Geology 212

Petrology

Prof. Stephen A. Nelson

Occurrence, Mineralogy, Textures, and Structures of Sedimentary Rocks

 

 
Rivers, oceans, winds, and rain runoff all have the ability to carry the particles washed off of eroding rocks. Such material, called detritus, consists of fragments of rocks and minerals. When the energy of the transporting current is not strong enough to carry these particles, the particles drop out in the process of sedimentation. This type of sedimentary deposition is referred to as clastic sedimentation. Another type of sedimentary deposition occurs when material is dissolved in water, and chemically precipitates from the water. This type of sedimentation is referred to as chemical sedimentation. A third process can occur, wherein living organisms extract  ions dissolved in water to make such things as shells and bones.  This type of sedimentation is called biogenic sedimentation.  Thus, there are three major types of sedimentary rocks: that can be grouped by the type of particle found in the rocks.
  1. Siliclastic sedimentary rocks form by the accumulation of mostly silicate mineral fragments.  These include most sandstones, mud rocks, conglomerates, and breccias.

  2. Biochemical sedimentary rocks consist of fragments of particles produced by precipitation from once living organisms.  Most of these rocks are limestones and cherts.

  3. Chemical sedimentary rocks are formed by direct chemical precipitation from water. While some limestones and cherts may form in this manner, evaporite deposits consisting of halite, gypsum, and other salts are the most common.
95% of all sedimentary rocks consists of sandstones (made up of sand sized fragments), mudrocks (made up of silt and clay sized fragments), and carbonate rocks (made up of mostly calcite, aragonite, or dolomite).  Of these, the mudrocks are most abundant, making up about 65% of all sedimentary rocks.  Sandstones make up 20 to 25% of all sedimentary rocks, and carbonate rocks account for about 10 to 15% of all carbonate rocks.  

The Occurrence of Sedimentary Rocks

Sedimentary rocks have an average thickness of about 1800 m on the continents.  This thickness is quite variable, however, with some areas, like the Canadian Shield having no cover of sedimentary rocks, and other areas, like the Louisiana and Texas Gulf coasts, having more than 20,000 m of sedimentary rock cover.  Still, about 66% of all continental areas have a cover of sedimentary rocks.
Because sediment gets buried, and if exposed gets eroded, older sedimentary rocks show less exposed outcrop area than younger sedimentary rocks. Over 40% of the exposed sedimentary rocks are younger than Cretaceous in age.   sedrockage.gif (5262 bytes)

Depositional Basins

In order for sediment to be deposited, a low lying area, called a basin, is required. The largest of basins are the ocean basins, which currently cover about 70% of the Earth's surface.  In the past, however, sea level has often changed, resulting in episodes where even the continents were covered by shallow seas, referred to as epiric or epicontinental seas.  When sea level rises to invade the continents it is referred to as a transgression.  Major transgressions occurred during the Cretaceous, and from the Early Cambrian through Mississippian (See your text for a more detailed discussion).

It is for these reasons that most of the sedimentary rocks preserved in the geologic record are marine sediments.

Basins result from plate tectonics, and even the large transgressions appear to be related to tectonic factors, as increased spreading rates in the ocean basins can result in changing the configuration of ocean basins that result in flooding of the continents. For a classification of the various types of basins see your text.

The Mineralogy of Sedimentary Rocks

Because of their detrital nature, any mineral can occur in a sedimentary rock. Clay minerals, the dominant mineral produced by chemical weathering of rocks, is  the most abundant mineral in mudrocks. Quartz, because it is stable under conditions present at the surface of the Earth, and because it is also a product of chemical weathering, is the most abundant mineral in sandstones and the second most abundant mineral in mudrocks. Feldspar is the most common mineral in igneous and metamorphic rocks. Although feldspar eventually breaks down to clay minerals and quartz, it is still the third most abundant mineral in sedimentary rocks. Carbonate minerals, either precipitated directly or by organisms, make up most biochemical and chemical sedimentary rocks, but carbonates are also common in mudrocks and sandstones.

 

Mineral Composition

Mudrocks  %

Sandstones %
Clay minerals 60 5
Quartz 30 65
Feldspar 4 10 - 15
Carbonate minerals 3 <1
Organic matter, hematite,  & others <3 <1

 

Minerals found in sedimentary rocks can be divided into 2 classes:
  • Allogenic minerals -  These are formed elsewhere and transported into the area of deposition.

