Lacustrine Environments - Trace Fossils as Indicators of Sedimentary Environments - page 384

continental realm ( Lucas et al., 2011; Melchor et al., 2006, 2012 ); (2) the recog-
nition of subaerial exposure in low-diversity assemblages containing simple
vertical burrows or simple surface trails and burrows, fromwhich the lake-basin
zone may otherwise be difficult to discern; and (3) the recognition of strati-
graphic surfaces that represent changing conditions in both lake margin (e.g.,
Prince and Lockley, 1989 ) and marginal-marine settings ( Phillips et al., 2007 ).
2.2.4 Terrestrial Trace-Fossil Assemblages
Terrestrial trace-fossil assemblages, typical of alluvial and eolian environ-
ments, may be present within the zone of lake-level rise and fall if water tables
are lowered sufficiently for terrestrial organisms to colonize the substrate. Com-
mon traces include vertebrate burrows, ant nests, termite nests and tunnels, bee-
tle burrows and brood balls, and burrows, nests, and cells of wasps and bees. In
lake basins, low-diversity assemblages of termite traces, exclusive of cells and
brood balls, may even occur in lake-margin to basin-center successions in arid
regions (e.g., Smith and Mason, 1998 ) or where lake levels have dropped for
extended periods of time (e.g., Scott et al., 2009 ). Assemblages dominated
by dung beetle nests (e.g., Coprinisphaera ) or bee cells (e.g., Celliforma )
appear to be present mainly near the basin margin or in deposits that have under-
gone moderate to extensive pedogenesis.
Carbonate palustrine environments are low-energy environments with fluctu-
ating water levels and groundwater tables, and are characterized by pedogenic
modification of shallow lacustrine to littoralmarsh carbonates (e.g., Alonso-Zarza,
2003; Platt, 1989 ). Theycontain trace-fossil assemblages typifiedbybee cells (e.g.,
Celliforma ) and other insect burrows and chambers (e.g., Skolithos , Taenidium ,
Teisseirei ; Edwards et al., 1998; Genise et al., 2010; Melchor et al., 2002 ).
Deposits from palustrine environments often contain both freshwater and terres-
trial gastropod assemblages, together with weakly developed paleosols and root
traces (e.g., Alonso-Zarza, 2003 ). Melchor et al. (2002) demonstrated the mutual
exclusion of very low-diversity assemblages of (1) Skolithos in fine-grained
carbonates that were relatively waterlogged, when compared to (2) the overlying,
pedogenicallymodified tuffaceous carbonateswith abundant Celliforma ,and(3)a
relatively well-developed paleosol with root traces and meniscate-backfilled
burrows. This example shows the transition from wetter to drier to vegetated
substrates in a vertical succession representing changing conditions in an environ-
ment characterized by fluctuating water tables ( Melchor et al., 2002 ).
3. AN INTEGRATED LAKE-TYPE BASIN AND
LACUSTRINE ICHNOLOGY MODEL
Depositional environments in lake basins are diverse and are distributed according
to the interaction of numerous abiotic factors (e.g., Cohen, 2003; Renaut and
Gierlowski-Kordesch, 2010; Wetzel, 2001 ). The lake-type model of Carroll and
Bohacs (1999) and Bohacs et al. (2000) permits the conceptual simplification of