Lacustrine Environments - Trace Fossils as Indicators of Sedimentary Environments - page 386

typically indistinct and are mainly progradational due to relatively constant lake
levels that are controlled by the sill over which lake waters flow out of the basin.
From a biological perspective, open lakes represent less stressful, more stable
ecosystems that tend to support a diverse fauna in relatively stable lakes with
low salinity and relatively high nutrient inputs (e.g., Buatois and M´ngano,
2004, 2007, 2009; Gierlowski-Kordesch and Park, 2004; Gore, 1989 ). Sedimen-
tary environments of overfilled basins are characterized by (1) alluvial systems
with channel avulsion, high water tables, and abundant vegetation (in older
sediments: peat or coal deposits); (2) deltaic deposits; and (3) relatively
well-oxygenated, sublittoral to profundal zones ( Bohacs et al., 2000 ).
The most important controls on trace-fossil distribution in permanent sub-
aqueous areas of overfilled lakes are relatively high oxygen contents of lake
waters, energy, stable food supply and abundant nutrients, and generally soft
to soupy substrates ( Buatois and M ´ ngano, 2007, 2009 ). Cold conditions
may also be an important control in glacial lakes. Although overfilled lakes
are not salinity stratified, deep overfilled lakes may be stratified due to temper-
ature differences between surface waters and deep waters ( Cohen, 2003 ).
Fluvial discharge into overfilled lakes may trigger underflow currents that oxy-
genate lake bottoms and which may contribute to the turnover of a stratified
water body. Resedimentation (e.g., slumping) of material supplied by the fluvial
system may also promote bottom oxygenation, which allows the establishment
of epifaunal and infaunal communities ( Buatois and M´ngano, 1998 ). Abundant
organic detritus derived from land plants is brought to overfilled lakes by fluvial
systems ( Bohacs et al., 2000; Carroll and Bohacs, 2001 ) and may be introduced
into deep lake waters by underflows and turbidity currents, increasing the
food supply for infaunal deposit-feeding and grazing communities. High sedi-
mentation rates, high water tables in subaerially exposed lake-marginal areas,
and unstable shallow lacustrine substrates are also expected to influence trace-
fossil assemblages by precluding the presence of trace types produced in firm
substrates ( Bohacs et al., 2007b; Buatois and M´ngano, 2004 ).
The Mermia Ichnofacies characterizes the low-energy, permanent subaqueous
zones (sublittoral, profundal) of overfilled lakes ( Figs. 5 and 6 ; Buatois and
M ´ ngano, 2009 ). Simple grazing (e.g., Gordia ) and sediment/water interface feed-
ing (e.g., Vagorichnus ) trace types are commonly dominant ( Fig. 6 E and F), with
fish swim traces (e.g., Undichna ), arthropod trackways (e.g., Diplichnites ,
Glaciichnium ), and shallow simple burrows and trails (e.g., Planolites , Palaeophy-
cus ) also being present ( Benner et al., 2009; Buatois et al., 1995, 1996; Netto et al.,
2012; Uchman et al., 2009 ). In oxygenated lakes, examples of the Mermia Ichno-
facies are typically of moderate to high diversity, with a range of behavior and
trophic levels represented (e.g., grazers, detritus feeders, deposit feeders, preda-
tors). The relative simplicity of ichnofaunas from low-energy, permanently sub-
aqueous lacustrine zones (i.e., Mermia Ichnofacies) is in sharp contrast with
fully marine ichnofaunas (e.g., Cruziana and Nereites ichnofacies), even in
well-oxygenated waters with abundant and consistent

food supplies. No