Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 352

wetland facies of the Maastrichtian of Canada ( Nadon, 1993 ). In this example,
herbivorous dinosaur tracks are deeply impressed (to 85 cm) and carnivorous
dinosaur tracks are shallowly impressed, suggesting that carnivorous dinosaurs
ventured into the wetlands only during the dry season.
Trace fossils display a better preservation potential in overbank areas, par-
ticularly in pond and crevasse-splay facies. These different preservational con-
ditions are probably linked to their particular sedimentary dynamics. After peak
discharge and consequent breaching of channel margins, large volumes of
sandy bedload are dumped on the flanking floodplains. The damp emergent
crevasse-splay sediments that cover a large area proximal to the river channel
are a very suitable substrate for track production and preservation. Trace pres-
ervation is enhanced by a muddy veneer that accumulates on the sand surface,
commonly on former swales where water ponded temporarily.
5.3 Trace Fossils in Channel-Belt Deposits
Most of the trace-fossil assemblages in channel facies occur on the top of, or in
the upper part of, fluvial bars, although trace fossils can also appear in inactive or
abandoned channels, bottom of channels, and between deposits of channel bars.
Many assemblages of inactive or abandoned channels have been interpreted as a
channel-related pond formed after abandonment of the active channel (e.g.,
Buatois et al., 2007; Głuszek, 1995; Kim et al., 2002; Tandon and Naug,
1984 ). The presence of rhizoliths and mud cracks at the bottom of the main chan-
nel indicates ephemeral or intermittent river discharge (e.g., Bridge, 2003, 2006 ).
However, other trace-fossil types than rhizoliths can be useful to infer discontin-
uous channel discharge, taken isolated or in conjunction with physical sedimen-
tary structures, if found at the bottom or between channel bars. For example,
diversemammal (rhinoceros, entelodontid, and camel) footprintswere described
from the base ofOligocene channel deposits of theUnited States ( Chaffee, 1943 ),
equoid footprints ( Plagiolophustipus ) occur at the bottom of Oligocene
meandering-channel deposits in Spain ( Astibia et al., 2007 ), and sauropod dino-
saur tracks on the sole of Maastrichtian anastomosed-channel sandstone in Utah,
United States ( Difley and Ekdale, 2002 ). In addition, the presence of burrows and
rhizoliths at the base of a volcaniclastics-rich channel-fill deposit from the lower
member of the Cretaceous Bajo Barreal Formation of Argentina ( Fig. 10 ) was
also used to suggest an ephemeral character ( Umazano, 2009 ). The lower part
of some channel-belt deposits of this formation exhibits a highly bioturbated
interval ( Fig. 10 A and B) that displays an ichnofabric dominated by simple bur-
rows ( Fig. 10 C) and small rhizoliths ( Fig. 10 D). These examples require an initial
river incision and subaerial exposure of sediment or ponding of water in the river
bed. In the case of the Cretaceous Bajo Barreal Formation, the ponded interval
allowed for the development of a paleosol in the river bed.
Trace-fossil assemblages that were preserved between channel bars are also
useful for paleoenvironmental interpretation of these facies ( Dalla Vecchia and
Rustioni, 1996; Melchor, 2009; Melchor et al., 2010; Williamson and Lucas,