  • Authigenic minerals - These are minerals that are formed at the site of deposition, either by direct chemical precipitation or by later diagenetic processes.
Any mineral can be an allogenic mineral, but some are more stable under the conditions present at the Earth's surface than are others.  Conditions that are present at the Earth's surface and differ from those where most minerals form are:
  • Low Temperature
  • Low Pressure
  • High free oxygen concentration
  • High amounts of free liquid water

Because these conditions differ from those under which most rocks form, allogenic minerals can be classified based on their stability under near surface conditions.  Such a classification, with minerals listed in order of increasing stability is as follows:

 

 

Stability Under Conditions Present at Surface Mineral
Unstable Olivines*
Pyroxenes*
Ca-rich Plagiocalse*
Hornblende*
Andesine - Oligoclase*
Less Unstable Sphene
Epidote
Andalusite
Staurolite
Kyanite
Sillimanite
Magnetite
Garnet
Very Stable Muscovite*
Albite*
Orthoclase/Microcline*
Clay Minerals
Quartz*
Tourmaline
Zircon

 

In this list, the igneous minerals have an asterisk (*).  Note that the order in which they occur is in the same order that occur in Bowen's reaction series. Igneous minerals that crystallize at the highest temperatures are most out of equilibrium at the Earth's surface, and are therefore the most unstable.
Minerals that are very stable at the Earth's surface are minerals that either form as a result of chemical weathering, or crystallize at the lowest temperatures.

Authigenic minerals can also be allogenic minerals, but some are formed during diagenesis but not very stable in the transportation cycle either because they dissolve readily in water or because they are easily abraded during transportation.  Thus we can divide authigenic minerals into those that are stable during diagenesis and transportation, and those that are unstable during transportation.

 

 

Unstable Authigenic Minerals

 Gypsum
Carbonate Minerals
Apatite
Gluaconite
Pyrite
Zeolites
Stable Authigenic Minerals Chlorite
Albite
Orthoclase
Muscovite
Quartz
Clay Minerals

 

The longer a mineral is in the weathering and transportation cycles of sedimentary rock forming processes, the more likely it is to break down to a more stable mineral or disappear altogether.  Thus, we can classify sediments on the basis to which they have achieved mineralogical maturity.
  • Mineralogically mature sediments and sedimentary rocks consist entirely of minerals that are stable near the surface.  Such sediment is considered to have been in the weathering and transportation cycle for a long amount of time. 

  • Mineralogically immature sediments and sedimentary rocks consist of a high proportion of unstable minerals. Because such minerals will not survive for a long time in the weathering and transportation cycles, sediments and rocks with high proportions of these minerals must not have been in the weathering/transportation cycle for long periods of time.

Textures of Sedimentary Rocks

Since most sedimentary rocks are derived by processes of weathering, transportation, deposition, and diagenesis, the textures we find in sediment and sedimentary rocks are dependent on process that occur during each stage.  These include:

  1. The nature of the source rocks.  This determines the original shape of the grains and the mineralogical composition of the original sediment.

  1. The strength of the wind or water currents that carry and deposit the sediment.  This determines whether or not grains are transported or deposited. The deposition process also controls structures that could be preserved in the sediment and thus give clues to the environment of deposition.

  2. The distance transported or time in the transportation process.  The longer grains are in the transportation process the more likely they are to change shape and become sorted on the basis of size and mineralogy. This also controls extent to which they break down to stable minerals during the transportation process.

  3. Biological activity with the sediment prior to diagenesis.  Burrowing organisms can redistribute sediment after it has been deposited, thus erasing some of the clues to the original environment of deposition.

  4. The chemical environment under which diagenesis occurs.  During diagenesis grains are compacted, new minerals precipitate in the pore spaces, some minerals continue to react to produce new minerals, and some minerals recrystallize.  What happens depends on the composition of fluids moving through the rock, the composition of the mineral grains, and the pressure and temperature conditions attained during diagenesis.
Grain Size
Clastic sediments and sedimentary rocks are classified on the basis of the predominant grain size of clasts in the rock as per the table below.

