Geology Reference
In-Depth Information
15.6.2 The Response of Carbonate Plat-
forms to Drowning
•
The death of the platform by repeated emergence
and submergence,
indicated by criteria pointing to sub-
aerial exposure (Schlager 1998; Wilson et al. 1998).
•
Environmental deterioration
(e.g. loss in water qual-
ity; decrease or increase in water temperature, nutri-
ents, salinity, water energy; see Erlich et al. 1990). This
explanation has the advantage that at least some of the
environmental factors can be traced by paleontologi-
cal and geochemical proxies; changes in the environ-
ment may be recorded in depositional textures of lime-
stones, paleocommunities and in changes in microfacies
types. Drowning induced by environmental factors can
lead to the decrease or loss of the growth potential of
reef and platform organisms. Nutrient-rich surface
waters of the equatorial upwelling zone may have been
responsible for the 'death-in-the-tropics' of Cretaceous
platforms (Wilson et al. 1998). Changes in nutrient
conditions affecting the conditions favorable for car-
bonate producing organisms may reduce the aggrada-
tion potential of the carbonate factory and lead to an
increase in accommodation space. The sedimentary sig-
nature of reduced carbonate production and the increase
in accommodation space is the formation of sedimen-
tary discontinuities separating neritic from deeper wa-
ter sediments. The increase in nutrient levels and the
decrease in sea-surface temperatures associated with
an acceleration of the biological pump and atmospheric
CO
2
draw-down during oceanic anoxic events have
been invoked to explain the drowning of Cretaceous
platforms (Hallock and Schlager 1986; Rougerie and
Fagerstrom 1994).
•
Oversteepening and self erosion of platform mar-
gins
. Drowning may occur by backstepping of the plat-
form rim and related partial multiple drownings (Erlich
et al. 1990; Betzler et al. 1995). Sediment bypass from
the platform to deeper water will result in a sediment
accumulation on the platform that is lower than the sea-
level rise, thus terminating platform growth.
•
Burial of carbonate platforms by prograding silici-
clastics
. Platforms may be suffocated by shelf sands or
prodelta shales (Schlager 1989).
Shallow-marine platforms and reefs are drowned when
rising sea level or tectonic subsidence outpaces car-
bonate accumulation and benthic carbonate production
ceases (Schlager 1981). Drowned platforms present a
fundamental paradox, because the growth potential of
healthy carbonate platforms is thought to be possibly
one or two orders of magnitude greater than long-term
rates of sea-level rise. Prolonged periods of platform
drowning events over a geographically restricted area
result in drowning unconformities (Schlager and Cam-
ber 1986; Erlich et al. 1990) that can be recognized
from geologic and/or seismic data. Drowning uncon-
formities are related to an abrupt shift from shallow-
water carbonate sedimentation to deeper-water clastic
or carbonate sedimentation.
A specific type of drowned platforms are
pelagic
carbonate platforms
(PCPs; Catalano et al. 1977) cor-
responding to submarine intrabasinal highs, which are
drowned fragments of ancestor shallow-marine plat-
forms and ramps (Sanantonio 1994). These platforms
are common at rifted continental margins bordered by
synsedimentary faults. PCPs are sites of condensed and
discontinuous pelagic sedimentation. They are recog-
nized by a pile of shallow-water limestones capped by
thin condensed pelagic sequence and bounded by paleo-
faults. Other terms used for these depositional sites are
seamount, swell, plateau, guyot or 'haut-fond péla-
gique'. The terms seamount and guyot should be re-
stricted to submarine highs built on oceanic crust (see
Sect. 15.7).
Drowning mechanisms
The death of carbonate platforms is caused by dif-
ferent mechanisms, including long- and short-term sea-
level fluctuations, tectonic processes, climate and re-
lated environmental changes. Long-term geodynamic
processes triggering sea-level history may lead to the
drowning of platforms and reefs, if the reduction of
growth with increasing time is largely caused by envi-
ronmental factors (Schlager 1999). In many cases,
drowning might have resulted from an interplay of dif-
ferently scaled factors as shown by the study described
in Sect. 15.6.2.2.
15.6.2.1 Microfacies Signals of Drowning
History
Capped by shales or tight limestones and often exposed
and leached prior to drowning, drowned platforms car-
bonates are stratigraphic traps for hydrocarbons (e.g.
Miocene platforms, Pearl River Mouth Basin, offshore
China). Understanding the origin of drowning allows
us to make better reservoir and seal predictions. Micro-
facies analysis of core material offers a possibility to
•
The increased and rapid pulses of relative sea-level
rise
caused by tectonic plus eustatic events (Schlager
1981; Dominguez et al. 1988; Purdy and Bertram
1993). This is the simplest explanation and may ex-
plain worldwide drowning events.
