The age-depth information summarized above indicates that a ~105-m-thick Pliocene-Pleistocene section and a ~165-m Miocene section are present in Hole 1138A (Fig. F6). Even though the lowermost Pliocene interval is missing, the lower Pliocene to upper Pleistocene section in Hole 1138A is one of the thickest recovered to date on the Kerguelen Plateau. The only comparable sections are those sections recovered during Leg 119 on the Northern Kerguelen Plateau at ODP Sites 736 and 737 and a deepwater section recovered on the Southern Kerguelen Plateau at Site 745 (Barron et al., 1991). The acquisition of a thick Quaternary section was a major objective of ODP Leg 120 (Sites 747-752); the designated Neogene site (751), however, yielded primarily Miocene rather than Pliocene-Pleistocene sediments (Schlich, Wise, et al., 1989).
Enhanced sedimentation rates through the Neogene at Site 1138 may, in part, be explained by the input of both ash and terrigenous clay material. A submarine canyon has been identified ~45 km west of Site 1138, and turbidity currents associated with this feature may have supplied distal overbank sediments to the site (Coffin, Frey, Wallace, et al., 2000). In addition to ash fall from periodic eruptions on Heard Island, this process may account for the expanded nature of Unit I and Subunit IIA in Hole 1138A (upper Miocene and lower Pliocene through upper Pleistocene).
The nearest comparable Neogene section to that of Hole 1138A lies at Site 747, also on the Central Kerguelen Plateau (Fig. F1). Hole 747A contains a ~29-m Pliocene-Pleistocene section and a ~98-m Miocene section (Harwood and Maruyama, 1992; Harwood et al., 1992; Ramsay and Baldauf, 1999), indicating that the Pliocene-Pleistocene section in Hole 1138A is expanded relative to nearby Hole 747A. An upper Miocene-lowermost Pliocene disconformity is present in both drill cores. The hiatus within Hole 747A, however, spans a much longer time interval, from ~5 to 9 Ma (Harwood et al., 1992; Ramsay and Baldauf, 1999). An upper Miocene-lowermost Pliocene disconformity is a common feature of many cores drilled in the Southern Ocean (see Wise et al., 1985, fig. 7; Ramsay and Baldauf, 1999, fig. 60). This may be related to cooling and intensification of the Antarctic Circumpolar Current through the late Miocene-earliest Pliocene interval.
Harwood et al. (1992) identify a possible hiatus near the Oligocene/Miocene boundary in Hole 747A. Ramsay and Baldauf (1999), however, consider the polarity sequence through this interval of Hole 747A to be complete. This discrepancy arises from the assignment of chron designations, which are not well constrained with biostratigraphy and are ambiguous for this core interval. Regardless, the possible hiatus at the Oligocene/Miocene boundary recognized in Hole 1138A is not unexpected considering the strong glacial event (Mi-1) postulated for this time interval (Zachos et al., 2001). A hiatus spanning this time interval is recognized at several Southern Ocean sites (Wright and Miller, 1993).