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SCAR Report No 16,
Appendix 7
ANTARCTIC SEISMIC STRATIGRAPHY:
STATUS, QUESTIONS, AND FUTURE PRIORITIES
Alan K. Cooper
U.S. Geological Survey, Menlo Park, CA, USA
Background
Seismic stratigraphy is a general technique for stratigraphic correlation.
The reflection characteristics (e.g., as amplitude, continuity, geometry,
etc.) of
regional unconformities and stratal surfaces are used to empirically estimate
rock properties, facies and chronostratigraphy, to infer structural evolution
and paleo-environmental histories. Seismic sequence stratigraphy, as defined
by Vail et al., 1977 is a relatively recent refinement of the general technique,
to also derive sea-level history from seismic sequence geometries. Both techniques
have been used on the Antarctic continental margin (i.e., shelf, slope and
rise) with success in regional mapping studies of acoustic-unconformities,
acoustic-units,
and seismic-sequences. Yet, because few continuous drill cores and down-hole
log data exist, it has not been possible to correlate seismic reflections
to sub-surface geology regionally, and thereby unequivocally decipher paleoenvironments
and ice/sea-level histories of the continental margin.
Seismic stratigraphic surveys have been done across nearly all accessible
regions of the Antarctic margin, collecting more than 300,000 km of small-airgun
single-channel
data (SCS) and nearly 200,000 km of large-airgun multichannel seismic reflection
data (MCS) (see Cooper et al., 1990, 1991, 1994 for many references). Various
seismic sources with different frequency bands and inherent resolutions have
been used (Table 1), and have imaged geologic features ranging from less
than a meter to kilometers in size, and buried at depths of a meter to up
to 10-15
km. In general, MCS (low to intermediate resolution) data image the deeply
buried and large features, and SCS (intermediate to very-high resolution)
data resolve
the shallow and small features.
Table 1: Ability of seismic-reflection systems to resolve geologic features.
Seismic System |
Center Frequency/ |
Approx. Resolution* |
Depth Penetration |
| Very-high resolution | 3500 Hz / 0.5 sec | 0.2 m / 3 m | up to 50-100 m |
| High resolution | 800 Hz / 1 sec | 0.8 m / 6 m | up to 250 m |
| Intermediate resolution | 150 Hz / 5 sec | 4 m / 30 m | up to 1-2 km |
| Low resolution | 60 Hz / 50 m | 10 m / 100 m | up to 10-14 km |
Assumptions: vertical = quarter wavelength with sediment velocity of 2.5 km/s and horizontal=2 times shot interval at ship speed of 6 kt
The regional
stratigraphic framework of the continental margin has been reasonably well
defined, with thick sedimentary sections of Paleozoic and younger age
covering many margin segments. Stratigraphic features common to many segments
of the Antarctic
margin include the thick Paleozoic and Mesozoic strata of pre- to post-rift
times that fill basement rift-structures, a thick ‘wedge’ of prograding
Cenozoic glacial sedimentary sequences that extend from the mid-shelf to
continental rise areas, thick oceanic sediments covered by large-scale sediment
mounds (i.e.,
drifts) on the continental rise, and numerous regional unconformities throughout
the sections across the margin. These features are believed to reflect the
general Paleozoic-Mesozoic continental breakup sequences (or Paleogene collision
sequences
in the Antarctic Peninsula region) overlain by the Cenozoic glacial sedimentary
units.
Seismic sections from all parts of Antarctica exhibit a large variety of
acoustic geometries and acoustic characteristics that are typical of today’s
non-glaciated and glaciated margins. These geometries are seen in outer shelf
sequences that
range widely from hundreds of meters (as local bank deposits) to hundreds
of kilometers (as regional submarine deltas/fans). The thickness can be meters
(topset
strata) to kilometers (continental slope sequences). The thickest prograding
sequences are commonly, but not always, located adjacent to the seaward end
of broad (up to 100 km wide) erosional, sea-floor or buried cross-shelf troughs.
