Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 160

omission suites. Creation of accommodation at rates higher than sediment sup-
ply leads to the return of more marine or more basinal conditions into a given
area (e.g., marine overlying brackish-water deposits or brackish and marine
deposits overlying continental successions). Sediment starvation in more
basinal positions, because of preferential aggradation in continental and
nearshore positions, leads to reduced deposition rates, reflected by higher bio-
turbation index values. Slower deposition in fully marine environments also
generally leads to increased ichnological diversity, and a tendency to shift
toward deposit-feeding- and grazing-dominated ichnological suites, as most
food arrives to the setting via suspension settling. Where the creation of accom-
modation is slower than sediment supply, progradation occurs, but shoreline
trajectories tend to be somewhat aggradational. Resulting successions typically
show suites reflecting such progradation, but the elevated sedimentation rates
commonly lead to reductions in bioturbation intensities, particularly in inshore,
higher energy settings. The specific characteristics of facies and ichnofacies
juxtaposition must, nevertheless, be evaluated on a case-by-case basis and will
vary spatially both down depositional dip and along depositional strike.
2. OVERVIEW OF SEQUENCE-STRATIGRAPHIC FRAMEWORKS
2.1 Siliciclastic Responses to RSL Changes
The sequence stratigraphy of siliciclastic systems has been discussed at length
in the literature (e.g., Bhattacharya, 2011; Catuneanu, 2003, 2006; Catuneanu
et al., 2011; Helland-Hansen and Gjelberg, 1994; Hunt and Tucker, 1992;
Posamentier et al., 1988, 1992; Van Wagoner et al., 1990 ). The response
of siliciclastic-dominated coastlines and deep-marine systems to changes in
RSL varies significantly as a function of a number of factors including, but
not limited to, sediment supply, tectonic regime, and relative hydraulic energy.
Sediment supply is a critical factor in determining shifts of siliciclastic shorelines.
On coastlineswhere the sediment supply is high, sediment is delivered to the coast-
line faster than it can be reworked by basinal processes and, hence, shorelines pro-
grade as deltas and delta-fed strandplains even during periods of RSL rise.
Correspondingly, a decline in sediment delivery may lead to local coastal retreat,
even when the rest of the system is characterized by normal regression.
The tectonic regime may affect deep-marine systems more profoundly than
it does shallow-marine systems. For example, shelf widths on active margins
are typically narrower than on passive margins (e.g., Eisma, 1988 ), and tecton-
ically active margins tend to have higher rates of hinterland uplift and corre-
spondingly, increased erosion and sedimentation. On tectonically active
coastlines (as well as passive-margin shelves where available accommodation
is largely filled), significant volumes of sediment bypass the shelf even during
periods of sea-level rise and relative highstand, such that basin-floor submarine