Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label sedimentary rock. Show all posts
Showing posts with label sedimentary rock. Show all posts

Geologic Contacts

A geologic contact is where one rock type touches another. There are three types of geologic contact:1. Depositional contacts are those where a sedimentary rock (or a lava flow) was deposited on an older rock
2. Intrusive contacts are those where one rock has intruded another
3. Fault contacts are those where rocks come into contact across fault zones.
Learn in detail about fault here

Following are the some pictures showing each type of geologic contact

Depositional Contacts

1. Angular Unconformity, Siccar Point, Scotland

This place is known as Siccar Point which is the most important unconformity described by James Hutton (1726-1797) in support of his world-changing ideas on the origin and age of the Earth.
Here 
gently sloping strata of 370-million-year-old Famennian Late Devonian Old Red Sandstone and a basal layer of conglomerate overlie near vertical layers of 435-million-year-old lower Silurian Llandovery Epoch greywacke, with an interval of around 65 million years.


2. Cretaceous Sandstone overlying Conglomerate    Kootenai Formation, SW Montana

Photo Courtesy: marlimillerphoto.com

3. Dun Briste Sea Stack, IrelandDun Briste is a truly incredible site to see but must be visited to appreciate its splendour. It was once joined to the mainland. The sea stack stands 45 metres (150 feet) tall.

Dun Briste and the surrounding cliffs were formed around 350 million years ago (during the 'Lower Carboniferous Period'), when sea temperatures were much higher and the coastline at a greater distance away.  There are many legends describing how the Sea Stack was formed but it is widely accepted that an arch leading to the rock collapsed during very rough sea conditions in 1393. This is remarkably recent in geological terms

Photo Courtesy: dunbriste.com 


Fault Contacts

1. Normal Faulting in the Cutler Formation near Arches National Park

Photo Courtesy: travelinggeologist.com

2. Normal Fault in Titus Canyon, Death Valley, California 

Photo Courtesy: travelinggeologist.com


3. 
Horst and Graben Structure in Zanjan, Iran

Photo Courtesy: Amazhda

Intrusive Contacts 

1. 
Pegmatite and aplite dikes and veins in granitic rocks on Kehoe Beach, Point Reyes National Seashore, California.


2. Spectacular mafic dyke from Isla de Socorro from Pep Cabré. The Isla de Socorro is a volcanic island off the west coast of Mexico and it is the only felsic volcano in the Pacific Ocean

Photo Courtesy: travelinggeologist.com

3. The margins of this Granite dyke cooled relatively quickly in contact with this much older Gabbro.
Photo near Ai-Ais Namibia

Photo Courtesy: travelinggeologist

Banded-iron formations (BIFs) - Evidence of Oxygen in Early Atmosphere

Our knowledge about the rise of oxygen gas in Earth’s atmosphere comes from multiple lines of evidence in the rock record, including the age and distribution of banded iron formations, the presence of microfossils in oceanic rocks, and the isotopes of sulfur.
However, this article is just focus on Banded Iron Formation.

BIF (polished) from Hamersley Iron Formation, West Australia, Australia

Summary: Banded-iron formations (BIFs) are sedimentary mineral deposits consisting of alternating beds of iron-rich minerals (mostly hematite) and silica-rich layers (chert or quartz) formed about 3.0 to 1.8 billion years ago. Theory suggests BIFs are associated with the capture of oxygen released by photosynthetic processes by iron dissolved in ancient ocean water. Once nearly all the free iron was consumed in seawater, oxygen could gradually accumulate in the atmosphere, allowing an ozone layer to form. BIF deposits are extensive in many locations, occurring as deposits, hundreds to thousands of feet thick. During Precambrian time, BIF deposits probably extensively covered large parts of the global ocean basins. The BIFs we see today are only remnants of what were probably every extensive deposits. BIFs are the major source of the world's iron ore and are found preserved on all major continental shield regions. 

