Before the Tasmanian Gateway opened, rifting and associated hinterland uplift and erosion allowed rapid deltaic sedimentation in the rifts. Evidence from seismic reflection profiles, drilling, and dredging indicates that as much as 4000 m of Cretaceous to Eocene, largely deltaic sediments were deposited in the offshore Tasmanian region (Exon et al., 1997; Hill et al., 1997, 2001; Hill and Exon, in press). For Leg 189 drill sites, late Maastrichtian through early late Eocene deposition (~75–36 Ma) was shallow-marine deltaic and siliciclastic. Moderately high depositional rates (~5–10 cm/k.y.) kept up with subsidence and compaction. Calcareous micropaleontological and palynological evidence indicates a cool to warm temperate climate through the late Maastrichtian to the late Eocene (Brinkhuis, Sengers, et al. and Brinkhuis, Munsterman, et al., both this volume).
Cretaceous (Maastrichtian) sediments were recovered only at Site 1172 on the ETP, where they are 70 m thick. The microflora indicate a humid and seasonally cool climate. Smectite completely dominates the clays (Robert, in press), suggesting a warm climate and extreme chemical weathering in the source area. The lower, thicker facies is dark claystone and silty claystone, which is essentially noncalcareous and rarely bioturbated (Exon et al., in press b). Dinocysts, spores, pollen, and diatoms are common, and the environment is interpreted as generally highly restricted and paralic. Occasional more marine beds contain molluskan debris, planktonic foraminifers, and nannofossils. The upper, thinner facies comprises brown, paralic, sideritic sandstone and sandy mudstone that contain 20%–55% sand consisting largely of either siderite micronodules or quartz and glauconite. The Cretaceous/Tertiary boundary is not preserved, and an iridium anomaly is lacking (Schellenberg et al., in press).
By the Paleocene, areas of continental crust that had been thinned by Cretaceous rifting—the future Bass Strait, the South Tasman Saddle between Tasmania and the STR, parts of the TASZ between Antarctica and the STR, and the East Tasman Saddle between Tasmania and the ETP—had subsided and were near sea level. Thereafter, very limited interchange of shallow-marine waters could have occurred between the AAG and the Pacific Ocean. Although plate tectonic reconstructions such as those of Royer and Rollet (1997), Lawver and Gahagan (2003), and Cande and Stock (in press) suggest the presence of a shallow-marine connection by the middle Eocene (Fig. F7A), the sedimentary evidence from Leg 189 suggests that this connection must have been very limited. Even in the late Eocene, the contrast between the poorly oxygenated, relatively warm shallow-marine waters of the AAG (Site 1168) and the better oxygenated, relatively cool shallow-marine waters of the southwest Pacific Ocean (Sites 1170–1172) suggests a lack of significant interchange between southern Indian and Pacific Ocean waters even at the shallowest depths.
Throughout the Paleocene and Eocene, water circulation in the AAG was probably a sluggish clockwise gyre, with contributions of some warmer waters from lower latitudes in the Indian Ocean. In contrast, and modifying precruise assumptions, micropaleontological evidence (Brinkhuis, Sengers, et al., this volume), with support from climate modeling, suggests that the eastern sites were influenced by cooler waters transported northwestward as a western boundary countercurrent. Evidence for glaciation is completely lacking at Leg 189 sites, with both marine and terrestrial microfossils indicating temperate conditions. Relatively diverse late Eocene calcareous nannofossil assemblages indicate slightly warmer conditions in the Tasmanian Gateway sector of the Southern Ocean than at comparable latitudes elsewhere, although most of the sites lack warmth-loving discoasters. The nannofossil diversity and the dinocyst and diatom assemblages confirm the absence of seasonal sea ice over the shelf.
Paleocene and Eocene organic-rich mudstones were deposited on a highly restricted, moderately tranquil broad shelf near the rift opening between Antarctica and Australia. Benthic foraminiferal assemblages indicate deposition at shelf depths. The lack of sedimentary characteristics indicating turbulence suggests deposition below wave base (which may have been shallow in the prevailing equable climatic conditions) and largely free of appreciable current or tidal influences.
The Paleocene was cored at Site 1172 on the ETP, and probably at Site 1171 on the STR. At Site 1172, a thin Danian (lowermost Paleocene) sequence is disconformably overlain by a thicker upper Paleocene sequence. The lowermost Danian (6 m thick) disconformably overlies the uppermost Maastrichtian and is brown noncalcareous muddy glauconitic quartz sandstone and sandy mudstone, deposited in paralic conditions. The disconformably overlying uppermost Paleocene (75 m thick) is dark glauconitic quartz-bearing mudstone; the water shoaled through time but remained paralic. Offshore dinocysts are much more common in the upper Paleocene than during the Danian. The microflora indicate a relatively warm and humid but weakly seasonal climate. Smectite continues to completely dominate the clays (Robert, in press), suggesting a warm climate and extreme chemical weathering.
