Geology Reference
In-Depth Information
the paleo-sea cliff and that of the sea cliff
itself can be projected to an intersection that
approximates the former shoreline elevation.
In contrast, the depth below sea level of the
outer edge of the abrasion ramp can vary by
10 m or more. Moreover, the position of the
preserved outer edge of the platform depends
on how much it has retreated due to erosion
and smoothing since it became emergent.
Consequently, the position of this outer edge
provides a much less reliable marker with
which to measure deformation, even though it
may be more readily preserved and observed
in the landscape.
Constructional marine terraces can form when
marine conditions are favorable for the growth
of corals and coralline algae. Typically, these
conditions occur where water temperatures in
winter remain above about 18
°
C, where clear
water permits penetration of sunlight to support
photosynthesis, and where the salinity is normal.
Under conditions conducive for growth, coral
reefs can be rapidly built upward toward the
sea surface (at rates exceeding 10 cm/yr) and
outward toward available space. When sea level
is stable for sufficiently long periods, corals
will build platforms that are closely tied to sea
level and, therefore, provide useful geomorphic
markers. During a relative rise in sea level, a
predictable upward and outward coral growth
occurs (Fig. 2.2A and B). If the rise is particularly
rapid, the rate of vertical growth of the crest
may lag behind the relative sea-level rise. Only
when the sea level stabilizes will the reef crest
attain its maximum height near the sea surface.
During a subsequent fall in relative sea level,
wave-cut notches may by cut into the forereef
during brief stillstands (Fig. 2.2C). If sea level
stabilizes at a new local level, corals will build a
reefal bench.
Because different coral species are adapted to
different marine conditions, a marked zonation
of species occurs within a reef. For example,
wave-resistant corals will be found in the
nutrient-rich waters on the leading edge of a reef,
whereas less robust forms occupy the backreef
(Chappell, 1974). Knowledge of this zonation
(Fig. 2.2A) and an ability to recognize various
coral species, some of which are more faithful
recorders of sea level, permits reconstruction
of the geometry of presently emerged reefs and
their relationship to former sea levels at the
time of their growth. For constructional terraces
comprising coral reefs, the reef crest and buttress
represent the leading edge of the reef and yield
the best approximation of sea level. Because this
edge is most subject to erosion if the reef is
uplifted above sea level, knowledge of the depth
below the sea surface at which various coral spe-
cies are found must often be combined with a
recognition of the preserved species zonation
within a reef (Fig. 2.2A) to estimate the position
of the former reef crest.
Along many tectonically rising coasts, flights
of marine terraces provide direct evidence for
multiple decreases in relative sea level. But when
do these terraces form, and how are they related
to sea-level changes? In order for a coastal
terrace to be generated, sea level must remain at
approximately the same relative position with
respect to the land so that corals can build
outward to form a reefal terrace or so that wave
attack is focused along the same abrasion
platform through time. Owing to rapid rates of
lateral erosion and of coral growth, it often takes
only a few thousand years to create a broad
terrace (Anderson
et al.
, 1999). Consequently,
we expect terraces to form during intervals
when rates of vertical movement of the land and
rates of sea-level change are nearly equivalent. If
we knew well the history of sea level and could
correlate between individual terraces and former
sea-level positions, we would have a basis for
calculating vertical deformation rates.
Fig. 2.2
(
cont'd
) These near-surface corals can be eroded, however, and commonly species that reside slightly below
the mean surface are better indicators of former sea levels. B. Idealized cross-section of the reef in A showing how the
topography of the reef is related to changing relative sea level. During the intervals of rapid sea-level rise (from sea
level 1 to 3, inset), the crest of the reef grows upward, but remains below the sea surface. As sea level stabilizes, the
reef reaches sea level and grows seaward. C. Wave-cut notches are a response to falling sea level. A new reefal
platform can be built once sea level stabilizes. Modified after Chappell (1974).
