SEDIMENTARY ROCK TYPES
3.2 Sandstones
quartz quartz arenite
5%
sa 25%
sl
arkosic arenite
lithic arenite
greywacke feldsp. lithic arenites
wackes quartz wacke mudrocks
> 75%
matrix
15%
increasing matrix feldspar rock fragments
sa: subarkose sl: sublitharenite
Figure 3.2 Classification of sandstones. Careful use of a hand-lens in the field should enable recognition of the main sandstone types: quartz arenite, arkose, litharenite and greywacke.
2%
25%
5%
33%
10%
50%
Figure 3.3 Percentage estimation charts. Use for giving a rough estimate of the percentage of grains or bioclasts (fossils) or crystals (etc.) present in the rock.
Figure 3.4 Close-up of surface of an aeolian sandstone (red-coloured), large- ly composed of quartz grains (clear, glassy). Note the light reflecting off some grain surfaces; these are crystal faces of the overgrowth cement. Millimetre scale. Permian, N.W. England.
flat crystal surfaces, reflecting the light (Fig. 3.4).Feldspar grains are com- monly slightly to totally replaced by clay minerals, so they do not have the fresh glassy appearance of quartz; they are usually white in colour, possibly pink. Cleavage surfaces and/or twin planes are usually visible on the fracture surfaces, as they reflect the light. In many sandstones at out- crop, the feldspar grains have been dissolved out, leaving a porous, quartz- dominated, usually friable sandstone.Lithic grains (rock fragments) can be recognised by their composite nature and may show alteration (to chlorite, for example). Of the micas, muscovite is recognised by its silvery-grey colour and flaky nature, and the less common biotite by its brown-black colour.
Some cements in arenites can be identified in the field. Apart from the acid test for calcite, many such cements are large poikilotopic crystals, several millimetres to even 1 cm across, enclosing several sand grains. The cleavage fracture surfaces of such crystals are easily seen with a hand-lens, or simply with the naked eye by getting the light to reflect off the cleavage surfaces of the calcite. Quartz cement usually takes the form of overgrowths on quartz grains. Such overgrowths commonly develop crystal faces and terminations, and these too can be seen with a hand-lens, or again when the sun reflects off the crystal faces, so the grains sparkle.
3.2.1 Quartz arenites
Compositionally supermature and clean, these sandstones are typical of, but not restricted to, high-energy shallow-marine environments, and also aeolian (wind-blown) sand-seas in deserts (e.g. Fig. 3.4). Sedimentary structures are common, especially cross-stratification, on small, medium and large scales (Section 5.3.3). Since only quartz is present, the colour of quartz arenites is commonly white or pale grey, especially those of shallow-marine environ- ments. Aeolian quartz arenites are commonly red through the presence of finely disseminated hematite which coats grains. Quartz and calcite cements are common.
Quartz arenites also form through leaching of a sediment, whence the unstable grains are dissolved out.Ganister, a type of soil occurring beneath coal seams and containing rootlets (black organic streaks), forms in this way.
3.2.2 Arkoses
Arkoses can be recognised by the high percentage of feldspar grains, although at outcrop these may be altered, especially tokaolinite(a white clay mineral).
Many arkoses are red or pink, in part due to the presence of pink feldspars but also through hematite pigmentation. Some coarse-grained arkoses look like granites until you see the bedding. In many, grains are subangular to subrounded and sorting is moderate; a considerable amount of matrix may be present between grains. Relatively rapid erosion and deposition under a semi-arid climate produce many arkoses. Fluvial systems (alluvial fan, braided stream) are typical depositional environments for arkoses, especially if granites and granite – gneisses are exposed in the source area.
3.2.3 Litharenites
Litharenites are very variable in composition and appearance, depending largely on the types of rock fragment present. In phyllarenites, fragments of argillaceous sedimentary rock are dominant, and incalclithites, limestone fragments predominate. Lithic grains of igneous and metamorphic origin are common in some litharenites. In the field it is usually sufficient to identify a rock as being a litharenite; a more precise classification would have to come from a petrographic study. Many litharenites are deltaic and fluvial sediments, but they can be deposited in any environment.
3.2.4 Greywackes
Greywackes are mostly hard, light to dark grey rocks with abundant matrix.
Feldspar and lithic grains are common and often clearly identifiable with a hand-lens. Although greywackes are not environmentally restricted, many were deposited by turbidity currents in relatively deepwater basins and so
show sedimentary structures typical of turbidites (sole structures, graded bedding and internal laminae: see Figs 8.3 and 8.10 – 8.12). Greywackes com- monly grade upwards into mudrocks.
3.2.5 Hybrid sandstones
These contain one or more components that are not detrital, such as the authigenic mineral glauconite or grains of calcite (ooids, bioclasts, etc.). The greensandscontain granules ofglauconite(a potassium iron aluminosilicate) in addition to a variable quantity of siliciclastic sand grains. Glauconite tends to form in marine-shelf environments starved of sediment.
Calcarenaceous sandstones contain a significant quantity (10–50%) of carbonate grains, usually skeletal fragments and/or ooids. With more than 50% carbonate grains, the rock becomes a sandy limestone. In calcareous sandstonesthe CaCO3is present as the cement.
For further information on the composition and mineralogy of a sandstone it is necessary to collect samples and study thin-sections made from them.
Petrofacies, that is sandstones distinct on petrographic grounds, can be of great importance in unravelling the source of the sediment and the palaeo- geography at the time. In a broad way, the composition of a sandstone does relate to the plate-tectonic setting of the depositional basin. See sedimentary petrology textbooks for further information.