GO 324A Rocks and Minerals
Susan Ward Aber

www.emporia.edu/earthsci/amber/go324/sediment.htm

Emporia State University
Emporia, Kansas USA
Earth Science Department

Introduction to Sedimentary Rock

Introduction    Group    Texture    Composition    Origins
Structures    Classification    References and Links

Introduction

Sedimentary is taken from Latin sedimentum, which means a settling. These rocks are formed at or near the surface by accumulating particles, which compact, consolidate, and cement into rock layers. As the thickness increases the pressure and rising temperatures compact and cement the particles into rock layers (e.g. mud becomes mudstone and then shale; sand becomes sandstone) (Chesterman, 1978, p. 603). Sedimentary rock usually has bedding or a layered structure and may have fossil remains. While sedimentary rocks and sediments may make up only 0.029% of the total volume of rocks on Earth, they account for two-thirds of the exposed rocks on the Earth's surface (Raymond, 1995, p. 263).

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Groups

All sedimentary rock can be divided into one of three groups, depending upon the place of origin or source area of sediments. These groups are detrital, biogenic, and chemical; within biogenic, there are two sediment classifications, organic and bioclastic. Detrital rock, also called clastic, is formed by the accumulation of sediment from preexisting rocks and minerals, using mechanical weathering and transportation by mechanical agents such as wind, water, ice, and gravity (Chesterman, 1978, p. 603). Biogenic rocks may have organic remains or not and form directly or indirectly by action of plant or animal organisms (p. 604). Chemical rocks form from precipitation from saline or freshwater solution; if the solution becomes concentrated by evaporation, rocks are classed as evaporites (Chesterman, 1978, p. 604). Each group is detailed below.

Detrital sediments, which are also called terrestrial or clastic, are composed of clastic sedimentary particles produced by the weathering and erosion of preexisting igneous, metamorphic, or sedimentary rocks, all derived from land. Detrital texture is described by size (see Wentworth Scale), shape, and composition or the sorting of constituent grains.

Biogenic sediments are derived from body parts of organisms. Hard skeletal parts comprise bioclastic biogenic sediments, and soft tissues produce organic biogenic sediments. Bioclastic biogenic sediments are produced by corals, mollusks, brachiopods, echinoderms, sponges, radiolaria, and other marine invertebrates which secrete calcite, aragonite, or silica shells. Such sediments are abundant in shallow, warm, clear seas, and are quickly cemented and may undergo mineralogical and textural diagenesis to create limestone, dolostone, or chert. Organic biogenic sediments are derived from decomposition of plant and animal tissues into carbon or hydrocarbon-rich sediment. Stagnant, oxygen-poor environments are necessary for accumulation of organic sediments. Lithification is accomplished mainly by compaction.

Chemical sediments are created by a precipitation of low temperature/pressure minerals from water solution onto a depositional surface or within sediment pores. Depending on the acidity, oxidation, temperature, or salinity, a variety of chemical sedimentary rock may result. Examples of chemical sediments include carbonates, evaporites, opal, chert (may form in other ways too), iron-oxides, and aluminum oxides. Chemical textures are usually crystalline with some special terms, such as oolitic or pisolitic.

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Texture

Texture is the size, shape, and arrangement of grains, and it provides a clue to the history of the sediment and its lithification into rock. There are two general categories: clastic and crystalline. Clastic texture is divided into coarse-grained gravel, medium-grained sand, and fine-grained mud based on the Wentworth Scale below. Clastic textures are referred to as well sorted or even, if grains are the same size; it is poorly sorted or uneven, if a there is a mix of sizes.