Banded-iron formation (BIF)
is 
consists of layers of iron oxides (typically either magnetite or hematite) separated by layers of chert (silica-rich sedimentary rock). Each layer is usually narrow (millimeters to few centimeters). The rock has a distinctively banded appearance because of differently colored lighter silica- and darker iron-rich layers. In some cases BIFs may contain siderite (carbonate iron-bearing mineral) or pyrite (sulfide) in place of iron oxides and instead of chert the rock may contain carbonaceous (rich in organic matter) shale.

It is a chemogenic sedimentary rock (material is believed to be chemically precipitated on the seafloor). Because of old age BIFs generally have been metamorphosed to a various degrees (especially older types), but the rock has largely retained its original appearance because its constituent minerals are fairly stable at higher temperatures and pressures. These rocks can be described as metasedimentary chemogenic rocks.



                     Jaspilite banded iron formation (Soudan Iron-Formation, Soudan, Minnesota, USA
Image Credits: James St. John



In the 1960s, Preston Cloud, a geology professor at the University of California, Santa Barbara, became interested in a particular kind of rock known as a Banded Iron Formation (or BIF). They provide an important source of iron for making automobiles, and provide evidence for the lack of oxygen gas on the early Earth.

Cloud realized that the widespread occurrence of BIFs meant that
the conditions needed to form them must have been common on the ancient Earth, and not common after 1.8 billion years ago. Shale and chert often form in ocean environments today, where sediments and silica-shelled microorganisms accumulate gradually on the seafloor and eventually turn into rock. But iron is less common in younger oceanic sedimentary rocks. This is partly because there are only a few sources of iron available to the ocean: isolated volcanic vents in the deep ocean and material weathered from continental rocks and carried to sea by rivers.


Banded iron-formation (10 cm), Northern Cape, South Africa.
Specimen and photograph: A. Fraser
Most importantly, it is difficult to transport iron very far from these sources today because when iron reacts with oxygen gas, it becomes insoluble (it cannot be dissolved in water) and forms a solidparticle. Cloud understood that for large deposits of iron to exist all over the world’s oceans, the iron must have existed in a dissolved form. This way, it could be transported long distances in seawater from its sources to the locations where BIFs formed. This would be possible only if there were little or no oxygen gas in the atmosphere and ocean at the time the BIFs were being deposited. Cloud recognized that since BIFs could not form in the presence of oxygen, the end of BIF deposition probably marked the first occurrence of abundant oxygen gas on Earth (Cloud, 1968).
Cloud further reasoned that, for dissolved iron to finally precipitate and be deposited, the iron would have had to react with small amounts of oxygen near the deposits. Small amounts of oxygen could have been produced by the first photosynthetic bacteria living in the open ocean. When the dissolved iron encountered the oxygen produced by the photosynthesizing bacteria, the iron would have precipitated out of seawater in the form of minerals that make up the iron-rich layers of BIFs: hematite (Fe2O3) and magnetite (Fe3O4), according to the following reactions:
4Fe3 + 2O2 → 2Fe2O3
6Fe2 + 4O2 → 2Fe3O4
The picture that emerged from Cloud’s studies of BIFs was that small amounts of oxygen gas, produced by photosynthesis, allowed BIFs to begin forming more than 3 billion years ago. The abrupt disappearance of BIFs around 1.8 billion years ago probably marked the time when oxygen gas became too abundant to allow dissolved iron to be transported in the oceans.
Banded Iron Formation
Source is unknown

It is interesting to note that BIFs reappeared briefly in a few places around 700 millionyears ago,during a period of extreme glaciation when evidence suggests that Earth’s oceans were entirely covered with sea ice. This would have essentially prevented the oceans from interacting with the atmosphere, limiting the supply of oxygen gas in the water and again allowing dissolved iron to be transported throughout the oceans. When the sea ice melted, the presence of oxygen would have again allowed the iron to precipitate.

