Geology Reference
In-Depth Information
Variable size, mixtures of m-sized and cm-sized clasts
are common. Poor sorting. Clast/matrix boundary sharp.
Microfacies variability of clasts high.
Groundmass:
Matrix common to abundant, consist-
ing of micrite or fine-grained rock debris. Several
phases of matrix infilling in fissures within the semi-
lithified breccia blocks are common.
Fossils:
Common in clasts
and as isolated, often
only poorly preserved fragments.
Case studies:
Prtoljab and Glovacju Jernej 1994;
Semeniuk and Johnson 1985; Sanders 1997; Thiedig
1975.
Case studies
: Aigner 1985; Craft and Bridge 1987;
Einsele and Seilacher 1982; Shinn 1986; Waters et al.
1989; Demicco and Hardie 1994.
Forereef breccia
Field observations:
Meter- to tens of meter thick
units consisting of coarsely bedded or massive brec-
cias. Lateral extension tens to hundreds of meters. Con-
siderable differences between proximal and distal parts
of the breccia with regard to size and sorting of clasts,
clast : matrix ratio, and the occurrence of inter-clast
cements.
Clasts
: Lithological composition monomict but
strong microfacies differentiation. Fabric clast-sup-
ported or mud-supported depending on the position on
the foreslope. Size strongly variable within millimeter
to decameter range. Sorting poor to moderate. Shape
variable, angular to apparently well-rounded. The lat-
ter clasts may have become round by repeated biogenic
encrustations. Low or no fitting. Fracturing or veining
of clasts prior to deposition. Clast/matrix boundary
sharp. Microfacies variability high because clasts are
derived from different parts of the reef complex in-
cluding reef flat, reef margin and various slope envi-
ronments. In addition, back-reef sands can be depos-
ited together with forereef clasts. Common microfa-
cies types of the clasts comprise boundstones, bio- and
lithoclastic wackestones and floatstones, packstones as
well as grainstones and rudstones.
Groundmass
: Fine-grained, usually poorly sorted
matrix (wackestone, packstone, grainstone), sometimes
graded. Submarine carbonate cements in interclast
voids, commonly several cement phases (Pl. 27/2). Dif-
ferential submarine cementation can result in down-
slope displacement of carbonate clasts and sands.
Fossils
: Reef builders and reef dweller are abun-
dant and often highly diverse within the clasts. Bio-
genic encrustations and borings are common on clast
surfaces. Isolated fossils occur in the interclast ground-
mass. Cryptic habitats in cavities between reef blocks
are characterized by specific microfossil associations.
Case studies:
Brachert and Dullo 1990, 1994; Enos
and Moore 1983; Hopkins 1977; Krebs 1969; Melim
and Scholle 1995; Stentoft 1994; Gischler 1995; Vecsei
2001.
Peritidal and shallow-marine breccia
Field observations:
Individual layers usually a few
decimeter thick. Layers discontinuous, ranging from
meter-sized lenses to a lateral extension of many tens
of kilometers. Flat-lying clast orientation, sometimes
caused by currents, sometimes caused by compaction.
True imbrication patterns are indicated by partial in-
filling of interstices of gravel clasts with sand after the
deposition of the gravel framework. Pseudo-imbrica-
tion may occur at the foresets of large-scale cross-strati-
fication. Cross-stratification and high-angle edgewise
structures are common.
Clasts:
Lithological composition monomict, but
strong microfacies differentiation. Fabric clast-sup-
ported to matrix-supported. Low fitting. Bimodal grain
size, with interstitial well-sorted sand-sized grains (pe-
loids, ooids, skeletal grains). Variable shape, many
clasts are highly rounded and form conglomerates.
These conglomerates comprise flat-pebble conglom-
erates caused by erosional rip-up of intertidal and su-
pratidal laminites during storm events, and subtidal
conglomerates. The latter originate as storm or swell
lags in shore-face environments, as lateral accretions
at the base of tidal channels (Pl. 132/4), or from a rip-
up of underlying older tidal flat sediments during rises
in sea level (Pl. 132/3). The last case results in a dis-
tinct polygenic composition of the conglomerate.
Microfacies
of clasts highly variable. Constituents
include desiccated and often laminated mud clasts de-
rived from modern intertidal and supratidal settings
(desiccation breccias), clasts derived from subaerial or
submarine cemented hardground (e.g. lithoclasts con-
sisting of reworked supratidal crusts or of blackened
soil crusts), and grainstone intraclasts. Clast/matrix
boundary usually sharp.
Groundmass
: Fine-grained carbonate matrix, often
dolomitized.
Fossils
: Rare in supra- and intertidal clasts, but com-
mon in subtidal clasts. Matrix with ostracods, foramin-
ifera and mollusk shells.
Non-depositional breccias
Caliche breccia
Field observations
: Related to exposure profiles.
Often superimposed on solution-collapse breccias. Bed-
ding dependent on primary clast distribution.
Clasts:
Strong compositional similarity of limestone
clasts and the limestone underlying beds. Polymict
