Geology Reference
In-Depth Information
same holds for other times of large ice bodies,
such as the Permo-Carboniferous and parts of the
Late Proterozoic.
300
Rates of erosion
STM
Discussion of erosion rates should separately
address marine and terrestrial environments
because the processes and their controls are vastly
different.
200
FST
100
Subaerial erosion of tropical carbonates
Tropical carbonates have a high content of
metastable aragonite and magnesian calcite.
These minerals dissolve rapidly in rain water
and re-precipitate as calcite cement. Thus trop-
ical carbonate sediments commonly lithify upon
exposure. This lithifi cation plus organic binding
and frame-building in the depositional envir-
onment reduce the rates of mechanical erosion.
Denudation rates of large terrestrial surfaces of
carbonate rocks strongly depend on dissolution.
Most data and theoretical models were contrib-
uted by studies of extant karst systems. There
is fairly good agreement between rates deduced
from fi eld observations and those predicted from
theoretical models based on reaction chemistry
(White, 1984; Dreybrodt, 1988). Denudation
rates of karst surfaces in temperate and cold
climates are in the range of 60-125 μm yr
1
, trop-
ical rates 90-200 μm yr
1
. Purdy & Winterer (2001)
postulate theoretical rates of up to 650 μm yr
1
assuming strong undersaturation of the waters
and no damping effect of evapo-transpiration;
both assumptions seem rather unlikely. Erosion
rates in cave conduits may exceed 1000 μm yr
1
,
i.e. an order of magnitude higher than rates of
surface denudation. The higher rates in con-
duits lead to effi cient subsurface drainage, thus
reducing surface denudation and often saving
the depositional surface from wholesale destruc-
tion. The Miami Oolite is a good example. The
formation was deposited as a carbonate sand
shoal during the last interglacial, about 120 ka.
Despite more than 100 kyr of karst erosion, the
oolite surface still shows the depositional pat-
tern of sand bars and intervening channels and
the relief of this pattern still is in the range of
modern analogues in the Bahamas (Halley &
Evans, 1983; Schlager, 2005). In sequence strati-
graphic terms one may conclude that the depos-
itional surface of the highstand systems tract
(HST) of the Miami Oolite essentially survived
0
0
1000
2000
m yr
−
1
)
Rate of fall (
μ
Fig. 16.
The FST case studies plotted with estimated rates
of sea-level fall and erosion in the fall-erosion plane of the
parameter space of Fig. 8. Brown line in upper right is apex
of brown hyperbola in Fig. 8a, separating stability domain
of FST and STM at production rates of 500
m yr
1
. See text
for data sources and justifi cation of error bars.
of observation (Harrison, 2002). Considering the
sea-level curves of the modelling runs, the most
critical observation interval for the present dis-
cussion is 0.5-500 kyr. This constraint makes the
Pleistocene Series the most important source of
data because suffi ciently accurate time control
for sea-level movements of the more distant past
is very rare. For the Pleistocene (and Holocene),
there exists a well-established relationship
between oxygen isotope ratios in calcareous
shells, ice volume and eustatic sea level. Sea-level
curves gleaned from oxygen-isotope data repeat-
edly show rapid falls in the range of 20,000-
40,000
m yr
1
(Labeyrie
et al
., 1987; Shackleton,
1987). These rates are in the range required to
suppress the FST and create the STM in tropical
carbonates without much additional contribution
from erosion. However, it should be kept in mind
that the extreme Pleistocene rates remain some-
what speculative. U-Th age data generally pro-
vide insuffi cient resolution and data coverage to
confi rm or reject the rapid sea-level falls derived
from oxygen isotopes. In summary, it seems
highly probable that rates required to suppress
the FST by rapid sea-level fall occurred in the
Pleistocene. By inference, it seems likely that the
