What Process Can Result in a Sedimentary Rock With Rounded Rock Fragments in Its Composition

Sedimentary Rocks: Typically tiresome and ugly compared to igneous or metamorphic, but they have great utility because they speak to us nigh the history of the Earth's surface, where life lives.

• Remember, any rock is a tape of the surroundings in which it formed. An igneous rock is, thus, a tape of the chemistry of its parent drape rock, the chemistry of the rocks through which information technology passed as magma, and the circumstances under which it solidified - i.e., they record events and conditions of the deep Earth. Sedimentary rocks, in dissimilarity, are records of weather condition at the Earth'south surface. Additionally, because sedimentary rocks tend to form in layers, they represent something more like a movie, in which each layer represents a frame, every bit opposed to a uncomplicated snap-shot.
• Furthermore, because sedimentary rocks form at the Globe'south surface, so they are like shooting fish in a barrel to locate and report and they reverberate ancient environmental conditions in environments that are of straight concern to humans.

Sedimentary rock - rock equanimous of the transported remains of pre-existing rocks, i.due east. sediment.

Sediment: material derived from the weathering of preexisting rock.

General life history of sediment: In order to make a sedimentary stone, four things need to happen:

Weathering: we discussed previously

Send: running water, gravity, ice, wind. Textile may be transported every bit fragments or in solution.

Deposition: The process of depositing the sediment. For materials in solution, this involves precipitation.

Cementation: The binding of grains through the precipitation of mineral cements (or, sometimes, through chemical reactions between the grains, themselves, that cause them to demark).
Each of these processes go out their signature on the resulting rock, with the effect that we tin acquire a bang-up deal near:

• The source rock from which sediments were weathered
• How far and by what means they were transported
• In what surroundings they were deposited
• The concrete changes that take occurred in the depositional environment since deposition.

Before we can practice this, nosotros have to institute a vocabulary that we can use to talk nigh sedimentary rocks. This is the major thrust of this lecture.

Nomenclature: Sediment may be particles such every bit gravel or sand, the remains of plants and animals, or chemicals in solution. This encompasses a great diversity of rock types. We try to make this variety manageable by classifying sedimentary rocks into the following types:

• Clastic sedimentary rocks:

  Rocks composed of solid fragments of preexisting rock. Clast: a single such fragment. Fragments may exist rock fragments including several minerals or individual mineral grains.
  

• Chemical sedimentary rocks:

  Rocks composed of material that was transported in solution and directly precipitated from solution.
  

• Biochemical (AKA "biogenic") sedimentary rocks:

  Precipitated first in the tissues of organisms, so deposited when the organism dies. E.1000.

  • limestone - calcite from the shells of marine organisms
  • coal - carbon from compressed remains of state plants.
    

Of course, within each type, grains and other materials tin be described further according to:

• Composition - what grains are fabricated of.
• Texture - grain size and sorting - the course in which the material was transported. The most fundamental division:

Clastic or detrital sediments: - made up of broken fragments of material. Sometimes you hear the term siliciclastic in connection with clastic rocks because their most mutual components, quartz & feldspar are silica (SiO₂) containing minerals. In that location are three distinct issues that we need to be aware of in describing clastic rocks:

• Clast size
• Rounding
• Sorting

Clast size:

• Clast size catagories:

  Clay	Finer than .0039 mm.
  Silt	.0039 - .062 mm
  Sand	2 - .062 mm.
  Pebble	2 - 64 mm.
  Cobble	64 - 256 mm.
  Boulder	Coarser than 256 mm.

  

• Conglomerate: Sedimentary rocks made primarily of pebble sized (ii mm.) clasts or larger.
  
• Sandstone: consists primarily of sand sized (1/16-2mm.) grains.
  
• Mudrocks: Consists of clasts < 1/16 mm. I.e. smaller than the center tin can readily distinguish. In this size range are two clast catagories: silt (1/16 - ane/256 mm.) and dirt (< 1/256 mm.) You are thinking that this is a difficult conclusion to make, but exist of good cheer. Silt feels gritty in your mouth, clay feels smooth. With this in mind, we distinguish three general types of mudrock that are really parts of a spectrum of variation.

  Siltstone	Mudstone	Claystone
  Mostly silt	Silt and clay	Generally dirt

When a claystone fractures along numerous thin laminae, we call information technology a shale. Notation that mudrocks tend to be much more vulnerable to weathering than sandstone.
Rounding:

Every bit a poorly rounded clast is transported, its jagged edges become worn downward, yielding a spheroidal well rounded fragment. The style and amount of rounding tells the states virtually how transport occurred and how long it lasted. There is one rock name to recall in connection with this:
Remember volcanic breccias? A clastic rock made of pebble sized clasts or larger that are angular and poorly rounded is as well called a breccia.