And, along several margin segments, high-standing (up to hundreds of meters)
drift deposits on the continental rise lie opposite the prograding shelf
sequences.
From these observations, investigators have speculated that Antarctic margin
sequences were influenced principally by sea-level changes (e.g., the Vail
et al., 1977 model) in pre-glacial times (e.g., Hinz and Block,1984; Bartek
et al.,
1991) and principally by severe fluctuations of the Antarctic Ice Sheet during
at least late Cenozoic times (e.g., Larter and Barker, 1989; Cooper et al.,
1991; Bart and Anderson, 1995). Before the Antarctic continental shelf areas
were overdeepened
by erosion to their current depths of 200-1400 m in early to late Miocene
times (Cooper et al., 1991), sea-level changes would have resulted in high-stand
and
low-stand acoustic geometries (e.g., Vail et al., 1977); however, after shelf
overdeepening other factors such as grounded ice sheets and bottom currents
have strongly influenced stratal geometries (e.g., ten Brink et al., 1995).
The outer-shelf
wedges, delineated largely by ANTOSTRAT studies, are thought to be sub-glacial
and glacio-marine deposits derived from polar and temperate sediment-ladened
glaciers with broad internal ice streams that move at up to meters per day
(e.g., Alley et al., 1989). These glaciers have fluctuated widely in size
since late
Eocene time, and have intermittently extended onto and across the continental
shelf and uppermost slope to their stable grounding position (e.g., Larter
and Barker, 1989; Eittreim et al., 1995).
The general concepts and models, noted above, about possible origins, processes,
and paleoenvironments for acoustic units, unconformities and seismic sequences
of the Antarctic margin are, however, largely untested by geologic sampling.
The comprehensive compilations of Antarctic seismic data have greatly extending
our knowledge of the geometric characteristics of the sedimentary sequences
at local to regional scales. However, the underlying factors and processes
(e.g.,
subsidence/uplift rates, sedimentation/erosion rates, eustacy, currents,
sediment delivery mechanisms, etc.) that have resulted in these characteristics
are still
poorly understood, and will remain so until adequate ground truth drilling
and coring information are collected.
The remainder of this report describes some limitations of our current models,
significant questions that remain unanswered, and topics of study that could
be addressed in the coming decade to build on our past ANTOSTRAT studies
and enhance our understanding of Antarctic paleoenvironments and processes,
from
seismic stratigraphy. Three broad topics are addressed: Technology and Data,
Geology and Glacial History, “Global” Connections.
Technology and Data
Our ability to correctly infer regional geologic processes from seismic
data (with or without drilling control) is strongly dependent upon the
consistency, resolution, and quality of the seismic data.
Consistency: Our perceptions and interpretations are based on what seismic
data we are given to analyze. Hence, it is critical that similar types of
data with
equivalent resolutions and similar trackline grids be compared from different
regions to accurately assess if the same acoustic features and processes
indeed occur. Today, the density of seismic tracklines is highly variable
around Antarctica,
and detailed comparisons of 2-dimensional (2-D) geometries and seismic character
of acoustic units for all but the largest (i.e., more than tens of kilometers)
scales cannot be made, for deriving processes and paleoenvironments. Now,
comparisons are principally made on combinations of 2-D profiles, and yield
only approximate
3-D real-world geometries. 3-D industry-type surveys would be most useful,
but are fiscally impractical.
Seismic resolution: Resolution is another fundamental attribute of seismic
studies that has not been uniformly applied in comparisons of geometric features,
and
hence the causative processes of acoustically-resolved features remains equivocal.
The direct comparisons of low-resolution and high-resolution data across
the prograding glacial sequences of the outer shelf (e.g., Antarctic Peninsula,
Ross
Sea) has led to long-raging debates about the underlying processes and depositional
environments of these sequences – an excellent example of how unjustified
comparisons lead to equivocal interpretations. With the advent of precision
navigation and multichannel low- to very-high-resolution systems, it is important
to establish
guidelines for vertical and horizontal sampling rates to more uniformly resolve,
than previously, the sub-surface stratigraphic features of the margin.
