Geology Reference
In-Depth Information
can be dissolved by organic acids and blackening is
induced by bitumen or pyrite.
vals of nondeposition (subsolution clasts: Hollmann
1964), but a multistage origin appears to be more reli-
able, including early lithification of some of the car-
bonate mud, followed by exhumation of the lithified
parts by weak currents, erosion and encrustation at the
surface, redeposition within the mud and additional con-
tinued accretionary growth in the sediment. In accor-
dance with this interpretation, the hardground lithoclasts
are therefore intraclasts. Partial lithification of modern
deep marine carbonates muds and subsequent forma-
tion of lithoclasts are known from the Mediterranean
(Müller and Fabricius 1974). Another interpretation is
reworking of lithified carbonate mud sediments by bio-
erosion or extreme water energy (Fürsich 1979). This
explanation is reliable if associated hardgrounds ex-
hibit burrows.
Hardground intraclasts
A specific type of lithoclasts are hardground intra-
clasts (Pl. 21/4, Pl. 139/10) found in deep shelf or basi-
nal settings, sometimes called 'subsolution clasts'.
These grains are mm- to cm-sized irregularly-shaped,
predominantly micritic clasts. The surface is impreg-
nated with Fe-Mn oxides (Pl. 23/8) and differs there-
fore in color from the matrix. Microborings as well as
biogenic encrustations (foraminifera, serpulids) are
common. The clasts reflect repeated phases of erosion,
encrustation/impregnation and accretion.
The clasts were originally interpreted as a result of
submarine dissolution at the sea bottom during inter-
Plate 16 Resediments: Intra, Extra and Lithoclasts
A clast is an eroded and redeposited particle. These processes can take place within the sedimentary basin
producing 'intraclasts' (-> 7, 8). Erosion of an older lithified sediment from outside the basin in which this
particle now becomes deposited produces an 'extraclast'. The term 'lithoclast' was originally coined for ero-
sional detritus brought from an outside source into the basin (= extraclast), but is now frequently used for both
intra- and extraclasts, when the genetic origin is unknown. Processes producing intraclasts are currents (e.g.
storms); desiccation of muds; local sliding; burrowing and grazing activities of organisms; redeposition of car-
bonates formed in association with coastal vegetation ('black pebbles' ->1, 3) and diagenetic changes in volume
during dehydration, compaction, or leaching.
'Clast analysis' (the study of microfacies, fossils and geochemical signatures of the clasts) has become an
essential approach in basin analysis. It provides data on the long-term history of sedimentation and basin devel-
opment.
1
Lithoclast limestone formed by storms (proximal tempestite). Clasts are intraclasts (black pebbles) reworked by storms
and deposited in floatstone and rudstone layers, alternating with micrite layers. Note the distinct black color of the pebbles.
Early Tertiary (Paleocene): Lower Austria.
2
Well-rounded carbonate extraclast distinctly differing in microfacies and diagenetic features from the host sediment.
Skeletal grains within the clast are algal spores and shells. Fossils in the host rock are fusulinid foraminifera (F). Note the
truncation of the gastropod shell at the boundary of the extraclast. Early Permian: Carnic Alps, Austria.
3
Poorly sorted intraclast wackestone. Note the differences in size, shape and orientation of the clasts, indicating strong
reworking of semi-lithified sediment. The dark clasts are 'black pebbles' originating from the erosion of pedogenic coastal
substrates overgrown by plants (e.g. seagrass or mangroves) and rich in organic matter. The arrow points to spar-filled
casts of rootlets. These intraclasts are indicative of the existence of subaerial environments and near-coast depositional
sites. Former seagrass zones can be recognized, former mangrove zones are more difficult to decipher (Plaziat 1995). Late
Jurassic: Switzerland.
4
Large storm-derived non-carbonate extraclasts (siltstone) together with carbonate peloids. Early Carboniferous (Late
Tournaisian): Czatkovice quarry, Cracow Upland, southern Poland.
5
Accumulation of planktonic foraminifera, occurring as clasts in pelagic wackestone containing identical foraminifera.
The clast may be produced as coprolite. Late Cretaceous: Northern Tunisia.
6
Fine-grained peloidal grainstone clast indicating submarine erosion of lithified carbonates adjacent to a reef zone re-
corded by debris with diagnostic reef organisms (
Tubiphytes
, arrows). Late Permian: Sosio, western Sicily, Italy.
7
Incipient tidal intraclasts. This coarse lithoclastic floatstone, characterized by unfossiliferous micrite clasts, formed as lag
deposit in tidal channels. Late Jurassic: subsurface, Kinsau, southern Germany.
8
Precambrian intraclasts, together with ooids (arrow). Altyn Formation: Logan Pass, Glacier Park, Montana, U.S.A.
