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Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
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Terrigenous Clastic Sediments: Summary 27
input increases. For example, maturity often increases redeposition are resistant minerals such as quartz and
downstream in a river and once the same sediment lithic fragments of chert. Certain heavy minerals (e.g.
reaches a beach the high wave energy will further zircon) are also extremely resistant and the degree to
increase the maturity. Care must be taken when which zircon grains are rounded may be used as an
comparing sediment from different sources as they index of the number of cycles of sedimentation mate-
are likely to have started with different grain size rial has been subjected to.
and shape distributions and are therefore not directly
comparable. Sediments may also be recycled from
older deposits, resulting in greater degrees of matur- 2.6 TERRIGENOUS CLASTIC
ity (2.5.4). SEDIMENTS: SUMMARY
Terrigenous clastic gavels, sands and muds are wide-
Mineralogical maturity
spread modern sediments and are found abundantly
Compositional maturity is a measure of the propor- as conglomerate, sandstone and mudrock in succes-
tion of resistant or stable minerals present in the sedi- sions of sedimentary rocks. They are composed
ment. The proportion of highly resistant clasts such as mainly of the products of the breakdown of bedrock
quartz and siliceous lithic fragments in a sandstone, com- and may be transported by a variety of processes to
pared with the amount of less resistant, labile, clast depositional environments. The main textural and
types present, such as feldspars, most other mineral compositional features of sand and gravel can be
types and lithic clasts, is considered when assessing readily determined in the field and in hand specimen.
compositional maturity. A sandstone is composition- For detailed analysis of the composition and texture of
ally mature if the proportion of quartz grains is very sandstones, thin-sections are examined using a petro-
high and it is a quartz arenite according to the Petti- graphic microscope. Investigation of mudrocks
john classification scheme (Fig. 2.11): if the ratio of depends on submicroscopic and chemical analysis of
quartz, feldspar and lithic fragments meant that the the material. Sedimentary structures formed in clastic
composition falls in the lower part of the triangle it is a sediments provide further information about the con-
mineralogically immature sediment. ditions under which the material was deposited and
provide the key to the palaeoenvironmental analysis
discussed in later chapters of this book.
2.5.4 Cycles of sedimentation
Mineral grains and lithic clasts eroded from an igneous FURTHER READING
rock, such as a granite, are transported by a variety of
processes (Chapter 4) to a point where they are depos- Adams, A., Mackenzie, W. & Guilford, C. (1984) Atlas of
ited to form an accumulation of clastic sediment. Mate- Sedimentary Rocks under the Microscope. Wiley, Chichester.
rial formed in this way is referred to as a first cycle Blatt, H. (1982) Sedimentary Petrology. W.H. Freeman and
deposit because there has been one cycle of erosion Co, New York.
transport and deposition. Once this sediment has been Blatt, H., Middleton, G.V. & Murray, R.C. (1980) Origin of
lithified into sedimentary rock, it may subsequently be Sedimentary Rocks (2nd edition). Prentice-Hall, Englewood
uplifted by tectonic processes and be subject to ero- Cliffs, New Jersey.
sion, transport and redeposition. The redeposited Chamley,H.(1989)Clay Sedimentology. Springer-Verlag, Berlin.
material is considered to be a second cycle deposit Leeder, M.R. (1999) Sedimentology and Sedimentary Basins:
from Turbulence to Tectonics. Blackwell Science, Oxford.
as the individual grains have gone through two
Lewis, D.G. & McConchie, D. (1994) Analytical Sedimentology.
cycles of sedimentation. Clastic sediment may go
Chapman and Hall, New York, London.
through many cycles of sedimentation and each Pettijohn, F.J., Potter, P.E. & Siever, R. (1987) Sand and
time the mineralogical and textural maturity of the Sandstone. Springer-Verlag, New York.
clastic detritus increases. The only clast types that Tucker, M.E. (2001) Sedimentary Petrology (3rd edition).
survive repeated weathering, erosion, transport and Blackwell Science, Oxford.