References:

1. Misra, K. (1999). Understanding Mineral Deposits Springer.
2. 
Cloud, P. E. (1968). Atmospheric and hydrospheric evolution on the primitive Earth both secular accretion and biological and geochemical processes have affected Earth’s volatile envelope. Science, 160(3829), 729–736.
3. 
James,H.L. (1983). Distribution of banded iron-formation in space and time. Developments in Precambrian Geology, 6, 471–490.

Siccar Point - the world's most important geological site and the birthplace of modern geology


Siccar Point is world-famous as the most important unconformity described by James Hutton (1726-1797) in support of his world-changing ideas on the origin and age of the Earth.

James Hutton unconformity with annotations - Siccar Point 



In 1788, James Hutton first discovered Siccar Point, and understood its significance. It is by far the most spectacular of several unconformities that he discovered in Scotland, and very important in helping Hutton to explain his ideas about the processes of the Earth.At Siccar Point, gently sloping strata of 370-million-year-old Famennian Late Devonian Old Red Sandstone and a basal layer of conglomerate overlie near vertical layers of 435-million-year-old lower Silurian Llandovery Epoch greywacke, with an interval of around 65 million years.
Standing on the angular unconformity at Siccar Point (click to enlarge). Photo: Chris Rowan, 2009
As above, with annotations. Photo: Chris Rowan, 2009





Hutton used Siccar Point to demonstrate the cycle of deposition, folding, erosion and further deposition that the unconformity represents. He understood the implication of unconformities in the evidence that they provided for the enormity of geological time and the antiquity of planet Earth, in contrast to the biblical teaching of the creation of the Earth. 

   
How the unconformity at Siccar Point formed.



At this range, it is easy to spot that the contact between the two units is sharp, but it is not completely flat. Furthermore, the lowest part of the overlying Old Red Sandstone contains fragments of rock that are considerably larger than sand; some are at least as large as your fist, and many of the fragments in this basal conglomerate are bits of the underlying Silurian greywacke. These are all signs that the greywackes were exposed at the surface, being eroded, for a considerable period of time before the Old Red Sandstone was laid down on top of them.
The irregular topography and basal conglomerate show that this is an erosional contact. Photo: Chris Rowan, 2009

The Siccar Point which is a rocky promontory in the county of Berwickshire on the east coast of Scotland.

Classes of sedimentary rocks

Classes of sedimentary rocks

Geologists divide sedimentary rocks into four major classes, based on their mode of origin. 
(1) Clastic sedimentary rock consists of cemented-together clasts, solid fragments and grains broken off of preexisting rocks (the word comes from the Greek klastos, meaning broken); (2) biochemical sedimentary rock consists of shells; (3) organic sedimentary rock consists of carbon-rich relicts of plants or other organisms; and (4) chemical sedimentary rock is made up of minerals that precipitated directly from water solutions. Let’s now look at these major classes in more detail.

Clastic Sedimentary Rocks Formation

Nine hundred years ago, a thriving community of Native Americans inhabited the high plateau of Mesa Verde, Colorado. In hollows beneath huge overhanging ledges, they built multistory stone-block buildings that have survived to this day. Clearly, the blocks are solid and durable they are, after all, rock. But if you were to rub your thumb along one, it would feel gritty, and small grains of quartz would break free and roll under your thumb, for the block consists of quartz sand grains cemented together. Geologists call such rock a sandstone. Sandstone is an example of clastic sedimentary rock. It consists of loose clasts, known as detritus, that have been stuck together to form a solid mass. The clasts can consist of individual minerals (such as grains of quartz or flakes of clay) or of fragments of rock (such as pebbles of granite). Formation of sediment and its transformation into clastic sedimentary rock takes place via the following five steps.

