Themes > Science > Earth Sciences > Geology > Rocks and Minerals > Sedimentology


Introduction

Sedimentary rocks usually made of bits of older rocks, either as clasts (chunks of rocks) or as mineral grains. They can also be chemically or biogenically precipitated. These types of sediments are dealt with later. The sedimentary rocks in this chapter are all clastic. They are principally classified on the basis of grain size and then further divided in terms of mineralogy. The most important thing sedimentary rocks tell us about is palaeoenvironments - that is ancient environments. This is done by looking at the sedimentary structures and the fossils contained within the rocks.

Classification

Classification of clastic sedimentary rocks based on grain size.
Diameter (mm) Sediment Name Rock Type
>256 Boulder Rudaceous Conglomerate or Breccia
Between 256 and 64 Cobble
Between 2 and 64 Pebble
Between 2 and 0.625 Sand Arenaceous Sandstone
Between 0.625 and 0.0039 Silt Argillaceous siltstone
<0.0039 Clay Claystone or shale

NB: A conglomerate has rounded clasts, a breccia has angular clasts. Put your pointer on the images below to get the rock name. The black and yelow scale is in centimetres. Pictures from Georgia College.
A conglomerate A breccia
Sandstone Siltstone

Textures

Textures in sedimentary rocks depend on the type of grains making up the rock.

Rounding of clasts
  • Roundness - the degree of rounding of a grain. Not to be confused with sphericity. Grains can be angular to well rounded. A well rounded grain has generally travelled further before deposition.
  • Sphericity - degree to which grain is a perfect sphere. Does NOT depend on roundness.
  • Sorting - the amount of different sized grains in a rock. Ranges from very poor to well sorted.
    Matrix and clasts from Georgia College
  • Matrix or cement - the finer grains in a rock or a chemical precipitate holding the rock together.
  • Competence - the "toughness" of a rock.

Other properties of a sedimentary rock are porosity and permeability. The ability to store fluid (e.g. oil, gas or water) is the porosity. The porosity is expressed as a percentage and depends on the amount of pore space in the rock. The ability to allow a fluid to pass through a rock is the permeability. Fluid can pass through using cracks, fissures or space between grains.

A high porosity rock can have a low permeability if the pore space do not connect in three dimensions

Structures

The structures in a rock tell us a great deal about the palaeoenvironment. This is where one of the great sayings in geology comes in use:

"The present is the key to the past" - the law of uniformatarianism

This essentially means if we can understand what processes occur today, for example, the forming of ripples in a tidal mud, then these principles can be applied to the geological record. Below are some examples of sedimentary structures and what formed them.

Cross Bedding Cross Bedding (or stratification). This shows some transport of grains by wind or water. These cross beds are formed by wind, forming dunes in a sand
Load casts and Flame structures Load casts and flame structures, formed by heavy sand squashing mud.
Load cast formation
Load cast formation
This is a way up structure
Mudcracks Mudcracks formed from the drying out of mud

A way-up structure tells us which way up the bed was originally deposited. Graded bedding usually occurs with the coarse grains at the bottom. If you find some graded bedding with coarse grains at the top, then the bed has probably been tectonically turned upside down (e.g. by folding).


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