Lacustrine Environments - Trace Fossils as Indicators of Sedimentary Environments - page 394

minerals) deposited in hydrologically closed, saline to hypersaline water bodies
( Bohacs et al., 2000 ). Siliciclastic lithofacies may be abundant toward the basin
margins, but lacking in basin-center successions (e.g., Smoot, 1983 ). Parase-
quences form mainly by aggradation rather than by shoreline progradation
( Bohacs et al., 2000 ). Accommodation within an underfilled lake itself is usu-
ally limited because of shallow lake waters. Lake levels fluctuate frequently in
response to climate variability and tectonic activity, so that shoreline deposits
may show extreme displacement across the basin. Lowstand deposition is char-
acterized by evaporite accumulation in the zones of maximum subsidence;
while in the remaining zones sediments that accumulated during the previous
highstand undergo desiccation, pedogenesis, and reworking ( Bohacs et al.,
2000, 2007a; Smoot, 1983 ). Well-mixed, shallow lakes in underfilled basins
may not experience density stratification, but deeper saline lakes are usually
stratified due to salinity or temperature differences between the epi-
limnion and the hypolimnion ( Renaut and Gierlowski-Kordesch, 2010 ). They
represent very stressful ecosystems, in which faunal diversity can be very
low (e.g., Hammer, 1986; Williams, 1998; Williams et al., 1990 ).
Salinity, which can also contribute to low oxygen levels, is the most impor-
tant limiting factor on biodiversity within and around lakes of underfilled
basins, leading to either the absence of, or impoverished expressions of, the
Mermia Ichnofacies in eulittoral to sublittoral zones ( Buatois and M´ngano,
2007 ). Trace fossils are not expected from profundal deposits. Suites of the
Mermia Ichnofacies are commonly of very low diversity, are mainly restricted
to grazing trails (e.g., Helminthoidichnites ), and are most common in short-
lived, ephemeral water bodies on low-energy lake-margin areas or in shallow
littoral to eulittoral lake waters ( Fig. 10 A; Scott, 2010 ). The patchy distribution
of high-density examples of the Mermia Ichnofacies is controlled by the local-
ized availability of relatively freshwater, either by spring-fed inputs or in
ephemeral pools fed by rain or overbank flows. Salinity-tolerant animals, that
survive within saline to hypersaline lakes (e.g., chironomid fly larvae), may pro-
duce high-density, low-diversity assemblages of diminutive vertical burrows
(e.g., Polykladichnus , Arenicolites , Skolithos ) and possibly meniscate-
backfilled burrows (small Beaconites ), in littoral to sublittoral zones with soft
substrates ( Ort ´ et al., 2003; Rodr ´ guez-Aranda and Calvo, 1998; Uchman and
´ lvaro, 2000 ). Deposits in littoral to eulittoral zones may also preserve high-
density, low-diversity assemblages of bird footprints and trample-grounds
produced within water-oversaturated, very shallow lacustrine substrates, attrib-
utable to the shorebird ( Lockley et al. 1994 ) or the Grallator Ichnofacies ( Hunt
and Lucas 2007 ).
Substrate consolidation is a main control on trace-fossil morphology in lake
margins of underfilled lake types. The presence of bioglyphs (e.g., striations)
and well-defined burrow margins and track morphology are evidence of firm-
ground development due to desiccation, early cementation of the substrate, or
exhumation of relatively well-indurated surfaces where net sedimentation rates