  • Weathering: Detritus forms by disintegration of bedrock into separate grains due to physical and chemical weathering. 
  • Erosion: Erosion refers to the combination of processes that separate rock or regolith (surface debris) from its substrate. Erosion involves abrasion, falling, plucking, scouring, and dissolution, and is caused by moving air, water, or ice. 
  • Transportation: Gravity, wind, water, or ice carry sediment. The ability of a medium to carry sediment depends on its viscosity and velocity. Solid ice can transport sediment of any size, regardless of how slowly the ice moves. Very fast-moving, turbulent water can transport coarse fragments (cobbles and boulders), moderately fast-moving water can carry only sand and gravel, and slow-moving water carries only silt and clay. Strong winds can move sand and dust, but gentle breezes carry only dust. 
  • Deposition: Deposition is the process by which sediment settles out of the transporting medium. Sediment settles out of wind or moving water when these fluids slow, because as the velocity decreases, the fluid no longer has the ability to carry sediment. Sediment is deposited by ice when the ice melts. 
  • Lithification: Geologists refer to the transformation of loose sediment into solid rock as lithification. The lithification of clastic sediment involves two steps. First, once the sediment has been buried, pressure caused by the weight of overlying material squeezes out water and air that had been trapped between clasts, and clasts press together tightly, a process called compaction. Compacted sediment may then be stuck together to make coherent sedimentary rock by the process of cementation. Cement consists of minerals (commonly quartz or calcite) that precipitate from groundwater and fill the spaces between clasts. 

Classifying clastic sedimentary rocks

Say that you pick up a clastic sedimentary rock and want to describe it sufficiently so that, from your words alone, another person can picture the rock. What characteristics should you mention? Geologists find the following characteristics most useful. 
  • Clast size. Size refers to the diameter of fragments or grains making up a rock. Names used for clast size, listed in order from coarsest to finest, are: boulder, cobble, pebble, sand, silt, and clay. 
  • Clast composition. Composition refers to the makeup of clasts in sedimentary rock. Clasts may be composed of rock fragments or individual mineral grains. 
  • Angularity and sphericity. Angularity indicates the degree to which clasts have smooth or angular corners and edges. Sphericity, in contrast, refers to the degree to which the shape of a clast resembles a sphere. 
  • Sorting. Sorting of clasts indicates the degree to which the clasts in a rock are all the same size or include a variety of sizes. Well-sorted sediment consists entirely of sediment of the same size, whereas poorly-sorted sediment contains a mixture of more than one clast size. 
  • Character of cement. Not all clastic sedimentary rocks have the same kind of cement. In some, the cement consists  predominantly of quartz, whereas in others, it consists predominantly of calcite. 

With these characteristics in mind, we can distinguish among several common types of clastic sedimentary rocks. This table provides common rock names specialists sometimes use other, more precise names based on more complex classification schemes. The size, angularity, sphericity, and sorting of clasts depends on the medium (water, ice, or wind) that transports the clasts and, in the case of water or wind, on both the velocity of the medium and the distance of transport. The composition of the clasts depends on the composition of rock from which the clasts were derived, and on the degree of chemical weathering that the clasts have undergone. Thus, the type of clasts that accumulate in a sedimentary deposit varies with location. To see how, let’s follow the fate of rock fragments as they gradually move from a cliff face in the mountains via a river to the seashore. Different kinds of sediment develop along the route. Each kind, if buried and lithified, would yield a different type of sedimentary rock.

To start, imagine that some large blocks of granite tumble off a cliff and slam into other blocks already at the bottom. The impact shatters the blocks, producing a pile of angular fragments. If these fragments were to be cemented together before being transported very far, the resulting rock would be breccia (a in above figure). Later, a storm causes the fragments (clasts) to be carried away by a turbulent river. In the water, clasts bang into each other and into the riverbed, a process that shatters them into still smaller pieces and breaks off their sharp edges. As the clasts get carried downstream, they gradually become rounded pebbles and cobbles. When the river water slows, these clasts stop moving and form a mound or bar of gravel. Burial and lithification of these rounded clasts produces conglomerate (b in above figure). If the gravel stays put for a long time, it undergoes chemical weathering. As a consequence, cobbles and pebbles break apart into individual mineral grains, eventually producing a mixture of quartz, feldspar, and clay. Clay is so fine that flowing water easily picks it up and carries it downstream, leaving sand containing a mixture of quartz and some feldspar grains this sediment, if buried and lithified, becomes arkose (c in above figure). Over time, feldspar grains in sand continue to weather into clay so that gradually, during successive events that wash the sediment downstream, the sand loses feldspar and ends up being composed almost entirely of durable quartz grains. Some of the sand may make it to the sea, where waves carry it to beaches, and some may end up in desert dunes. This sediment, when buried and lithified, 
becomes quartz sandstone (d figure below). Meanwhile, silt and clay may accumulate in the flat areas bordering streams, regions called floodplains that become inundated only during floods. And some silt and mud settles in a wedge, called a delta, at the mouth of the river, or in lagoons or mudflats along the shore. The silt, when lithified, becomes siltstone, and the mud, when lithified, becomes shale or mudstone (e figure below).

