Geology Reference
In-Depth Information
intertidal and subtidal zones; most boring chips are silt-
sized (Futterer 1974, Moore and Shedd 1977). Clion-
ids are common in tropical regions, but also occur at
high latitudes. Boring bivalves (e.g.
Lithophaga
, Pl. 52/
8) and snails occur preferentially in shallow seas.
Of particular importance are heterotroph bacteria,
cyanobacteria, green and red algae as well as fungi (Pl.
52/1-4) producing microborings and causing micrit-
ization (Pl. 12/2, Pl. 13/6). Bacterial attacks of organic
matter in shells lead to the disintegration of skeletal
grains into ultrastructural units. Borings of bacteria and
fungi have diameters <4
m, often 1-2
m. Microbor-
ers produce species-specific boring patterns are used
as paleobathymetrical markers (see Sect. 9.3.4).
Bioerosion is responsible for significant carbonate
production in reefs (Peyrot-Clausade et al. 1995; Van
Treek et al. 1996). Abrasion and bioerosion of totally
micritized skeletal grains provide the dominant source
of Mg-calcite lime mud in some modern lagoonal en-
vironments (Reid et al. 1992). Carbonate mud produc-
tion triggered by bioerosion is equally effective in tropi-
cal and non-tropical environments (Farrow and Fyfe
1988; Gaillard et al. 1994).
Grazing and rasping organisms (e.g. mollusks, echi-
noderms, fishes) produce large amounts of fine-grained
carbonate sediment in intertidal and subtidal environ-
ments. A combination of bioerosion caused by micro-
borers, predominantly cyanobacteria, and subsequent
grazing by gastropods and sea-urchins on endolithic
microflora within the inter- and supratidal zone is one
of the main causes of the erosion of limestone coasts
(Torunski 1979).
but chalk is white in color, less dense, and less com-
pact than micrite. The sediment consists of diageneti-
cally altered coccolith plates, smaller micrite grains and
large euhedral calcite crystals resulting from pressure
solution overgrowth. The low cementation potential of
shelf chalks due to the primary Low-calcite mineral-
ogy of most skeletal grains and the lack of meteoric
pore fluids are responsible for the high reservoir quali-
ties of the North Sea chalks (Scholle 1977; Zijlstra
1995).
Planktonic foraminifera micrite (Pl. 6/6)
Although only a small part of the original life plank-
tonic assemblages become incorporated in the sediment,
tests of planktonic foraminifera contribute significantly
to the particle flux from shallow to deep waters and to
the composition of pelagic carbonate muds (Berger
1971, 1975). The transition from death assemblage to
sediment assemblage is strongly controlled by dissolu-
tion of calcite on the sea floor which in turn is a func-
tion of the fertility of the overlying waters and the ag-
gressiveness of the bottom waters. Both factors depend
on oceanic circulation patterns. Planktonic foraminifera
are deposited with nanno-organisms and other calcare-
ous plankton, e.g. calciodinoflagellate cysts (Pl. 73/2).
Nannofossil micrite (nannomicrite, nannoagorite, ultra-
micrite; Pl. 7/3-5, 7)
The contribution of calcitic nanno-organisms to the
formation of very fine-grained micrites depends on pri-
mary productivity, depositional conditions and the
course of diagenesis. Nannofossils include coccolitho-
phorids as well as other nanno-organisms (cone-shaped
nannoconids, schizospherids, spherical thoracosphae-
rids and others) belonging to systematically different
planktonic algal groups. Modern coccolithophorids
flourish in open-marine oceanic waters but also occur
in lagoonal environments (Kling 1975) and reefs
(Conley 1979). The cone-shaped nannoconids were
abundant in Jurassic and Lower Cretaceous strata, but
also occur in Late Cretaceous open-marine microcrys-
talline carbonates. They are commonly associated with
radiolaria, calpionellids, and ammonites and may form
almost pure nannoconid limestones (Pl. 77/23).
Schizo-
sphaerella
(Pl. 7/7) is a common rock building fossil
in Liassic pelagic carbonates (Kälin and Bernoulli 1984;
Bombardiere 1993). Although there are a few reports
of isolated coccolith-like fossils from Paleozoic sedi-
ments and the Middle Triassic as well (Vecsei and
Duringer 1998), the earliest quantitatively important
coccolithophorids appear in Late Triassic (Late Car-
nian) Tethyan marginal seas (Di Nocera and Scandone
1977; Bellanca et al. 1995).
(9) Accumulation of calcareous plankton on the deep
shelf and in basins:
Modern pelagic carbonate muds
deposited largely in water depths of several hundred to
several thousand meters are dominated by (1) accumu-
lations of calcitic planktonic foraminifera associated
with tests of nannoplankton (foraminiferal or
Globi-
gerina
mud), (2) accumulations of calcitic nannoplank-
ton consisting predominantly of planktonic algae (coc-
colithophorids; see Winter and Siesser 1994 for an over-
view; coccolith mud), and (3) deposition of pelagic gas-
tropods (aragonitic pteropods and heteropods; ptero-
pod mud). Planktonic foraminifera evolved at the be-
ginning of the Mesozoic and diversified through the
Cretaceous and Cenozoic. The earliest quantitatively
important calcareous nannofossils appeared in Late Tri-
assic Tethyan marginal seas. Pteropods have been
known since the Tertiary (Eocene).
A pelagic sediment consisting of calcareous plank-
ton is
chalk
(Pl. 7/3) deposited in Cretaceous deep shelf
and basinal settings. The texture is similar to micrite,
