Environmental Engineering Reference
In-Depth Information
During periods when lake levels are near but below the sill height, shore-
lines around balanced-fill lakes may fluctuate, leading to the frequent flooding
and subaerial exposure of the eulittoral zone and the presence of the Scoyenia
Ichnofacies in low-energy areas ( Fig. 8 ). In contrast to underfilled basins,
these settings are typically near the basin margins. Horizontal simple and
branching structures produced in softgrounds (e.g., Planolites , Taenidium )
and firmgrounds (e.g., Scoyenia , Spongeliomorpha ) are common (e.g., Brom-
ley and Asgaard, 1979; Gierlowski-Kordesch, 1991 ), and vertebrate footprints
may be preserved. During periods when lake levels are well below the sill
height, desiccation of lacustrine and lake-margin sediments is pervasive
and firmgrounds become widespread. These deposits tend to host the Scoyenia
Ichnofacies. In particular, the firmground suite of this ichnofacies, containing
striated trace fossils or vertically oriented structures, may help to delineate
parasequences through the recognition of subaerial exposure ( Fig. 8 F).
High-energy, wave-influenced carbonate shorelines of balanced-fill lake
types preserve stromatolitic bioherms, which may be encrusted by caddisfly
cases ( Leggitt et al., 2007 ), and/or preserve embedment structures of organisms
dwelling within the bioherms ( Lamond and Tapanila, 2003 ). Wave-influenced
siliciclastic littoral zones may preserve high-density examples of simple bur-
rows ( Planolites ), which appear to be mainly present during relatively
freshwater phases or in well-mixed areas of balanced-fill lakes. Similar litho-
facies of the littoral to sublittoral zones that were deposited during saline phases
or in poorly mixed lake waters do not preserve evidence of infaunal activity.
During highstand progradation of deltaic systems, elements of the Skolithos Ich-
nofacies may occur in delta mouth-bar deposits ( Bromley and Asgaard, 1979;
MĀ“ngano et al., 1994, 2000 ).
Although freshwater conditions and softgrounds are more common during
high lake levels, dysaerobic conditions may prevail in poorly mixed, deeper
lake waters, imparting a stress factor on lacustrine biotas. As a result, ichnofaunas
from turbidite systems in balanced-fill lakes are less abundant and diverse than
those from overfilled lake turbidites, if present at all (e.g., Uchman et al., 2007 ).
Low-diversity, low-abundance examples of simple grazing trails or fish swim
traces ( Undichna ; Fig. 8 A) of the Mermia Ichnofacies are present in laminated,
fine-grained lacustrine sediments, which reflect low-oxygen conditions of the
sublittoral to profundal zones ( Magyar et al., 2006; Uchman et al., 2007 ). Delta
front deposits in these stratified lakes may preserve a low-diversity assemblage
of the Mermia Ichnofacies ( Melchor, 2004 ). During periods of better oxygenation
of lake waters, the Mermia Ichnofacies may increase in diversity, with a variety of
trace types including grazing trails, deposit-feeding burrows, and arthropod
trackways and trails associated with the delta front ( Melchor, 2004 )todistal
prodelta ( Fig. 8 B; de Gibert et al., 2000 ). In some cases, vertical burrows may
occur in sandy tempestites interbedded with mudstones and appear to repre-
sent the temporary oxygenation of the lake bottom due to storms ( Magyar
et al., 2006 ).
Search WWH ::




Custom Search