Cross cutting relationships

Where a fault cuts across a sequence of sedimentary rock, the relative ages of the fault and the sedimentary sequence can be determined. The fault is younger than the rocks it cuts. The sedimentary rocks are older than the fault which cuts them, because they had to be there first, before they could be faulted.

When observing a faulted sequence of sedimentary strata, always look to see how the beds on either side of the fault have been displaced. You might be able to locate a "key bed" which has been offset by the fault. If so, you will be able to determine the type of fault (normal fault, reverse fault, etc.).

(1) Normal fault (2) Reverse fault

Examples of faults to illustrate cross-cutting relationships.

(1) Unit A is the oldest, followed by B and C. Fault D is the youngest.

(2) Unit A is the oldest, followed by B and C. Fault D is younger than C, but older than unit E.

Crosscutting Relationships (Charles Lyell, 1797-1875). Any event that cuts across an existing rock unit is younger than that unit. This law is self-evident. Clearly, the older rock unit must be in place before something can happen to it. Common events that can cut across existing rock units are unconformities, intrusions and faults.

A) Unconformities. Unconformities are surfaces of nondeposition or surfaces that have been subject to erosion. There are three basic types.

Nonconformities are erosional surfaces between older crystalline rocks and younger sedimentary rocks. In this case there will be erosional fragments of the older crystalline rock in the younger sedimentary beds (components).

Angular unconformities are erosional surfaces that separate older tilted or folded beds the younger flat lying beds. This type is the easiest to recognize since the lower beds are tilted and the erosional surface cuts across multiple beds.

Disconformities are erosional surfaces between parallel sedimentary beds. This is the hardest to recognize. Geologists look for the erosional material of the older lower bed in the younger upper bed (components). They also look for cut and fill structures and an irregular erosional surface.

B) Igneous intrusions. Igneous intrusions are called dikes if the intrusion cuts across the bedding plane of the existing rock or sills if the intrusion is parallel to the bedding plane of the existing rock. Both types of intrusions can be recognized by a zone of contact metamorphism (aureole) surrounding the intrusion in the preexisting rock.

C) Faults. Faults are stress (tension, compression or shear) induced fractures in the Earth’s crust. The crust then moves in opposite directions along and parallel to the fault plane.

Normal faults result from tensional stresses and are recognized be the downward movement of the hangingwall block in relation to the footwall block. The hangingwall block is the block above the fault plane and the footwall block is the block below the fault plane.

Reverse faults result from compressive stresses and are recognized by the upward movement of the hangingwall in relation to the footwall block.

Thrust faults are low angle reverse faults.

Strike-slip faults are faults in which the relative movement of one block is lateral to the other in the horizontal plane.

 

feature which cuts another is the younger of the two features. For example, in the instance of an igneous dike cutting through a layer of sandstone, the dike must be younger than the sandstone.

Cross-cutting relationships are of several basic types. There are structural cross-cutting relationships wherein a fault or fracture cuts through older rock. Stratigraphic cross-cutting relationships occur where an erosional surface (or unconformity) cuts across older rock layers, geological structures, or other geological features. Sedimentologic cross-cutting relationships occur where currents have eroded or scoured older sediment in a local area to produce, for example, a channel filled with sand. Paleontologic cross-cutting relationships occur where animal activity or plant growth produce truncation. This happens, for example, where animal burrows penetrate into pre-existing sedimentary deposits. Geomorphic cross-cutting relationships occur where a surficial feature, such as a river, flows through a gap in a ridge of rock. In a similar example, an impact crater excavates into a subsurface layer of rock.

Cross-cutting relationships may be seen cartographically, megascopically, and microscopically. In other words, these relationships have various scales. A cartographic crosscutting relationship might look like, for example, a large fault dissecting the landscape on a large map. Megascopic crosscutting relationships are features like igneous dikes, as mentioned above, which would be seen on an outcrop or in a limited geographic area. Microscopic cross-cutting relationships are those that require study by magnification or other close scrutiny. For example, penetration of a fossil shell by the drilling action of a boring organism is an example of such a relationship.