At Site 1171, we follow Röhl et al. (in press a) in distinguishing a 44-m-thick upper Paleocene section that consists of laminated dark mudstone with almost no sand or carbonate. Sporomorphs are abundant, but other diagnostic fossils are rare; diverse pollen and spore assemblages indicate a strong terrigenous influence. The microflora and abundant smectite suggest a relatively warm, humid, but weakly seasonal climate. Brinkhuis, Sengers, et al. (this volume) and Röhl et al. (in press a) suggest that very shallow marine conditions prevailed. However, in the late Paleocene, Site 1171 had more restricted marine conditions than Site 1172.
The tectonic setting in the middle Eocene is shown in Figure F7A. Lower and middle Eocene siliciclastic sediments were cored at Sites 1170–1172:
At all sites in the early and middle Eocene, pervasive pollen and spore assemblages and abundant and continuous low-diversity assemblages of dinocysts indicative of eutrophic and brackish surface waters point to highly restricted nearshore conditions. Sporadic occurrences of well-preserved calcareous nannofossils suggest that their rarity is due to limited access to the restricted coastal setting and to high sedimentation rates, rather than to dissolution. Water mass ventilation was generally poor to limited, judging by the high organic carbon content, limited bioturbation, and benthic foraminiferal assemblages dominated by agglutinated forms and nodosariids.
Distinct cycles in physical properties, sediment type, and microfossil assemblages are well documented in middle and upper Eocene sediments at Site 1172 (Röhl et al., in press b) and are also evident at Sites 1170 and 1171. The cycles are between dark, poorly bioturbated, dinocyst-rich but nannofossil-poor sediments lacking glauconite and lighter, dinocyst-bearing, and more nannofossil-abundant, bioturbated sediments containing glauconite. Röhl et al. (in press b) conclude that the sediment cycles were produced under the influence of orbital perturbations of the Earth relative to the sun (Milankovitch cycles), which affected sea level and climate and, in turn, changed siliciclastic sediment supply, upwelling and nutrient supply, and associated bottom water ventilation. White (in press) argues that cycles observed in geochemical parameters in lower to middle Eocene sediments at all sites resulted from the influence of glacioeustasy in these very shallow marine environments.
At Site 1172, the plant microflora suggest a cool and uniformly humid climate in the hinterland, whereas abundant smectite suggests a warm climate and intense weathering. Exon et al. (in press b) showed that the lowermost Eocene (35 m) is dark, noncalcareous, variably glauconitic, quartz-rich shallow-marine mudstone with abundant siderite micronodules toward the top, suggesting paralic deposition. The overlying lower to middle Eocene dark, noncalcareous mudstone (83 m) is more fossiliferous and slightly more marine. The overlying middle Eocene is dark, noncalcareous diatomaceous mudstone (69 m), representing very different shelfal conditions in which siliceous organisms thrived and were preserved. The upper middle Eocene is somewhat calcareous, dark, diatom-bearing to diatomaceous mudstone (64 m) containing more sand. Deposition probably occurred on an open continental shelf. Overall, the Eocene trend was toward more open marine conditions as waters became slightly deeper, with less reducing conditions with time, as indicated by increasing carbonate and diatom components in the sediments in addition to changes in dinocyst assemblages (Brinkhuis, Sengers, et al., this volume).
At Site 1171, marine mudstones contain dinocysts, indicating restricted, eutrophic, and neritic conditions throughout, with open marine taxa being relatively rare (Shipboard Scientific Party, 2001d). The microflora indicate a relatively warm, humid, but weakly seasonal early Eocene climate and a cooler and uniformly humid middle Eocene. The lower Eocene (45 m) consists of greenish noncalcareous and quartz-rich mudstone. The mixed clay mineral assemblage suggests intense erosion of steep relief areas (Robert, in press). The middle Eocene (142 m) is greenish gray mudstone with nannofossil and carbonate content moderate at the bottom, negligible in the middle, and moderate at the top. The return to dominance of smectite suggests a warm climate and decreasing erosion and intense weathering in the hinterland. At Site 1170, middle Eocene dark mudstones (241 m) contain abundant dinocysts that indicate somewhat restricted, euphotic, neritic conditions (Shipboard Scientific Party, 2001c). Some microfloral evidence suggests a relatively cool, humid climate, but smectite dominates the clays, suggesting warm climate and intense weathering in the hinterland. The nannofossil distribution suggests periods of somewhat more open marine, less restricted conditions.
In the Eocene, both the AAG and the southern Proto-Pacific Ocean were under the influence of temperate climate, but the currents were warmer in the AAG than in the proto-Pacific (Fig. F7A). Ventilation of the waters increased toward the developing seaway on the Pacific side of the Tasmanian land bridge. At Site 1170, laminations are periodically absent and there is some bioturbation. Farther east, the well-bioturbated shelf sediments at Site 1171 and other evidence indicate more ventilated conditions and the absence of an oxygen minimum zone. Within the open Pacific Ocean at Site 1172, the water mass was better ventilated than at either of the sites to the west.