Geology Reference
In-Depth Information
(1) The lysocline, defined as the depth that sepa-
rates well-preserved from poorly preserved planktonic
assemblages and where a noticeable decrease in the per-
centage of carbonate occurs. Because calcareous mi-
crofossils dissolve at different rates, three types of
lysoclines are used: the pteropod or aragonite lysocline
(ALy), defining significant dissolution of aragonite
material occurring in relatively shallow depths; and the
foraminiferal lysocline and the coccolith lysocline (cal-
cite lysocline, CLy), defining the significant dissolu-
tion of calcite material in greater water depths.
(2) The calcite compensation depth (CCD) is defined
as the depths at which calcite microfossils and carbon-
ate are largely absent. The depth of the CCD in modern
oceans varies between about 4 200 and 5 400 m, with a
mean depth of about 4500 m. At that level, the rate of
dissolution balances the rate of accumulation.
-> A cautionary note: The depth position of the CCD
in Phanerozoic oceans has varied during geological time
owing to fluctuations in the amount of carbonate being
brought into the oceans from the land, variations in hy-
drothermal activity, or increased carbonate production
on the shelves.
Because the calcareous tests of planktonic organ-
isms differ in dissolution susceptibilities, dissolution
types can be used to record steps in the formation of
diagenetic patterns. Present-day foraminifera species
exhibit a dissolution ranking that is strongly controlled
by the thickness, size and shape of the tests. Most coc-
coliths are more resistant than foraminifera with regard
to dissolution. First evidence of coccolith dissolution
occurs in depths above 3000 m, but changes in the com-
position of the assemblages become evident in depths
between 3000 and about 5000 m.
onstrated by excellent examples from the Holocene Ba-
hamian slopes. The resulting fine-grained 'periplatform
carbonates' (Schlager and James 1978; Boardman and
Neumann 1984; Boardman et al. 1986; Pilskaln et al.
1989) form a sequence of resedimented shallow-ma-
rine carbonates intercalated within pelagic carbonates.
The Bahamian carbonate slopes (Straits of Florida,
Little Bahama Bank) offer the possibility of studying
the interplay between allochthonous and pelagic sedi-
mentation, and submarine cementation, resulting in the
formation of hardgrounds. The periplatform ooze of the
upper slope consists of a mixture of bank-derived ma-
terial and pelagic debris. Lower slope and base-of-slope
seiments are turbidites and mud- or grain-suported de-
bris flow deposits, interbedded with periplatform ooze,
and forming large carbonate aprons.
Cyclic variations in the prevailing mineralogy (ara-
gonitic and Mg-calcitic platform-derived grains vs cal-
citic pelagic grains) of periplatform carbonates and dif-
ferences in the abundance of platform-derived and pe-
lagic carbonates signalize climate changes, sea-level
fluctuations and changes in the rate of carbonate pro-
duction on the platforms which are reflected by micro-
facies analysis (Sect. 16.1). Another promising ap-
proach of microfacies studies is the evaluation of the
controls on the different declivities of carbonate slopes
which depends greatly on the primary depositional tex-
ture.
Basics: Deepmarine carbonates
Berger, W.H. (1991): Produktivität des Ozeans aus geologi-
scher Sicht: Denkmodelle und Beispiele. - Zeitschrift der
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Cook, E., Enos, P. (eds., 1977): Deep water carbonate envi-
ronments. - Soc. Econ. Paleont. Min. Spec. Publ.,
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, 336 pp.
Cook, H.E., Hine, A.C., Mullins, H.T. (1983): Platform mar-
gin and deep water carbonates. - Soc. Econ. Paleont. Min.
Short Course,
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, 563 pp.
Cook, H.E., Mullins, H.T. (1983): Basin margin environment.
- In: Scholle, P.A., Bebout, D.G., Moore, C.H.(eds.): Car-
bonate depositional environments. - Amer. Ass. Petrol.
Geol. Mem.,
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, 539-617
Coniglio, M., Dix, G.R. (1992): Carbonates slopes. - In:
Walker, R.G., James, N.P. (eds.): Facies models. Response
to sea level change. - 349-373, Ottawa (Geol. Ass. Canada)
Crevello, P.D., Harris, P.M. (1985): Deep water carbonates:
buildups, turbidites, debris flows and chalks. - Soc. Econ.
Paleont. Min. Core Workshop,
6
, 527 pp.
Doyle, L.J., Pilkey, O.H. (eds., 1979): Geology of continen-
tal slopes. - Soc. Econ. Paleont. Min. Spec. Publ.,
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, 374 pp.
Enos, P., Moore, C.H. (1983): Fore-reef slope environment.
- In: Scholle, P.A., Bebout, D.G., Moore,C.H. (eds.): Car-
bonate depositional environments. - Amer. Ass. Petrol.
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Hsü, K.J., Jenkyns, H.C. (eds., 1974): Pelagic sediments: on
land and under the sea. - Spec. Publ. Int. Ass. Sedimentol.,
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2.4.5.7 Carbonate Slopes, Periplatform
Carbonates and Carbonate Aprons
Gravity flows, particularly turbidites and debris flow
deposits, are common on carbonate slopes, and also
occur in basins (Cook et al. 1983; McIlreaü992).
These sediments are derived from shallow-water
platform and platform-edge settings, sometimes also
from the upper slope, and transported to upper, mid
and lower slope settings.
Carbonate slopes were categorized as: (1) 'by-pass
slopes' on which sediment is transported from shallower
to deeper water without significant deposition on the
slope: gravity flows bypass the slopes nd accumulate
in adjacent basins. (2) less steep 'depositional or ac-
cretionary slopes' where gravity flow sediments are pre-
dominantly deposited in lower slope settings, as dem-
