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fili'. ICHNOFABRIC APPROACH TO FRESHWATER U HNOFAUNA#

tocustriric areas. Avnilable Information sugggstft that (hcre are no archetypal tracę fossil associations that dearly distinguish shallow and deep lacustrine sellings (Table 17.3). For example, the Palaeophycus ichnocoenoais documented by Pickerill (1992) from Carboniferous shoreface lacustrine successions of the Albert formation In New Brunswick consists of horizontai shallow burrows and trails produced by mobile predators and deposit feeders (e.g., Helminthopsis tenuis, Cordia marina, Cochlichnus an%u-ineus). It is extremely similar to ichnofaunas recorded in dceper lacustrine settings affectcd by turbidity currents and other density flows (e.g., Buatois and Mdngano, 1993b; Buatois et al., 1996b). Both exampl.es should be allocated to the same ichnofacies. Variation in tracę fossil content from one lacustrine basin to the other most likely reflects the wide variability of lacustrine systems. However, in large, deep lakes, depth-related tracę fossil zones can be established. Walter and Suhr (1998) documented a bathymetric zona tion in Pleistocene glacial lakes of Germany. In these lakes, shallow-lacustrine tracę fossil assem-blages are dominated by arthropod trackways (e.g., Waruichnium, Glaciichnium, Lusatichnium), while grazing trails are abundant in deeper zones (e.g., Cochlichnus, Cordia, Helminthoidichniteś). Metz (1996) noted that in Triassic lacustrine deposits of the Newark Basin, elements of the Mermia ichnofacies arereplaced by typical representatives of the Scoyenia ichnofacies during shoreline regression. Similarly, Melchor et al. (2003) and Melchor (2004) documented the replacement of the Mermia ichnofacies by the Scoyenia ichnofacies due to progradation of lacustrine deltas in different Triassic redbed units from Western Argentina. Although there are no archetypal, recur-rent ichnofacies that clearly distinguish shallow- vs deep-lacustrine settings, zonations prove to be useful at the scalę of individual lacustrine basins.

THE ICHNOFABRIC APPROACH TO FRESHWATER ICHNOFAUNAS

Ichnofabric comprises all aspects of the texture and intemal structure of a substrate that result from bioturbation and bioerosion at all scales (Ekdale and Bromley, 1983; Bromley and Ekdale, 1986; Taylor et al., 2003, this volume). The ichnofabric approach became very popular during the Jast two decades, but still Jittlc is known about the naturę and genesis of Continental ichnofabrics, and review chapters are almost exclusively based on marinę examples (e.g., Taylor et al., 2003;. Notably, a conceptunl and methodological framework for th<^J analysłs of paleosol ichnofabrics has been advanced by Cienise et al. (2004a) recently. These authors noted that Soli feature* that disrupt the primary fabric of terrestrial deposits may be formed without the intervenłłon of bioturba-tion (pedofabric). In addition, soi! processes may also disrupt ichnofabrics. Genise et al. (2(K)4a) noted that ichnofabric analysis in paloosols recjuire inc)difi catlons to the standard methodology developed from marinę examples. They suggested construction of tiering diagrams, evaiuation of the pedofabric independent of the ichnofabric, and construction of ternary diagrams showing percentages of bioturbation, pedofabric, and original bedding. These nuthors illustrated their methodology with examples from Mesozoic and Cenozoic paleosols from Argentina, Uruguay, and Egypt. The rest of our discussion in this chapter deals with freshwater ichnofabrics that are commonly much simpler than paleosol and marinę ichnofabrics.

Buatois and Mangano (1998) addressed some of the potential and limitations of the ichnofabric approach in Continental successions. These authors noted that evolutionary innovations of the terrestrial and freshwater biotas constrained the development of Continental ichnofabrics. Paleozoic ichnofaunas are dominated by bedding-plane, very shallow tracę fossils, mostly grazing trails and arthropod trackways (Miller, 1984; Maples and Archer, 1989; Buatois and Mangano, 1993a, 1998; Buatois et al., 1998a). The dominance of epifaunal and shallow infaunal tracę fossils result in little or no bedding disruption and, therefore, most Paleozoic Continental deposits tend to be unbioturbated. Grazing trails, such as Mermia, Cordia or /-/elminthopsis, and arthropod trackways, such as Diplichnites or Umfolozia, which are very common in freshwater deposits, do not produce ichnofabrics. Accordingly, trail- and trackway-bearing deposits are commonly seen in cores as unbioturbated, fine-grained, thinly laminated rocks (Buatois et al., 1998b). This is particularly true for Paleozoic lacustrine deposits.

However, Paleozoic fluvial deposits may contain distinct ichnofabrics characterized by monospecific assemblages of meniscate tracę fossils that have been variably assigned to Beaconites or Taettidium (Fig. 17.5A) (e.g., Allen and Williams, 1981; Graham and Pollard, 1982; Morrissey and Braddy, 2004). Notably, large specimens (up to 250 mm wide) seem to be typical of Silurian-Carbonifermis rocks (Keighley and Pickerill, 1994) and bioturbation depth may reach 1.44in in examples from the Deyonian Old Red Sandstone (Lance Morrissey, written communieation, 2004). Based on their recurrent associatinn with large