From Greenhouse to Icehouse – The Eocene/Oligocene - UMass ...

Author's personal copy
From Greenhouse to Icehouse – The Eocene/Oligocene in Antarctica 315
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, B43–34 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 o5 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