Cross-cutting relationships may be compound in nature. For example, if a fault were truncated by an unconformity, and that unconformity cut by a dike, we can say, based upon compound cross-cutting relationships that the fault is older than the unconformity and that the unconformity is older than the dike. Using such rationale, the sequence of geological events can be better understood.

Cross-cutting relationships can also be used in conjunction with radiometric age dating to effect an age bracket for geological materials that cannot be directly dated by radiometric techniques. For example, if a layer of sediment containing a fossil of interest is bounded on the top and bottom by unconformities, where the lower unconformity truncates dike A and the upper unconformity truncates dike B (which penetrates the layer in question), this method can be used. A radiometric age date from crystals in dike A will give the maximum age date for the layer in question and likewise, crystals from dike B will give us the minimum age date. This provides an age bracket, or range of possible ages, for the layer in question.

The principle of cross-cutting relationships, like the principles of superposition and inclusions, is one of the most basic tools used by geologists to understand relative age relationships on Earth and on planetary and satellite surfaces in our solar system.

Arkose and sandstone

Arkose (pronounced /ˈɑrkoʊz/) is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar.[1],[2] Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose. The other mineral components

may vary, but quartz is commonly dominant, and minor mica is often present. Apart from the mineral content, rock fragments may also be a significant component. Arkose usually contains small amounts of calcite cement, which causes it to effervesce slightly in dilute hydrochloric acid; sometimes the cement also contains iron oxide. Arkose is typically grey to reddish in colour. The sand grains making up an arkose may range from fine to very coarse, but tends toward the coarser end of the scale. Fossils are rare in arkose, due to the depositional processes that form it, although bedding is frequently visible.

Arkose is generally formed from the weathering of feldspar-rich igneous or metamorphic rocks, most commonly granitic rocks, which are primarily composed of quartz and feldspar. These sediments must be deposited rapidly and/or in a cold or arid environment such that the feldspar does not undergo significant chemical weathering and decomposition; therefore arkose is designated a texturally immature sedimentary rock. Arkose is often associated with conglomerate deposits sourced from granitic terrain and is often found above unconformities over such granitic terrain.

PROPERTIESDistinctive features:   Sandstone rich in feldspars. Bedding is sometimes present, but fossils are rare. It effervesces slightly in dilute hydrochloric acid, which indicates calcite cement.

Colour:  Buff to brownish-grey or pink.

Texture and granularity:  Usually medium-grained (2 mm / 1/16 in on average), but can be fine-grained.

Composition:  Quartz sandstone containing over a quarter feldspar with calcie or iron oxide cement. Micas may also be present.

Field associations:  Derived from rapid weathering, trasnportation and deposition of granitic rocks (SEE granite rocks

Arkose

An arenaceous rock that contains a high proportion of feldspar in addition to quartz and other detrital minerals. Arkose is also known as feldspathic sandstone. Although there is no universal agreement, many geologists consider a minimum of 25% feldspar a requisite for calling sandstone an arkose. Other geologists accept a lower value. Arkoses may contain a high proportion of other nonquartz detritus, such as igneous and metamorphic rock fragments, micas, amphiboles, and pyroxenes. Frequently the accessory heavy mineral suite consists of a variety of species.

Sedimentary structures of arkoses are similar in kind to those of the orthoquartzites. Cross-bedding, the major feature, may be displayed on a huge scale, some cross-bedded units being many feet thick. Arkoses are associated with a variety of clastic rocks, dominantly conglomerates, and reddish-colored shales. Arkoses also are found with basic lava flows. Most arkoses are found in geosynclinal areas, but the thin, reworked, granite wash arkoses can be found on stable continental platforms. See also Geosyncline.

