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ichnofacies on top of channel bars, although these ichnofacies are also present in
floodplain settings (crevasse splays and floodbasin ponds). An alternative interpre-
tation for the Skolithos Ichnofacies is that it represents colonization of fluvial bars in
a high-energy setting.
The Hatcherichnus and Chelonipus ichnocoenoses may be indicative of per-
manent subaqueous substrates deposited under moving water (lotic settings),
particularly in channel facies. No change in fluvial discharge is implied by
the presence of these swim-trace fossils in main channel-fill deposits. The
meaning of the Brontopodus Ichnofacies for sedimentological analysis of
fluvial successions is at present unclear.
Retallack (1990, 2001) , Bockelie (1994) , Kraus and Hasiotis (2006) , and
Genise et al. (2011) identified different morphologies of root traces that can
be linked to well drained (deep water table), more poorly drained (intermediate
water table), and poorly drained (shallow water table) paleosol profiles (see also
Cohen, 1982 ). Different types of root traces would increase the value of a poten-
tial rhizolith ichnofacies as a paleoenvironmental indicator.
4.1 Composite Ichnofacies
The recognition of significant participation of elements of two ichnofacies in a
specific sedimentary facies may be useful for the refinement of the facies anal-
ysis in fluvial successions. These examples can be considered as a mixture of
two ichnofacies (e.g., the Mermia and Scoyenia ichnofacies) or a composite
ichnofacies ( Buatois and MĀ“ngano, 2002, 2004; Keighley and Pickerill, 2003;
Kim et al., 2005 ). Another possible example is the change or overprinting of
trace-fossil assemblages that characterize the Scoyenia and Celliforma
ichnofacies with increasing degree of subaerial exposure ( Genise et al.,
2010 ). In a floodplain pond, short periods of emergence punctuated by flooding
are common. In this scenario, a trace-fossil assemblage typical of the Scoyenia
Ichnofacies may develop. If the time of subaerial exposure was long enough to
provide the well aerated and bare soils that hymenopterans need to nest ( Genise,
2004 ), the previous assemblage might be replaced or overprinted by that typical
of the Celliforma Ichnofacies ( Genise et al., 2010 ).
These composite or mixed assemblages hold the potential of reflecting sub-
tle environmental changes that commonly cannot be inferred from physical
sedimentary structures alone. The recognition of ichnocoenoses potentially
representative of composite Mermia - Scoyenia Ichnofacies has been documen-
ted in modern pointbars and bars in braided channels ( Lawfield and Pickerill,
2006; Martin, 2009 ), which are essentially composed of simple horizontal
trails ( Mermia Ichnofacies representatives) and vertebrate tracks (purported
Scoyenia Ichnofacies component). The potential of preservation of these
ichnocoenoses is low, as they can be destroyed due to desiccation after expo-
sure or fluvial reworking during periods of peak-flow discharge. A similar
trace-fossil assemblage was described from Oligocene fluvial ponds of
Switzerland ( Uchman et al., 2004 ) and type 1 paleosurfaces from Permian
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