Terrigenous Sedimentary Rocks

Clastic terrigenous sedimentary rocks consist of rock and mineral grains, or clasts, of varying size, ranging from clay-, silt-, and sand- up to pebble-, cobble-, and boulder-size materials. These clasts are transported by gravity, mudflows, running water, glaciers, and wind and eventually are deposited in various settings (e.g., in desert dunes, on alluvial fans, across continental shelves, and in river deltas).

Because the agents of transportation commonly sort out discrete particles by clast size, terrigenous clastic sedimentary rocks are further subdivided on the basis of average clast diameter. Coarse pebbles, cobbles, and boulder-size gravels lithify to form conglomerate and breccia; sand becomes sandstone; and silt and clay form siltstone, claystone, mudrock, and shale. Because most of the clasts are rich in silica, they are also referred to as siliciclastic sedimentary rocks. Siliciclastics are further subdivided on the basis of clast diameter into conglomerate and breccia, sandstone, siltstone, and finer-than-silt-sized mudrock (shale, claystone, and mudstone).


Subdivision of Terrigenous Sedimentary Rocks.

A prominent physical feature of terrigenous clastic rocks is texture—that is, the size, shape, and arrangement of the constituent grains. These rocks have a fragmental texture: discrete grains are in tangential contact with one another. Terrigenous clastic sedimentary rocks are further subdivided on the basis of the mean grain diameter that characterizes most fragments, using the generally accepted size limits. Granules, pebbles, cobbles, boulders, and blocks constitute the coarse clastic sediments; sand-size (arenaceous) clasts are considered medium clastic sediments; and fine clastics sediments consists of silt- and clay-size materials.

Grain Size The size of particles is rigorously described with the Wentworth scale and similar tools. But the four gross divisions of the size scale are easy to use: clay, silt, sand and gravel. Clay is microscopically small, smooth to the touch and teeth. Silt is gritty but invisible, with particles up to 1/16th millimeter in size. Sand is familiar, and gravel is anything larger than 2 millimeters. The coarse, medium and fine subdivisions of those categories can be learned too with a little practice. Particle size is the primary way of assigning the right name to siliciclastic rocks: shale, siltstone, sandstone and conglomerate.

The Wentworth scale was published in 1922 by Chester K. Wentworth, modifying an earlier scale by Johan A. Udden. Wentworth's grades and sizes were later supplemented by William Krumbein's phi or logarithmic scale, which transforms the millimeter number by taking the negative of its logarithm in base 2 to yield simple whole numbers. The size fraction larger than sand (granules, pebbles, cobbles and boulders) is collectively called gravel, and the size fraction smaller than sand (silt and clay) is collectively called mud.


The Wentworth scale.

Sedimentary rocks are made of grains in a mixture of sizes. Sorting refers to the range of sizes in that mixture (officially, the standard deviation of grain sizes); for instance, a rock that is mostly medium sand, with a small size range, is well sorted. Sorting is pretty easy to gauge once you're aware of it, but careful observations can tease out subtleties. For instance, this conglomerate is better sorted than this one. Sorting can provide information about the history of a sediment before it settled down and became rock.

Grain Shape
The most obvious thing about the shape of grains is their roundness. There are three angular divisions (very angular, angular and sub-angular) and three rounded ones (sub-rounded, rounded and well rounded). Roundness can tell us how far away the sediment is from the place it formed, and how it was carried away from there.


Classification diagram for Terrigenous Sedimentary Rocks.