Slopes - Trace Fossils as Indicators of Sedimentary Environments - page 614

5. DISCUSSION AND CONCLUSIONS
Beyond the shelf-break, the deep-water realm is characterized by a variable sea-
scape that consists of subaqueous canyons, sinuous channels, slump scars, and
minibasins, which contribute to substantial topographic rugosity ( Fig. 1 ). The sea-
scape morphology is influenced by numerous factors, including tectonic setting,
oceanic currents, fluctuations in sea-level, and sediment input. Sedimentological
and stratigraphical analyses of slope deposits over the past decade have led to the
development of increasingly realistic and complex facies models (e.g., Hodgson
and Flint, 2005; Hubbard et al., 2011; Wynn et al., 2007 ). Recent ichnological
work has tended toward helping to better establish the utility of trace-fossil anal-
ysis for recognizing the deposits of these highly variable slope subenvironments
in the rock record (e.g., Heard and Pickering, 2008 ). Although there have been
substantial advances to date, we have only just started and undoubtedly there
are numerous discoveries yet to be made that will help future sedimentary geol-
ogists to better resolve ancient slope successions. Slope ichnology arguably can
be considered a field of research with tremendous potential at this time.
The trace-fossil assemblages of slope deposits show a marked variability,
controlled by physico-chemical parameters such as current energy, sedimenta-
tion rate, slope instability, the character of the substrate, oxygen and food avail-
ability, and water turbidity. Background (ambient) slope deposits are dominated
by deposit-feeding and grazing structures, although diversities appear to vary
from setting to setting ( Figs. 5 and 6 ). Ambient slope environments are subject
to slow, continuous suspension settling of fine-grained material, with associated
oceanic currents, turbidity currents (low- or high-density), and/or mass-wasting
events. Bioturbation can be thorough and pervasive in successions typified by
gradual sedimentation ( Fig. 6 ), but considerably lower and sporadically distrib-
uted in event bed-dominated intervals ( Fig. 5 ). An additional challenge, asso-
ciated with characterizing slope trace-fossil assemblages, derives from the fact
that many slope settings are typified by bedding-parallel trace fossils. Hence,
cross-sectional perspectives of slope deposits, such as those observed in cores,
will likely result in underrepresented trace-fossil diversity measurements,
affecting forms attributed to grazing behaviors, as well as graphoglyptids
(cf. Knaust, 2009; MacEachern et al., 2010 ).
Conduits that cross-cut the slope, including canyons, gullies, and channels,
are commonly characterized by increased and replenished oxygenation and
more abundant food resources (e.g., Blake and Hilbig, 1994; Rhoads and
Hecker, 1994 ). Although large gravity flows are expected to significantly
(fu) and Macaronichnus ( Ma ). The underlying siltstone contains abundant Phycosiphon ( Ph ), and
locally, Chondrites and Planolites as well. (F) Levee sandstone erosionally overlying pervasively
burrowed siltstone. The siltstone shows BI ¼ 3-4 with abundant Phycosiphon ( Ph ). The overlying
turbiditic sandstone shows convolute bedding and fecal-pellet filled meniscate burrow tentatively
identified as Compaginatichnus ( Co ?).