Geology Reference
In-Depth Information
The appearance of calcareous plankton microorgan-
isms in the Mesozoic has markedly moved the location
of major carbonate deposition towards an oceanic set-
ting. Nannofossils were abundant in Late Jurassic-Early
Cretaceous fine-grained carbonates formed in shallow
epicontinental and shelf settings (e.g. Solnhofen lime-
stone, Flügel and Franz 1968; Keupp 1978; Noel et al.
1993), and also were deposited in deep-sea basins (Fig.
4.3). During the latest Jurassic the sink for carbonate
deposition shifted from shallow seas to deep oceans.
This is reflected by the common association of calpio-
nellid limestones, ribbon cherts and pure nannofossil
carbonates, first recognized by Steinmann (1925). Nan-
nofossil productivity was sufficiently high in the pe-
lagic regime to depress the carbonate compensation
depth in the Tethyan Ocean. Owing to the Late Creta-
ceous transgressions, the pelagic carbonate sedimenta-
tion (Chalk facies) spread to epicontinental seas oc-
curred. Many Cretaceous and Early Tertiary chalks con-
sist almost exclusively of coccolithophorids. Post-Ter-
tiary oceanic sediments have a smaller percentage of
coccoliths and larger percentage of pelagic foramin-
iferal remains.
The evidence of nannofossil contribution to micrite
genesis requires detailed SEM studies of the 'nanno-
facies' (Noel 1969) as well as more sophisticated in-
vestigations of the orientation patterns of coccolith el-
ements (Ratschbacher et al. 1994). Sedimentation and
diagenesis of nannomicrites (Fig. 4.4) have been stud-
ied by many authors (e.g. Noel and Melguen 1978,
Flügel and Keupp 1979). The good correlation of the
coccolithophorid assemblages within the water masses
and those found in surface sediments provide a pow-
erful tool for the reconstruction of paleo-oceanographic
patterns. Preservation patterns of coccolithophorids are
important proxies for paleo-bottom currents and dif-
ferential solution at the sea floor and within the sedi-
ment (McIntyre and McIntyre 1971; Schneidermann
1973).
on the sea floor which in turn is a function of the fer-
tility of the overlying waters and the aggressiveness
of the bottom waters. Both factors depend on oceanic
circulation patterns.
The disintegration of plankton foraminiferal skel-
etons is poorly understood (Palmer-Julson and Rack
1992). Some planktonic foraminifera disarticulate into
individual chambers and the chamber walls may dis-
sociate into crystallites (Lowenstam and Weiner 1989).
But most constituents of foraminiferal carbonate muds
range from silt- to sand-size. The microcrystalline com-
ponent of the foraminiferal mud consists of accumu-
lations of coccoliths. In limestones and modern coc-
colith ooze, coccoliths are commonly observed as dis-
articulated plates and are rarely intact. The plates them-
selves may disassociate into crystallites.
(10) Mechanical erosion of limestones:
In high-en-
ergy environments limestone coasts are prone to me-
chanical erosion causing abrasion of clay- and silt-sized
carbonate particles. Examples have been described
from the Adriatic coast (Schneider 1977) and from the
coasts of the Persian Gulf and Puerto Rico (Pilkey and
Noble 1966; Kukal and Saadallah 1973). Abrasion is
facilitated by coeval bioerosional loosening of rock
surfaces. Rivers as well as wind transport the fines.
Interpretation of limestones: Indications for micrites
formed by abrasion are poor sorting and the associa-
tion of fine-grained carbonate and non-carbonate par-
ticles (Lindholm 1969), a very small crystal size
(<2.4
m), as well as rounding and good sorting (Peszat
1991). Because of the lack of calcareous plankton in
the Paleozoic, some authors have argued that Paleo-
zoic micrites formed in deeper ramp, slope and basi-
nal positions, may represent fine-grained carbonate
eroded and winnowed from shallower platforms or
carbonate coasts.
The ten modes of possible origins of micrite de-
scribed above correspond more or less to the original
definition given by Folk (1959, 1962) which focuses
on crystal size and inferred sedimentary origin. Both
crystal size and the different clues to the origin of mi-
crites are more successfully studied by means of scan-
ning electron microscopy and geochemical data than
solely by investigating thin sections alone.
How do plankton skeletons become pelagic ooze?
The sedimentation of calcareous plankton occurs
by settling of individual tests, fecal pelletization, and
inorganically formed flocs of biogenic material ('ma-
rine snow': Lampitt 1996). At the sea bottom plank-
ton is affected by bottom currents and bioturbation.
Although only a small part of the original life plank-
tonic assemblages become incorporated in the sedi-
ment, tests of planktonic foraminifera contribute sig-
nificantly to the particle flux from shallow to deep
waters and to the composition of pelagic carbonate
muds. The transition from death- to sediment-assem-
blage is strongly controlled by dissolution of calcite
Diagenetic micrites (pseudomicrites)
The recognition of the diagenetic character of mi-
crocrystalline carbonate rocks is of crucial importance,
particularly for interpreting ancient reefs and specu-
lating on the energy level during the deposition of fine-
grained sediments on platforms and ramps.
