Geology Reference
In-Depth Information
4.2.8.2 Extraclasts: Strange Foreigners
Basics: Carbonate grains
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The presence of extrabasinal carbonate and non-car-
bonate clasts in limestones is often regarded as an un-
lucky mishap despite the high potential of extraclasts
in reconstructing paleobasins and paleosource areas
(Zuffa 1980).
Common features of extraclasts seen in thin sec-
tions are:
•
extraclasts are often either conspicuously round or
angular;
•
fossils and other grains within the extraclast are trun-
cated at the extraclast boundary;
•
extraclasts should contain stratigraphically 'older'
fossils than the matrix;
•
many extraclasts exhibit criteria pointing to recrys-
tallization, dolomitization or tectonic deformation
(e.g. calcite-filled veins) prior to their deposition;
•
carbonate extraclasts often occur together with non-
carbonate clasts such as sandstone or chert fragments
(Pl. 114/1) or together with volcanic material (Pl.
26/2).
Extraclasts occurring in limestones originate from
the drainage of river systems in near-coastal regions,
rock-falls at coasts, deposition of erosional material by
debris and turbidity flows but also by eolian transport.
Less common are transport as stomach stones from rep-
tiles, or ice-transport (dropstones; e.g. Hladil 1991).
More common are pebbles transported by seaweeds or
drift wood and incorporated within a root framework.
Examples were described from Mesozoic shelf and pe-
lagic settings (Mayr 1953; Birkenmaier et al. 1960;
Massari and Savazzi 1981; Leinfelder 1994). A sum-
mary was given by Etzold and Maus (1990). Of high
interest for the reconstruction of paleogeographic and
paleoceanographic patterns are exotic clasts (e.g. crys-
talline rock fragments) found in limestones (Jansa and
Carozzi 1970; Lantschner et al. 1996; Rajchel and
Myszkowska 1998).The accumulation of extraclasts
may result in the formation of breccias (see Sect. 5.3.3).
Diagenetic clasts
Pressure solution can result in the formation of di-
agenetic lithoclasts (Huber 1987).
An example is shown in Pl. 47/6: The continued
solution of the micritic matrix of a former lime mud-
stone has led to the formation of a marly matrix yield-
ing solution-resistant particles which correspond in
shape, size variation and sorting to lithoclasts. The di-
agenetic origin, however, is shown by stylolites and
argillaceous solution residues.