Name of Particle 

Size Range 

f Scale

Loose Sediment 

Consolidated Rock 
Boulder  >256 mm  <-8  

Gravel 

 

 Conglomerate or Breccia
(depends on rounding) 
Cobble  64 - 256 mm  -6 to -8
Pebble  4 - 64 mm  -2 to -6
Granule 2 - 4 mm -1 to -2
Very Coarse Sand 1 - 2 mm  0 to -1 Sand  Sandstone 
Coarse Sand 0.5 - 1 mm 1 to 0
Medium Sand 0.25 - 0.5 mm 2 to 1
Fine Sand 0.125 - 0.25 mm 3 to 2
Very Fine Sand 0.0625 - 0.125 mm 4 to 3
Coarse Silt 0.031 - 0.625 mm 5 to 4  

Silt

 

 Siltstone 
Medium Silt  0.016 - 0.031 mm 6 to 5
Fine Silt 0.008 - 0.016 mm 7 to 6
Very Fine Silt 0.004 - 0.008 mm 8 to 7
Clay  <0.004 mm >8 Clay  Claystone, Mudstone, Shale
Note how the particle sizes small than pebble size are defined.  The lower size limit of granules is 1/2 the lower size limit of pebbles, the lower size limit of coarse sand is 1/2 the lower size limit of granules, etc.  For this reason, grain size is often given in units called f (phi) units, where f is defined as follows:

 

 

f = -log2(d)

where d is the grain diameter in millimeters.

 

This is convenient for grain size analyses of sediment, or rocks if they can be disaggregated, because sieves can be constructed where the opening of each sieve is 1/2 the size of the sieve above.  Sediment put through such a set of sieves, will be trapped in different sieves, and sorted by grain size.  The amount of grains in each sieve can then be weighed to give a quantitative measure of the size distribution of sediment.

One precaution- note how the large grain sizes have negative f units.
Sorting - Sorting refers to the uniformity of grain size in a sediment or sedimentary rock.  Particles become sorted on the basis of density because of  the energy of the transporting medium.  High energy (high velocity) currents can carry larger fragments.
As the energy or velocity decreases, heavier particles are deposited and lighter fragments continue to be transported.  This results in sorting due to density.  If the particles have the same density, such as all grains of quartz, then the heavier particles will also be larger, so the sorting will take place on the basis of size.  We can classify this size sorting on a relative basis: well-sorted to poorly-sorted.
In a more quantitative sense, sediment can be sieved to determine the distribution of grain size, as noted above.  By weighing the amount of sediment or disaggregated sedimentary rock that occurs in each sieve then re-summing to 100%, a histogram can be constructed to show the size distribution of the sediment.  To simplify matters, we can also construct a frequency curve by drawing a smooth curve through the midpoints of the histogram. frequency.gif (6495 bytes)
In a quantitative sense, sorting is a measure of the standard deviation from the mean grain size.  Thus, on a histogram or frequency curve, a narrow peak has a low standard deviation and would represent well sorted material.  A broad, spread-out frequency curve would indicate a poorly sorted sediment. 
beachsandsort.gif (6445 bytes) sortglacial.gif (6029 bytes)
Beach sands and dune sands tend to be well-sorted because the energy of the waves or wind is usually rather constant.  The coarser grained sediment is not carried in because the wave or wind velocity is too low to carry such large fragments, and the finer grained sediment is kept in suspension by the waves or wind.

Mountain streams, because they have many turbulent eddies where the velocity of the stream changes suddenly usually show poorly-sorted sediment on the bottom of the stream channel.  Similarly, glacial till, because it is deposited in place as glacial ice melts, and is not transported by water, tends to show poor sorting. 

 
Rounding - During the transportation process, grains may be reduced in size due to abrasion.  Random abrasion results in the eventual rounding off of the sharp corners and edges of grains.  Thus, the degree of  rounding of grains gives us clues to the amount of time a sediment has been in the transportation cycle.  Rounding is classified on relative terms as well.  Note that rounding is not the same a sphericity.  Sphericity is controlled by the original shape of the grain.
Porosity-  Porosity is the percentage of the volume of the rock that is open space (pore space). This determines the amount of water or other fluids, like petroleum, that a rock can contain.  