Sorting:

In a deposit of sediment, there is always a range of clast size. If that range is narrow, we say the deposit is well sorted. If information technology is wide, we call it poorly sorted. Sorting is very revealing about methods of ship. For case, current of air blown sediments are ever very well sorted. Sediments transported by glaciers are very poorly transported.

Thus, a full description of a sedimentary stone might exist something like, "A well sorted, well rounded, quartz sandstone."

Diagenesis: The chemic alteration of sedimentary rock subsequently its degradation. Let'southward flesh out this concept, at present that we have a basic sedimentological vocabulary:

Chemic sedimentary rocks: Besides called evaporites These precipitates may take the texture of a crystalline crust or may form around small particles that accept a clastic texture, i.e. they resemble clastic rocks.

Crystalline chemical sedimentary rocks:

• Evaporites: If sea water is bars, certain common minerals precipitate out in the following order:
  • Calcite and aragonite (polymorphs!)
  • Gypsum
  • Halite
  • Other exotic halides
  Normal sea water contains 3.five% NaCl by weight. When we confine a body of sea water, the water evaporates, leaving increasingly concentrated alkali. Somewhen the concentration of common salt exceeds the solubility limit and salt precipitates. Fifty-fifty fresh water has some salts in solution. Ephemeral lakes often evaporate to leave playas, dry lake beds marked by crusts of these minerals.

  

• Precipitated limestone: Rocks composed of calcite and aragonite are called limestone. The bulk of limestone is biochemical, but in some cases, it forms as a direct precipitate, such as in:
  • Speleothems Cave deposits (such equally stalagtites and stalagmites.)
  • Tufa - Direct precipitates of CaCO₃ from bodies of fresh water.
  • Travertine - precipitates of CaCO₃ deposited near hot springs (such as the Jemez River Soda Dam, right).
    

"Pseudoclastic" chemical sedimentary rocks:
Biochemical sedimentary rocks (AKA biogenic) - clasts are remains of organisms.

By far the near of import are the remains of marine organisms made of CaCO_three . A vast taxonomic diversity brand shells or frameworks of calcite and aragonite (polymorphs of CaCO_three). These range from critters every bit big as behemothic clams to the teaming hoardes of microscopic organisms that inhabit the oceans. We will return to them in detail afterwards.



• SiO₂.nH₂O - Opal: Secreted equally skeletons of some sponges and unicellular diatoms and radiolarians. Deposits of these serve as an important source of silica which may precipitate every bit cement of Chert - microcrysatlline SiO_two. This ofttimes appears every bit nodules and concretions in rocks, as in image shown here. (A familiar grade of chert is flint, used to make stone tools).
  
• Carbonized organismal remains: In some depositional environments, the soft parts of organisms are buried earlier they tin be decomposed. Due east.G:
  • Neato fish from Eocene Greenish River Shale.
  • coal - carbonized plant parts. This is volumetrically the biggest reservoire of carbonized material, and also the most economically significant.

1 last headache - concretions: Quite often, because of local variations in groundwater chemistry, a small region (usually pea to watermelon sized) will precipitate a harder or less soluble cement than the surrounding area during cementation. These regions for nodules called concretions. Quite often, these remain intact subsequently the surrounding rock has weathered away. Information technology's not uncommon for naive people to mistake these for exotic items similar fossils or meteorites, or alien artifacts.

---

Key concepts and vocabulary:

• Sedimentary rock
• Sediment
• Sediment life history
  • Weathering
  • Ship
  • Deposition
  • Cementation
• Sedimentary stone types
  • Clastic
  • Chemic
  • Biochemical
  • Clastic rock descriptors
• Clast size
• Rounding
• Sorting

Clast size categories:

• Clay
• Silt
• Sand
• Pebble
• Cobble
• Boulder

Clastic rock clast-size names

• Mudrock
• Sandstone
• Conglomerate

Diagenesis

• Degradation
• Compaction
• Cementation

Common cements

• Calcite and aragonite
• Silica
• Hematite

Chemical sedimentary rocks

Common mineral precipitates

• Calcite and aragonite
• Gypsum
• Halite
• Exotic halides

Chemic sedimentary rock types

• Speleothems
• Tufa
• Travertine

Pseudoclastic rocks

• Ooids
• Pisolites

Biochemical sedimentary rocks

Common minerals

• Calcite and aragonite
• Opal

Chert

Carbonized organismal remains

Concretions

alexanderwipt1982.blogspot.com

Source: https://www.geol.umd.edu/~jmerck/geol100/lectures/13.html

Share this post