Quality of seismic data: As herein used, quality is the variable appearance
of seismic data from similar systems due to natural-geologic, instrumental-noise,
and variable-data-processing factors. Because seismic data are strongly susceptible
to the above factors, quality is variable and in turn has led to widely different
interpretations of processes from the same environment. The interpretation
of
sub-glacial and marine-glacial deposits, a fundamental difference, is commonly
based on the lack or presence of internal reflections along seismic profiles
--- in digital processing, this may, for example be the difference solely
between applying or not applying an AGC filter, or may be a function of different
system
gain settings. For accurate comparisons and interpretations of seismic and
geologic data, criteria are needed to assure that uniform data collection
and processing
techniques are used or otherwise attainable.
Technology and data factors (noted above, and others) are significant fundamental
parameters that must be “normalized” when developing a seismic stratigraphic
model for the Antarctic margin, to accurately discern local features, processes
and environments from circum-Antarctic ones. The first decade of ANTOSTRAT
studies focused on existing data compilations. In the next decade, uniform standards
of resolution, trackline density, processing parameters, etc. should be established
and applied to seismic surveys for at least several select margin transects
around
Antarctica. Along these transects, all current acoustic systems should be
used to image the full suite (small to large) of acoustic features. These transects
would define “type sections” to be drilled/cored and compared in
detail to derive a high-resolution seismic stratigraphic process-model for
the Antarctic margin, like that of the Vail et al. (1977) sea-level model for
low-latitude
margins. Very high priority should be given to conducting the acoustic surveys
needed to define the “type sections” around Antarctica.
Geology and Glacial History
Ground-truth information: The fundamental objective of
Antarctic seismic stratigraphic analyses – to decipher regional processes,
paleoenvironments and chronostratigraphy – can
only be attained by directly relating seismic reflectors to geology. Without
geologic samples and in-situ information, the inferences and uncertainties
inherent in nearly all existing studies cannot be documented and clarified.
The near absence
of cores that penetrate below the ubiquitous glacial diamicton of the last
glacial advance on the shelf, and below a few meters of Pleistocene and younger
strata
on the continental slope and rise has not allowed seismic reflections to
be tied directly to subsurface geology. Instead, the geology below most parts
of the
Antarctic continental margin has necessarily been inferred from comparison
of Antarctic seismic records with those from (a) Antarctic regions hundreds
to thousands
of kilometers away where deep drill cores exist (e.g., Prydz Bay, Ross Sea,
Weddell Sea) and (b) northern high-latitude regions where seismically-defined
units have
been drilled and cored. Highest priority should be directed to acquiring
continuous geologic cores and down-hole logs from all possible seismic units
and sequences
around Antarctica.
Greater resolution of geologic features: As noted above, our ability to resolve
the 3-dimensional shape and internal geometry of seismic sequences, which
by convention contain the key geologic features that characterize an inter-related
suite of depositional environments, will determine the degree to which we
can
understand the structural- and facies-relationships of each particular environment.
Even though greater resolution will be possible principally for glacial sedimentary
sequences (i.e., pre-glacial sequences are commonly too deeply buried, except
on the inner shelves, to be reached by high-frequency seismic energy), the
improved definition will help answer critical questions such as: what is
the seismic signature
of a single glacial advance across the shelf, if such is preserved? What
are the internal geometries of thin-bed topset strata, and can these geometries
reliably
be related to subglacial and open-water environments? What are the seismic
facies relationships within individual foreset strata, and can they be traced
reliably
onto the continental rise, to directly link the shelf and abyssal paleoenvironments?
