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Textures in terrigenous clastic rocks

blue pore stain in sandstone

The texture of a rock is the relationships between the components of the rock - how they are arranged.

Texture of sedimentary rocks

Grain size

Grain size is described according to the Wentworth scale, where terms such as fine sand or cobble have a very specific meaning. Use a grain size comparator for accuracy in the laboratory and in the field. It is also important that you know the width of the field of view of your microscope at each magnification (measure it, if you don't know) to estimate grain size.

Wentworth scale for the description of grain size of sediments: click to enlarge
Grain size comparator: click to enlarge

Sorting

Sorting describes the range of grain size. Samples in which most of the grains fall into one size class on the Wentworth scale are well sorted. We can illustrate sorting by plotting the grain-size distribution of our sample.

Textural maturity of a sediment

Boggs, 1995, pp. 79-93: Grain size & sorting; phi scale (p. 80)

Tucker, 1991, pp.

 

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.


Grain shape

Grain shape can be described in terms of:

 

Boggs, 1995, pp. 94-102: Grain shape

Tucker, 1991, pp. 15-16


Support

Sedimentary rocks may be either grain-supported or matrix supported. This applies equally to sandstones, and to coarser rocks such as conglomerates and breccias.

conglomerate - 13 kb Grain-supported (or clast-supported) conglomerate, with a sandstone matrix. All of the pebbles are in contact in 3 dimensions, forming a framework.
conglomerate - 7 kb Section cut through a grain-supported conglomerate with a silica cement. All the pebbles are in contact in 3 dimensions, but in most cases the actual contact is not seen here - it occurs above or below the plane of the section. When the grains appear as close together as this, it is almost certain that the rock is grain-supported. Beware of the apparent lack of grain support in sandstone thin sections where the grains are close together.
conglomerate - 13 kb A matrix-supported breccia. Here, the small, angular pebbles are separated by several pebble diameters. Most of the rock is composed of mud-sized matrix, in which the pebbles 'float'.

A warning

In thin section, it is important to remember that we are seeing a 2-dimensional section through a 3-dimensional rock - not all the grain contacts may be seen, and we may get a false impression of grain size and sorting:

wood balls in resin - 12 kb This artificial 'sediment' consists of wooden spheres, embedded in clear resin. In this 3-D view, we can see that the spheres are of uniform size, and are in contact with their neighbours (point or tangential contacts). Packing approximates to cubic close packing.

Model by Richard D'Lemos

wood balls in resin - 8 kb These are cuts through the same 'sediment'. On the cut surface, note that:
  • many of the 'grains' do not appear to be in contact (but we can see these contacts through the resin below the plane of the cut)
  • the spheres appear to show a range of sizes, according to where the plane of the section intersects each 'grain'
wood balls in resin - 10 kb Beware of the problems of studying a thin section which may give a distorted impression of the rock texture:
  • the rock may appear to have a smaller grain size than it really has
  • sorting may appear poorer than it really is
  • grain-supported rocks may appear not to be grain-supported in thin section

Boggs, 1995, pp. 94-102: Grain shape

Tucker, 1991, pp. 15-16


Grain contacts

Boggs, 1995, p. 107: Grain contacts

Tucker, 1991, p 18


Porosity

Porosity is an aspect of texture which is of particular interest to the economic geologist, because it is in the pore spaces that economically valuable fluids such as oil, gas or water may be contained.

Primary porosity is formed at or before the time of deposition: it occurs between and within the grains.

Secondary porosity is formed during diagenesis, usually by solution/dissolution of components of the rock.

Both primary and secondary porosity are progressively destroyed as sediments are buried, mainly by processes of compaction and cementation.


This page was written by Roger Suthren

Last updated 20 January, 2013 6:53 PM