The granite-wash arkoses appear to have formed as the result of a transgression of the sea over a land area underlain by granite. The fragmented granite in the soil and mantle rock is incorporated in the basal sediment. In some areas the original granite is changed so slightly that the arkose is called recomposed granite and may be almost indistinguishable from the original granite. Since high relief and climatic extremes generally are associated with orogenic movements, arkoses are usually interpreted as sediments that result from tectonically active regions. See also Arenaceous rocks; Feldspar; Graywacke; Sandstone; Sedimentary rocks.

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-size mineral or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, gray and white. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions.

Some sandstones are resistant to weathering, yet are easy to work. This makes sandstone a common building and paving material. However, some that have been used in the past, such as the Collyhurst sandstone used in the north of England, have been found less resistant, necessitating repair and replacement in older buildings.[1] Because of the hardness of the individual grains, uniformity of grain size and friability of their structure, some types of sandstone are excellent materials from which to make grindstones, for sharpening blades and other implements. Non-friable sandstone can be used to make grindstones for grinding grain, e.g., gritstone.

Rock formations that are primarily sandstone usually allow percolation of water and are porous enough to store large quantities, making them valuable aquifers. Fine-grained aquifers, such as sandstones, are more apt to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestones or other rocks fractured by seismic activity.

Origins of Sandstone

Sandstones are clastic in origin (as opposed to organic, like chalk and coal, or chemical, like gypsum and jasper). They are formed from cemented grains that may either be fragments of a pre-existing rock or be mono-minerallic crystals. The cements binding these grains together are typically calcite, clays and silica. Grain sizes in sands are in the

range of 0.1 mm to 2 mm (clays and rocks with smaller grain sizes including siltstones and shales are typically called argillaceous sediments; rocks with larger grain sizes including breccias and conglomerates are termed rudaceous sediments).

Red sandstone interior of Lower Antelope Canyon, Arizona, worn smooth due to erosion by flash flooding over millions of years

The formation of sandstone involves two principal stages. First, a layer or layers of sand accumulates as the result of sedimentation, either from water (as in a river, lake, or sea) or from air (as in a desert). Typically, sedimentation occurs by the sand settling out from suspension, i.e., ceasing to be rolled or bounced along the bottom of a body of water (e.g., seas or rivers) or ground surface (e.g., in a desert or sand dune region). Finally, once it has accumulated, the sand becomes sandstone when it is compacted by pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains. The most common cementing materials are silica and calcium carbonate, which are often derived either from dissolution or from alteration of the sand after it was buried. Colors will usually be tan or yellow (from a blend of the clear quartz with the dark amber feldspar content of the sand). A predominant additional colorant in the southwestern United States is iron oxide, which imparts reddish tints ranging from pink to dark red (terra cotta), with additional manganese imparting a purplish hue. Red sandstones are also seen in the Southwest and West of England and Wales, as well as central Europe and Mongolia. The regularity of the latter favors use as a source for masonry, either as a primary building material or as a facing stone, over other construction.

The environment where it is deposited is crucial in determining the characteristics of the resulting sandstone, which, in finer detail, include its grain size, sorting and composition and, in more general detail, include the rock geometry and sedimentary structures. Principal environments of deposition may be split between terrestrial and marine, as illustrated by the following broad groupings:

Terrestrial environments

1. Rivers (levees, point bars, channel sands) 2. Alluvial fans 3. Glacial outwash 4. Lakes 5. Deserts (sand dunes and ergs)

Marine environments

1. Deltas 2. Beach and shoreface sands 3. Tidal flats

4. Offshore bars and sand waves 5. Storm deposits (tempestites) 6. Turbidites (submarine channels and fans)

Types of sandstone

Once the geological characteristics of a sandstone have been established, it can then be assigned to one of three broad groups: arkose or arkosic sandstones, which have a high (>25%) feldspar content and a

composition similar to granite. quartzose sandstones, also known as "beach sand", which have a high (>90%)

quartz content. Sometimes these sandstones are termed "orthoquartzites", e.g., the Tuscarora Quartzite of the Ridge-and-valley Appalachians.

argillaceous sandstones, such as greywacke or bluestone, which have a significant clay or silt content.

