Slopes - Trace Fossils as Indicators of Sedimentary Environments - page 596

The pioneering work of Hayward (1976) featured the recognition of bioglyph-
bearing (scratch-marked) firmground burrows (e.g., Rhizocorallium , Skolithos
[ Tigillites ], and Thalassinoides ) excavated into the walls of an Early Miocene
submarine canyon succession. Similar firmground suites have been identified
from submarine canyon margins in the West Ahken Field of the Nile Delta,
and in the Mississippian Canyon, Gulf of Mexico (e.g., MacEachern et al.,
2007b ). Additionally, contour currents were deemed important in the genera-
tion of a firm substrate on the Pliocene Atlantic-margin continental-slope off
the east coast of North America, showing that like marginal-marine manifesta-
tions of the ichnofacies, bathyal examples could be associated with widespread,
sequence-stratigraphically significant discontinuities ( Savrda et al., 2001 ).
Hubbard and Shultz (2008) proposed a list of potential mechanisms for wide-
spread firmground generation in the deep sea, with substrate exhumation most
likely linked to (1) widespread turbidity current erosion, (2) large-scale slumps,
and (3) contour currents (cf. Savrda et al., 2001 ).
Channelized turbidity currents and slumps are commonly focused, leading
to localized effects on basin-margin slopes. Larger spatial extents may be antici-
pated with interconnected slope-channel systems. Nevertheless, erosion and
colonization of firm substrates may impact the sea floor over limited areas, such
that the stratigraphic significance of the Glossifungites Ichnofacies is likely
to be minimal. Indeed, submarine canyon examples of this ichnofacies are,
themselves, spatially limited (e.g., Anderson et al., 2006; Di Celma et al.,
2010; Hayward, 1976 ).
An alternative utility of passively filled firmground burrows in the bathyal
realm could be as indicators of sediment bypass. Specifically, sand-infilled bur-
rows (in some instances) record the passage of coarse-grained material through
an erosional conduit such as a submarine canyon or channel, without deposition
of a coarse-grained lag. The slope is commonly not a locus of deposition for
coarse-grained detritus; background strata and conduit fill typically consist
of conformable siltstone and mudstone. Alternatively, channels may be filled
with siltstone- and mudstone-dominated mass-transport deposits ( Fig. 3 ).
In either case, the only evidence of coarse-grained sediment in the system
may be recorded within the burrow fills ( Fig. 3 ). As such, such trace fossils
could be considered “tubular turbidites”, in a manner similar to the record of
storm events preserved in “tubular tempestites” ( Tedesco and Wanless, 1991 ).
3.4 Cruziana Ichnofacies
The archetypal Cruziana Ichnofacies is typical of fully marine settings character-
ized by unconsolidated cohesive to semi-cohesive muddy substrates generated in
areas of low to moderate energy. Common depositional settings include the distal
lower shoreface, offshore, and inner shelf ( MacEachern and Bann, 2008; MacEa-
chern et al., 2007a; Pemberton et al., 1992 ). The archetypal expression of the