Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 356

facies seems to have a better preservation potential of vertebrate tracks (e.g.,
Fuglewicz et al., 1990; Melchor et al., 2010; Smith, 1993 ). In a detailed study
on the ichnotaxonomy and facies distribution of vertebrate tracks from Early
Triassic fluvial rocks of Poland, Fuglewicz et al. (1990) recognized that 64%
of the footprints appear in a distal crevasse-splay facies, 23% in a proximal
crevasse-splay facies, and only 13% in channel bars.
Some of the trace-fossil assemblages from crevasse-splay deposits contain
only, or are dominated by, arthropod trackways (mostly Diplichnites ). These
examples range in age from Devonian to Triassic (e.g., Briggs et al., 1984;
Lucas et al., 2006; Neef, 2004 ). Smith (1993) found that the arthropod trackway
Umfolozia was characteristically linked to proximal crevasse-splay settings in
the Permian Beaufort Group, but this possible link between facies and a definite
ichnotaxon has not been recorded in other lithostratigraphic units.

5.5 Comparison of Trace-Fossil Assemblages from
Different River Systems
In this chapter, we have adopted a division of fluvial systems in braided, meander-
ing, anastomosed, and ephemeral rivers. Braided and meandering channels may
be recognized objectively as end members of a fluvial channel spectrum. Since
braided and meandering rivers may have a permanent or intermittent discharge,
the division of fluvial systems adopted herein, especially the consideration of
ephemeral rivers as a distinct category, is not completely objective but reflects
the characterization of the paleoenvironment proposed by the various authors.
Architectural element analysis of fluvial successions accompanying the descrip-
tion of trace-fossil assemblages is very rare in the literature, Zavala et al. (1993)
and Krapovickas et al. (2009) being some exceptions.
The database includes only nine trace-fossil assemblages from anastomosed
river-deposits (e.g., Difley and Ekdale, 2002; Johnson, 1986; Marriott et al., 2009;
Nadon, 1993 ), and for this reason, they are analyzed separately. The majority of
trace fossils in anastomosed river-deposits occurs in channel and floodbasin-pond
deposits, and there is no description of assemblages from pedogenized-overbank
deposits. Channel deposits contain simple trails and horizontal and meniscate
burrows, but the most remarkable components are tetrapod tracks, particularly
bird tracks and large dinosaur (sauropod) tracks. Nadon (1993) suggested that
large tetrapod tracks have a higher potential of preservation in anastomosed
river-deposits because of the abundance of soft substrates to record the tracks
and the occurrence of annual flooding that rapidly buries the footprints.
A comparison of the trace-fossil assemblages from meandering, braided,
and ephemeral river-facies ( Fig. 12 ) allows recognizing some general differ-
ences between them. The trace-fossil assemblages that were not assigned orig-
inally to a specific river system were excluded from this analysis ( n

¼

42; see
Supplementary Material in http://booksite.elsevier.com/9780444538130 ) .
There are a number of simple trace-fossil morphologies that exhibit a roughly
similar proportion in every river type, namely, simple horizontal trails,