Geology Reference
In-Depth Information
clasts Pl. 26/7), (b) sediment infilled ('sucked in') af-
ter the deposition of the clasts, (c) the residual matrix
of beds which have been affected by solution (stylo-
breccias, Pl. 26/1), or (d) fine erosional detritus con-
sisting of rock debris (Pl. 26/5) The major aim of the
study of matrix composition is to find out if clasts and
matrix are different with regard to the environment and
timing of breccia formation.
Cement minerals:
Common chemical precipitates
of carbonate breccias are calcite cements. The miner-
alogical composition, cement types and spatial distri-
bution of pore-filling carbonate cements are important
criteria for understanding the course of the diagenetic
and tectonic history of breccias.
The following summary of common breccia types
is based on the suggestions of Norton (1917), Blount
and Moore (1969), Keith and Friedman (1977) and
Richter and Füchtbauer (1981). Major genetic catego-
ries include depositional, non-depositional, tectonic and
diagenetic breccias (Fig. 5.16). Depositional, non-
depositional and tectonic breccias as well as conglom-
erates differ in composition and the role of transport
processes (Fig. 5.15).
Depositional breccias
Mass-flow breccia
Field observations:
Bedded, primary sedimentary
structures, stratigraphically concordant, stratabound.
Variable thickness, large lateral extension. Lens-shaped
bodies, massive laterally extended beds. Variable grad-
ing (inverse and normal). Upper and lower boundary
sharp or undulated, lower boundary often erosive or
channelized.
Clasts:
Lithologic composition: often polymict (Fig.
5.21). Fabric usually clast-supported, no primary fit-
ting but common boundaries due to pressure solution
(Pl. 26/1). Close to open packing. Variable shape, all
degrees of roundness, subangular and rounded clasts
common (Pl. 26/1, 3), Pl. 27/1). Variable size, poor to
moderate sorting (Pl. 114/1, 3). Fracturing or veining
of clasts (if present) predates breccia formation. Clast/
matrix boundary sharp. Microfacies variability very
Fossils
The fossils occurring within the matrix and in the
clasts are the main criteria in
(a) reconstructing the environment and facies pat-
terns in the source area of depositional breccias and
conglomerates (Pl. 27), and
(b) evaluating age and timing of breccia and con-
glomerate formation.
In basin analysis, the microfacies data of isolated
pebbles and of breccia clasts assists significantly in rec-
ognizing erosion events connected with the breakdown
of carbonate platforms (e.g. Flügel and Kraus 1988)
and of allochthonous sedimentation on slopes and in
deeper basins (e.g. Herbig 1984).
Abundant stratigraphically useful microfossils seen
in microfacies thin sections of Phanerozoic conglom-
erates and breccias include foraminifera, calcareous al-
gae and many microproblematica (see Chap. 10). The
combined use of thin-section biota and microfossils rec-
ognized in solution residues of limestone pebbles (e.g.
conodonts) is highly recommended (e.g. Pohler and
James 1989).
Because fossils can be reworked several times and
set free from surrounding rock material, difficulties can
arise in evaluating 'isolated' microfossils found within
the matrix. Taphonomic criteria (preservation) should
help overcome these pitfalls (Pl. 9/7).
5.3.3.3 Carbonate Breccia Types: Origin,
Classification, Criteria
Fig. 5.15.
Relations between the composition of breccias and
conglomerates and the relative motion of clasts.
Note the
differences between non-depositional breccias (shrinkage and
solution/collapse breccias) and tectonic breccias (internal, fis-
sure-fill and shear breccias) as compared with depositional
breccias. Conglomerates, including synsedimentary soft-clast
conglomerates, appear to be distinctly separated from brec-
cias, but may act as precursor stages of breccia formation.
Modified from Richter and Füchtbauer (1981).
Carbonate breccias are of different origins. Their clas-
sification is essentially genetic and differs conceptu-
ally from those of limestones. Classification systems
are based either on descriptive criteria or inferred from
brecciation processes (Spence and Tucker 1997).
