Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 344

Another example comes from the Early Permian of New Mexico ( Hunt
et al., 2005 ). In this case, different paleoenvironmental settings are represented
by ichnocoenosis assigned to the Batrachichnus and Brontopodus ichnofacies.
Three ichnocoenoses are distinguished on the basis of the dominant ichnotaxon,
namely Ichniotherium in inland/distal alluvial-fan settings, Amphisauropus in
alluvial-plain settings, and Batrachichnus and Dimetropus in coastal/tidal-flat
settings ( Hunt et al., 2005 ).
Recognition of trace fossils typical of the Scoyenia and other ichnofacies has
been used in large-scale stratigraphic studies since its original proposal by
Seilacher (1967) . In an example that involves fluvial facies, Siggerud and
Steel (1999) recognized a number of marine and continental ichnofabrics in
an Eocene fan-delta complex from Spain. The identification of an ichnofabric
dominated by meniscate ( Scoyenia , Taenidium ) and vertical burrows
( Skolithos ) in red siltstone, mudstone, and fine-grained sandstone helped in
the identification of sequence boundaries.
The application of ichnology in sequence stratigraphy of fluvial succes-
sions is still in its infancy, although a general model has been proposed
( Buatois and M ´ ngano, 2004, 2007 ). According to this model, the distribu-
tion of trace fossils in fluvial successions most likely reflects changes at the
scale of depositional systems which, in turn, may be related to the stacking
of system tracts. During the early lowstand systems tract, widespread ero-
sion, high energy, and high sedimentation rates are conducive to channel
amalgamation and extensive reworking of fluvial deposits, conditions that
are detrimental for the formation and/or preservation of biogenic structures
in fluvial settings. Interfluve areas may be delineated by paleosols, contain-
ing root and insect trace fossils, and potentially represented by the Coprini-
sphaera , Celliforma , and Termitichnus ichnofacies. During the late low-
stand, higher accommodation leads to the presence of increasingly isolated
fluvial channels encased in fine-grained material. The development of wide-
spread overbank areas tends to increase the preservation potential of bio-
genic structures, which are typically represented by the Scoyenia
Ichnofacies and associated vertebrate ichnofacies, namely, the Grallator
and the Batrachichnus ichnofacies. Stagnant water bodies (e.g., marshes,
floodbasins) are widespread if the rate of accommodation exceeds the sedi-
ment supply, signaling transgressive conditions; the Mermia Ichnofacies
may develop at this time. Shanley et al. (1992) noted that tidal influence
and brackish-water conditions may extend for tens of kilometers inland dur-
ing a transgression. If this is the case, fluvial deposits may become inter-
bedded with marginal-marine strata displaying ichnological evidence of
marine influence (e.g., Ophiomorpha , Teichichnus , logs containing Teredo-
lites ). During the subsequent highstand, increased sediment supply and
decreased fluvial accommodation are conducive to deltaic progradation
and increased establishment of channel bodies, leading to the reestablish-
ment of the Scoyenia Ichnofacies.