dsc08831 (2)

THE ICHNOFABRIC APPROACH TO FRESHWATER ICHNOFAUNAS

Diplichnites and their similar size rangę, Morrissey and Braddy (2004) convincingly argued for a myriapod (e.g., arthropleurid) producer for these large meniscate tracę fossils. Variants of the Beaconites-Taenidium ichnofabric seem to be common in abandoned channel and overbank deposits and represent probably the earliest Continental ichnofab-rics. Detailed analysis by Morrisey and Braddy (2004) suggests that the Beaconites-Taenidium ichnofabric records colonization of subaerially exposed sediment in response to seasonal desiccation with animals excavating into the substrate to aestivate or mould at water table level. This ichnofabric represents en masse colonization during a single bioturbation event and is, therefore, a simple ichnofabric. Since then Triassic meniscate ichnofabrics became common not only in fluvial environments but also in lakę margin deposits (e.g., Johnson and Graham, 2004). In desic-cated floodplain and lakę margin deposits, meniscate ichnofabrics are commonly associated with desiccated substrates (Fig. 17.5B) and composite ichnofabrics, induding both softground (Beaconites-Taenidium) and firmground (Scoyenia) suites, are common (Fig. 17.50 (Buatois et al., 1996a). The omamented burrow system Spongeliomorpha is also present in this environmental situation (Metz, 1993, 1995), but the associated ichnofabrics are still unknown.

An ichnofabric dominated by vertical burrows seems to be present in freshwater deposits, represent-ing an analogue to the marinę Skolithos and Arenicolites ichnofabrics (Fig. 17.5D-F). Commonly, these ichnofabrics contain only one ichnotaxa. However, slightly morę diverse examples are known from the Triassic of Greenland, where Skolithos and Arenicolites commonly coexist with Polykladichnus (Bromley and Asgaard, 1979, 1991; Bromley, 1996). Escape tracę fossils may also be present (Sarkar and Chaudhuri, 1992). ecorded examples are present in moderate to high energy settings, such as active channels, wave-dominated lacustrine shorelines, lacustrine delta mouth bars and storm deposits (Bromley and Asgaard, 1979, 1991; Bradshaw, 1981; Zawiskie et al., 1981; PHzgerald and Barrett, 1986; Woolfe, 1990; ^rkar and Chaudhuri, 1992; Mdngano et nl., 1994; Bromley, 1996; MeJrhor et al., 2003). This ichnofabric ^rf'fk'(.'tr> the ernplacement of moderafely cleep to deep burrows, whkh may form dense assemblnges in the 'd Skul i t fum pjpororks (Fit/geralil and Barrett, This »<hnofabri< most likely records opporlu-^r"'^łu aiUmi/.alUm of rapldly etnplaeed sancls ^fBrf'mley, |0%y

Afiother diMm*tive Ichnofobfk in aKimluned ^rbrt_fłrł#,|    _;in<|    margin deposits,

^twuUriy in Momt/.nk and C eno/oU strata^ i" 309

dominated by the crayfish burrow Camborygma (Hasiotis and Mitchell, 1993; Hasiotis et al, 1993, 1998). Camborygma is commonly the only ichnogenus present in this ichnofabric, but several of its ichnos-pecies may occur. Burrow architecture is related to the position of the water table; complex architectures with many branches and chambers are constructed by primary burrowers in areas of high water table (Fig. 17.5G), while deep simple burrows are dominant in areas of Iow and/or highly fluctuating water table (Fig. 17.5H) (Hobbs, 1981; Hasiotis and Mitchell, 1993). Burrowing depth of Im seems to be fairly common (Hasiotis et al., 1998). The establishment of deep burrows in freshwater deposits led to consider-able disruption of the sedimentary fabric (e.g., Hasiotis, 2002, p. 91).

The Fuersichnus ichnofabric represents intense bioturbation in ephemeral lacustrine and fluvial deposits (Fig. 17.51) (Bromley and Asgaard, 1979, 1991; Gierlowski-Kordesch, 1991; Bromley, 1996). Fuersichnus is commonly the only ichnotaxa present in the ichnofabric, but under Iow bioturbation intensities, Lockeia may be present as a subordinate element (Bromley and Asgaard, 1979,1991; Bromley, 1996). The Fuersichnus ichnofabric is locally character-ized by very high degrees of bioturbation, as illustrated by dense occurrences of Fuersichnus com-munis from Triassic successions of East Greenland (Bromley and Asgaard, 1979, 1991; Bromley, 1996). According to these authors, the Fuersichnus ichnocoe-nosis records the activity of shallow tier deposit feeders leading to common obliteration of all physical structures and, in most cases, of all other biogenic structures. It is most likely a rapidly produced structure reflecting the activity of opportunistic colonizers (Bromley, 1996). This is consistent with sedimentologic data, which suggest stressful environ-ments subject to episodic flows and rccurrent desiccation in ephemeral fluvial systems and playa lakę complexes (Gierlowski-Kordesch, 1991; Dam and Stemmerik, 1994; Clemmensen et al., 1998).

Mesozołc and Cenozoic permanent sub«H]ueous lacustrine deposits may be nioderately to intonsoly bioturbated and commonly contain a mottled toxtiuv thnt may be referred to as Plunoliłes ichnofabric (Fig. I7.5J). Hxnmptes of bioturbated lacustrine depos-its reflecting the activłty of a middle-tier, dej?0»H> feeder In fauna are parlicularly common sińce the Crelaceous (e.g., Whatoloy and Jordan, 1989; Flint et al., 1,989; Buatois and Mrtngano. 1998), Fht# ichnotabric iippnreułlv relleets the acttvtty ot the Irtirweather lainslrlne łnlauna, allhough iehnodiwr-■ilI u In commonly Iow, eomprłstng mouospecillc lehiuwoenose*. Bumiwing by the Planolitcs producer