In sediments or sedimentary rocks the porosity depends on grain size, the shapes of the grains, the degree of sorting, and the degree of cementation.
  • Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit together well.  Angular grains of fine grained sediment can be compacted to fit together better, and thus porosity is reduced. Mudrocks, because of fine grain size, usually have very low porosities.
  • Poorly sorted sediments usually have lower porosity because the fine-grained fragments tend to fill in the open space
  • Cementation that takes place during diagenesis tends to fill in the pore space.  Thus, highly cemented sedimentary rocks have lower porosity than do poorly cemented sedimentary rocks.
Packing of Grains- Packing refers to the arrangement of clastic grains entirely apart from any authigenic cement that may have later crystallized between them. If the clastic grains touch each other throughout, the rock is said to be grain supported. If the rock is poorly sorted and the grains are separated by a mud or silt matrix, the rock is said to be matrix supported.

Matrix- If the rock is poorly or moderately sorted, the percentage of matrix and texture of the matrix should also be described.

Induration- Induration refers to the hardness of the rock or how easily it breaks apart.  Well indurated rocks are difficult to break with a hammer.  Moderately indurated rocks can be easily broken with a rock hammer.  Poorly indurated or friable rocks break apart easily in your hand.  The term non-indurated would describe a sediment that has not undergone any cementation.
Textural Maturity
The longer sediment is involved in the transportation cycle, the more time it has to become well-sorted.  Similarly, the longer the sediment is transported, the more time is available for grains to lose their rough edges and corners by abrasion.  Thus, we consider a texturally mature sediment to be sediment that is well-sorted and well-rounded.  Note that sediment tends to become both texturally and mineralogically mature the longer it is in the transportation cycle. 
Descriptions of Texture
A complete description of the texture of a sedimentary rock should include statements about each of the factors discussed above.  To summarize, these are:
  1. Size of the grains.
  2. Sorting.
  3. Degree of roundness and sphericity of the grains.
  4. An estimate of the porosity of the rock.
  5. Packing of the grains.
  6. A description of the matrix.
  7. Induration of the rock.
  8. A statement about the textural maturity of the rock.

Structures of Sedimentary Rocks

The process of deposition usually imparts variations in layering, bedforms, or other structures that give clues to environment in which deposition occurs. Such things as water depth, current velocity, and current direction can be sometimes be determined from sedimentary structures.  Thus, it is important to recognize various sedimentary structures so that we can interpret the clues that they offer to these conditions. 

Also, because sedimentary rocks can be deformed by folding and faulting long after the deposition process has ended, it is important to be able to determine which was up in the rock when it was originally deposited, especially if one is going to use the rocks to determine the sequence of events that occurred or interpret the geologic history of an area.   Features that tell us which way was up are often referred to as top and bottom indicators.

Here we explore some the structures that occur in sediments and sedimentary rocks and provide information on how to interpret these structures.

Stratification and Bedding
One of the most obvious features of sedimentary rocks and sediment is the layered structure which they exhibit.  The layers are evident because of differences in mineralogy, clast size, degree of sorting, or color of the different layers. In rocks, these differences may be made more prominent by the differences in resistance to weathering or color changes brought out by weathering. 

Layering is usually described on the basis of layer thickness as per the table below.  Other distinctive types of layering are described below.

 

 

Layer Thickness Names
> 300 cm Massive
100-300 cm Very thickly bedded
30 - 100 cm Thickly bedded
10 - 30 cm Mediumly bedded
3 - 10 cm Thinly bedded
1 - 3 cm Very thinly bedded
0.3 - 1 cm Thickly laminated
<0.3 cm Thinly laminated

 

  • Rhythmic Layering - consists of alternating parallel layers having different properties.

 

    This is sometimes caused by seasonal changes in deposition (Varves). i.e. lake deposits wherein coarse sediment is deposited in summer months and fine sediment is deposited in the winter when the surface of the lake is frozen.
  • Cross Bedding - consists of sets of beds that are inclined relative to one another.  The beds are inclined in the direction that the wind or water was moving at the time of deposition.  Boundaries between sets of cross beds usually represent an erosional surface. Cross bedding is very common in beach deposits, sand dunes, and river deposited sediment.   Individual beds within cross-bedded strata are useful indicators of current direction and tops and bottoms.  Note how the beds become asymptotic to the lower boundary on which they were deposited.
  • Graded Bedding  - As current velocity decreases, the larger or more dense particles are deposited first, followed by smaller particles.  This results in bedding showing a decrease in grain size from the bottom of the bed to the top of the bed.  This gives us a method for determining tops and bottoms of beds, since reverse grading will not be expected unless deposition occurs under unusual circumstances.   Note that reverse graded bedding cannot occur as current velocity increases, because each layer will simply be removed as the current achieves a velocity high enough to carry sediment of a particular size.
  • Non-sorted Sediment - Sediment showing a mixture of grain sizes results from such things as rockfalls, debris flows, mudflows, and deposition from melting ice.
  • Imbricate bedding - Elongated grains can sometimes pile up on each other to form what is called imbricate bedding.  Note that imbricate bedding can be a current direction indicator if some other means is present to provide top/bottom directions.
 imbricate.gif (4425 bytes)
Surface Features
  • Ripple Marks  - Ripple marks are characteristic of shallow water deposition.  They are caused by waves or winds piling up the sediment into long ridges.  Asymmetrical ripple marks can give an indication of current direction.  Symmetrical ripples form when the water moves back and forth

