Geology Reference
In-Depth Information
6.1.2 Grain-Size Studies of Modern and
Ancient Carbonates
•
Oncoids: < 2 mm to > 40 millimeter, commonly
< 20 mm; calcirudite; granules to very coarse pebble
size.
The following chapter summarizes the still rather in-
complete information about grain-size distributions of
recent marine carbonate sands, and discusses the ap-
plication of grain-size studies to the microfacies analy-
sis of carbonate rocks.
•
Rhodoids: about 5 to 40 mm, commonly < 100 mm;
calcirudite; fine to very coarse pebble size.
•
Aggregate grains: 0.5 to several millimeters, com-
monly < 1 mm; calcarenite and calcirudite; coarse
sand to medium pebble size.
•
Intraclasts: < 0.10 to about 10 mm; calcarenite and
calcilutite; fine sand to coarse pebble size.
6.1.2.1 GrainSize Studies of Modern
Carbonate Sediments
•
Carbonate extraclasts: < 1 mm to several tens of mil-
limeters; calcarenite and calcirudite.
How does sorting of carbonate sediments compare with
sorting of siliciclastic sediments?
Controls on the sizes of carbonate grains
If the grain size of carbonate rock is taken as a guide
to energy levels in the environment of deposition, sev-
eral points must be born in mind:
•
Differences between siliciclastic and carbonate
sands:
The grain-size distribution of siliciclastic sands
depends on the source, weathering, erosion and trans-
port processes. Marine carbonates are predominantly
biogenic sediments (Sect. 2.1.2). Therefore, the size of
carbonate particles is highly controlled by the original
size and decay of the skeletons, and by size-reduction
processes which take place during transport. In sub-
tidal carbonate environments, however, the areal effect
of larger transport processes is rather restricted and the
sediment is produced nearby or at the site of final depo-
sition.
Larger transport processes occur if storms and strong
currents cause bottom erosion and redeposition of car-
bonate sands. Many bioclastic carbonates exhibit a bi-
modal grain-size distribution consisting of silt- and
sand-sized grains and mud.
size and sorting of skeletal grains may reflect only
growth sizes as for example, shown by foraminiferal
sands in back-reef settings;
different sizes of microfossils can by caused by hy-
draulic sorting as described from calciturbidites
(Herbig and Mamet 1994);
•
calcareous organisms have different architecture and
will therefore disintegrate in different ways; corals
and
Halimeda
, for example, have an inbuilt bimo-
dality (Folk and Robles 1964; Hoskin et al. 1983;
Fig. 6.4);
•
skeletal grains have unique hydrodynamic proper-
ties and are commonly porous or hollow and con-
tain organic matter (Braithwaite 1973);
•
the effect of the hydraulic modification on the depo-
sitional grain size of skeletal grains depends on the
shape, bulk density, size, angularity and structure of
bioclastic grains. The variability in the shapes of skel-
etal grains causes variations in the settling veloci-
ties of particles with identical diameters;
•
Common size range of carbonate grains:
The sizes
of carbonate grains vary between <0.10 mm and sev-
eral centimeters. Skeletal grains, oncoids, rhodoids,
aggregate grains and extraclasts exhibit wide size
ranges, peloids, ooids, pisoids and intraclasts rather nar-
row ranges:
skeletal grains, ooids, peloids and intraclasts can dif-
fer in their susceptibility to transport depending on
the differences in grain shape and density of the
grains (Matthews 1991);
•
•
Skeletal grains: Commonly within the range of
< 0.250 mm to about 20 mm; coarse calcisiltite, calc-
arenite and calcirudite; medium silt, sand, granules
and to coarse pebble size.
sand-sized skeletal grains and intraclasts often ex-
hibit similar grain-size and sorting patterns;
•
carbonate fecal pellets differ in their hydrodynamic
modification from other carbonate grains (Wanless
et al. 1981);
•
•
Peloids: Commonly about 0.10 to 0.20 mm (except
for fecal pellets which can be larger); fine to me-
dium-sized calcarenite; fine and medium sand size.
ooids are susceptible to long-distance transport by
currents: sorting of ooids is a measure for ooids de-
posited more or less in place or allochthonous trans-
ported ooids (Anwar et al. 1984; Sect. 4.2.5);
•
•
Ooids: < 2 mm, commonly 0.5 to 1 mm; coarse and
very coarse calcarenite.
•
Pisoids: > 2 mm to several tens of millimeters, but
sometimes < 2 mm; calcirudite; granules to coarse
pebble size.
specific grain sizes can be produced by organisms
(fishes, crabs) that crush hard parts in their search
for food;
•
