Environmental Engineering Reference
In-Depth Information
The recognition of substantial intraslope sand accumulations has led to a
revolution in slope sedimentology and stratigraphy over the past two decades,
spawned in large part by the development of predictive models by researchers in
the hydrocarbon industry (e.g., Prather et al., 1998 ). Intraslope minibasins have
been shown to be characterized by sand-rich fills in many instances and formed
from a combination of processes such as salt withdrawal, mass-wasting, exten-
sional rifting, and/or growth-fault development. Conduits, such as submarine
canyons and gullies, also have been shown to contain ponded sandstone in many
cases (e.g., Anderson et al., 2006; Paull et al., 2005 ). Additionally, channel-
levee complexes and other channel subenvironments are common on the slope,
representing the proximal loci of sand deposition associated with expansive
basin-floor fan complexes ( Fig. 1 ).
Acknowledging the variability in continental slopes and their deposits, this
chapter focuses on areas of the slope associated with high current energies,
many of which are sand prone ( Fig. 1 ). This approach fosters the incorporation
of high-quality subsurface and outcrop data. Furthermore, the integration of
ichnological observations and interpretations complements developing sedi-
mentological models and aims to be particularly useful to hydrocarbon explora-
tionists and sedimentologists attempting to discern slope deposits and their
stratigraphic architecture in the rock record. Of note, Wetzel et al. (2008)
focused on bioturbation associated with contour-current-dominated slope
deposits and correspondingly, this topic is not addressed in detail herein.
2. THE DISTRIBUTION OF INFAUNA ON SLOPES:
INSIGHTS FROM MODERN BASIN MARGINS
The distribution of sea-bottom-dependant (epibenthic and endobenthic) organ-
isms, both on the seabed and within the substrate in the Earth's oceans and
peripheral seas, has been extensively documented (e.g., Heezen and Hollister,
1971; Sanders, 1968 ). Although not all of these organisms leave distinctive
traces, their distribution provides insights into physical and chemical conditions
across the bathyal realm. Tracemakers are variable, including holothurians,
echinoids, polychaetes, echiurians, enteropneusts, gastropods, molluscs, seapens,
and decapods ( Heezen and Hollister, 1971 ). Overall faunal densities are typically
observed to decrease with increasing water depth across basin margin slopes (e.g.,
Flach and Thomsen, 1998; Rowe et al., 1982; Stora et al., 1999 ), although in some
instances, perturbations in this trend are notable (e.g., Gage et al., 2000 ). Species
diversities can mimic this downward-decreasing trend of biomass abundance
(e.g., Stora et al., 1999; Vetter and Dayton, 1998 ), but there are numerous excep-
tions. In certain instances, species diversity increases with water depth (e.g.,
Rowe et al., 1982; Vetter and Dayton, 1998 ), and other slopes are characterized
by a gradual increase in overall species diversity to the mid-slope position, fol-
lowed by a persistent decrease in this trend toward the abyssal depths (e.g., Gage
et al., 2000; Stuart et al., 2003 ).
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