Environmental Engineering Reference
In-Depth Information
lake-level rise and fall, is evidenced by the traces (e.g., de Gibert and S´ez, 2009;
Uchman and ´ lvaro, 2000 ). These substrates are particularly valuable for rec-
ognizing stratigraphic surfaces (e.g., sequence boundaries, flooding surfaces) in
lacustrine successions (e.g., Buatois and M´ngano, 2009; Scott et al., 2009 ).
4. CONCLUSIONS
Trace-fossil assemblages show overlap between the overfilled, balanced-fill,
and underfilled lake types. It is the diversity within ichnofacies, the
substrate-controlled characteristics of the traces, and the lateral and vertical
distributions of ichnofacies and trace-fossil suites that provide important infor-
mation for the interpretation of sedimentary processes, depositional settings,
environmental conditions, hiatuses, and the architecture of the lake-basin
fill. By considering an integrated lake-type model, the understanding of the
depositional processes operating within a lake basin, and how they influence
the composition, distribution, and behavior of biological communities is
improved. Importantly, the lateral distribution and relative areal extent of
lake-basin zones ( Table 1 ) varies between the different lake-type basins, as does
the degree of lateral heterogeneity of environmental conditions and the
frequency of environmental change in an area through time.
The position within a basin, whether it is a basin-margin or basin-center
locality for example, is an essential consideration for applying ichnology to
lake-basin stratigraphy, and also for a better understanding of the potential lim-
iting controls (e.g., oxygenation, salinity) that influence the precise makeup of
an ichnofacies or suite. Although balanced-fill and underfilled lake types pre-
serve both softground and firmground expressions of the Scoyenia Ichnofacies
in lake margins, for example, these types of depositional environments are com-
monly present in basin-center areas if the basin is underfilled, but more typically
in basin-margin areas if it is a balanced-fill basin. Knowledge of the structural
setting of the studied locality can be important for helping to explain the var-
iability in environmental controls represented by different compositions of an
ichnofacies along strike, due to the influence of structural features on the dis-
tribution of springs and ephemeral streams or perennial rivers, for instance. The
frequency and extent of lake-level fluctuations varies between the lake types,
which influences not only the distribution of biogenic structures emplaced dur-
ing or shortly after sedimentation but also the degree of overprinting within
palimpsest substrates and the frequency of lake marginal to terrestrial trace
types present within lacustrine sediments.
The integration of the lake-type model with the ichnology of lake basins pro-
vides a framework from which to better understand the various expressions of
ichnofacies, including their composition, diversity, and distribution. Likewise,
the applicability of ichnology to lake-basin stratigraphy appears to be extensive,
through helping to recognize changing conditions and hiatuses as well as impor-
tant sequence-stratigraphic surfaces. More case studies that consider lake-basin
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