Biochemical Sedimentary

Rocks The Earth System involves many interactions between living organisms and the physical planet. Numerous organisms have evolved the ability to extract dissolved ions from seawater to make solid shells. When the organisms die, the solid material in their shells survives. This material, when lithified, comprises biochemical sedimentary rock. Geologists recognize several different types of biochemical sedimentary rocks, which we now describe.

Limestone (biochemical)

A snorkeler gliding above a reef sees an incredibly diverse community of coral and algae, around which creatures such as clams, oysters, snails (gastropods), and lampshells (brachiopods) live, and above which plankton float (a figure above). Though they look so different from each other, many of these organisms share an important characteristic: they make solid shells of calcium carbonate (CaCO3). The CaCO3 crystallizes either as calcite or aragonite. (These minerals have the same composition, but different crystal structures.) When the organisms die, the shells remain and may accumulate.  
Rocks formed dominantly from this material are the biochemical version of limestone. Since the principal compound making up limestone is CaCO3, geologists refer to limestone as a type of carbonate rock. Limestone comes in a variety of textures, because the material that forms it accumulates in a variety of ways. For example, limestone can originate from reef builders (such as coral) that grew in place, from shell debris that was broken up and transported, from carbonate mud, or from plankton shells that settled like snow out of water. Because of this variety, geologists distinguish among fossiliferous limestone, consisting of visible fossil shells or shell fragments (b figure above); micrite, consisting of very fine carbonate mud; and chalk, consisting of plankton shells. Experts recognize many other types as well. Typically, limestone is a massive light-gray to darkbluish-gray rock that breaks into chunky blocks it doesn't look much like a pile of shell fragments (c figure above). That’s because several processes change the texture of the rock over time. For example, water passing through the rock not only precipitates cement but also dissolves some carbonate grains and causes new ones to grow.

Chert (biochemical).

If you walk beneath the northern end of the Golden Gate Bridge in California, you will find outcrops of reddish, almost porcelain-like rock occurring in 3- to 15-cm-thick layers (a figure above). Hit it with a hammer, and the rock would crack to form smooth, spoon-shaped (conchoidal) fractures. Geologists call this rock biochemical chert; it’s made from cryptocrystalline quartz (crypto is Greek for hidden), meaning quartz grains that are too small to be seen without the extreme magnification of an electron microscope. The chert beneath the Golden Gate Bridge formed from the shells of silica-secreting plankton that accumulated on the sea floor. Gradually, after burial, the shells dissolved, forming a silica-rich gel. Chert then formed when this gel solidified. 

Organic Sedimentary Rocks 

We've seen how the mineral shells of organisms (CaCO3 or SiO2) can accumulate and lithify to become biochemical sedimentary rocks. What happens to the “guts” of the organisms the cellulose, fat, carbohydrate, protein, and other organic compounds that make up living matter? Commonly, this organic debris gets eaten by other organisms or decays at the Earth’s surface. But in some environments, the organic debris settles along with other sediment and eventually gets buried. When lithified, organic-rich sediment becomes organic sedimentary rock. Since the dawn of the industrial revolution in the early 19th century, coal, one type of organic sedimentary rock, has provided the fuel of modern industry and transportation, for the organic chemicals in the rock yield energy when burned. Coal is a black, combustible rock consisting of over 50 to 90% carbon. The remainder consists of oxygen, nitrogen, hydrogen, sulphur, silica, and minor amounts of other elements. Typically, the carbon in coal occurs in large, complex organic molecules made of many rings note that the carbon does not occur in CaCO3. Coal forms when plant remains have been buried deeply enough and long enough for the material to become compacted and to lose significant amounts of volatiles (hydrogen, water, CO2, and ammonia); as the volatiles seep away, a concentration of carbon remains  (b figure above).

