SUMMARY OF RESULTS (continued)
Downhole logging was conducted at four of the sites drilled during Leg 189 (Sites 1168, 1170, 1171, and 1172). The sequences logged included a range of siliciclastic and biogenic sediments deposited in a variety of sedimentary environments spanning from the Late Cretaceous to the late Neogene. The results from all sites show that there are two distinct Cenozoic logging units: (1) an upper unit characterized by low natural gamma, magnetic susceptibility, resisitivity, and an ~4 photoelectric value associated with the Oligocene to Quaternary pelagic carbonate deposits and (2) a lower unit with higher natural gamma, higher magnetic susceptibility, and lower photoelectric values associated with the Paleocene to late Eocene shallow-water siliciclastic deposits (Fig. 20).
Log parameters in the upper unit are fairly uniform with only small variations apparent in magnetic susceptibility, natural gamma, density, and resistivity indicating that relatively homogeneous, carbonate-rich, pelagic sediments have accumulated since the Oligocene because of opening and deepening of the Tasmanian Gateway. The small fluctuations in the natural gamma and susceptibility logs imply that, despite the predominance of pelagic carbonates, variations in the sediment terrigenous content occurred throughout the Neogene. Minor changes in the terrigenous component could result from variations in the supply and delivery of terrigenous material to the site or, alternatively, may represent changes in the overlying production of biogenic carbonates, which dilute a relatively constant terrigenous supply. Discriminating between these various influences, each of which is related to different physical mechanisms with their own relationship to climate changes, will require detailed postcruise sediment geochemical studies.
In contrast to the relative stability of log parameters in the Neogene pelagic sequences, pronounced variability is evident in downhole logs within the underlying shallow-water Paleocene to Eocene siliciclastic deposits. This lower sequence is characterized by a general increase in the downhole natural gamma logs with depth, suggesting the sediment terrigenous fraction increases with age at every site, although the trend is most pronounced at the STR Site 1171. Likewise, the general increase in U-spectral gamma logs with depth suggests that the shallow-water siliciclastic Paleogene deposits have higher organic carbon content than the overlying open-ocean Neogene deposits. Yet, the Th/U >2 suggests that the depositional environment, although less ventilated than the overlying pelagic carbonate sequence, was dysoxic or oxic. Superimposed on this general trend of increasing natural gamma values with depth are intervals containing higher frequency cyclic variations in the spectral gamma, magnetic susceptibility, and resistivity logs at Sites 1170, 1171, and 1172. The regional persistence of these cyclic variations in the Th spectral gamma and magnetic susceptibility logs suggest a common influence on sedimentation, either through changes in sea level or regional climate, which influenced sediment supply and/or delivery. Shipboard biostratigraphic results suggest that the period of these sediment cycles is near 20 or 40 k.y. using nannofossil and dinocyst datums, respectively.
The transition interval between the relatively homogeneous upper, and highly variable lower log unit varies in thickness by site occurring abruptly at Sites 1171 and 1172 but more gradually at Site 1168. In addition, correlative peaks in the density, resistivity, PEF, and K spectral gamma logs, associated with glauconite-rich sediments (typical of sediment-starved environments) are common in late Eocene sediments just below the transition to the overlying deeper, open-ocean carbonate deposits.
Summary of Results-Geochemistry | Table of Contents