The Cretaceous paleovegetation of the Kerguelen Plateau (KP), today located in the southern Indian Ocean between the Australian-Antarctic and the African-Antarctic Basins, was part of the southernmost (Antarctic) floral province. The plateau, today more or less completely submerged, exhibits only in the northern and central parts a few subaerial islands: the volcanic Kerguelen, Heard, and McDonald Islands. The Southern Kerguelen Plateau (SKP) lies generally at a water depth between 1000 and 2000 m.
The drilling campaigns of Ocean Drilling Program (ODP) Legs 120 and 183 (Schlich, Wise, et al., 1989; Coffin, Frey, Wallace, et al., 1999) focused partly on the development of the basaltic basement and the subsequent subsidence history of the central Kerguelen Plateau. The age for the basement and immediately overlying sediments at the southern central Kerguelen Plateau seems to be late Early Cretaceous, or 110 Ma (Whitechurch et al., 1992). There, purely terrestrial sediments directly overlying the basalt contain a rich paleoflora (Francis and Coffin, 1992; Mohr and Gee, 1992a). The preliminary stratigraphy of the overlying marine Cretaceous strata has been elucidated partly by nannoplankton and foraminifers and also by organic walled microfossils, including sporomorphs and dinocysts (Watkins et al., 1992; Mohr and Gee, 1992b).
The basement age of the central Kerguelen Plateau (CKP) seems to be slightly younger. Previous studies reported ages of the oldest sediments at the central plateau as late Cenomanian to early Turonian (Mohr and Gee, 1992b). Newly gathered geochemical data show an age of ~95 to 102-103 Ma (Coffin, Frey, Wallace, et al., 1999), which is equivalent to the latest Albian and earliest Cenomanian.
Paleobotanical and palynological data from Hole 1138A on the CKP, on which this paper is focused, give an even more detailed picture of the mid-Cretaceous geological and paleovegetational history and add new information to the previous stratigraphic and paleogeographical analyses of Upper Cretaceous strata recovered at Site 747 (Mao and Mohr, 1992; Mohr and Mao, 1997). Studies of Cretaceous southern high-latitude terrestrial and marine floras, to which the Kerguelen Plateau vegetation and marine phytoplankton belong, began during the 1950s with the classic taxonomic descriptions of Australian material by Cookson and co-workers, later also by Dettmann (e.g., Cookson and Eisenack, 1958, 1960, 1974; Cookson and Manum, 1964; Dettmann, 1963). Backhouse, Burger, and Helby and co-authors (Backhouse, 1988; Burger, 1980; Helby et al., 1987) completed comprehensive studies on Australian sections and Couper (1953, 1960) and Wilson (1984) on New Zealand strata. During the last decades, studies on late Early and early Late Cretaceous palynomorphs and dinocysts were also conducted on material from the Antarctic Peninsula area (Dettmann and Thomson, 1987; Keating et al., 1992; Mohr, 1990).
The KP forms a large topographic high located in the southern Indian Ocean (Fig. F1). The plateau stretches ~2300 km between 46° and 64°S toward the Antarctic continental margin. The structure is between 200 and 600 km wide. Today, most of the Kerguelen Plateau lies south of the present-day Polar Front and its northern and central parts are situated in >1000 m water depth. The southern parts are located mostly at a depth of 1000 to 2000 m below sea level. The northern Kerguelen Plateau (NKP) and CKP includes the plateau's only subaerial parts: Kerguelen, Heard, and McDonald Islands.
The Kerguelen Plateau province has been divided into five distinct geological domains: northern, central, and southern Kerguelen Plateau, Elan Bank, and the Labuan Basin. A lack of rocks older than ~40 Ma from the Kerguelen archipelago and plate reconstructions suggest that the age of this structure is indeed not older than 40 Ma. The central and southern domains of the submarine Kerguelen Plateau are, however, of Cretaceous age (see above).
The CKP, 50° to 55°S, is relatively shallow, contains a major sedimentary basin, the Kerguelen-Heard Basin, and includes the volcanically active Heard and McDonald Islands. During ODP Leg 120, an extended section in the Central Kerguelen Basin was drilled at Site 747, where the oldest sediments, nannofossil chalk, have been dated as late Cenomanian/early Turonian by palynomorphs. The upper Upper Cretaceous sequence consists of upper Santonian to Maastrichtian sediments with a possible large gap during the early to mid-Campanian (Watkins, 1992; Watkins et al., 1992, 1996).
Site 1138, on which this paper is focused, lies on the CKP, 180 km east-southeast of Heard Island (Fig. F1), ~150 km north-northwest of Site 747. The site was drilled in a water depth of ~1150 m. Hole 1138A was rotary cored continuously to a depth of ~840 m. The marine sedimentary section above the volcanic basement, which was reached at 698 meters below seafloor (mbsf), consists of upper Pleistocene to Upper Cretaceous strata (Fig. F2). The upper Pleistocene to Pliocene is represented by a relatively expanded section (110 m) and high sedimentation rates and consists of predominantly foraminifer-bearing diatom clay with interbedded foraminifer-bearing diatom ooze (Unit I). Unit II, foraminifer-bearing nannofossil clay, is dated as late to early Miocene. Unit III is Oligocene to Campanian and consists of white to light greenish gray foraminifer-bearing nannofossil chalk. Core 183-1138A-52R contains the Cretaceous/Tertiary boundary, where no lithologic changes were discovered.
Units IV to VI contain Upper Cretaceous sections. The interval 183-1183A-64R-1, 0 cm, to 69R, 112 cm (Unit IV), consists of cyclic alternations of light gray foraminifer-bearing chalk with gray to greenish gray to black intervals of nannofossil claystone. The clay-rich intervals also contain a planktonic foraminifer assemblage that can be placed in the Whiteinella baltica Zone, which is considered to be of Turonian age (Section 183-1138A-67R-4). The oldest calcareous microfossils encountered on board ship are of early Turonian age. They are present in dark gray and green horizons within highly bioturbated organic-rich chalks and clays (Unit V). These strata immediately overlie an ~1-m-thick marine black shale unit with 2.2 wt% organic carbon (for measurement methods, see Coffin, Frey, Wallace, et al., 1999). Nannofossil and planktonic foraminifer biostratigraphy suggest that this might represent the Cenomanian-Turonian anoxic event (OAE2, the "Bonarelli" horizon). The black terrestrial sediments of sedimentary Unit VI (dark brown silty claystone to siltstone) are rich in fossil plant remains and are discussed in detail in this chapter.