Chemical Sedimentary Rocks 

The colourful terraces, or mounds, that grow around the vents of hot-water springs; the immense layers of salt that underlie the floor of the Mediterranean Sea; the smooth, sharp point of an ancient arrowhead these materials all have something in common. They all consist of rock formed primarily by the precipitation of minerals from water solutions. We call such rocks chemical sedimentary rocks. They typically have a crystalline texture, partly formed during their original precipitation and partly when, at a later time, new crystals grow at the expense of old ones through a process called recrystallization. In some examples, crystals are coarse. In others, they are too small to see. Geologists distinguish among many types of chemical sedimentary rocks, primarily on the basis of composition.

Evaporites: the products of salt-water evaporation.  


In 1965, two daredevil drivers in jet-powered cars battled to be the first to set the land speed record of 600 mph. On November 7, Art Arfons, in the Green Monster, peaked at 576.127  mph; but eight days later Craig Breedlove, driving the Spirit of America, reached 600.601 mph. Travelling at such speeds, a driver must maintain an absolutely straight line; any turn will catapult the vehicle out of control. Thus, high-speed trials take place on extremely long and flat racecourses. Not many places can provide such conditions the Bonneville Salt Flats of Utah do. The salt flats formed when an ancient salt lake evaporated. Under the heat of the Sun, the water turned to vapour and drifted up into the atmosphere, but the salt that had been dissolved in the water stayed behind. Salt precipitation occurs where salt-water becomes supersaturated, meaning that it has exceeded its capacity to contain more dissolved ions. In supersaturated salt-water, ions bond to form solid grains that either settle out of the water or grow on the floor of the water body. Supersaturated salt-water develops where evaporation removes water from a water body faster than the rate at which new water enters. This process takes place in desert lakes and along the margins of restricted seas (figure above). For thick deposits of salt to form, large volumes of water must evaporate. Because salt deposits form as a consequence of evaporation, geologists refer to them as evaporites. The specific type of salt minerals comprising an evaporite depends on the amount of evaporation. When 80% of the water evaporates, gypsum forms; and when 90% of the water evaporates, halite precipitates. 

Travertine (chemical limestone).  

Travertine is a rock composed of crystalline calcium carbonate (CaCO3) formed by chemical precipitation from groundwater that has seeped out at the ground surface either in hot- or cold-water springs, or on the walls of caves. What causes this precipitation? It happens, in part, when the groundwater degasses, meaning that some of the carbon dioxide that had been dissolved in the groundwater bubbles out of solution, for removal of carbon dioxide encourages the precipitation of carbonate. Precipitation also occurs when water evaporates, thereby increasing the concentration of carbonate. Various kinds of microbes live in the environments in which travertine accumulates, so biologic activity may also contribute to the precipitation process. Travertine produced at springs forms terraces and mounds that are meters or even hundreds of meters thick, such as those at Mammoth Hot Springs (a in figure above). Travertine also grows on the walls of caves where groundwater seeps out (b in figure above). In cave settings, travertine builds up beautiful and complex growth forms called speleothems.

Dolostone.  

Another carbonate rock, dolostone, differs from limestone in that it contains the mineral dolomite (CaMg[CO3]2), which contains equal amounts of calcium and magnesium. Where does the magnesium come from? Most dolostone forms by a chemical reaction between solid calcite and magnesium-bearing groundwater. Much of the dolostone you may find in an outcrop actually originated as limestone but later changed into dolostone as dolomite crystals replaced calcite. This change may take place beneath lagoons along a shore soon after the limestone formed, or a long time later, after the limestone has been buried deeply.

