtedly varied through each sea-level cycle. Moreover, the succession on the whole records a progressive increase in sedimentation rate as the fronts of successive prograding clinoforms moved closer to the site. Net accumulation rates apparently increased slowly through the Tertiary (from <1 cm/ky to several cm/ky), and then escalated markedly in the Pleistocene (dm/ky). Changes in softground ichnofabrics within individual depositional sequences and through the succession document the control of sedimentation rate on the development and preservation of bioturbate textures vs. distinct ichnofossils, on burrow densities, and on the distribution of some common ichnotaxa (e.g., Zoophycos and Chondrites).