Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 359

The current knowledge on fluvial ichnofabrics is very limited, although
ichnofabric studies on continental settings have a large potential for aiding
paleoenvironmental, stratigraphical, and paleoecological studies. Application
of a definite descriptive methodology may be used to recognize particular ich-
nofabrics in different fluvial subenvironments and to link the composing trace
fossils with their potential producers. Paleosol ichnofabrics exhibit a greater
complexity and variety of trace fossils than in other fluvial subaquatic suben-
vironments. In consequence, they hold a larger potential for paleoenvironmen-
tal and paleoecological studies.
Channel-belt and floodplain facies exhibit a dominance of very simple trace-
fossil morphologies, essentially of invertebrate origin. The abundance and
diversity of trace fossils is much higher in overbank facies than in channel-belt
deposits. These differences are linked to contrasting taphonomical conditions in
both settings. Coarse-grained channel deposits are mostly devoid of trace fos-
sils, which are concentrated in specific intervals and bedding planes (mostly bar
tops and abandoned-channel fills). Rhizoliths and/or tetrapod tracks (that might
be associated with mud cracks and/or mud veneers) found at the bottom of chan-
nel deposits or between channel bars are good clues to intermittent or ephemeral
channel discharge. However, the low preservation potential of these features in
a modern ephemeral creek suggests that their absence does not imply permanent
discharge, as commonly assumed.
Floodplain ichnofaunas are more abundant and diverse than channel-belt
ichnofaunas. Trace-fossil assemblages produced under subaerial conditions,
such as those of pedogenized floodplains, are quite different from those
showing no or minimal subaerial exposure, such as pond and crevasse splays.
Pedogenized floodplain (paleosol) trace-fossil assemblages are typified by
chambers and chambered burrow systems, rhizoliths and other root structures,
burrows with pelletal fill, large vertebrate burrows, and vertebrate coprolites,
whereas vertebrate tracks are rare or absent. Pond deposits commonly contain
simple horizontal trails, complex sinusoidal trails, tetrapod swim traces, bird
tracks, arthropod trackways, trough-like reliefs, and bilobed burrows/trails.
Crevasse-splay trace-fossil assemblages do not show clear distinguishing fea-
tures, although tetrapod-track preservation is enhanced in this setting.
Trace-fossil assemblages from anastomosed, braided, meandering, and
ephemeral rivers display some differences in composition and abundance, espe-
cially when considering floodplain ichnofaunas. Anastomosed rivers appear to
preserve trace fossils preferentially in channel facies, where bird tracks and large
dinosaur (sauropod) tracks are more common. The trace fossils from ephemeral
and braided deposits can be distinguished from those of meandering rivers by the
larger proportion of structures preserved on bedding planes, including arthropod
trackways and other arthropod-produced traces (bilobed burrows/trails, trough-
like reliefs), Q 4/5, Q 5/5 sl, Q 5/5 st, Q 2/2, and chirotheriid tracks. Meandering
rivers contain a larger proportion of trace fossils related to pedogenized flood-
plain settings (e.g., rhizoliths, burrows with pelletal fills, large vertebrate