Chert (replacement).  

A tribe of Native Americans, the Onondaga, once lived off the land in eastern New York State. Here, outcrops of limestone contain layers or nodules (lenses or lumps) of a black chert (a in figure above). Because of the way it breaks, the tribe’s artisans could fashion sharpedged tools (arrowheads and scrapers) from this chert, so the Onondaga collected it for their own toolmaking industry and for use in trade with other people. Unlike the deep sea (biochemical) chert described earlier, the chert collected by the Onondaga formed when cryptocrystalline quartz gradually replaced calcite crystals within a body of limestone long after the limestone was deposited; geologists call such material “replacement chert.” 
Chert comes in many colours (black, white, red, brown, green, gray), depending on the impurities it contains. Petrified wood is chert that forms when silica-rich sediment, such as ash from a volcanic eruption, buries a forest. The silica dissolves in groundwater, and then later precipitates as cryptocrystalline quartz within wood, gradually replacing the wood’s cellulose. The chert deposit retains the shape of the wood and the growth rings within it. Some chert, known as agate, precipitates in concentric rings inside hollows in a rock and ends up with a striped appearance, caused by variations in the content of impurities incorporated in the chert (b in figure above).

Basics of Geology, Rocks and Minerals


Sedimentary Rocks

Sedimentary Rocks:

Sediments are loose rock particles, produced by one of three mechanisms:

1) Weathering of preexisting rocks, followed by transportation and deposition (produce clastic sedimentary rocks such as sandstones, breccias, and conglomerates).
2) Chemical precipitation of minerals from water (produce chemical sedimentary rocks such as limestone and evaporites).
3) Accumulation of biological matter such as shells and plant fragments (such as coal).

Sediments that are buried may harden into sedimentary rock through a process know as lithification (a combination of compaction and cementation)

From Sediment to Sedimentary Rock

Prior to lithification, sediment experiences two major events:

1)  Transportation- The movement of sediment away from its source rock by water, wind, or ice

Rounding of particles occurs due to abrasion during transport.  Rounding increases as transport distance increases.

Size sorting occurs by transport agents, especially running water.  Sediment size decreases as transport distance increases.

2)  DepositionThe environment of deposition is the location where deposition occurred.  Examples of environments of deposition are:

River Channel
Lake Bottom
Delta
Beach
Deep Ocean
Desert Dunes




Classification of Sedimentary Rocks by Origin

1) Clastic (or detrital) sedimentary rocksForm from the cementation of sediment grains that come from pre-existing rocks.  This is the most common sedimentary rock type.  Clastic sedimentary rocks are classified by grain size, and to a lesser extent by chemical composition.

Boulder - >256 mm
Cobble - 64 to 256 mm
Pebble - 2 to 64 mm
Sand - 1/16 to 2 mm
Silt - 1/256 to 1/16 mm
Clay - <1/256 mm


Breccia- A coarse-grained clastic rock composed of angular rock fragments cemented together (poor rounding).

Breccia© Marli Miller, University of Oregon.


Conglomerate- A coarse-grained clastic rock made of rounded gravel cemented together (good rounding).

Conglomerate, © Marli Miller, University of Oregon.



Sandstone- A medium-grained clastic rock.

Sandstone, Public Domain Image, USGS (Minerals in your World Project).

Sandstones are subclassified based on the chemical composition:

Quartz sandstone - Contains >90% quartz grains.
Arkose - Contains mostly feldspar and quartz grains mixed together.
Graywacke - Contains sand grains surrounded by dark, fine-grained matrix (they are "dirty" sandstones).



Siltstone- A fine-grained clastic rock.  Siltstones have a gritty feel, and the individual grains are visible with a hand lens.

Siltstone, Public Domain Image, USGS (Minerals in your World Project).



Shale- A fine-grained clastic rock containing silt and clay-sided grains.  Shale spits into thin layers, a behavior known as fissile.  Shales feel smoother than siltstones.