Wentworth Sediment Size Scale
Millimeters Class Clastic texture terms
>256 boulder gravel- coarse grained
64 cobble gravel- coarse grained
4 pebble gravel- coarse grained
2 granule gravel- coarse grained
1.0 very coarse sand sand- medium-grained
0.5 coarse sand sand- medium-grained
0.25 medium sand sand- medium-grained
0.125 fine sand sand- medium-grained
0.0625 very fine sand sand- medium-grained
0.031 coarse silt mud- fine-grained
0.0156 medium silt mud- fine-grained
0.0078 fine silt mud- fine-grained
<0.0039 clay mud- fine-grained

Crystalline texture terms are used for sedimentary rock in chemical and biogenic groups. These terms include rocks that contain whole crystals, fragments of crystals, or fine-grained crystals , which are called smooth or dense such as microcrystalline or cryptocrystalline. For example, cryptocrystalline varieties of quartz are known as chert or chalcedony. Other specialized textural terms include oolitic and pisolitic. For unconsolidated sediment such as sand, the grain shape is also recorded. Shape may be well-rounded, sub-rounded, sub-angular, or angular.

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Composition

Minerals present in sedimentary rock are largely the remains of or derived from mechanical and chemical transformations of preexisting rock. The minerals may be hard to isolate and identify. Therefore, the composition of the sedimentary rock is given in type terms. Argillaceous is a rock composition primarily made up of clay minerals, while arkosic composition contains grains of feldspars. A sedimentary rock can be primarily feldspar and termed feldspathic. Calcareous contains calcite, and dolomitic contains some calcite as well as calcium magnesium carbonate minerals such as dolomite. A composition rich in carbon and organic matter is termed carbonaceous, while on rich in iron oxides is termed ferruginous. Rocks containing quartz are quartzose, or if free silica, siliceous. Rocks containing nodular chert or chert as replacemnt lenses is simply called cherty.

Some specific minerals found in different sedimentary rock are shown below.

Minerals from solution in chemical sedimentary rock include:
sulfate: gypsum, thenardite
carbonate: calcite, dolomite
halide: halite, sylvite, and anhydrite
borate: borax
nitrate: trona.

Minerals formed from preexisting minerals or secondary minerals, oxidized or reduced, and in clastic sedimentary rock include:
silicate: chrysocolla, hemimorphite
sulfate: anglesite
vanadate: carnotite, vanadinite
carbonate: malachite, azurite, cerussite, smithsonite
oxide: cuprite
native element: gold, silver, copper.

Minerals formed from organic activity include:
native element: sulfur
carbonate: aragonite, calcite
phosphate: apatite
hydroxide: oolitic limonite.

Insoluble residue or resistant minerals found in sedimentary rock include:
silicates: chalcedony, clay minerals
hydroxides: limonite, bauxite.

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Origins

The environment of deposition is responsible for the character of the sediments that are deposited in a region. Sedimentary depositional environments are classified in two main categories: marine and continental. Continental deposits may be either on dry land such as deserts, gravity, landslides, glacial deposits, or subaqueous such as lake, swamp, steam bed. Sedimentary rocks found within the chemical group may have precipitated origins, such as cave formations and evaporites or hydrothermal replacement deposits such as geysers. Marine deposits include various oceanic environments such as deep ocean, shallow ocean, barrier beach, or coastline with pounding waves. Delta, lagoon, and estuary deposits are transitional between marine and continental.

Different terms are assigned to different environments. If the sedimentary rock is deposited on land it is continental, but if it is derived from land, terrigenous. If the rock is deposited by water it is aqueous or deposited in sea water, marine. Lacustrine is a rock resulting from lake deposition, and eolian, rock transported and deposited by wind. Hydrogenic is reserved for rock and sediment preciptated from water by inorganic processes.

Chert is sedimentary rock that can be grouped under all three categories. Various origins of chert include:

  • bioclastic biogenic when chert contains siliceous skeletond from a biochemical precipitation or a process by which living organisms induce crystallization of solids from solution
  • detrital when chert is formed by compaction of deposits resulting from erosion, transportation, and deposition of previously formed siliceous materials
  • chemical when chert results from hydrogenous precipitation from low temperature water-based solutions that become supersaturated with silica such as vein or cavity fillings in continental or marine rocks, especially volcanics
  • chemical when chert results from hydrothermal precipitation in thermal waters as the solubility of silica increases and silica precipitates upon cooling in hot springs, geysers, and seafloor spreading along mid- ocean ridges
  • chemical when silica replaces calcite in limestones
  • chemical when chert is derived from opaline silica into quartz, nodular or bedded, as a result of diagenesis or chemical conversion

