Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 354

Not all trace fossils that occur within or at the base of fluvial channel
deposits are indicative of ephemeral flow. Two trace-fossil assemblages from
the Upper Jurassic of United States ( Foster and Lockley, 1997; Lockley and
Foster, 2006 ), involving essentially swim traces in channel facies, are good
examples of alternative interpretations. Foster and Lockley (1997) described
purported crocodile tracks ( Hatcherichnus sanjuanensis ) associated with pos-
sible tail-drag traces at the base of a cross-bedded sandstone set that lies 6 m
above the base of a 12-m-thick channel deposit. In this example, inferences
about the producer (habitat of crocodiles) and indications of possible swim-
ming or basking behavior, in addition to the absence of emergence and/or des-
iccation features, suggest that river discharge was essentially uninterrupted.
The remaining example from the Upper Jurassic of the United States includes
vertebrate tracks from the base and lower part of a channel complex ( Lockley
and Foster, 2006 ).
Another example, where trace fossils of fluvial-channel deposits give some
indication about changing paleoenvironmental conditions, was described by
Hubert and Dutcher (2010) . These authors found several tens of oriented subhori-
zontal meniscate burrows at the top of a trough cross-bedded set of pebbly sand-
stone from the Triassic of the United States, which were interpreted as reflecting
escape traces, probably in response to a rising water table.
5.4 Floodplain Subenvironments
Most of the analyzed overbank ichnofaunas are assigned to three settings: cre-
vasse splay, floodbasin pond, and the pedogenized floodplain. There are a few
cases of assemblages from levee deposits ( Groenewald et al., 2001; Hasiotis and
Honey, 2000; Kim et al., 2002; Tandon and Naug, 1984 ), which are herein sub-
sumed under pedogenized floodplain. The last setting is represented by a range
of floodplain and channel-belt deposits that exhibit different degrees of devel-
opment of pedogenic features.
The floodplain ichnofaunas share various simple invertebrate burrow types
with a roughly similar proportion in each subenvironment (horizontal burrows,
burrow systems, meniscate burrows, simple vertical burrows, and J-, U-, or
Y-shaped burrows; Fig. 11 ). However, the floodplain ichnofaunas of these
subenvironments can be easily discriminated.
Pedogenized floodplain trace-fossil assemblages are particularly distinctive.
They are characterized by chambers and chambered burrow systems (e.g., Copri-
nisphaera , Rebuffoichnus , Cellicalichnus , Termitichnus ), rhizoliths and other
root structures, burrows with pelletal fill (e.g., Edaphichnium , Castrichnus ), large
vertebrate burrows (e.g., Alezichnos , Daimonelix , L, and G-type burrows), and
vertebrate coprolites. These trace fossils are characteristic of ichnofacies that
occur in paleosols ( Coprinisphaera , Celliforma , Termitichnus , and other potential
ichnofacies discussed previously). J-, U-, or Y-shaped vertical burrows ( Lolo-
ichnus , Camborygma , Capayanichnus , Macanopsis ) and perforations in the wall
of chambered trace fossils ( Lazaichnus and Tombownichnus ) can also be present.