Lacustrine Environments - Trace Fossils as Indicators of Sedimentary Environments - page 389

Higher energy shoreface to foreshore and fluvio-deltaic areas of overfilled
lakes are characterized by low-density, indistinct vertical burrows ( Skolithos ),
escape traces, and U-shaped burrows ( Arenicolites ) produced in soft substrates
( Figs. 5 and 6 A-D; Bohacs et al., 2007b; Buatois and M´ngano, 2004; Voigt
and Hoppe, 2010 ). Areas of rapid sedimentation near the mouth of distributary
channels tend not to be burrowed or are sparsely bioturbated. This assemblage is
closely comparable to the Skolithos Ichnofacies of marine nearshore settings.
High-density simple burrows ( Planolites , Palaeophycus ) may be common
along horizons representing short-term periods of quiescence associated with
the deposition of plant material ( Fig. 6 A-C). Gastropod and bivalve coquinas
are also common components ( Bohacs et al., 2000 ). Sediments deposited in quie-
ter waters of the littoral zone may also preserve meniscate-backfilled burrows
(e.g., Taenidium ), arthropod trackways (e.g., Diplichnites ), bilobate trails
( Cruziana ), and bivalve resting traces ( Lockeia ).

3.2 Balanced-Fill Lake-Type Basins
Balanced-fill lake-type basins ( Fig. 7 ) are characterized by dominantly “fluc-
tuating profundal” facies associations, representing low-energy sublittoral to
profundal zones with fine-grained carbonate and siliciclastic deposits and high
levels of organic carbon preserved in stratified lake deposits (e.g., oil shale)
( Bohacs et al., 2000; Carroll and Bohacs, 2001 ). In balanced-fill lake-type
basins, the input of sediment and water is approximately equal to the accommo-
dation potential. This relationship fluctuates depending on climatic and tectonic
controls, and with changing drainage captures from upstream basins ( Carroll
et al., 2008 ). These types of lakes periodically overflow into an adjacent basin,
with lake levels typically at or near the elevation of the basin-bounding sill
( Bohacs et al., 2003 ). During periods when the lake is hydrologically closed
and lake levels are below the sill, salinity may become elevated, the lake waters
may become stratified and oxygen depleted, and carbonate and siliciclastic muds
accumulate in aggradational packages in the basin center. Lake levels can fluc-
tuate due to minor climatic variations, with widespread subaerially exposed areas
(depending on the gradient) and rarely, balanced-fill lakes may dry up almost
completely. During periods when the lake is hydrologically open, lake waters
become fresher, better oxygenated, and progradational packages of sediments
are deposited at the basin margins where lake levels are stable.
Trace fossils produced in balanced-fill basins appear to reflect these variable
abiotic conditions and may be closely associated with particular lithofacies asso-
ciations ( Fig. 7 ). Salinity, substrate, and oxygen are the main limiting factors in
balanced-fill lakes. Salinity plays a main role during periods when the lake is
hydrologically closed. Saline conditions (

20-50 g/L) are detrimental to lacus-
trine faunas (e.g., benthic invertebrates, fish), and density stratification between
bottom waters and freshwater inflow is more likely, leading to low-oxygen con-
ditions in sublittoral to profundal zones (e.g., Cohen, 2003; Hammer, 1986 ).