Geology Reference
In-Depth Information
Microlithoclasts
Limestones predominantly consisting of small an-
gular to subangular micritic grains (sometimes de-
scribed as 'peloids', Sect. 4.2.2) appoximately equal to
or smaller than 0.1 mm are common in deeper-marine
shelf, slope and basin settings (Pl. 114/2, Pl. 137/1).
These 'small intraclasts' (Wilson 1967) differ from pe-
loids in their low sphericity and greater variation in
size and shape.
Black pebbles
The term refers to special types of carbonate clasts
and grains predominantly characterized by the distinc-
tive gray or black color of the particles. Although origi-
nally summarized as 'black pebbles' (cailloux noirs)
two types should now be differentiated: (a)
black grains
in the sand-size range and (b) dark gray to black angu-
lar limestone pebbles (
black lithoclasts
) millimeters or
centimeters in size. Both types occur together with non-
blackened counterparts and may be found within the
same limestone bed. Black pebbles and black grains
are common in lithoclastic packstones (limestone brec-
cias and conglomerates) and oncolitic limestones (form-
ing nuclei of oncoids) but also occur in mud- and wacke-
stones.
Reef clasts
Destruction of lithified reef carbonate leads to the
production of reef-derived detritus ranging from milli-
meter-sized debris to huge blocks up to several tens of
meters in size. The smaller fraction consists of mostly
angular and subangular lithoclasts, which are commonly
deposited on fore-reef slopes or transported to lower
slope and basinal settings (e.g. calciturbidites). Because
the distribution of many reef organisms (framebuild-
ers, foraminifera, algae) is limited to specific parts of
the reefs, the biotic composition of the lithoclasts can
be used to reconstruct the source area (Pl. 16/6, Pl. 27/
2, Pl. 114/3, Pl. 115/2).
Black grains:
Modern and ancient black grains have
been recorded from intertidal and subtidal environ-
ments. The grains are usually rounded (whole or bro-
ken fossils, peloids, ooids). Together with nonblackened
grains they exhibit an often graded salt-and-pepper tex-
ture and are common above unconformities and di-
astems. Intertidal and subtidal blackening of carbonate
Fig. 4.29.
'Black pebbles':
Black-colored limestone clasts are a valuable tool for identifying partial or complete subaerial
exposure of limestones (Strasser and Davaud 1983). The figure displays models for blackening (top) and black-pebble
deposition (bottom), based on the study of the Upper Jurassic Ota limestone of the Lusitanian Basin, Portugal (Leinfelder
1987). Blackening is mostly due to plant material. Organic material infiltrates soft or slightly lithified sediments as well as
solution cavity fillings within cemented limestones. Black pebbles are in part transported and found in alluvial fans, beach
conglomerates and lagoonal settings represented by oncolitic limestones and lithoclastic packstones. Other black pebbles are
parautochthonous relicts of brecciated black crusts that were reworked during a transgression.
