Geology Reference
In-Depth Information
The deformed patterns of uplifted Pleistocene
terraces are an obvious and readily observed
consequence of variable bedrock uplift rates
in  coastal regions. Moreover, comparisons of
the shapes of deformed terraces of different
ages allow one to assess spatial and temporal
variability in rock uplift rates. Commonly,
the faults that are responsible for uplift of the
terraces are either blind faults or break the Earth's
surface below sea level. In such circumstances,
these warped terraces take on added signifi-
cance, because they may permit testing of
hypotheses about the behavior of unexposed
faults or folds that are thought to be responsi-
ble for their uplift. For example, comparisons of
coseismic coastal uplift patterns with patterns
of warped terraces can be used to  assess
whether a long succession of characteristic
earthquakes on one or two local faults could
have generated the observed terrace pattern.
Such a situation was previously described (see
Fig. 4.8) on the California coast in the vicinity
of Santa Cruz, where uplifted and broadly
warped marine terraces are well preserved.
Alternatively, the geometry of warped terraces
on the flanks of a growing anticline can be used
to deduce the orientation and typical slip direc-
tion along the buried fault(s) responsible for
terrace uplift (Ward and Valensise, 1994). The
terraces are like bathtub rings around a grow-
ing fold: each one originally formed a horizon-
tal surface (see Box  9.1). But now, in their
deformed positions, their geometry can be
inverted to estimate the depth, dip, and slip on
underlying faults.
Most marine abrasion platforms vary from
100 to 500 m wide. The depth to wave base and
the  requirement that the seaward slope of the
platform permits removal of debris from the
shoreface appears to control the maximum
platform width. In some areas, however,
platforms greater than or equal to 1 km wide are
preserved (e.g., Fig. 9.5A). Unusually wide
terraces are likely to result from one of two
conditions: the presence of very weak, readily
eroded rocks; or successive reoccupations of a
given terrace level. If the amount of bedrock
uplift between two highstands is just a bit less
in magnitude than, but in the same direction as,
the vertical difference in sea level between the
two highstands, then a previously formed
terrace can be reoccupied by a slightly higher
sea level and laterally extended (Kelsey and
Bockheim, 1994).
It is important here to note that there exists a
“terrace survival problem” analogous to the
glacial moraine survival problem (see Box 2.3).
A sequence of elevations corresponding to a
terrace flight at one location along a coastline
might be different from another nearby sequence
in that one or more terraces might be missing
(Anderson et  al. , 1999). This mismatch results
from the fact that: (i) platform width is depend-
ent on very local variables such as lithology,
structure, and orientation of the coastline
relative to the dominant wave energy (Adams
et al. , 2005); and (ii) a younger terrace platform
grows in width at the expense of older platforms,
whose outer edges are progressively nibbled
away by the cliff at the back of the younger
terrace. Hence, younger terraces can locally
eliminate older ones. In addition, this geomorphic
reality raises a cautionary flag against using
platform width as a correlation tool.
Until recently, only emergent platforms were
used in defining uplift patterns. This limitation
has changed with the advent of increasingly
available and detailed bathymetry and with
new drilling methods that allow collection of
geological materials from the sea floor or from
beneath younger sediments. An example of the
use of sub-sea-level platforms utilizes corals
dredged from about 2 km deep on a submerged
platform in the Huon Gulf of the Solomon Sea
(Galewsky et al. , 1996). These ancient platforms
were first visualized using sidescan sonar and
detailed bathymetry, where both anomalously
flat surfaces and striking spires (interpreted to
be coral pinnacles) were identified as potential
targets for dredging. Samples retrieved from
these surfaces were dated at approximately
340 ka using uranium-series dating and, thereby,
revealed a long-term subsidence rate of about
6 mm/yr, one of the first documentations of
such sustained rates. With the explosion of
high-resolution sea-floor data, the use of sub-
merged terrace platforms promises to become
more routine.
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