Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 350

participation of arthropod trackways, and the apparent restriction of bird tracks
and complex sinusoidal trails to overbank facies ( Fig. 8 B).
5.2 Contrasting Trace-Fossil Preservation in Channel-Belt and
Overbank Facies
The differences in ichnodiversity and abundance of trace fossils from channel
and overbank facies are related to contrasting taphonomic pathways. Channels
are the loci of strong currents that typically erode previously deposited sedi-
ments producing truncation surfaces of different hierarchy that are typical of
channel belts. Even in ephemeral rivers or creeks, the traces formed in channel
facies have almost zero preservation potential, as shown by observations in the
ephemeral Atuel River near Algarrobo del ´ guila town, La Pampa province,
Argentina ( Fig. 9 A). The low-sinuosity channel belt shows point and longitu-
dinal bars ( Fig. 9 A-C). The climate is temperate semiarid, and the river is dry
for most of the year. The observations were conducted every 2 weeks from
September 2008 to March 2009, at the surface and in shallow pits dug in the
channel bottom and a longitudinal bar. The observations include two intervals
with water currents (September to October 2008 and December 2008 to January
2009) and two dry intervals (end of November 2008 and February to March
2009). During the dry intervals, the discharge decreased and the remaining
water was concentrated in small ponds in the river bed. The generation of shal-
low horizontal mole-cricket burrows, bird and tetrapod tracks, and plant colo-
nization of channel floors ( Fig. 9 E) was observed in channel floors and
longitudinal bars. Wind ripples ( Fig. 9 F), laminated mudstones in pools, mud
cracks, and raindrop imprints were also recorded. In the section logged inMarch
2009, there were no deposits of the dry intervals in the longitudinal bar pit and
an erosive surface marked the erosion that occurred during the January 2009
current. In the neighboring channel floor pit, a thin mudstone intercalation is
the only indication of the dry periods ( Fig. 9 D). None of the observed biogenic
structures were preserved in the analyzed sections. When buried under younger
sediments, an adequate interpretation of this channel belt will be aided by an
architectural-element analysis. This example also suggests that the absence
of trace fossils in fluvial channel floors does not necessarily imply a permanent
discharge.
In a compilation of the facies distribution of different track types from 50
localities of the Jurassic Morrison Formation, Foster and Lockley (2006) found
different abundances of sauropod and theropod tracks. Sauropod tracks are
more abundant in floodplain mudstone, whereas theropod tracks are dominant
on the upper surfaces of sandstone formed as bars or crevasse-splay deposits.
Sauropod tracks are preserved in floodplain mudstone as natural sandstone casts
(some deeply impressed) connected to sandstone overhangs that cover flood-
plain mudstone. Although a minor paleoenvironmental distinction between
both types of tracks is possible, Foster and Lockley (2006) suggested that