Geology Reference
In-Depth Information
'Deep sea' as used in marine biology to mean that
part of the marine environment that lies below the level
of effective light penetration for phytoplankton syn-
thesis in the open ocean (
photic zone
), and deeper than
the depth of continental shelves (corresponding approxi-
mately to > 200 m). The upper part of the deep sea
retains some light (
disphotic zone
). The permanently
dark region below sunlit waters is the
aphotic zone
which starts in temperate waters at rather shallow depths
(about 100 m), and in tropical waters at deeper depths
(about 600 m).
2.4.3.1 Pericontinental vs Epicontinental
Shallow Seas
Most modern shallow seas cover shelves around the
margins of the continents. These pericontinental seas
differ strongly from the epicontinental (epeiric) setting
of many ancient shallow seas. Epeiric seas occupied
extensive areas of the continental interior or the conti-
nental shelf. They differ from pericontinental seas with
regard to wave and current regimes, sediment input and
sea level controls. Modern examples of epeiric shelf
seas are rare in contrast to the geological record, where
epeiric carbonate platforms and ramps were common.
Fig. 2.5.
Rimmed and non-rimmed carbonate shelves
(platforms) and ramps
(slightly modified after James and
Kendall 1992). Note the different position of the 'carbonate
factories' at the seafloor.
Carbonate factories
(James 1979) are
subtidal areas characterized by high carbonate production by
predominantly benthic organisms. The optimum autochthonous
carbonate production on rimmed platforms occurs near the
platform margin and behind the margin. On ramps optimum
areas of carbonate production are distributed over the entire
extension of the ramp. Rimmed and unrimmed platforms are
common in tropical and subtropical sunlit waters, today
approximately 30° N and S of the equator, where the carbonate
factory is primarily controlled by high water temperatures
favoring phototroph carbonate-secreting organisms.
Ramps are common in cool-water zones, extending
polward from the limit of the tropical factory to polar latitudes,
and characterized by the dominance of heterotroph organisms
(Sect. 16.4).
Schlager (2000) differentiated a third carbonate factory
(mud mounds), characterized by the in-situ production of
biotically induced and abiotic carbonate mud.
2.4.3.2 Carbonate Shelves, Ramps and Plat-
forms
Carbonate depositional models developed in the 1960s
and early 1970s were strongly based on modern ana-
logues from the Bahamas, Florida, the Yucatan and the
Persian Gulf (Wilson 1975). These models describe
sedimentation patterns on shelves characterize by the
deposition of shallow-water sediments on flat platform
tops, and by significant rims marked by reefs or shoals
near the shelf break. Ahr (1973) noted that the 'rimmed
shelf' model does not fit many ancient carbonate suc-
cessions, and that a different model was necessary, es-
pecially for the epeiric shallow-water carbonates (Irwin
1965).
The term
carbonate ramp
was adopted to describe a
gently sloping depositional surface which passes gradu-
ally without slope break from a shallow, high-energy
environment to a deeper, low-energy environment. A
ramp is attached to a shoreline at one end and to cor-
relative basinal beds at the other end. Fig. 2.5 exhibits
the basic differences between rimmed and unrimmed
shelves (platforms) and ramps with respect to the posi-
tion of the optimum carbonate production (carbonate
factory) and sediment transport.
trouble doing so. Microfacies analysis assists in over-
coming these difficulties (see Sect. 12.3).
Common criteria used by marine geologists and bi-
ologists in distinguishing 'shallow-marine' from 'deep-
marine' environments in modern oceans are the shelf
break and the lower boundary of the well-illuminated
zone. Another important boundary level is the storm
wave weather base (see Box 2.1). Shallow seas occur
above the continental shelf and include marginal-ma-
rine and marine environments, both in pericontinental
and epicontinental settings. Deep-marine sea bottom
environments are located on the slope, at the toe-of-
slope and within basins.
