Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 341

by root traces that show more recurrence in time and space than any other ter-
restrial one. The indication of subaerial exposure of this ichnofacies (except for
root structures of swampy vegetation), and other inferences that can be made
from the type of roots, will be very valuable.
4. APPLICATION OF ICHNOFACIES AND ICHNOFABRICS TO
THE INTERPRETATION OF FLUVIAL SEDIMENTARY FACIES
The identification of key elements of an ichnofacies may help with the
sedimentological and paleoenvironmental interpretation of a lithofacies. Some
of the ichnofacies recognized in fluvial deposits (particularly archetypal
invertebrate ichnofacies) are useful for distinguishing processes or subenvir-
onments. The Scoyenia Ichnofacies develops on a substrate that is peri-
odically submerged and exposed, which is also reflected by the associated
physical sedimentary structures (e.g., mud cracks, raindrop imprints, water-
level marks, ripple marks, rill structures). The presence of the Scoyenia Ich-
nofacies may be used to delineate channel-bar tops or the margins of flood-
basin ponds, including temporary pools associated with crevasse-splay
processes. Buatois and M ´ ngano (2002, 2004) proposed that the Scoyenia Ich-
nofacies includes a “predesiccation suite” with structures lacking ornamenta-
tion (e.g., Taenidium , Beaconites ) and developed in a soft substrate and a
“desiccation suite” typified by trace fossils with striated walls (e.g., Scoyenia ,
Spongeliomorpha ) that cross-cut the former, suggesting firmer substrates. Pre-
desiccation and desiccation suites can be reliably distinguished only using
cross-cutting relationships between different ichnotaxa, whereas criteria such
as the relative degree of definition of burrow margins are less useful ( Melchor
et al., 2006 ).
The identification of the Mermia Ichnofacies, and probably also the Charac-
ichnos ichnocoenosis, suggests low-energy water-lain substrates that never
were exposed to air, a situation that is restricted in fluvial settings to floodbasin
ponds. Associated sedimentary structures reflect settling from suspension
(parallel-laminated mudstone) or weak currents (current-rippled fine-grained
sandstone or siltstone).
The identification of the Coprinisphaera , Celliforma , and Termitichnus
ichnofacies is indicative of soil development related to herbaceous communi-
ties, scrubs and woodlands, and closed forests, respectively. However, the
assignment of a trace-fossil assemblage to any of these ichnofacies requires
a detailed ichnotaxonomical assessment. The identification of an insect trace
fossil (chambers and chambered burrow systems) is an evidence that the hosting
sedimentary deposit was exposed subaerially ( Genise, 2004 ) and, consequently,
that it can be recognized as a paleosol ( Retallack, 2001 ).
The Skolithos Ichnofacies in fluvial settings can be related to the tops of
emergent channel bars or sandy crevasse splays that were colonized by insects.
In consequence, it may mark variations in fluvial channel discharge. A similar
interpretation can be applied to occurrences of the Grallator and Batrachichnus