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Types of Sedimentary Rock Help (page 2)

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

Sand And Sandstone

Sandstone is made up of mineral grains (mostly quartz) cemented together by silica, iron oxide, or calcium carbonate. Sandstones are commonly white, gray, brown, or red. Iron oxide impurities give the red and brown color to the darker colored sandstones. Most sandstones are gritty, while some are easily crushed ( friable ) and break apart to form sand. See Table 7-2 for the different types of sandstone.

Table 7-2 Sandstone is classified according to the levels of its mineral content.

Majority of grains

Type of sandstone

Quartz

Quartz sandstone or quartz arenite

Feldspar

Arkose

Diverse rock fragments

Lithic sandstone (litharenite or greywacke)

 

The pores or spaces between the separate grains of sand in sandstones controls how porous the sandstone is. The amount and size of this spacing is called porosity . The porosity of sandstone allows sandstone to serve as good reservoirs for oil and natural gas. Petroleum engineers and geologists often look for these natural resources in sandstone areas. Sandstone is made up of one or more of the following:

  • Silt (grain size 1/256 to 1/16 mm (gritty)),
  • Siltstone ,
  • Clay (grain size less than 1/256 mm (smooth)),
  • Shale (most abundant of sedimentary rock types),
  • Claystone ,
  • Mudstone (a mixture of silt and clay or mudshale if it fractures along sedimentary lines), and
  • Ironstone (clay ironstone).

Sandstones are very resistant to erosion and form bluffs, cliffs, ridges, rapids, arches, and waterfalls. Loose sands have many colors, but are commonly seen as white to light brown. Silica (quartz-rich) sands and sandstones of high purity (white color) are used widely in the glass industry for making window glass, light bulbs, vases, and utility containers. Tightly cemented sandstone is often used as a building stone.

Sand sediments are moved along by medium-speed currents like those of rivers, shoreline waves, and the wind. These can be rounded, which tells a geologist that they have traveled far (probably by water), or rough edged which usually points to shorter treks.

The amount of sand grains sorting in one area is another clue as to sediment origins. The average size of grains tells a lot about the strength of the current that carried them to a new spot and the type of parent crystal the grain was originally part of. If the grain sizes in sandstone are all the same, they are well sorted. If many grains are larger, with a lot of smaller and in-between grains, then they are poorly sorted. Sorting takes place during the sand’s travels. Well-sorted sand grains commonly come from beaches, while poorly sorted grains are often the result of glacial travels.

Rock Asphalt

Rock asphalt is a medium- to coarse-grained sandstone with asphalt ( bitumen ) filling the pore spaces. It is squishy to solid, brown to black, has a pitchy to resinous luster, and is very sticky when completely saturated.

Bitumen is made up of various mixtures of liquid, viscous, flammable, or solid naturally occurring hydrocarbons, excluding coal, that are soluble in carbon disulfide.

Rock asphalt deposits were formed when erosion of the surface rocks exposed oil-bearing rocks and allowed the more volatile hydrocarbons to escape. The asphalt-based crude gradually thickened until only a heavy tar or asphalt remained. The bitumen content of commercial rock asphalt varies from 3 to 15%. When asphalt is produced as part of some petroleum refining processes, it is called artificial asphalt .

Rock asphalt was once mined widely in Kentucky in the eastern United States. Large deposits were found in several areas. During the early 1980s, attempts were made to recover the petroleum in the rock asphalt by heat treatment, distillation, and other processes. Rock asphalt is used most commonly for surfacing streets and roads. It is also used for roofing, waterproofing, and mixing with rubber.

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