Geology Reference
In-Depth Information
sedimentation. In geological sections they are recog-
nized by a pile of shallow-water limestones capped by
a condensed pelagic sequence and bounded by paleo-
faults. Instructive examples were described by Wendt
(1988) from the Devonian, and Winterer and Bosellini
(1981) and Santantonio (1993) from the Jurassic. Other
terms often used for pelagic platforms are seamount,
swell, plateau, guyot or haut-fond pélagique. The terms
seamount and guyot, however, should be restricted to
submarine highs built on oceanic crust.
Box 15.4. Selected references on the criteria and micro-
facies of ancient pelagic and deep-water basinal carbon-
ates. References on ancient hardgrounds are listed in
Box 5.5.
Cretaceous and Tertiary
Chalk
: Bromley and Ekdale
1987; D'Heur 1984; Feazel et al. 1985; Feazel and Farrell
1988; Hakansson et al. 1974; Hardman 1982; Hardman
and Kennedy 1980; Kennedy 1980; Maliva and Dickson
1992; Neugebauer 1973, 1974; Schatzinger et al. 1985;
Scholle 1977; Van den Bark and Thomas 1980
Jurassic:
Alvarez 1989; Bernoulli 1971; Bernoulli and
Jenkyns 1970; Blendinger 1988; Castellarin 1970;
Catalano et al. 1977; Bice and Stewart 1990; Böhm 1992;
Böhm et al. 1999; Ebli 1997; Evans and Kendall 1977;
Garrison 1967; Garrison and Fischer 1969; Gawlick et
al. 1999; Germann 1971; Hollmann 1962, 1964; Jenkyns
1970, 1971; Kuhry et al. 1976; O'Dogherty et al. 2001;
Sanantonio 1993; Seyfried 1980; Soussi and Ben Ismail
2000; Wierczbowski et al. 1999; Wilson 1969; Winterer
and Bosellini 1981
Triassic
: Bachmann and Jacobshagen 1974; Blendinger
1988; Dürkoop et al. 1986; Krystyn et al. 1971; Schlager
1974; Wendt 1969, 1973; Zankl 1971
Carboniferous:
Bissell and Parker 1977; Yurewicz 1977
Devonian:
Bandel 1974; Tucker 1973, 1974; Matti et
al. 1975; Tucker and Kendall 1973; Vai 1980; Wendt
and Aigner 1985; Wendt 1988
Ordovician:
Cook and Taylor 1977; Ferretti and Kriz
1995; Jaanusson 1955, 1973; Lindström 1963, 1971,
1979; McBride 1970.
Controls
on the facies of pelagic carbonates are the
sedimentation rate determined by the organic produc-
tivity, sea-floor currents and circulation patterns, and
the water depth relative to the calcite compensation
depth. These parameters control sea-floor cementation
and dissolution.
Pelagic carbonates can be attributed to
five major
lithofacies categories
(Tucker 1990; see Fig. 15.31).
Another spectrum of pelagic lithofacies is reflected
by the increasing
clay content
ranging from pure lime-
stones through limestones with shaley partings and
pressure solution seams to nodular limestones and then
to shales (marls) with dispersed calcareous nodules of
early diagenetic origin. This limestone to shale spec-
trum may result from
•
topography: pelagic limestone preferentially depos-
ited on topographic highs, shales with nodules in
topographic lows;
•
increasing clastic input and decreasing carbonate
productivity;
'
Pelagic carbonate platforms'
(Catalano et al. 1977)
corresponding to type B of Fig. 15.30, are submarine
intrabasinal highs representing drowned fragments of
ancestor shallow-marine platforms and ramps (Sant-
antonio 1993). They correspond to horsts or crests of
rotated blocks and are common at rifted continental
margins bordered by synsedimentary faults. These plat-
forms are sites of condensed and discontinuous pelagic
•
increasing depth with loss of CaCO
3
as the CCD is
approached.
Biota:
Common planktonic and nectonic
constitu-
ents of Paleozoic pelagic limestones
are radiolarians,
goniatite cephalopods (Pl. 90/2), nautilid cephalopods,
ostracods, tentaculitids (Pl. 91/3), and styliolinids (Pl.
Fig. 15.30.
Range of depositional sites for pelagic carbonates.
After Tucker (1990). Pelagic sediments are deposited in
relatively shallow to very deep environments.
A
: Pelagic sediments occurring in neptunian dikes of shallow platforms.
B
: Pelagic sediments occurring on isolated platforms caused by isolated fault-blocks and on seamounts; generally little
pelagic sediment is deposited. Hardgrounds and Fe-Mn crusts are common.
C
: Thick sequences of pelagic carbonates may
develop upon subsiding platforms if there is little winnowing and dissolution.
D
: The pelagic facies on slopes is character-
ized by thin or thick resediments (Sect. 15.7.2).
E
: Thick pelagic carbonates will accumulate in basins. Marly intercalations
and calciturbidites are common.
