Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 343

crevasse-channel deposits of South Africa ( Smith, 1993 ). In these cases, the
vertebrate tracks (purported Scoyenia component) are best compared with a
vertebrate ichnofacies (like the Grallator or Brontopodus Ichnofacies),
because vertebrate tracks alone are not considered herein as diagnostic of
the Scoyenia Ichnofacies. The Mermia components are represented by simple
horizontal trails, arthropod trackways, and horizontal burrows. Additional fos-
sil examples of composite Mermia - Scoyenia Ichnofacies from fluvial facies
are characterized by a similar participation of simple horizontal trails or bur-
rows and complex sinusoidal trails, the Mermia components, associated with
meniscate burrows, arthropod trackways, trough-like relief, and simple verti-
cal burrows, the Scoyenia components ( Głuszek, 1995; Kim et al., 2002; Metz,
1992 ; ichnocoenosis F of de Gibert and S ´ ez, 2009; Keighley and Pickerill,
2003 ). In these cases, the overprinting of significant numbers of components
of the Mermia and Scoyenia ichnofacies suggests transient subaqueous
deposits with periodical subaerial exposure.
Another example of mixed or composite assemblages was described by
Smith et al. (1993) from the Late Pleistocene of Namibia. These authors iden-
tified a soft-sediment assemblage composed of Planolites , Skolithos , and Tae-
nidium , which can be compared to the Scoyenia Ichnofacies ( Arenicolites
Ichnofacies of those authors), overprinted by three types of rhizoliths that might
indicate the presence of the potential rhizolith ichnofacies ( Termitichnus Ichno-
facies of those authors). A detailed example of the utility of a trace-fossil analy-
sis combined with physical sedimentary structures and observation of modern
processes is the description of fluvial paleosurfaces from the Permian Beaufort
Group by Smith (1993) .
4.2 Stratigraphical Applications
The Neogene, dominantly fluvial Vinchina Basin of north-western Argentina
provides a good example of contrasting ichnocoenoses found in fluvial and
shallow-lacustrine intervals. The most common trace fossils in the basin are
vertical I, or J-shaped large burrows with striated walls (ichnogenus Capayan-
ichnus ) that are recognized in channel-belt and crevasse-splay deposits of a
variety of river systems. Capayanichnus is commonly accompanied by burrow
systems ( Spongeliomorpha ), meniscate burrows ( Scoyenia , Taenidium ), and
vertebrate trackways, composing assemblages that can be easily compared with
the Scoyenia Ichnofacies ( Melchor et al., 2010 ). A 65-m-thick interval of
shallow-lacustrine, wave-dominated deposits, that are intercalated in fluvial
deposits of the upper member of the Vinchina Formation, exhibits a different
trace-fossil assemblage dominantly composed of palmate bird tracks
( Phoenicopterichnum ) and bird tracks without web traces, and subordinate
trough-like reliefs ( Lockeia , Rusophycus )( Melchor et al., 2012 ). The lacustrine
assemblage can be compared with the Grallator Ichnofacies or the shorebird
ichnofacies of Lockley et al. (1994) .