J Bret Bennington

Updated 9/99

Terrigenous Clastic Sediments and Sedimentary Rocks

Materials derived from weathering and fragmentation of pre-existing rocks.

Aggregates - loose, uncemented sediments (unlithified)

Stone - cemented (lithified) sediments

Classification:

Based primarily on grain size and grain composition.

Also important is the ration of different types of grains in the rock - particularly the ratio of clasts (large particles that define the rock type) to matrix (particles smaller than the clasts).

Grain Size - Three-fold division with subcategories.

Udden - Wentworth Scale: base 2 logarithmic progression where each grain size category is half as large as the previous.

Gravel - boulders, cobbles, pebbles, granules

Sand - very course, course, medium, fine, very fine

Mud - course silt, medium silt, fine silt, clay

Phi (f) = -log₂ (grain diameter in mm)
 
 

Gravel / Conglomerate

Size category + conglomerate (also called a rudite or rudaceous)

Breccia - if clasts are angular

Monomict - one type of clast

Oligomict - two or three types of clast

Polymict - many types of clast

Matrix - sand and mud surrounding the clasts

Orthoconglomerate - clast supported

Paraconglomerate - matrix supported
 
 

Sand / Sandstone

Arenite - sandstone with < 15% matrix

Mineral components

Quartz

Feldspar (rapidly weathered in humid climates)

Mica (primarily muscovite and biotite - concentrated on bedding planes)

Heavy minerals (density greater than 2.85 g/cc)

Iron oxides

Lithic fragments (derived from fine-grained rocks)

Biogenic particles (shell fragments)

Glauconite (authigenic mineral)

Pettijohn classification for sandstones ('Toblerone plot')

This classification is based the proportion of clasts that are quartz, feldspar, or lithic fragments and on the proportion of clasts relative to muddy matrix. Arenites contain less than 15% matrix. Sandstones with between 15% and 75% matrix are called greywacke. Rocks that are more than 75% matrix are mudrocks.
 
 

Mudrocks: Traditionally, mudrocks have not been studied as intensively as other terrigenous rocks. This is because the small size of the particles makes them difficult to identify and analyze. Also, mudrocks are generally less well exposed and contain fewer types of sedimentary structures than more coarse-grained rocks.

Mudstone - a mixture of clay and silt-sized particles

Siltstone - a rock composed of silt-sized particles

Claystone - a rock composed of clay-sized particles

Shale - a mudstone with fissility (the tendency to split in thin sheets parallel to bedding)

Most silt is made out of quartz. However, glaciers produce a silt composed of lithic fragments called rock flour. Windblow deposits of rock flour are called loess. Clay-sized particles are composed mostly of clay minerals, which are produced by the chemical weathering of silicate minerals such as feldspars.

Clay particles often clump together into larger particles that settle out of suspension at a faster rate. This process is called flocculation and it commonly occurs when freshwater with clay in suspension enters an estuarine body of saltwater.
 
 

Textures of terrigenous rocks

Sorting - defined as the distribution of particle sizes in a rock. Sorting is often measured by calculating the standard deviation of sediment particle diameters. A well-sorted rock is composed of particles of similar size. A poorly-sorted rock consists of particles with a wide range of sizes.
 
 

Roundness - the degree to which the sharp edges of a particle have been smoothed.

Sphericity - the degree to which a clast approaches the shape of a sphere.
 
 

Maturity of terrigenous sediments

Textural - related to the transport history of the sediment

Mineralogical - related to the transport history and source region of the sediment

Textural maturity is a function of three variables: mud content, sorting, and clast shape.

Mineralogical maturity is shown by the proportion of mineralogically stable grains in the rock.