    • Just as chert can have a variety of origins, sandstone and conglomerate are derived from different environments of of deposition. Sandstone types include:

  • Quartzose sandstones are nearly pure quartz (>90%), well sorted deposits with rounded quartz grains, and are usually lithified by cementation with silica, calcite, iron-oxide, glauconite. The environmental conditions could be marine or a coastal environment.
  • Graywacke sandstones are a mixture of quartz, feldspar, mica, rock fragments, and more. They are poorly sorted dirty sandstones with angular grains, lithified by compaction of clayey matrix with an oceanic trench environment.
  • Arkose sandstones are a mixture of quartz and feldspar, variable sorting and angular grains, lithified by cementation with calcite, iron-oxide, or silica. The environmental conditions could be continental alluvial fan or river.

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    Sedimentary Structures

    Sedimentary rocks are easily recognized in the field by characteristic structural features. If the structures are preserved from the time of deposition, they are called primary structures. These are often related to flowing agents which laid down the sediment or to biological activity at the depositional site. Sedimentary rock is layered or bedded, and this is referred to as strata or stratification. Thin strata is called lamination (e.g., thin-bedded or laminated shale). Other structures include: cross-bedding, ripple marks, cut-and-fill channels, mudcrack casts, concretions, oolites, pisolites, imbrication or an alignment of pebbles, grooves and furrows, rain-drop impressions, fossils in growth position, root casts, and animal burrows.

    Hey, another opportunity for you! If you are enrolled in the course, find an online image demonstrating a sedimentary structure in an outcrop or geologic setting. Send the image and proper citation plus URL to saber@emporia.edu for one substitution point on exam 3. Place GO 324 point for sed exam in the subject line of the email, and I must receive it by November 20.

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    General Classification

    Texture Cement Clastic Chemical Bioclastic Biogenic Organic Biogenic
    Size/Shape/Sorting Composition Quartz/Chert
    Quartz/Chert/Mica/Rock
    Quartz/Feldspars/Clay     		Silica Gel
    Calcite/Dolomite
    Gypsum
    Halite     		Marine Skeletal Organic
    Carbon
    Crystalline Limestone/Dolostone
    Gypsum
    Rock Salt
    Coarse grain size >2mm Silica
    Carbonate
    Ferruginous     		Breccia/Conglomerate Gypsum
    Rock Salt
    Concretions
    Medium grain size >1/16-2mm Silica
    Carbonate
    Ferruginous     		Clean Sandstones
    Greywacke Sandstones
    Arkose Sandstones     		Concretions
    Gypsum
    Rock Salt
    Fine/Very Fine <1/16mm Silica
    Carbonate
    Ferruginous     		Siltstone
    Claystone
    Shale     		Opal/Chert
    Limestone/Dolostone
    Anhydrite/Rock Salt     		Chalk
    Fossiliferous Limestone
    Open Fibrous
    Fossil Remains     		Carbonate
    None     		Chalk
    Fossiliferous Limestone
    Coquina     		Peat
    Dense Anhydrite
    Rock Salt     		Coal: Lignite/
    Bituminous/Anthracite

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    Recommended References and Notable Links

    To the beginning!

    Petrology Introduction
    www.emporia.edu/earthsci/amber/go324/intro.htm     		Minerals
    www.emporia.edu/earthsci/amber/go324/mineral.htm
    Rocks
    www.emporia.edu/earthsci/amber/go324/rock.htm     		Igneous
    www.emporia.edu/earthsci/amber/go324/igneous.htm
    Sedimentary Rock
    www.emporia.edu/earthsci/amber/go324/sediment.htm     		Metamorphic Rock
    www.emporia.edu/earthsci/amber/go324/metamor.htm
    Course Field Trip
    www.emporia.edu/earthsci/amber/go324/field_trip.htm     		Course Syllabus
    www.emporia.edu/earthsci/amber/go324/syllabus.htm

    This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. The curriculum is © by the author, 2001-2009. Last update 18 July 2009. For more information contact the course instructor, Dr. S. W. Aber, e-mail: saber@emporia.edu

    To understand copyright, visit lcweb.loc.gov/copyright/. All rights reserved. Susan Ward Aber.