Geology Reference
In-Depth Information
form margins (Spence and Tucker 1997). Diagnostic
criteria of megabreccias are (a) the association of mega-
breccias with various types of mass-flow deposits, (b)
mixtures of extrabasinal clasts, intrabasinal clasts and
tabular intrabasinal blocks with dimensions of tens of
meters to several hundreds of meters, and (c) the com-
mon disorientation of geopetal fabrics within blocks
(Pl. 26/7) or from one block to another block. Spec-
tacular examples are known from toe-of-slope depos-
its (e.g. Gullo and Vitale 1987; Flügel et al. 1991; Haas
1999) and in accretionary settings (Sano and Tamada
1994). Decameter-sized constituents of megabreccias
should not be mistaken for autochthonous formations.
Boundaries and relationships to adjacent rocks
: The
genetic interpretation of breccias needs data concern-
ing the basal and upper boundaries and the relations to
the underlying and adjacent rocks. Lower contacts can
be erosional and sharp, gradual or passing into frac-
tures and fissures (e.g. internal breccias). Breccias
formed by sliding and slumping often exhibit multiple
brecciation phases and reverse gradation of the clasts.
Differences in the color of clasts and groundmass
:
The color of the clasts and the fine-grained matrix is
important in recognizing extra- or intrabasinal origins
of the material.
Clasts
Lithologic composition:
The composition of clasts
is differentiated with regard to (a) homogeneity or dif-
ferences in lithology (monomict and polymict breccias,
pure carbonate breccias, lithologically mixed breccias),
and (b) the quantitative proportions of different litholo-
gies. Lithology is commonly described in terms of field
observations only (e.g. limestone, dolomite or chert
clasts) but should be evaluated by thin sections. Com-
monly, very small clasts are attributed to the matrix
during field work, but thin sections might provide evi-
dence that precisely these clasts are the only relicts of
stratigraphically older rocks that have suffered repeated
erosion and redeposition. Quantitative analysis needs
statistical sampling based on line-, ribbon-, area- or
point-counting (Sect. 6.2.1). Quantification must con-
sider not only the frequency of lithologic types but also
the frequency of clasts which catch the eye by their
particular rock color, rounding or size.
Compositional clast analysis offers key informa-
tion on (a) extra- or intrabasinal sources, (b) the envi-
ronmental controls in the provenance area, (c) multiple
redeposition, (d) depositional or non-depositional for-
mation of the breccias and (e) the site of breccia for-
mation (terrestrial, marginal-marine, marine).
Fabric
: The fabric of breccias is characterized by
matrix-support or clast-support. Matrix-supported brec-
cias exhibit clasts floating within fine-grained matrix
(Pl. 26/2) or a calcarenitic groundmass. Clast-supported
breccias consist of clasts supporting each other (Pl. 26/
3). This differentiation resembles the distinction be-
tween mud-support and grain-support used to charac-
terize depositional structures of limestones (Dunham
1962), but should not be regarded as strictly genetic,
because matrix- and clast-support fabrics are caused
by depositional as well as non-depositional processes
(e.g. solution, brecciation, tectonic shear).
Size and sorting
: The size of carbonate clasts is
highly variable. Clasts can range from arenite to boul-
der size. Shallow-marine breccias and mass-flow brec-
The timing of breccia or conglomerate formation
can be indicated by the adjective 'intraformational'.
Intraformational breccias
are formed by synsedimen-
tary processes and consist of clasts believed to have
formed within the depositional basin. The clasts origi-
nate from penecontemporaneous reworking and rede-
position fairly close to the site of formation. Breaking-
up takes place subaerially (e.g. drying of mud on tidal
flats) or in a submarine setting by tidal currents (Pl.
132/4), storm waves (Pl. 132/2), or gravity flows
(Walcott 1894; Assaruri and Langbein 1987).
5.3.3.2 How to Describe Carbonate Breccias?
Basic criteria relevant to the microfacies analysis of
carbonates breccias include:
Field relationships
Geometry, thickness and lateral extension
: Geom-
etry as well as local or regional extension of breccia
bodies are crucial in understanding breccia genesis.
Shallow-marine breccia and carbonate conglomerates
are characterized by: individual layers usually a few
decimeters thick, discontinuous layers ranging from
meter-sized lenses to a lateral extension of many tens
of kilometers (Grotzinger 1986). Mass-flow breccias
can extend laterally several hundreds of meters to tens
of kilometers.
Stratification, grading and sedimentary structures
:
Breccias are massive or stratified. Stratification should
be described in terms of distinctness, dimensions and
sedimentary structures (cross-stratification, high-angle
edgewise structures, common in modern beach depos-
its, occurring in lenses and pockets). Edgewise con-
glomerates have various origins, they occur as erosional
scours in supratidal flats, as discontinuous layers near
linear mounds, and on beach ridges along eroded shore-
lines. Breccia grading can be normal or inverse.
