5.3 Clastic Sedimentary Rocks
A clast is a fragment of rock or mineral, ranging in size from less than a micron[1] (too small to see) to as big as an apartment block. Various types of clasts are shown in Figure 5.2.1 and in Practice Exercise 5.2. The smaller ones tend to be composed of a single mineral crystal, and the larger ones are typically composed of pieces of rock. As we’ve seen, most sand-sized clasts are made of quartz because quartz is more resistant to weathering than any other common mineral. Many of the clasts that are smaller than sand size (less than 1/16 or 0.063 mm) are made of clay minerals. Most clasts larger than sand size (greater than 2 mm) are actual fragments of rock, and commonly these might be fine-grained rock like basalt or andesite, or if they are bigger, coarse-grained rock like granite or gneiss. Sedimentary rocks that are made up of clasts are called clastic sedimentary rocks. A comparable term is “detrital sedimentary rocks”.
Grain-Size Classification
Geologists that study sediments and sedimentary rocks use the Udden-Wentworth grain-size scale for describing the sizes of the grains in these materials (Table 5.2). There are six main grain-size categories; five are broken down into subcategories, with clay being the exception. The diameter limits for each successive subcategory are twice as large as the one beneath it. In general, a boulder is bigger than a toaster and difficult to lift. There is no upper limit to the size of boulder.[2] Sand ranges from 2 millimetres down to 0.063 millimetres, and its key characteristic is that it feels “sandy” or gritty between your fingers—even the finest sand grains feel that way. Silt is essentially too small for individual grains to be visible, and while sand feels sandy to your fingers, silt feels smooth to your fingers but gritty in your mouth. Clay is so fine that it feels smooth even in your mouth.
| Type | Description | Size range (millimetres, mm) | Size range (microns, μm) |
|---|---|---|---|
| Boulder | large | 1024 and up | |
| medium | 512 to 1024 | ||
| small | 256 to 512 | ||
| Cobble | large | 128 to 256 | |
| small | 64 to 128 | ||
| Pebble (Granule) | very coarse | 32 to 64 | |
| coarse | 16 to 32 | ||
| medium | 8 to 16 | ||
| fine | 4 to 8 | ||
| very fine | 2 to 4 | ||
| Sand | very coarse | 1 to 2 | 1000 to 2000 |
| coarse | 0.5 to 1 | 500 to 1000 | |
| medium | 0.25 to 0.5 (1/4 to 1/2 mm) | 250 to 500 | |
| fine | 0.125 to 0.25 (1/8 to 1/4 mm) | 125 to 250 | |
| very fine | 0.063 to 0.125 (1/16 to 1/8 mm) | 63 to 125 | |
| Silt | very coarse | 32 to 63 | |
| course | 16 to 32 | ||
| medium | 8 to 16 | ||
| fine | 4 to 8 | ||
| very fine | 2 to 4 | ||
| Clay | clay | 0 to 2 |
If you drop a granule into a glass of water, it will sink quickly to the bottom (less than half a second). If you drop a grain of sand into the same glass, it will sink more slowly (a second or two depending on the size). A grain of silt will take several seconds to get to the bottom,