The lower unit in the sequence at this site appears to have been deposited during the Late Cretaceous or, more precisely, Turonian to possibly Santonian (see Table T3). An unconformity representing the Campanian-early Eocene is inferred to occur at ~267 mbsf. This record is an important one, given that sediments of Turonian-Santonian age have not previously been reported from Antarctica. Samples below 267 mbsf yielded very low frequencies of marine dinocysts and low to medium recoveries of spores and pollen. The microfloras support close floristic (both marine and terrestrial) links with southeastern Australia during the Late Cretaceous. Detailed distribution of the taxa recovered is given in Table T6.
A reference framework for age control is provided by the time distribution of species in southeastern Australia (Stover and Partridge, 1973, 1982; Helby et al., 1987; Marshall, 1988, 1989; A.D. Partridge and M.K. Macphail, unpubl. data) and by regional comparisons with sequences in West Antarctica and adjacent offshore regions (Truswell et al., 1999).
Most samples between 296.06 and 343.00 mbsf preserve fragments of the long-ranging Late Cretaceous species Heterosphaeridium heterocanthum (Pl. P8, fig. 1) and chitinous material lining the internal chambers of planktonic foraminifers (trochospiral test linings). Index species of Late Cretaceous dinoflagellate zones established by Helby et al. (1987) are generally absent. Exceptions are (1) Isabelidinium variable, described from ?Santonian (Tricolporites apoxyexinus Zone) dredge samples from the Gippsland Basin (Marshall, 1988), and (2) a dinocyst closely resembling Wuroia corrugata, recorded in the P. mawsonii Zone in the Gippsland and Bass Basins (Marshall, 1989; M.K. Macphail and A.D. Partridge, unpubl. data) (Pl. P8, figs. 2-4).
Microfloras in this interval are wholly dominated by gymnosperms (average = 88%) and cryptogams (average = 11%). Frequent to common taxa include (with modern equivalents in parentheses) Baculatisporites (Osmundaceae), Cyathidites (Cyatheaceae), Araucariacites australis (Araucariaceae), Cupressacites (Cupressaceae, Taxodiaceae), Dilwynites (Agathis/Wollemia), Microcachryidites antarcticus (Microcachrys), P. mawsonii (Lagarostrobus), Podocarpidites (Podocarpus-Prumnopitys), and T. subgranulosus (?Microcachrys) (Pl. P10, figs. 1-14). The only angiosperm type that is constantly recorded is A. obscurus, a species whose nearest living relative is the freshwater aquatic herb Callitriche. Peat-forming mosses such as Stereisporites (Sphagnum) are rare.
The miospore component indicates that the maximum age is the Turonian P. mawsonii Zone equivalent, based on the indicator species, P. mawsonii, which is present to frequent in all samples. It is also notable that two long-ranging species, Cupressacites and Dilwynites sp. A, which reach their maximum relative abundances in the P. mawsonii Zone in the Gippsland Basin (A.D. Partidge, unpubl. data), attain values of 15% and 4%, respectively, in this interval.
The minimum age is less certain because of the persistent presence of species which (1) are confined to the P. mawsonii Zone (e.g., Laevigatosporites sp. A and Verrucosisporites sp. A), or (2) typically first appear in the ?Santonian T. apoxyexinus Zone in southeast Australia (e.g., C. bullatus, Dacrycarpites australiensis, and Lygistepollenites florinii).
Unless better evidence for a T. apoxyexinus Zone equivalent age is found, for example, the presence of Ornamentifera sentosa, Latrobosporites amplus, Forcipites stipulatus, Tricolpites confessus, and T. apoxyexinus, we prefer to correlate the interval with the P. mawsonii Zone but broaden the age limits to encompass Turonian-?Santonian time. The lowest sample, at 362.03 mbsf, cannot be dated. It should be noted that some samples assigned to the P. mawsonii Zone equivalent in Prydz Bay include taxa whose ranges rarely or never overlap in the Gippsland Basin, for example, C. bullatus, Laevigatosporites sp. A, and D. australiensis. We have pointed out (M.K. Macphail and E.M. Truswell, unpubl. data) that some degree of diachronism is expected because of environmental contrasts between the two regions. For the same reason, the P. mawsonii Zone equivalent should be considered provisional, although the Turonian-?Santonian age range is considered to be reliable.
The sparse yield of dinoflagellate cysts (or their absence, as in the interval between 288.26 and 276.60 mbsf) suggests a limited marine influence. The combined data indicate that the interval between 276 and 343 mbsf accumulated in a marginal basin with a diminishing influence of the sea. Whether this was due to falling relative sea levels (regression) or progradation of the shoreline during a marine high stand is unknown. A summary of depositional environments is presented in Table T4.
Relative abundance data point to the coastal plain vegetation being dominated by conifers. Ferns formed a diverse understorey or perhaps heathland in open areas. Physiological constraints imposed by low light during winter months imply that trees and taller shrubs will have been widely spaced to form a woodland rather than forest (Jefferson, 1982; Specht et al., 1992; del Valle et al., 1997). Two woody species (P. mawsonii and T. subgranulosus complex) indicate the presence of freshwater swamps, but both are uncommon relative to dry land gymnosperms.
The suggested vegetation formation is termed Austral Conifer Woodland or heath because of its broad resemblance to modern boreal communities growing close to the limits of forest tree growth in the Northern Hemisphere (Table T4). The presence of gymnosperms points to year-round moderate humidity and temperatures that were mild relative to the present day. The low relative abundance of probable or suspected hydrophytes such as Stereisporites, Podosporites, and Phyllocladidites indicates conditions were not sufficiently cold or uniformly wet to sustain extensive freshwater swamps or raised peat bogs.