Geology Reference
In-Depth Information
Fig. 15.31.
Lithofacies categories of pelagic limestones
resulting from variations in depth, current activity and clay content.
After Tucker (1990).
A:
Thick sequences of parallel, thin-bedded limestones, sometimes with some lamination and bioturba-
tion. Nearly continuous, out-of-suspension deposition. Common on deep-water slopes and subsided platforms.
B:
Bioturbated
and nodular limestones. Slower sedimentation rates allow burrowing and destruction of lamination. Common on deep-water
slopes and subsided platforms.
C:
Bedded limestones showing evidence of cementation in the form of intraclasts and hard-
grounds with surfaces encrusted and bored. Common on topographic highs.
D:
Bedded limestones with evidence of sea-floor
dissolution in the form of corrosional hardgrounds and molds of aragonitic bioclasts. Common on topographic highs.
E:
Pelagic
skeletal sands originating in places where current winnowing removes lime mud, leaving larger pelagic skeletal grains (e.g.
foraminifera, thin-shelled bivalves, styliolinids). Common on topographic highs. Fe-Mn encrustations and nodules are com-
mon in the categories C and D associated with discontinuity surfaces.
91/6). Common planktonic and nectonic
constituents
of Mesozoic pelagic limestones
are coccolithophorids
(Pl. 7/3, 5), planktonic foraminifera (Pl. 73), calci-
spheres (Pl. 73/2), radiolarians (Pl. 19/2; Pl. 76/1),
calpionellids (Fig. 10.31), ammonites (Pl. 90/1), thin-
shelled bivalves ('filaments', Pl. 87/3), ostracods, and
free-swimming crinoids (Fig. 10.56).
(Fig. 15.32) and contain abundant nautilid cephalopods
and shells of trilobites, echinoderms, mollusks, ostra-
cods, and bryozoans. Glauconite, hardgrounds and evi-
dence of submarine lithification associated with non-
deposition are abundant. Similar limestones but also
dark, laminated pelagic lime mudstones are known from
North America, associated with pelagic and hemipe-
lagic black graptolite shales (Matti et al. 1975; Cook
and Taylor 1977).
15.8.2 Examples and Case Studies of
Pelagic Carbonates
The following section presents examples displaying
common microfacies types of Paleozoic and Mesozoic
pelagic limestones formed in epeiric seas, at the mar-
gins of continents and in open ocean basins.
15.8.2.1 Microfacies of Paleozoic Basinal
Carbonates
Widely distributed pelagic Paleozoic microfacies types
common in the Early Paleozoic are gray and red, often
burrowed bioclastic wackestones and packstones with
abundant hardgrounds. Paleozoic pelagic facies occur
on epeiric ramps and at the margins of continents.
Cambrian and Ordovician cephalopod limestones of
the Baltic Shield
Fig. 15.32.
Nodular bioclastic wackestone,
an example of
Early Paleozoic open-marine limestones deposited in deep
shelf environments and regarded as pelagic sediments. The
micritic matrix is very fine-grained. The source of the matrix
is believed to be finely comminuted macrofossil debris
(Lindström 1979). The sample is a burrowed skeletal wacke-
stone. Most of the shells are trilobite fragments associated
with some echinoderm plates. The picture is a detail of Pl.
19/1. Ordovician: Öland Island, Sweden.
Cephalopod limestones, informally named '
Ortho-
ceras
limestones' because of the occurrence of nautilid
cephalopods, were formed in deep shelf environments
of epeiric seas (Ferretti and Kriz 1996). These lime-
stones are variously colored, often strongly burrowed