What are the characteristic seismic attributes, that can be reliably used
to discriminate between sub-glacial and glacial-marine strata, and between
sub-glacial
deposits derived from temperate glaciers and polar glaciers? Seismic variability
in Antarctic glacial sequences is well known to be large, laterally and vertically,
but at what seismic resolution (cm to m?) can characteristic universal seismic
facies be defined, if at all, to provide greater help in interpreting local
and regional glacier systems? In any case, the greater the seismic resolution,
the
greater the potential for accurate geologic assessments. High priority should
be placed on conducting high- and very-high-resolution seismic surveys to
attain precise lateral- and vertical-resolution of geologic features, for
better ascertaining
the processes by which they formed and their relationship to features elsewhere
on the shelf, continental slope and rise.
Origin of unconformities and sequences: The regional unconformity is the
fundamental building block of seismic stratigraphic studies, and prior investigators
(e.g.,
Anderson, 1984; Hinz and Kristoffersen, 1987; Cooper et al., 1991) have attributed
many processes (e.g., shelf currents, grounded ice, slope boundary-currents,
etc.) to their origins in Antarctica. Yet, few (if any) detailed studies
have been done to decipher and document formative processes of unconformities,
and
hence, the evolution of, and linkages between, depositional paleoenvironments
for the interleaved sedimentary sequences of the continental margin. Prior
to glaciation, it is commonly assumed that Antarctic unconformities formed
by processes
similar to those of today’s non-glaciated margins. But, did the processes
change with the initiation of glaciation or with overdeepening of the shelf?
Which processes are similar? And, what are the new processes, if any, resulting
from extensive glaciation of the continental shelves? Can the effects of
glaciation be separated from the effects of shelf overdeepening? How have
sea-level fluctuations
affected the development of Antarctic unconformities during the initiation
of Cenozoic Antarctic glaciation, and since then with the overdeepening of
the Antarctic
continental shelves? High priority should go to detailed seismic and geologic-core
studies of seismically-defined regional unconformities, to ascertain the
relationships between the formation of these unconformities with their interleaved
sedimentary
sequences, and changes in global sea levels and other paleoceanographic factors.
“Global” Connections
The term “global” herein is used to mean the continental shelf to abyssal basin environment of the entire circum-Antarctic region. Seismic stratigraphy has previously been used to attempt “global” connections via mapping of unconformities and acoustic units across the continental margin (e.g., Wannesson et al., 1985; Kuuvas and Leitchenkov, 1992; Larter and Cunningham, 1993) and via comparison of unconformity progressions on different parts of Antarctica (e.g., Hinz and Kristoffersen, 1987). Such correlations, if they can be confirmed by drilling, would provide important clues about the “global” processes that resulted in those features with common acoustic properties over thousands of kilometers. Sea-level change has been the only process yet identified (e.g., Vail et al., 1977) at this scale, although bottom-water currents have been widely implicated. Expansions of the Antarctic Ice sheet, with coeval carving of continental shelf unconformities has been widely suggested as a mechanism for “global” connections, but is not documented. What are the geologic processes that could result in similar acoustic properties in rock units to allow seismic unconformities to form and be traceable over hundreds to thousands of kilometers? Is this realistic? Or, are we seeing an aggradation of many separate processes over these same distances that provide the appearance of acoustic continuity? At what seismic resolutions do we see, and do we lose, the acoustic continuity needed for “global connections”? Priority should be given to investigating in detail the acoustic and geologic attributes of regional seismic features to ascertain how seismic stratigraphy in Antarctica can best be used, if at all, to make “global” connections.
Summary
ANTOSTRAT studies have significantly advanced our knowledge of the stratigraphic framework of the Antarctic margin. In the next decade, stratigraphic studies should focus primarily on acquiring drill/core data to “groundtruth” the known acoustic stratigraphy. Secondarily, the emphasis should be on collecting more-detailed acoustic images of “type sections” (within the Cenozoic sequences that underlie all segments of the Antarctic margin), to derive a unified model that accurately inter-relates Antarctic glaciation, sea levels, and other “global” processes that control sediment deposition across the entire margin.
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