Geology Reference
In-Depth Information
particles can not be directly compared with quartz grains
or pebbles with regard to morphometric characteris-
tics, because the particles vary in their ability to resist
mechanical destruction and chemical solution, and be-
cause carbonate grains and quartz grains differ in their
initial shapes. However, changes in grain shapes in as-
sociation with changes in grain sizes, sorting and modal
composition allow one to identify different hydrody-
namic levels (Plumley et al. 1962; Catalov 1972; Skupin
1973; see Sect. 12.1.1) and the maturity of calcareous
sands (Folk 1962; see Fig. 8.6). A morphometric index
alone generally does not allow environments to be
clearly distinguished, but the roundness and sphericity
values of bioclasts coupled with grain size and sorting
offer good results in differentiating environments with
different hydrodynamic controls, e.g. reef platforms.
The presence of well-rounded extraclasts in limestones
points to material subjected to fluvial transport or
rounded in coastal environments prior to final deposi-
tion (Pl. 16/2).
Diagenesis:
Variously shaped skeletal grains react
differently with regard to sedimentary packing. Knowl-
edge of the inherent grain volume of carbonate sands
composed of differently shaped grains allows the dy-
namic and inhibitory factors that control grain volumes
in grainstones to be studied quantitatively. These data
assist in understanding relations between burial diage-
netic factors, compaction and porosity development.
Fig. 4.31.
Roundness scale for calcareous shell material
(af-
ter Pilkey et al. 1967). Class 1 fragments exhibit no apparent
rounding of edges or corners. Classes 2 and 3 are gradations
between the fresh fragments and the highly rounded, essen-
tially cornerless fragments of class 4. Note that the images
shown in this chart not only correspond to transport processes
but also reflect preservational bias and various taphonomic
controls. Grains in class 3 can be the result of the activity of
predators, such as crabs, or of the destruction of shells by
storms. Class 4 may indicate multiple reworking or rounding
in a high-energy environment.
Basics: Morphometry of carbonate grains
Barrett, P.J. (1980): The shape of rock particles. A critical
review. - Sedimentology,
27
, 291-303
Depenbroek, M., Bartolomä, A., Ibekken, H. (1992): How
round is round? A new approach to the topic 'roundness'
by Fourier grain shape analysis. - Sedimentology,
39
,
411-422
Folk, R.H. (1955): Student operator error in determination of
roundness, sphericity, and grain size. - Journal of Sedi-
mentary Petrology,
25
, 297-301
Folk; R.H. (1962): Spectral subdivision of limestones types.
- In: Ham, W.E. (ed.): Classification of carbonate rocks.
- American Association of Petroleum Geologists, Mem-
oir,
1
, 62-84
Folk, R.H., Robles, R. (1964): Carbonate sands of Isle Perez,
Alacran Reef Complex, Yucatan. - Journal of Geology,
73
, 255-292
Hilbrecht, H. (1995): Computergestützte Methoden in der
Morphometrie. - Berliner Geowissenschaftliche Abhand-
lungen,
E16
, 765-779
Hofmann, H.J. (1994): Grain-shape indices and isometric
graphs. - Journal of Sedimentary Petrology,
A64
, 916-920
Krumbein, W.C. (1941): Measurement and geological sig-
nificance of shape and roundness of sedimentary particles.
- Journal of Sedimentary Petrology,
22
, 64-72
Kuenen, Ph. H. (1956): Experimental abrasion of pebbles. 2.
Rolling by current. - Journal of Geology,
68
, 427-449
Mills, H.M. (1979): Downstream rounding of pebbles, a
remains unbroken. In contrast, a distinct energy-round-
ness relationship does not exist for sand-sized calcare-
ous bioclasts. This can be explained by the effect of
gradual wearing down rather than fragmentation of
coarser material. An increase in rounding of skeletal
grains is also known in traverses crossing lagoonal to
high-energy environments of modern reef tracts (Folk
and Robles 1964, Swinchatt 1965) and ancient reefs.
4.3.2 Significance of Morphometric Data for
Carbonate Grains
Morphometric data assist in evaluating paleoenviron-
mental conditions (e.g. in Fig. 4.32) and diagenetic pro-
cesses, and can additionally be used in the descriptive
discrimination of samples, of both geological and ar-
chaeological material (e.g. temper grains in ceramics,
Sect. 19.5).
Environmental indicators:
Sand-sized carbonate
