Page 25 - Sedimentology and Stratigraphy

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26.2.2009 8:14pm Compositor Name: ARaju
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Terrigenous Clastic Sediments: Gravel, Sand and Mud
12 Nichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 12 26.2.2009 8:14pm Compositor Name: ARaju
of these fragments of rock usually requires petro- eous fragments make up over 50% of the bulk of the
graphic analysis by thin-section examination (2.3.5) rock it would be considered to be a limestone (the
to identify the mineralogy and fabric. nature and occurrence of calcareous biogenic frag-
Grains of igneous rocks such as basalt and rhyolite ments is described in the next chapter: 3.1.3). Frag-
are susceptible to chemical alteration at the Earth’s ments of bone and teeth may be found in sandstones
surface and are only commonly found in sands from a wide variety of environments but are rarely
formed close to the source of the volcanic material. common. Wood, seeds and other parts of land plants
Beaches around volcanic islands may be black may be preserved in sandstone deposited in continen-
because they are made up almost entirely of lithic tal and marine environments.
grains of basalt. Sandstone of this sort of composition
is rare in the stratigraphic record, but grains of vol- Authigenic minerals
canic rock types may be common in sediments depos-
ited in basins related to volcanic arcs or rift volcanism Minerals that grow as crystals in a depositional envi-
(Chapter 17). ronment are called authigenic minerals. They are
Fragments of schists and pelitic (fine-grained) meta- distinct from all the detrital minerals that formed by
morphic rocks can be recognised under the micro- igneous or metamorphic processes and were subse-
scope by the strong aligned fabric that these quently reworked into the sedimentary realm. Many
lithologies possess: pressure during metamorphism carbonate minerals form authigenically and another
results in mineral grains becoming reoriented or important mineral formed in this way is glauconite/
growing into an alignment perpendicular to the stress glaucony (11.5.1), a green iron silicate that forms in
field. Micas most clearly show this fabric, but quartz shallow marine environments.
crystals in a metamorphic rock may also display a
strong alignment. Rocks formed by the metamorph-
Matrix
ism of quartz-rich lithologies break down to relatively
resistant grains that can be incorporated into a sand- Fine-grained material occurring between the sand
stone. grains is referred to as matrix (2.2.2). In sands and
Lithic fragments of sedimentary rocks are generated sandstone the matrix is typically silt and clay-sized
when pre-existing strata are uplifted, weathered and material, and it may wholly or partly fill the spaces
eroded. Sand grains can be reworked by this process between the grains. A distinction should be drawn
and individual grains may go through a number of between the matrix, which is material deposited
cycles of erosion and redeposition (2.5.4). Finer- along with the grains, and cement (18.2.2), which
grained mudrock lithologies may break up to form is chemically precipitated after deposition.
sand-sized grains although their resistance to further
breakdown during transport is largely dependent on
the degree of lithification of the mudrock (18.2). 2.3.3 Sandstone nomenclature
Pieces of limestone are commonly found as lithic and classification
fragments in sandstone although a rock made up
largely of calcareous grains would be classified as a Full description of a sandstone usually includes some
limestone (3.1). One of the most common lithologies information concerning the types of grain present.
seen as a sand grain is chert (3.3), which being silica Informal names such as micaceous sandstone are
is a resistant material. used when the rock clearly contains a significant
amount of a distinctive mineral such as mica. Terms
such as calcareous sandstone and ferruginous
Biogenic particles
sandstone may also be used to indicate a particular
Small pieces of calcium carbonate found in sandstone chemical composition, in these cases a noticeable pro-
are commonly broken shells of molluscs and other portion of calcium carbonate and iron respectively.
organisms that have calcareous hard parts. These These names for a sandstone are useful and appro-
biogenic fragments are common in sandstone priate for field and hand-specimen descriptions, but
deposited in shallow marine environments where when a full petrographic analysis is possible with a
these organisms are most abundant. If these calcar- thin-section of the rock under a microscope, a more