TYPES OF SEDIMENTARY ROCKS

Sedimentary rocks are broadly divided into four categories:

1. Clastic sedimentary rocks are composed of fragments of weathered rocks, called clasts, that have been transported, deposited, and cemented together. Clastic rocks make up more than 85 percent of all sedimentary rocks. This category includes sandstone, siltstone, and shale.

2. Organic sedimentary rocks consist of the remains of plants or animals. Coal is an organic sedimentary rock made up of decomposed and compacted plant remains.

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

4. Bioclastic sedimentary rocks. Most limestone is composed of broken shell fragments. The fragments are clastic, but they form from organic material. As a result, limestone formed in this way is called a bioclastic rock.

CLASTIC SEDIMENTARY ROCKS

Clastic sediment consists of grains and particles that were eroded from weathered rocks and then were transported and deposited in loose, unconsolidated layers at the Earth’s surface.

Clastic sediment is named according to particle size. Gravel includes all rounded partices larger than 2 millimeters in diameter. Angular particles in the same size range are calledrubble. Sandranges from 1/16 to 2 millimeters in diameter. Siltvaries from 1/256 to 1/16 millimeter. Clay is less than 1/256 millimryrt in diameter. Mud is wet silt and clay.

TRANSPORT OF CLASTIC SEDIMENT

After weathering creates clastic sediment, flowing water, wind, glaciers, and gravite erode it and carry it downslope. Streams carry the gratest proportion of clastic sediment. Because most streams empty into the oceans, most sediment accumulates near continental coastlines. However, some streams deposit their sediment in lakes or in inland basins.

Streams and wind modify sediment as the carry it downslope. The rubble becomes rounded as the stream carries 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 do not round as effectively because they are so small and light that water and even wind, to some extent, cushion them as the bounce along, minimizing abrasion. Glaciers do not round clastic particles because the ice prevents the particles from abrading each other.



Weathering breaks bedrock into particles of all sizes, ranging from clay to boulders. Yet most clastic sediment and sedimentary rocks are well sorted – that

is, the grains are of uniform size. Some sandstone formations extend for hundreds of square kilometers and are more than a kilometer thick, but they consist completely of uniformly sized sand grains.

Sorting depends of three factors: the viscosity and velocity of the transporting medium and the durability of the particles. Viscosity is resistance to flow; ice has high viscosity, air has low viscosity, and water is intermediate. Ice does not sort effectively because it transports particles of all sizes, from house-sized boulders to clay.

In contrast, wind transports only sand, silt, and clay and leaves the larger particles behind. Thus, wind sorts particles according to size.

A stream transports only small particles when it flows slowly, but larger particles when it picks up speed, but only small particles during normal flow.

Finally, durability of the particles affects sorting. Sediment becomes abrades as it travels downstream. Thus a stream may transport cobbles form the mountains toward a delta, but the cobbles may never complete the journey because they wear down to smaller grains along the way.

LITHIFICATION

Lithification refers to processes that convert loose sediment to hard rock. Two of the most important processes are compaction and cementation.

If you fill a container with sand, the sand grains do not fill the entire space. Small voids, called pores, exist between the grains. When sediment is deposited is water, the pores are usually filled with water. The proportion of space occupied by pores depends on particle size, shape, and sorting. Commonly, freshly deposited clastic sediment has about 20 to 40 percent pore space, although a well-sorted and well-rounded sand may have up to 50 percent pore space. Clay-rich mud may have as much as 90 percent pore space occupied by water.

As more sediment accumulates, its weight compacts the buried sediment, decreasing pore space and forcing out some of the water. This process is called compaction. Compaction alone may lithify clay because the platy grains interlock like pieces of a puzzle.

TYPES OF CLASTIC ROCKS

Water normally circulates through the pore space in buried and compacted sediment. This water commonly contains dissolved calcium carbonate, silica, and iron, which precipitate in the pore spaces and cement the clastic grains together to form a hard rock.

The speed of lithification depends mainly on the availability of cementing material and water to carry the dissolved cement through the sediment.

Conglomerate and Breccia

Conglomerate and breccia are coarse-grained clastic rocks. They are the lithified equivalents of gravel and rubble, respectively. In a conglomerate the particles are rounded, and in a sedimentary breccia they are angular.

Gravel typically has large pores between the clasts, because the individual particles are large. These pores usually fill with the finer sediment such as sand or silt. As a result, most conglomerates have fine sediment among the large clasts.

Sandstone

Sandstoneconsists of lithified sand grains. Most sandstones consist predominantly of rounded quartz grains.

The word sandstone refers to any clastic sedimentary rock comprising primarily sand-sized grains. Most sandstones are quartz sandstones and contain more they 90 percent quartz. Arkose is a sandstone comprising 25 percent of more feldspar grains, with most of the remaining grains being quartz. The sand grains in arkose are commonly coarse and angular. Graywacke is a poorly sorted sandsonte with considerable quantities of silt and clay in its pores. Graywacke is commonly dark in color because of fine clay that coats the sand grains. The grains are usually quartz, feldspar, and fragments of volcanic, metamorphic, and sedimentary rock.

Claystone, Shale, Mudstone, and Siltstone

Claystone, shale, mudstone, and siltstone are all finegrained clastic rocks. Claystone is composed predominantly of clay minerals and small amounts of quartz and other minerals of clay size. Shale consists of the same material but has a finely layered structure called fissility. Mudstone is a nonfissile rock composed of clay and silt. Siltstone is lithified silt. The main component of most siltsones is quartz, although clays are also commonly present.

Shale, mudstone, and siltstone make up 70 percent of all clastic sedimentary rocks. Shale is usually gray to black due to the presence of partially decayed remains of plants and animals commonly deposited with clay-rich sediment. This organic material in shales is the source of most oil and natural gas.

ORGANIC SEDIMENTARY ROCKS

Organic sedimentary rocks, such as chert and coal, form by lithification of the remains of plants and animals.

Chert

Chert is a rock composed of pure silica. It occurs as sedimentary beds interlayered with other sedimentary rocks and as irregularly shaped lumps called nodulesin other sedimentary rocks. Chert is made up of the remains of tine marine organisms that make their skeletons of silica rather that calcium carbonate. In contrast, some nodular chert appears to form by precipitation form silica-rich ground water, most often in limestone. Chert was one of the earliest geological resources. Flint, a dark gray to black variety, was frequently used for arrowheads, spear points, scrapers, and other tools chipped to hold a fine edge.

Coal

When plants die, their remains usually decompose by reaction with oxygen. However, in warm swamps and in other environments where plant growth is rapid, dead plants accumulate so rapidly that the oxygen is used up long before the dacay process is complete. The undecayed or partially decayed plant remains form peat. As peat is buried and compacted by overlying sediments, it converts to coal, a hard, black, combustible rock.


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