Glacial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 307

supports the shallow condition of these ancient glacial lakes ( Netto et al., 2009;
Noffke, 2010 ). This model was compared by Netto et al. (2009) with modern
postglacial landscapes in Alaska and Antarctica and reinforced by the common
occurrence of freshwater ponds and lakes that develop in shallow depressions
excavated by ice-mass movements in marginal-marine plains in high-latitude
settings ( Horne and Goldman, 1994 ). Several of these lakes overlie permafrost,
which melts out during summer, promoting lake drainage. According to Talbot
and Allen (1996) , cool glacial conditions in polar regions create arid conditions
and low lake levels in high-latitude temperate settings. Lake-level changes are
controlled dominantly by climate oscillation, which controls the hydrologic
budget. Thus, in shallow lakes, reduction of water input and concomitant
increases in water loss may reduce the water level, giving rise to wetlands,
or cause them to dry up entirely.
Thus, rather than a distinctive “glacial ichnofacies”, ichnofaunas from gla-
cial and periglacial settings characterize particular suites of the Mermia Ichno-
facies. Suites of Scoyenia Ichnofacies, dominated by arthropod trackways
(typical of margins of closed lakes, Buatois and M ´ ngano, 2004 ), also occur,
being well documented in the Gondwana record ( Buatois et al., 2010; Netto
et al., 2009 ; Fig. 7 ).
3.4 Glaciomarine Trace-Fossil Assemblages
The ichnofauna of shallow glaciomarine Gondwanaland is preserved in fine- to
very fine-grained sandstones and siltstoneswith flaser andwavybedding; ismainly
composed of ? Arenicolites , Chondrites , Diplocraterion , Palaeophycus , Phyco-
siphon , Planolites , Rhizocorallium ,and Thalassinoides (see Supplementary
Table 2 in http://booksite.elsevier.com/9780444538130 ; Fig. 6 G-J); and is well
represented in the Rio do Sul (Paran´ Basin) and San Gregorio (North Uruguayan
Basin) Formations ( Balistieri, 2003; Buatois et al., 2006, 2010 ).Massive siltstones
locally contain sharply bounded burrows ( Thalassinoides isp., Diplocraterion isp.,
Palaeophycus isp., P. striatus , ?Rhizocorallium isp., and Gyrolithes- like burrows;
Fig. 6 A-F) representative of the Glossifungites Ichnofacies ( Balistieri and Netto,
2002 ), suggesting transgressive erosional exhumation and firmground coloni-
zation in fjord valley flanks. Glaciomarine trace fossils are small if compared
with equivalent ichnofaunas from normal-marine settings, and comprise non-
specialized feeding burrows produced by trophic generalists. The overall structure
and composition of the assemblage are consistent with impoverished Cruziana
Ichnofacies suites normally found in brackish-water settings (e.g., Buatois et al.,
2005 ). Dropstones and diamictites with faceted clasts occur throughout most of
the succession and thick deposits of fossiliferous marine shales record periods
of maximum flooding in the Gondwanaland glacial environment.
The most diverse marine ichnofauna reported in glacially influenced
Gondwanaland deposits is preserved in lower shoreface storm sediments from
the Talchir Formation in central and eastern India ( Bhattacharya and