 

    Symmetrical ripple marks occur in environments where there is a steady back and forth movement of the water.  Such ripple marks can still be used as top and bottom indicators.

 symripples.gif (7403 bytes)
  • Mudcracks - result from the drying out of wet sediment at the surface of the Earth.  The cracks form due to shrinkage of the sediment as it dries. In cross section the mudcracks tend to curl up, thus becoming a good top/bottom indicator.   The presence of mudcracks indicates that the sediment was exposed at the surface shortly after deposition, since drying of the sediment would not occur beneath a body of water.
  • Raindrop Marks - pits (or tiny craters) created by falling rain. If present, this suggests that the sediment was exposed to the surface of the Earth, and also are good top/bottom indicators..
  • Casts and Molds-  Any depression formed on the bottom of a body of water may become a mold for any sediment that later gets deposited into the depression.  The body of sediment that takes on the shape of the mold is referred to as a cast.
    • Load Casts - These are bulbous protrusions that are formed when compaction causes sediment to be pushed downward into softer sediment.

    • Flute Casts (Sole Marks) - Flutes are elongated depressions that form on the bottom of a body water as the current erodes.  The flutes may form a mold into which new sediment is deposited.  Preservation of the overlying sediment as a cast result in flute casts, which are sometimes referred to as sole marks.  Flute casts are excellent indicators of current direction and tops/bottoms of beds.
  • Tool Marks- Any object carried along by a current my gouge or scrape the sediment on the bottom of the body of water.  Depressions, scratches, or gouges are called toolmarks and can be useful top/bottom indicators.
  • Tracks and Trails - These features result from organisms moving across the sediment as they walk, crawl, or drag their body parts through the sediment.
  • Burrow Marks- Any organism that burrows into soft sediment can disturb the sediment and destroy many of the structures discussed above.  If burrowing is not extensive, the holes made by such organisms can later become filled with water that deposits new sediment in the holes.  Such burrow marks can be excellent top and bottom indicators.
 burrows.gif (4316 bytes)

Sedimentary Facies

A sedimentary facies is a group of characteristics that reflect a sedimentary environment different from those elsewhere in the same deposit. Thus, facies may change vertically through a sequence as a result of changing environment through time. Also, facies may change laterally through a deposit as a result of changing environments with distance at the same time.

The structures and textures found in sedimentary rocks give clues to the environment of deposition, as discussed above, and thus allow geologists to assign parts of a deposit to a particular sedimentary facies. Individual facies are generally described in terms of the environment is which deposition occurred.  The following are facies or environments are among those possible:

I.   Terrigenous or Non-marine Facies

II. Near Shore Facies

  1. Alluvial Fan

  2. Floodplain

  3. Lacustrine (lakes)

  4. Fluvial (streams)

  5. Eolian (wind deposited, sand dunes)

  6. Swamp

  7. Glacial

  1. Deltaic

  2. Estuarine

  3. Bay or Lagoon

  4. Marsh

  5. Intertidal

  6. Beach or Barrier Island

  7. Glacial Marine

III. Shallow Marine (depths < about 200 m)

IV. Deep Marine Facies

  1. Continental Shelf

  2. Carbonate bank and shelf

  3. Forearc basin

  4. Evaporite basin

  1. Submarine canyon

  2. Submarine fan

  3. Back arc basin

  4. Deep ocean basin
Each of these could be subdivided further.  A description of each of these facies is beyond the scope of the present course and will be covered in more detail in your future course in sedimentology and stratigraphy.
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