Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 345

4.3 Ichnofabric Studies of Fluvial Successions
In recent years, the ichnofabric approach has slowly expanded frommarine case
studies to continental examples, including fluvial successions. Much progress
has been attained in the field of paleosol ichnofabrics, and a conceptual and
methodological framework for their analysis is now available ( Bedatou et al.,
2009; Genise et al., 2004 ). Bedatou et al. (2009) described four recurrent cray-
fish and earthworm-dominated complex paleosol ichnofabrics from Late Juras-
sic to Early Cretaceous volcaniclastic deposits of Patagonia. In comparison with
paleosol ichnofabrics, those subaquatic ichnofabrics from channels, crevasse
splays, and floodbasins are much simpler. The most typical fluvial ichnofabric
consists of meniscate burrows (typically Beaconites or Taenidium ) in aban-
doned channel and overbank deposits. Morrissey and Braddy (2004) provided
a detailed study in the Devonian of Wales and suggested that this ichnofabric
records colonization of subaerially exposed sediment as a response to seasonal
desiccation, with animals excavating the substrate to aestivate or molt at water-
table level. Buatois et al. (2007) documented this ichnofabric in channel and
crevasse-splay sandstone and floodplain mudstone, the latter being pervasively
bioturbated. In this case, the depth and intensity of bioturbation of the main-
channel and crevasse sandstones are a reflection of the time between deposi-
tional events (i.e., the colonization window). Main-channel and crevasse sand-
stones underlying thick packages of bioturbated overbank mudstone are
thoroughly bioturbated, as a response to prolonged periods of low-energy sedi-
ment fallout between crevassing events. On the other hand, the lowest degree of
bioturbation is detected in amalgamated channel sandstones underlying thin
intervals of overbank mudstone, revealing short colonization windows due to
high-frequency depositional episodes. The ichnofabric of meniscate burrows
has been recognized in both outcrop and core ( Fig. 7 A-C).
5. DEPOSITIONAL ENVIRONMENTS
In this section, we will first examine the broad differences between channel-belt
and floodplain trace-fossil assemblages and discuss the possible reasons for this
contrast. We will then examine the composition of trace-fossil assemblages of
channel-belt and floodplain subenvironments, and different river types.
5.1 Comparison of Channel-Belt and Floodplain
Trace-Fossil Assemblages
The existing information on trace-fossil assemblages of fluvial channels and
floodplain deposits suggests a dominance of very simple trace-fossil types,
mainly of invertebrate origin ( Fig. 8 ). This dominance of invertebrate trace fos-
sils may be a reflection of greater diversity of invertebrate tracemakers. In both
settings, the trace-fossil types are essentially similar: more than half of the
recorded trace-fossil types are (in approximate order of abundance) meniscate