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Types of Sedimentary Rock Help

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By — McGraw-Hill Professional
Updated on Aug 31, 2011

Types Of Sedimentary Rocks

Unlike igneous rock, most sedimentary rocks have a fine-grained texture. Since a lot of the reason they have layered or settled in one place is due to water or wind, the particles of sediment are usually small and fine.

The way that sedimentary rock is deposited can also be related to size. Since wind can’t blow or carry away boulders (well, maybe tornadoes can), generally it is the lighter, finer grains of silt that are transported by the wind.

In contrast to that, water tumbles rocks of different sizes. With the water deposit of sedimentary rock, current plays a big part. The stronger the current, the larger the rock and the farther it is carried. The relationship between current and particle size is the reason why many beds have the same types of particles. They sort and group according to size when flowing in the same current stream. So you see sand together with sand, river pebbles with other river pebbles.

Clastic

Clastic or detrital sedimentary rocks are formed from the weathering of existing rocks, which have been carried to a different spot from where they were originally and then turned into rock. They have a clastic (broken) texture made up of clasts (bigger pieces, like sand or gravel) and are grouped according to their grain size. Table 7-1 lists the various clastic particles and their associated sizes.

Table 7-1 Clastic sediments and rocks are named by size and shape.

Size of particle (mm)

Sediment

Rock

<1/256th

Clay

Claystone or shale

1/256th–1/16th

Silt

Siltstone or shale

1/16th–2

Sand

Sandstone

2–64

Pebble

Conglomerate or breccia

64–256

Cobble or gravel

Conglomerate or breccia

>256

Boulder

Conglomerate or breccia

 

Detritus is igneous, sedimentary, or metamorphic rock that has been moved away from its original location.

Clastic sedimentary rocks are made up of pieces of other rocks. These pieces of rock are loosened by weathering , and then carried to some low area or crack where they are trapped as sediment. If the sediment gets buried deeply enough, it becomes compacted and cemented, forming sedimentary rock.

Clastic sedimentary rocks have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. Beginning with the smallest grains, there are clay , then silt , then sand . Grains that are larger than 2 mm are called pebbles .

Shale is a rock made mostly of clay, siltstone is made up of silt-sized grains, sandstone is made of sand-sized clasts, and a conglomerate is made of pebbles surrounded by a covering of sand or mud. Figure 7-4 compares the different sedimentary rock types and their different proportions.

Sedimentary Rock Clastic

 

Fig. 7-4. Siltstone, mudstone, and shale are found in much higher amounts than other sedimentary rock types.

  • Coarse-grained clastics
  • Gravel (grain size greater than 2 mm; rounded clasts = conglomerate; angular clasts = breccia)

  • Medium-grained clastics
  • Sand (grain size from 1/16 to 2 mm)

    Sandstone (mostly quartz grains = quartz sandstone (also called quartz arenite ); mostly feldspar grains = arkose ; mostly sand-sized rock fragment grains = lithic sandstone (also called litharenite or greywacke ))

  • Cement (the glue that holds it all together) like calcite, iron oxide, silica
  • Fine-grained clastics
  • Silt and siltstone (grain size from 1/16 to 1/256 mm)

    Mud (clay), mudstone (claystone), and shale mud (grain size <1/256 mm)

The deposit of these sedimentary rock types by different currents is as you might guess. The larger gravel, rocks, and pebbles are only carried along by strong currents. These are rushing mountain streams, rocky beaches with high waves, and glaciers’ melt water. Strong glacial currents also carry sand. That is why you usually see sand between the gravel and pebbles. Pebbles and small stones are tumbled along and become smooth very quickly while bouncing along the land or in the water. Beach gravels and broken bits of glass, constantly rolled back and forth in the surf, also get smooth and rounded.

The coarse-grained clastic rock that doesn’t easily smooth or erode is not a conglomerate, but instead a breccia . These sharp-edged rock fragments are found close to their source where sedimentary rock has been layered on top of them before they travel very far. Although some breccias are sedimentary in origin, others come from igneous rock and volcanic beginnings. They were deposited onto a sedimentary rock layer after first being shot out during an eruption or broken away from igneous rock along a fault during an earthquake.

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