BIOSTRATIGRAPHY AND BIOFACIES

Papers by McCarthy and Gostlin (2000), McCarthy et al. (2000, in press), Katz et al. (in press), and Savrda et al. (2001a, 2001b) extend shipboard work on palynology, benthic foraminiferal biofacies, and ichnofabrics. McCarthy and Gostlin (2000) and McCarthy et al. (in press) studied Pleistocene palynomorphs (pollen and dinocysts) from Sites 1072 and 1073 as a means for determining ages in shelf sediments lacking nannofossils and as a proxy for environmental change. Cluster analysis shows that the highest degree of dissimilarity in terrestrial and marine palynomorph records in Hole 1072A is around 90 mbsf, and the second highest degree of dissimilarity is above ~127 mbsf. This allowed the identification of three palynomorph zones informally labeled, from youngest to oldest, P1, P2, and P3. Surface pp4(s) is found within Zone P3 at Site 1072 and between zones P2 and P3 at Site 1073, consistent with a hiatus of up to 0.8 m.y. at Site 1073 and with evidence for the relative ages of shelf and slope sections of the overlying sequence. Surface pp3(s) lies within Zone P1, but uncertainty in its location and, hence, age at Site 1073 can be considered with reference to palynological data. The surface has been traced with seismic data to the interval between 243 and 325 mbsf at Site 1073, an uncertainty that reflects complexities in physical stratigraphy on the slope. Its age, based on nannofossils, depends critically on whether it is located above or below 324.86 mbsf (younger or older than 0.46 Ma). A marked change in palynological character at 285 mbsf at Site 1073 led McCarthy and Gostlin (2000) to interpret surface pp3(s) at that level. The high ratio of terrestrial to marine palynomorphs observed at that horizon is consistent with downslope transport of neritic sediments from a subaerially exposed continental shelf.

McCarthy et al. (in press) formulated a general model for how palynological records may relate to sequence development and sea level change at a siliciclastic-dominated passive margin. During times of falling sea level and progradation, recycling of palynomorphs is expected to produce a taphonomically altered and ecologically mixed palynological assemblage, an increase in the ratio of terrestrial to marine palynomorphs, and a lowering of palynomorph concentration. During times of marine transgression, the opposite trends are anticipated, with pollen assemblages dominated by bisaccate pollen adapted for long-distance transport at interglacial sea level highstands. The main uncertainty in this reasoning concerns whether observed palynological changes reflect the timing of particular sequence boundaries such as surface pp3(s) directly or only indirectly. The horizon interpreted by McCarthy and Gostlin (2000) as surface pp3(s) is close to an oxygen isotopic minimum, a time of relatively warm climate and, hence, elevated global sea level (see McHugh and Olson, 2002). The marked palynological change may therefore indicate a threshold in the long-term progradation of the margin or perhaps in its climatic history. Sedimentological evidence of glaciation in maritime Canada and New England dates back to oxygen isotope Stage 14 (~550 ka) (Fullerton, 1986).

McCarthy et al. (2000) showed that changes in Pleistocene palynomorphs from the New Jersey margin (Site 1072) are approximately synchronous with similar changes observed at the Iberian margin (ODP Leg 149, Site 898) (Sawyer, Whitmarsh, Klaus, et al., 1994). Sediments older than ~1.4 Ma contain relatively few terrestrial palynomorphs and a dinocyst flora rich in Operculodinium israelianum and other taxa characteristic of relatively warm surface water. Pollen-rich sediments with a cooler-water "modern" dinocyst flora rich in Operculodinium centrocarpum, Bitectatodinium tepikiense, Spiniferites spp., and Brigantedinium spp. are found in sediments younger than ~1.15 Ma. The change was attributed by McCarthy et al. to a climate-driven intensification of the subtropical gyre in response to global cooling.

Katz et al. (in press) evaluated late Miocene to Pleistocene benthic foraminiferal biofacies and planktonic foraminiferal abundances at shelf Sites 1071 and 1072 in the context of an interpretation of borehole lithology and the seismically defined sequence stratigraphic framework. Providing more detail with respect to the preliminary benthic foraminiferal interpretation quoted by Metzger et al. (2000), Katz et al. interpreted paleowater depths ranging from near zero to >100 m. The most obvious feature of these tantalizing data is the very considerable within-sequence variability. This makes it difficult to interpret systems tracts, especially because the assignments suggested by Katz et al. (in press) are without specific reference to internal sequence geometry. None of the lowstand designations can be defended conceptually for backstepping stratigraphic elements overlying prominent offlap surfaces in a shallow shelf setting, nor repetitions of transgressive and highstand systems tracts within a single sequence.

In a review of stratigraphic age constraints, Katz et al. (in press) used an assessment of sedimentation rates between dated horizons to estimate the ages of two sequence boundaries. Surface pp4(s) is said to be ~1.8 Ma, an age that is slightly older than the range quoted by Metzger et al. (2000) (1.7 to 1.4 Ma) or inferred here on the basis of nannofossils (see Wei, Chap. 5, this volume, and below) (1.6-1.4 Ma). The age of surface m0.5(s) was interpreted by Katz et al. (in press) as ~8.6 to ~7.6 Ma, a range that is within the uncertainty of the more conservative interpretations of Austin, Christie-Blick, Malone, et al. (1998) and Metzger et al. (2000). The greatest difficulty in dating this sequence boundary relates to uncertainty in its location in boreholes owing to poor recovery of sands. Austin, Christie-Blick, Malone, et al. (1998) argued that its age is >7.4 Ma and probably >8.6 Ma.

Papers by Savrda et al. (2001a, 2001b) deal, respectively, with Eocene to Pliocene and Pleistocene deepwater ichnofabrics at Site 1073. Sixteen erosional surfaces are recognized in the 144-m-thick highly condensed section of Eocene-Pliocene age (Savrda et al., 2001a). Most of these correspond, at least approximately, to stratigraphic discontinuities inferred on the basis of nannofossil and strontium isotope data. These age data are generally consistent, although with some notable exceptions in the Oligocene to lower Miocene. All of the discontinuities separate clay or biogenic muds below from authigenic glauconitic sandy muds and sands above, and they define the bases of upward-fining successions. The entire interval studied is thoroughly bioturbated and dominated by ichnotaxa representing softground conditions. In contrast, most of the discontinuities are marked by firmground Thalassinoides, burrow systems that penetrate up to 2 m into underlying clays and are characterized by extremely sharp walls. According to Savrda et al. (2001a), many of the surfaces correlate with sequence boundaries on the shelf. However, available seismic data lack the resolution and reflection continuity needed for critical evaluation of this possibility, and specific discontinuities may relate instead to sediment starvation or to marine erosion unrelated to sequence boundary development.

The upper Pleistocene section at Site 1073 is characterized by two texturally defined sedimentary facies (Savrda et al., 2001b). Clay-rich sediments are inferred to have been deposited rapidly from turbidity currents and suspended plumes during times of cooler climate and lowered sea level. The low-diversity Cruziana ichnofacies is dominated by deposit-feeding worms. Sand-rich sediments are interpreted to represent overall slower sedimentation by off-shelf spillover and winnowing during times of warmer climate and higher sea level (cf. McHugh and Olson, 2002). Biogenic disruption is greatest in sandy sediments, predominantly Thalassinoides (crustacean burrows in firmground substrates).

NEXT