Geology Reference
In-Depth Information
Limestone turbidites, allodapic limestones and the
Meischner Sequence
Carbonate and siliciclastic turbidites: Limestone
turbidites differ from siliciclastic turbidites in
Lithoclasts are often concentrated at the base, bio-
clasts in higher parts of the turbidite bed.
Skeletal grains include benthic organisms derived
from shallow-marine and slope environments, frag-
ments of reef organisms, and some pelagic fossils.
the size of the bioclastic particles; it is predominantly
controlled by ecological constraints in the source
area and by taphonomic criteria, and not by the range
of grain transport as for siliciclastic grains;
Secondary silicification is common. Late diagenetic
silification occurs along sedimentary structures due
to the high porosity. Pore waters are enriched in silica
because of the rapid burial of the sediment. Silicif-
ication is often preceded by calcite cementation
(Hesse 1987).
the abundance of lithified particles. Platform and
shelf carbonates are rapidly cemented. Lithoclasts
are therefore much more common than in siliciclas-
tic turbidites;
the variability of grains contributing to limestone
turbidites; it is much higher than the rather uniform
composition of siliciclastic turbidites. Transport and
settling of skeletal grains is influenced by differences
in size, shape, microstructure, porosity and density.
Hydraulic sorting is common.
Source areas of limestone turbidites are shallow-
water environments (platforms, platform-margins,
banks, reefs) and slope environments. Limestone tur-
bidite beds can be followed over several hundred meters
to several kilometers. Bed thickness ranges between
< 1 cm and several meters. Amalgamation of thick beds
is common.
Unlike siliciclastic turbidites, complete Bouma Se-
quences are rare in carbonate deposits. Some of these
differences may be caused by the relatively weak
thixotropy of calcareous muds as compared to clay
muds.
Allodapic limestones: This term, suggested by
Meischner (1964), denotes limestone beds consisting
of graded carbonate debris, transported and redepos-
ited by turbidity currents. The term 'allodapic' refers
to detrital material 'which originated elsewhere'. Origi-
nally coined for limestone turbidites yielding grains
derived from platform rims or reefs, the term is now
used more or less as a synonym for carbonate turbid-
ites.
Criteria of limestone turbidites: Carbonate turbid-
ites occur in deep-marine and shallow-marine, but also
in lacustrine settings. Sequences containing turbidites
are interpreted as fan deposits, or described by apron
models.
Common criteria are:
Internal sequence: The Meischner Sequence (Fig.
15.18) summarizes the criteria of an ideal allodapic
bed. The composition and sequence of allodapic beds
is determined by the amount of transported mate-
rial, the distance from the source area, the rate of
accumulation, the intensity of background sedimen-
tation.
Thick or thin allochthonous limestone beds interca-
lated within micritic carbonates, marls and argilla-
ceous sediments.
Graded calcarenites or calcisiltites; calcirudites.
Lower bed boundary sharp, but sometimes with bot-
tom marks (groove and flute casts, erosional marks).
Gradual transition of top surfaces into overlying
beds.
Geometry: Each bed forms a lenticular body. The
coarser basal parts reach their maximum thickness
upstream. The fine-grained upper parts have their
maxima further down stream.
Vertical sequence consisting of sedimentary units
(zones) characterized by specific sedimentary struc-
tures (Meischner Sequence).
Fossils: The coarse-grained parts of allodapic lime-
stones usually contain benthic biota, both macro-
fossils and microfossils. Mixing of ecologically dif-
ferent types may occur. Microfossils can exhibit hy-
draulic sorting resulting in a fractionation of fora-
minifera depending on size, density and settling
potential (Herbig and Mamet 1994). Planktonic fos-
sils occurring in fine-grained parts are sometimes
strikingly well preserved.
Grading and lamination common.
Cross-bedding and convolute lamination relatively
rare.
Internal size grading.
Sizes of lithoclasts range from silt to boulders, sizes
of skeletal grains between coarse sand and silt.
Grain size decreases parallel to the decrease of bed
thickness.
Sorting in coarse parts good, in laminated parts
moderate to good.
Diagenetic overprint, e.g. compaction, can destroy
primary depositional boundaries. Care must be taken
in differentiating depositional lamination and lami-
nation caused by stylolitization. Pressure solution
Particles are skeletal grains, peloids and ooids as
well as lithoclasts and sometimes also extraclasts.
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