Slopes - Trace Fossils as Indicators of Sedimentary Environments - page 615

influence canyon environments, it is important to consider that major events
often recur at a frequency of decades or centuries (e.g., Romans et al., 2009 ),
rates at least an order of magnitude lower than bed recolonization by benthic
organisms (e.g., Grassle, 1977 ). For long periods, submarine conduits are com-
monly associated with focused and sustained sediment-poor and oxygen- and
nutrient- rich currents (e.g., upwelling currents). Therefore, it is likely that these
conduits are ideal areas for infaunal activity for extended periods. The paucity
of trace fossils in many ancient channel-fill successions probably reflects their
preservation potential, in part owing to pronounced erosion from the rare, out-
sized events ( Fig. 8 ). Furthermore, although numerous interpretations of ancient
deep-water strata consider punctuated sedimentation as a major deterrent to bio-
genic reworking, many of the deposit-feeding ethologies recorded in ancient
slope deposits suggest that organisms were likely able to withstand, or recover
(as a community) relatively quickly from turbidity current sedimentation. As a
result of all these factors, trace-fossil assemblages (mostly impoverished) that
are more typical of shallow-marine environments are commonly observed in
bathyal deposits (e.g., Figs. 7-9 ). Although it is possible that some organisms
residing on the slope have been transported there via turbidity currents
(cf. Crimes, 1977; F ¨ llmi and Grimm, 1990; Wetzel, 1984 ), it is more likely
that the environmental conditions themselves can sustain a so-called
shallow-water community in many instances. Deep erosion, associated with
submarine conduits, can expose semi-consolidated substrates, and therefore,
substrate-controlled trace-fossil associations attributable to the Glossifungites
Ichnofacies are also common and should be expected in channel fill and margin
deposits (e.g., Figs. 3 and 4 ; Anderson et al., 2006; Callow et al., 2012a;
Hayward, 1976; MacEachern et al., 2007b ).
Channel-levee complexes are an important component of submarine fan and
slope systems, and the ichnology of levees is a burgeoning research topic
(e.g., Fig. 10 ; Callow et al., 2012a,b; Cummings and Hodgson, 2011; Heard
and Pickering, 2008; Hubbard and Shultz, 2008; Kane et al., 2007 ). In general,
levee deposits are characterized by higher trace-fossil diversities relative to those
of many other slope environments, including abundant graphoglyptids in some
instances. For example, in Cretaceous-Eocene levee units of the Basque Basin in
northern Spain, Cummings and Hodgson (2011) recognized 30 distinct ichno-
taxa, including 17 pre-depositional forms and 12 post-depositional forms. Of
these, 12 forms were considered graphoglyptids. Although trace-fossil suites
with common graphoglyptids are not featured in this review, it is apparent that
the lower bathyal realm is host to depositswhere they are at least locally abundant
(e.g., Heard and Pickering, 2008 ). In comparison to basin-floor fan deposits,
however, graphoglyptids are commonly lacking in slope deposits due to unstable
environmental conditions (e.g., currents, sedimentation) and the presence of rel-
atively abundant food resources on the slope. Organisms that adapt specialized
feeding strategies, likemany that are attributed to the construction of graphoglyp-
tids, are out-competed on the slope by more mobile organisms, many of which