Lacustrine Environments - Trace Fossils as Indicators of Sedimentary Environments - page 378

and changing sedimentary processes where sediments themselves are not record-
ed. With increasing frequency and/or duration of periods of subaerial exposure,
the production of overprinted ichnofacies (or suites) within a single substrate
becomes more likely. In contrast to the “composite ichnofacies” model of
Keighley and Pickerill (2003) , the recognition of overprinted ichnofacies in a
single substrate aids the recognition of dynamic, changing environmental
conditions (e.g., de Gibert and S´ez, 2009; Metz, 1996; Scott et al., 2009; Uchman
and ´ lvaro, 2000 ). Where sedimentation rates are higher, changing sedimento-
logical and ichnological features through a stratigraphic succession signify
changing environmental conditions through time.
2.2 The Ichnofacies Model in Lake Basins
Three main ichnofacies that characterize continental environments through
geological time are widely accepted ( Mermia , Scoyenia , Coprinisphaera ). In
addition, the recently proposed Celliforma Ichnofacies qualifies as a Seilacher-
ian ichnofacies ( Genise et al., 2010 ). The Termitichnus Ichnofacies sensu
stricto requires additional examples ( Genise et al., 2010 ), while the eolian
Octopodichnus - Entradichnus Ichnofacies is in need of refinement considering
the shared features in Paleozoic and post-Paleozoic examples ( Buatois and
M ´ ngano, 2011; Ekdale et al., 2007; Hunt and Lucas, 2007 ). Determining
the significance of each of the continental ichnofacies and recognizing new
ichnofacies remain major goals for continental ichnology, and the state of
the ichnofacies model in continental environments presently is in flux.
Although ichnofacies are associated with sets of environmental controls char-
acteristic of particular depositional settings, trace-fossil types may be present in
several environments of a lake basin. The disjunction in time between deposition
and biogenic activity by terrestrial organisms in pedogenically modified sub-
strates causes the traces produced to be more closely associated with paleosol fea-
tures than a particular depositional environment ( Genise et al., 2000, 2004;
Hasiotis, 2007 ). In subaerially exposed fluvio-lacustrine and lake-margin settings,
which may be frequently flooded, pedogenesis is typically very weak. Thus, the
ichnofacies that characterize this zone are typically more closely associated with
the depositional conditions and particular lithofacies.
2.2.1 The Mermia Ichnofacies
The Mermia Ichnofacies ( Fig. 2 ), formally proposed by Buatois and M ´ ngano
(1995, 1998) , dominantly comprises simple grazing trails and shallow, simple
infaunal deposit-feeding burrows associated with permanently subaqueous,
oxygenated, low-energy, freshwater lakes, represented by thinly laminated,
sometimes heterolithic, fine-grained deposits. Subordinate features include
insect trackways and bilobate structures (e.g., Cruziana ). This ichnofacies
has subsequently been recognized in other environments, including short-lived