Glacial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 295

3. TRACE-FOSSIL ASSEMBLAGES FROM ANCIENT
GLACIAL ENVIRONMENTS
3.1 Bioturbated Deposits in Glacial Settings
Diamictites and rhythmites (e.g., varves) are themost characteristic and dominant
modes of sedimentation in past glacial environments (e.g., Donovan and Picker-
ill, 2007; Eyles, 1993; Eyles and Eyles, 1992; Jensen andWulff-Pedersen, 1996;
Thunell et al., 1995 ). These deposits are directly related to glaciers, which deliver
a large volume of sediment to marine and terrestrial/lacustrine basins, forming
glacioterrestrial and glaciomarine systems tracts ( Eyles and Eyles, 1992 ). The
glacioterrestrial realm is composed of subglacial, supraglacial, glaciolacustrine,
and glaciofluvial depositional systems, which are mainly expressed in low-relief
glacioterrestrial settings. Glaciomarine settingsmay be dominated by glacial pro-
cesses (ice proximal) or marine processes (ice distal). The influence of meltwater
input in glaciomarine settings depends on the regional climate regime and con-
trols chemical and biogenic processes. Most glacial deposits in low-relief glacio-
marine settings are concentrated on continental shelves, the ice-proximal zone
bearing the coarse-grained facies and the fine-grained facies deposited in the
ice-distal zone. Fjords characterize the high-relief glaciomarine settings, where
the ice-proximal deposits remain on the continental slope and the ice-distal
deposits extend to the basin as turbidites ( Eyles and Eyles, 1992 ). Stratigraphic
discontinuities are common, as erosion plays an important role in these systems.
Laminated rhythmites are associated with glaciolacustrine deposits and
shallow tide-dominated and prodeltaic glaciomarine deposits (e.g., Brodzikowski
and van Loon, 1991; Domack, 1984; Mackiewicz et al., 1984; Powell, 1981,
1983; Smith et al., 1990 ). Millimeter- to centimeter-scale depositional layers, that
represent annual cycles known as varves, are the best-known glacigenic rhyth-
mites. However, rhythmites formed in glacially influenced environments do
not necessarily represent annual seasonality, but rather result from recurrent
episodic meltwater discharge events that form distal turbidite deposits in distinct
settings (e.g., Brodzikowski and van Loon, 1991; Eyles and Eyles, 1992; Talbot
and Allen, 1996 ).
Trace fossils in ancient glacial deposits have been recorded chiefly in fine-
grained rhythmic deposits and, more rarely, in massive mudstones. They
represent colonization after ice retreat, possibly in response to meltwater pulses,
being well preserved in regular, varve-like rhythmites from the Upper
Carboniferous-Lower Permian of Gondwana and Northern Hemisphere Pleis-
tocene glacial deposits. They represent the bulk of the known glacial trace-fossil
record.
The Late Paleozoic Gondwana Ice Age is well represented in the sedimen-
tary record of Bolivia, in the Andean basins of Argentina (Tarija, Calingasta-
Uspallata, and Paganzo Basins), in Brazil (Paran´ Basin), in South Africa
(Karoo Basin), and also in Antarctica and Australia (see Buatois et al., 2006,