Shale, Public Domain Image, USGS (Minerals in your World Project).

Mudstone/Claystone- Mudstone is made of silt and clay-sized grains.  They are blocky (non-fissile).



2) Chemical (and biochemical) sedimentary rocksChemical sedimentary rocks form by the precipitation of minerals from water (this process may or may not involve the actions of organisms).  In contrast to the clastic textures of the rocks discussed above, chemical sedimentary rocks have crystalline textures.


LimestoneA chemical sedimentary rock composed mainly of calcite (CaCO3).

Fossiliferous limestone, Public Domain Image, USGS (Minerals in your World Project).

Most limestones are biochemical, but many are inorganic.

They often contain easily recognizable fossils (fossiliferous).

Chemical alteration of limestone in Mg-rich water can produce dolomite, CaMg(CO3)2.


ChertA hard, compact, fine-grained chemical sedimentary rock composed almost entirely of silica (SiO2).

 
Chert, Public Domain Image, USGS (Minerals in your World Project).

Chert can occur as layers or as lumpy nodules within other sedimentary rocks, especially within limestone.

Some cherts are primary precipitates, but others formed by replacement of pre-existing material by silica.



Evaporates- Evaporites are chemical sedimentary rocks that grow upward from seas and salt-rich lakes due to water evaporation.

Delicate evaporites
© Larry Fellows, Arizona Geological Survey.

Common evaporite deposits are gypsum, CaSO4•2(H2O), and halite (NaCl).



Organic sedimentary rocksFossil fuels are sedimentary rocks with a biological origin.

Coal- Coal is a sedimentary rock formed from the compaction of partially decayed plant material (requires stagnant water and rapid burial).

Oil and natural gas- “Cooking” below Earth's surface can change organic solids into oil and natural gas.  These fossil fuels rise and accumulate in porous overlying rocks.



Sedimentary Structures

Sedimentary structure- A features within a sedimentary rock that provides clues about the environment of deposition.

1) Bedding: Series of horizontal layers within an outcrop of rock.  The most common sedimentary structure.
Shale with interbedded limestone. © Marli Miller, University of Oregon.


2) Ripple marks: Small ridges formed on the surface of sediment layer by moving wind or water.  Symmetrical ripples represent water wave ripples, whereas assymetrical ripples represent water current or wind current ripples.

Sand dunes of Death Valley, © Marli Miller, University of Oregon.
Ripples on sandy beach in southern Alaska, © Marli Miller, University of Oregon.

Ripple marks on sandstone of the Triassic Chinle Formation, © Marli Miller, University of Oregon.

3) Cross-bedding- Series of thin, inclined layers within a horizontal bed of rock that represent the preservation of migrating dunes.  You are looking at the side of a bed rather than the top of the bed.

Cross-bedding structures in the Navajo Sandstone of Zion National Park, © Marli Miller, University of Oregon.

4) Graded bedding- A type of bedding where grain size gets smaller from bottom to top due to sorting of grains under water.  This type of bedding results from underwater landslides (called turbidity currents) initiated by earthquakes.

Graded bedding in matrix supported conglomerate from the Pliocene Copper Canyon formation, © Marli Miller, University of Oregon.

5) Mud Cracks/Dessication Cracks- Polygonal cracks formed in mud that dries upon exposure to air (common in dry lake beds and tidal flats).

Mud cracks in the bed of the Amargosa River in California's Death Valley National Park, © Michael Collier.

Ancient mudcracks (shrinkage cracks) preserved in red-brown mudstone near the base of the Watahomigi Formation, Public Domain Image, United States Geological Survey.

6) Fossils- Any evidence of past life preserved in rock.  Hard parts (shells, bones) are the most easily preserved parts of organisms.
Fossils can give detailed information about the environment of deposition.
Trilobite. © Oklahoma University

Animal footprints in Coconino Sandstone of Aubrey Cliffs, Arizona- Courtesy United States Geological Survey.