Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 326

In particular, the Heteropolacopros and Dicynodontocopros ichnofacies were
proposed for fluvial successions ( Hunt et al., 2007 ).
In the next paragraphs, the expression in fluvial deposits of the previously
mentioned ichnofacies or ichnocoenoses is analyzed using selected fluvial
trace-fossil assemblages from the literature ( Table 2 ; Fig. 1 ). Trace-fossil
assemblages are included within ichnofacies using the dominant trace fossils
and not by means of a single or subordinate component (opposite to the proposal
by Keighley and Pickerill, 2003 ). Some trace-fossil assemblages are too simple
to be assigned with confidence to a single ichnofacies or cannot be satisfactorily
fitted within the existing ichnofacies scheme (both accounting for about 20% of
the analyzed trace-fossil assemblages). Some of the trace-fossil assemblages
can be considered the work of a coeval community of organisms or ichnocoe-
noses in the sense of Bromley (1990, 1996) , but in most cases, this is uncertain.

3.1 Ichnofacies

Most of the trace-fossil assemblages surveyed (62 cases, 33%) can be assigned
with confidence to the Scoyenia Ichnofacies ( Fig. 1 A). These assemblages have
been identified as belonging to this ichnofacies because of the dominance of
meniscate burrows ( Fig. 2 A and F) and/or the presence of arthropod trackways
( Fig. 2 B), burrow systems ( Fig. 2 C and E), trough-like reliefs ( Fig. 2 D), and
bilobed burrows/trails (e.g., Bromley and Asgaard, 1979; MacNaughton and
Pickerill, 1995; Melchor et al., 2006; Morrissey and Braddy, 2004; Stanley
and Fagerstrom, 1974 ). Assemblages containing only vertebrate tracks, but
lacking any of the diagnostic components mentioned above, cannot be com-
pared satisfactorily with the Scoyenia Ichnofacies and are treated separately
in this chapter. Individual Scoyenia assemblages usually exhibit a low ichno-
diversity, averaging 5 and ranging from 1 to 12 trace-fossil types. They occur
in similar proportions in both in fluvial channel belts and overbank settings
(mainly crevasse splay and floodbasin ponds). Considering the different river
types, these assemblages are more common in braided and ephemeral river sys-
tems. The Scoyenia Ichnofacies reflects a superposition of suites of trace fossils
associated with changes in substrate consistency and water saturation, generally
coupled to water-level changes (compare Fig. 2 E). Additionally, assemblages
dominated or only composed of meniscate burrows from poorly developed
and/or Paleozoic paleosols (e.g., ichnocoenosis G of Keighley and Pickerill,
2003 ) can also be included in this ichnofacies.

Scoyenia

3.2 Ichnofacies

Some of the assemblages that were used to propose the Coprinisphaera Ichno-
facies are from fluvial deposits ( Genise et al., 2000 ), including the well-known
examples from the Eocene Asencio Formation of Uruguay (e.g., Genise et al.,
2004 ), the Paleocene-Eocene Claron Formation of United States ( Bown et al.,

Coprinisphaera