1 TYPES OF SEDIMENTARY ROCKS

䊳 7.1 TYPES OF SEDIMENTARY ROCKS

Table 7–1 • SIZES AND NAMES OF SEDIMENTARY PARTICLES AND

Sedimentary rocks are broadly divided into four cate-

CLASTIC ROCKS

gories:

1. Clastic sedimentary rocks are composed of frag- CLASTIC

SEDIMENTARY ments of weathered rocks, called clasts, that have been

DIAMETER

ROCK transported, deposited, and cemented together. Clastic

(mm)

SEDIMENT

rocks make up more than 85 percent of all sedimentary Conglomerate 256– Boulders Cobbles Gravel rocks (Fig. 7–1). This category includes sandstone, silt- (rounded particles)

(rubble) or breccia stone, and shale.

Pebbles

(angular particles)

Sandstone ⁄ of plants or animals. Coal is an organic sedimentary –

2. Organic sedimentary rocks consist of the remains

1 16 Sand

Siltstone rock made up of decomposed and compacted plant

Silt

or shale }

Mud Mudstone remains.

3. Chemical sedimentary rocks form by direct precipi- tation of minerals from solution. Rock salt, for exam- ple, forms when salt precipitates from evaporating sea- water or saline lake water.

same size range are called rubble. Sand ranges from

4. Bioclastic sedimentary rocks. Most limestone is 1/16 to 2 millimeters in diameter. Sand feels gritty when composed of broken shell fragments. The fragments are rubbed between your fingers, and you can see the grains clastic, but they form from organic material. As a re- with your naked eye. Silt varies from 1/256 to 1/16 mil- sult, limestone formed in this way is called a bioclas- limeter. Individual silt grains feel smooth when rubbed tic rock. between the fingers but gritty when rubbed between your

teeth. Clay is less than 1/256 millimeter in diameter. It

䊳 7.2 CLASTIC SEDIMENTARY ROCKS

is so fine that it feels smooth even when rubbed between your teeth. Geologists often rub a small amount of sedi-

Clastic sediment consists of grains and particles that ment or rock between their front teeth to distinguish be- were eroded from weathered rocks and then were trans-

tween silt and clay. Mud is wet silt and clay. ported and deposited in loose, unconsolidated layers at the Earth’s surface. The sand on a beach, boulders in a

TRANSPORT OF CLASTIC SEDIMENT river bed, and mud in a puddle are all clastic sediments.

Clastic sediment is named according to particle size After weathering creates clastic sediment, flowing water, (Table 7–1). Gravel includes all rounded particles larger

wind, glaciers, and gravity erode it and carry it down- than 2 millimeters in diameter. Angular particles in the

slope. Streams carry the greatest proportion of clastic

Sandstone 15%

Limestone 10%

Shale and siltstone 70%

Other, less than 5%

Figure 7–1 Relative abundances of sedimentary rock types.

Clastic Sedimentary Rocks 111

Glacier

Sand dunes

Delta

Stream deposits

Figure 7–2 Sediment and dissolved ions are transported by water, gravity, wind, and glaciers. They may be deposited tem- porarily in many different environments along the way, but eventually most sediment reaches the ocean.

sediment. Because most streams empty into the oceans, most sediment accumulates near continental coastlines (Fig. 7–2). However, some streams deposit their sedi- ment in lakes or in inland basins.

Streams and wind modify sediment as they carry it Figure 7–3 Rounded cobbles in the West Fork of the downslope. The rounded cobbles shown in Figure 7–3

Bitterroot River just below Trapper Peak. originally formed as angular rubble in the Bitterroot

Range of western Montana. The rubble became rounded as the stream carried it only a few kilometers. Water and wind round clastic particles as fine as silt by tumbling them against each other during transport. Finer particles

In contrast, wind transports only sand, silt, and clay and do not round as effectively because they are so small and

leaves the larger particles behind. Thus, wind sorts par- light that water and even wind, to some extent, cushion

ticles according to size.

them as they bounce along, minimizing abrasion. Glaciers

A stream transports only small particles when it do not round clastic particles because the ice prevents the

flows slowly, but larger particles when it picks up speed. particles from abrading each other.

For example, a stream transports large and small parti- Weathering breaks bedrock into particles of all sizes,

cles when it is flooding, but only small particles during ranging from clay to boulders. Yet most clastic sediment

normal flows. As a flood recedes and the water gradually and sedimentary rocks are well sorted—that is, the grains

slows down, the stream deposits the largest particles first are of uniform size. Some sandstone formations extend

and the smallest ones last, creating layers of different- for hundreds of square kilometers and are more than a

sized particles.

kilometer thick, but they consist completely of uniformly Finally, durability of the particles affects sorting. sized sand grains.

Sediment becomes abraded as it travels downstream. Sorting depends on three factors: the viscosity and

Thus a stream may transport cobbles from the mountains velocity of the transporting medium and the durability of

toward a delta, but the cobbles may never complete the the particles. Viscosity is resistance to flow; ice has high

journey because they wear down to smaller grains along viscosity, air has low viscosity, and water is intermedi-

the way. This is one reason why mountain streams are ate. Ice does not sort effectively because it transports

frequently boulder choked but deltas are composed of particles of all sizes, from house-sized boulders to clay.

mud and sand.