Eocene Environments in the Ross Sea Region

There is an intriguing record of Eocene pre-glacial environments in the Ross Sea region, around McMurdo Sound. Although the location of the Eocene outcrop is presumably under the ice sheet, there are no major exposures. A glimpse of Eocene environments is, however, provided by several hundred erratic boulders and cobbles recovered from coastal moraines around the shores of Mount Discovery, Brown Peninsula and Minna Bluff (Fig. 8.1).

These fossiliferous glacial erratics (called the McMurdo Erratics), recovered from moraine around the northwest coast of Mount Discovery and Minna Bluff in southern McMurdo Sound, provide a window into the environment that may have existed along the coast of the gradually rising Transantarctic Mountains during the Eocene. The erratics are most likely derived from sub-glacial basins, such as Discovery Deep, that lie along the coast of the Transantarctic Mountains or basement highs situated to the east of the discovery accommodation zone (Wilson, 1999; Wilson et al., 2006). The erratics were distributed into their distinctive pattern of terminal and lateral retreat moraines during relatively recent advance and retreat of grounded ice into southern McMurdo Sound (Wilson, 2000). Subsequent basal adfreezing and surface ablation has transported the erratics to the surface of the McMurdo Ice Shelf. Although currently out of their original stratigraphic position, this suite of erratics provides us with a mechanism to obtain geologic data that are otherwise buried beneath the Antarctic Ice Sheets and fringing ice shelves.

The McMurdo Erratics comprise a range of lithotypes and ages. Eocene rocks contain a rich suite of fossil floras and faunas including marine and terrestrial palynomorphs, diatoms, ebridians, marine vertebrates and invertebrates, terrestrial plant remains and a bird humerus. Biostratigraphic data from dinoflagellate cyst (dinocyst), ebridian and mollusc assemblages recovered from many of the erratics indicate that the majority of fossiliferous rocks range from Middle to Late Eocene, ~ 43-34 Ma. Erratics collected between 1993 and 1996 (Stilwell and Feldmann, 2000) include several hundred samples of Oligocene, Miocene and Pliocene sediment. Although relatively rich dinocyst assemblages have been described from Oligocene Miocene Sequences from the Cape Roberts Cores, assemblages in post Eocene erratics comprise few taxa (typically < 5 species). This general paucity of dinocyst species in late Palaeogene and Neogene sequences is observed in several other sites from the southern high latitudes (Wilson, 1989; Mao and Mohr, 1995) and may be a reflection of geographical and thermal isolation of Antarctica (McMinn, 1995; Williams et al., 2004) or preservation.

The majority of the Eocene erratics record a suite of lithofacies that were deposited in coastal-terrestrial to inner shelf marine environments (Levy and Harwood, 2000a,b). These sediments were probably deposited within fan deltas that formed along the rugged coastline of the rapidly rising Transantarctic Mountains. Abundant macroinvertebrate faunas, including bivalves, gastropods, scaphopods, cirripeds, bryzoans, decapods and brachiopods, indicate that many of these sediments were deposited in a spectrum of predominantly shallow marine environments. The presence of terrestrial plant material and palynomorphs also suggests that the majority of the rocks were formed in nearshore environments. However, the occurrence of open marine dinocyst species and the absence of benthic diatom taxa in many of the fine-grained lithofacies indicate that outer shelf open marine environments were also present in the source region. The Eocene erratics contain no direct or unequivocal sedimentological evidence for the presence of ice close to the basins in which the sediments were originally deposited. It is notable that erratics composed of diamictites recovered from the coastal moraines are all Oligocene and younger in age.

Although rare, fossil leaves, wood and pollen recovered from several erratics provide a glimpse of the Eocene climate for the region. One erratic contains wood and leaves from Araucaria and Nothofagus trees, which suggests that the climate was not extreme. Cool temperate conditions with some winter snow may have been possible but temperatures were probably not cold enough to allow extensive ice at sea level (Francis, 2000; Pole et al.. 2000). Spore and pollen assemblages recovered from the erratics reflect Nothofagus-podocarpaceous conifer-Proteaceae vegetation with other angiosperms growing in temperate climate conditions (Askin, 2000). Oligocene and younger erratics show a major drop in species richness, which is also noted in sequences recovered in CIROS-1 and the Cape Roberts Project (CRP) cores (Mildenhall, 1989; Raine and Askin, 2001).

Fossil invertebrate remains recovered from the erratics include a humerus shaft from a pseudodontorn (giant bony-toothed sea bird) (Jones, 2000), a probable crocodile tooth (Willis and Stilwell, 2000) and teeth from two species of shark (Long and Stilwell, 2000). The small but significant record of East Antarctic invertebrate fauna indicates a temperate to cool temperate marine environment.

0 0

Post a comment