Composition of terrigenous clastic sediments & rocks

dark sand - 15 kb

Please note


Introduction

Four main groups of sediments and sedimentary rocks may be defined:

  1. terrigenous clastics (sandstones, conglomerates, mudstones) - this page
  2. volcaniclastics
  3. carbonates (limestones) - biochemical sediments
  4. other chemical sediments (e.g. evaporites)

What are the terrigenous clastic rocks? They are mixtures of mineral grains & rock fragments derived by weathering & erosion of pre-existing rocks. These processes are just one small part of the Rock Cycle.


Factors which determine the mineralogy of terrigenous clastic sediments and rocks

These include:

  1. the nature of the source area(s) (provenance) - which minerals are available to form a sediment? The composition of a sediment may give evidence of tectonic setting.
  2. the mode of weathering at the source (mechanical weathering simply breaks up existing minerals for transport; chemical weathering creates new minerals)
  3. mode & length of transport and reworking: when these processes operate over a long time, the less stable and less durable minerals are broken down and destroyed, leaving only the more stable minerals
  4. depositional environment - physical sorting, and chemical and biological attack may further change the composition of the sediment
  5. climate at site of deposition - controls rates and types of chemical and physical processes
  6. diagenesis - chemical, physical and biological changes to the sediment after deposition

2. and 3. are particularly important in modifying the mineralogy of the sediment produced from the original source rock. For example, common changes at earth surface temperatures and pressures are:

feldspars clay minerals
mafic minerals chlorites + ions in solution

Unstable minerals include: pyroxenes, olivines, amphiboles, some plagioclase feldspars. In fact, many high temperature and/or high pressure igneous and metamorphic minerals are not stable under the low temperatures and pressures typical of sedimentary environments. Many igneous minerals, particularly the mafic ones and Ca-rich feldspars, are susceptible to chemical weathering: they break down, and are not commonly found in sedimentary rocks. So, if you think you have found pyroxene or olivine in a sandstone, double check your identification!! It is possible to find such minerals in sands and sandstones close to their source areas (e.g. black sand beaches on volcanic islands), but not common.

Stable minerals include quartz, K-feldspar, Na-feldspar, muscovite, clay minerals, some 'heavy minerals' (e.g. some garnets, zircon, tourmaline etc.).

Mechanical durability is also important. Minerals which are hard, or which have no cleavage, are likely to survive longer: they are more resistant to abrasion during transport.

Quartz is by far the most common of these stable, durable minerals, and the commonest component in clastic sediments. It survives because of its resistance to chemical attack (it is soluble only in highly alkaline solutions), and its mechanical durability (hardness 7, no cleavage).

The concept of mineralogical maturity: a mineralogically mature sediment or sedimentary rock is one which consists largely of the stable minerals, particularly quartz. Mineralogically mature rocks are those which have undergone prolonged weathering, transport and reworking.



Components Of Terrigenous Clastic Rocks

These rocks can be regarded as having four components:

sandstone thin section - 11 kb This is a cartoon view of a sandstone in thin section. It illustrates the four components:
  • grains
  • matrix - mud-sized sediment between the grains. May be primary (deposited at same time or soon after grains), or secondary (formed by diagenetic alteration of grains.
  • cement - chemical precipitates in pore spaces
  • pore space - primary or secondary

Grain Mineralogy

You will already be familiar in thin section with most of the grains that you are likely to come across in sandstones: many of them are also found in igneous rocks. In some cases, it may also be possible to recognize different types of the same mineral, such as varieties of quartz.

A problem with sedimentary grains is that, because of transport & abrasion, they rarely show crystal faces, so we often do not have crystal shape or straight or inclined extinction as identifiable properties.

Mineralogy of clastic rocks

This page contains detailed information on the occurrence and recognition of minerals and other components in sedimentary rocks. It is also available as a printed handout.

Boggs, 1995, pp. 158-162, 168-173: Sandstone mineralogy

Tucker, 1991, pp. 38-47

Boggs, 1995, p. 183-185: Provenance

Tucker, 1991, pp. 53-55

 

Note: all references to Boggs, 1995 and Tucker, 1991 in these pages refer to:

BOGGS, S. 1995. Principles of Sedimentology and Stratigraphy (2nd edition). Prentice Hall.

TUCKER, M.E. 1991. Sedimentary Petrology (2nd edition). Blackwell.


Matrix

So far, we have considered the composition of the grains. Now let us move on to the matrix - the mud-sized sediment between the grains in many clastic sediments and rocks. Surprisingly, it is often possibly to determine the mineralogy of this fine-grained material by careful examination under high magnification. It may contain silt-sized quartz grains, and one or more clay minerals. The properties of the common clay minerals are described in Mineralogy of clastic rocks, and in Clay Minerals.

Clays are seen better under the scanning electron microscope (SEM).

Clays minerals such as chlorite, illite and kaolinite are of great economic interest because of their effects (usually detrimental) on porosity and permeability of sedimentary rock

 


Cement

Chemically precipitated material, whether a new mineral, or an addition to an existing mineral, may form a cement, which binds the grains of the sediment together to form a rock.


Sandstone Classification

Sandstones are named and classified according to their composition. This is usually defined in terms of the percentage of matrix present, and the proportions of the different grain types (commonly quartz, feldspar and rock fragments). We shall use Dott's classification, which involves grain composition and percentage matrix, but ignores cement.

Dott classification


This page was written by Roger Suthren

Last Modified: 20 January, 2013