Geology Reference
In-Depth Information
within the basin and redeposited within the same area
(Pl. 16/7, 8). An
extraclast
is a fragment of carbonate
rock derived from the erosion of an exposed ancient
limestone on land outside the depositional basin in
which it is found (Pl. 16/2, 4). These definitions look
succinct, but are in practice somewhat tricky because
of the difficulties involved in the precise recognition
of the two categories in thin sections. Many authors,
therefore, use the term
lithoclast
(Folk 1962) to char-
acterize both, erosional detritus brought into the depo-
sitional area from an outside source (= extraclasts) as
well as pieces of semilithified sediment which have
been reworked syndepositionally (= intraclasts). The
term
limeclast
(Wolf and Conolly 1964) which desig-
nates 'limestone rock fragments and grains' has the
same comprehensive meaning. The term was introduced
to replace the genetic term 'intraclast'. Genetically,
limeclasts embrace however, 'intraclasts' as well 'ex-
traclasts'. It should be noted, that the term lithoclast
referred in the original definition only to angular frag-
ments of reworked non-contemporaneous particles of
extrabasinal lithified carbonate sediment.
which are derived from the erosion of older lithified
limestones (Folk 1959). The grain size of these rocks
ranges from silt to gravel, but is commonly formed in
the sand or conglomerate sizes. Many calclithites are
typically stream or alluvial fan deposits. Calclithites
record rapid erosion and signify a high-relief source
area, as shown by the high content of carbonate extra-
clasts in the lithic sandstones of the Alpine Molasse
basins.
4.2.8.1 Intraclasts: Origin and Facies
Diagnostic Types
Intraclasts are commonly found in shallow-marine en-
vironments, but also are transported to deepwater. Shal-
low-marine environments in which intraclasts are
formed are characterized by wave-dominated regimes
and tides that continuously rework carbonate. 'Rip-up
clasts' are common in inter- and supratidal settings but
also occur in subtidal environments where scouring
undermines lithified mud beds (Exuma Sound, Baha-
mas). Intraclast grainstones are often interpreted as de-
posits formed by storm wave erosion and reworking of
various sediment types occurring in shallow-marine en-
vironments. The occurrence in cross-bedded channel
fills supports this interpretation. Intraclasts can also
originate by desiccation of carbonate muds in supratidal
environments (Ainardi and Champetier 1976).
Intraclasts formed by seismic events are discussed
in Sect. 12.4.
Other terms that describe genetic subcategories of
intraclasts are plasticlasts, algal intraclasts, pseudo-in-
traclasts, and protointraclasts. The last term as well as
caliche intraclasts refer to diagenetic products.
Plasticlasts
(Folk 1959) result from the reworking
of weakly consolidated lime mud at the sea or lake bot-
tom by waves or currents; the grains correspond to small
mud flakes and mud pebbles (Pl. 16/7). These micritic
particles are usually small (< 0.5 mm); they have the
same composition as that of the micritic matrix.
Algal intraclasts
resulting from reworking of lime
mud covered by microbial and algal mats are common
constituents of inner ramp and platform carbonates.
Pseudo-intraclasts
(Wobber 1965) are produced by
bioturbation. The breakup of the sediment by burrow-
ing organisms causes variously sized, often angular mi-
critic clasts, which are more or less still in place.
Protointraclasts
(Bosellini 1964) or autoclasts
(Sander 1936) can be formed by early diagenetic move-
ment of sediment related to submarine or subaerial de-
hydration of the sediment, but also to seismic shocks.
The common characteristic is that the boundaries of
the separated clasts still fit like pieces of a jigsaw puzzle.
Diagenetically formed intraclasts which are still in place
also occur in calcareous paleosols.
These
caliche intraclasts
(Siesser 1973) are subae-
rially broken pieces of caliche crusts (Esteban and
Klappa 1983; Francis 1986; Pl. 128/6).
Calclithite
is a terrigenous or carbonate-cemented
rock consisting of more than 50% carbonate extraclasts,
Box 4.21.
Selected records of ancient and modern 'black
pebbles'.
'Black pebbles' are known from ancient paleo-
sols, karstic sinkholes and reworked speleothems, lacus-
trine ponds, beachrocks as well as from tidal and sub-
tidal carbonates. A model for blackening and black-
pebble sedimentation was described by Leinfelder
(1987): See Fig. 4.29.
Holocene:
Barthel 1974; Houbolt 1957; Maiklem 1967;
Shinn 1973; Shinn et al. 1969; Strasser 1984; Strasser
and Davaud 1986; Sugden 1966; Van Straaten 1954;
Wagner and van der Togt 1973; Ward et al. 1970.
Pleistocene:
Beach and Ginsburg 1980; Montenat 1981.
Tertiary:
Freytet 1982; Häfeli 1966; Platt 1992; Shinn
and Lidz 1988.
Cretaceous:
Barthel 1974; Cotillon 1960; Francis 1986;
Lang and Tucci 1997; Rey 1972; Strasser and Davaud
1983.
Jurassic:
Enay 1980; Seyfried 1980; Leinfelder 1987.
Triassic:
Bechstädt 1975, 1979; Bechstädt and Döbler-
Hirner 1983; Eppensteiner 1965; Henrich 1984; Piller
1976; Wignall and Twitchett 1999.
Carboniferous:
Wilson 1967.
Ordovician:
Sardeson 1914.