Aeolian sandstone is a term used for a rock which is composed of sand grains that show signs of significant transportation by wind. These have usually been deposited in desert environments.

According to the USGS, U.S. sandstone production in 2005 was 192,000 metric tons worth $24.3 million, the largest component of which was the 121,000 metric tons worth $9.75 million of flagstone or dimension stone.

Conglomerates

PROPERTIESDistinctive features:   Boulders, pebbles, or shingle, set in fine-grained matrix, sometimes resembling coarse concrete. 

Colour:  Variable, depending on the type of rock fragments. 

Texture and granularity:  Variable. 

Composition:  Rounded rock fragments set in a fine-grained matrix. 

Field associations:  Derived from beach, lake and river deposits of boulders, pebbles and gravel. Often found near deposits of sandstone and arkose. 

 

OCCURRENCEWorldwide.

 

USES

Aggregate, ornamental when highly compacted forms are cut and polished.

Conglomerate rock is a common sedimentary rock.  It forms in many different environments and settings where the energy of transport is high enough to move large grains.  The sediment from which it forms is much courser than other clastic sedimentary rocks except for breccias.  The only difference between conglomerates and breccias is the roundness of the grains.  In conglomerates, the grains are rounded and usually indicate that they have been transported or worked more than the angular grains found in breccias. Distinguishing between breccias and conglomerates is usually very easy as the grains are mostly large enough to see with the unaided eye.  If the rock has a smaller grain size (< 2.0mm) which is almost too small to see, then the rock is a sandstone.

Like sandstone and breccias, conglomerates are cemented by various minerals.  Normal cementing agents include calcite, quartz (silica), clays and gypsum.  When the sediment is first deposited there are lots of open spaces or pores. Cement can affect the amount of pore space that is left in a rock as it solidifies.  Conglomerates usually have significant pore space and they are generally a good rock to act as a reservoir for ground water, natural gas and petroleum.

Conglomerates form in environments that are generally not to far from the source of the sediments and high in energy.  The grains of a breccia are found closer to the source of the sediments since they have not been rounded like the grains of a conglomerate.  If the deposit is farther from the source, then the sediment is more likely to be a sandstone with all the large grains left behind.  Prehistoric glacial deposits are a great source of conglomerates as are alluvial fans.  Anywhere that pebbles are found is a possible source of a conglomerate.  Generally conglomerates are made up of fragments of other rocks, but at times large quartz or feldspar crystals can also make up a significant percentage of the conglomerates components.  These crystals are of course lacking in crystal faces and are just rounded grains.

Conglomerates with their interesting pebbled and fine matrix textures are often used as ornamental rocks for buildings, monuments, grave stones, tiles and many other ornamental uses. However their irregular grain sizes contribute to less durability than that of sandstone and therefore fewer uses in building construction.

A conglomerate (pronounced /kɒnˈglɒmərət/) is a rock consisting of individual stones that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts.[1] Both conglomerates and breccias are characterized by clasts larger than sand (>2 mm).

Paraconglomerates consist of a matrix-supported rock that contains at least 15% sand-sized or smaller grains (<2 mm), the rest being larger grains of varying sizes.[2]

Orthoconglomerates are defined by texture. They are a grain-supported rock that consists primarily of gravel-sized grains (~256 mm), with less than 15% matrix of sand and finer particles.[3]

In rock types such as paraconglomerates and orthoconglomerates, were the matrix to be removed, the rock would collapse. This is because the larger grains are supported by the matrix and, without it, there is nothing to hold the grains together. Therefore, the higher the percentage of matrix, the more unstable the boulder. Breccias are a particular class of conglomerate that differ only in that the clasts are angular.

A spectacular example of conglomerate can be seen at Montserrat, near Barcelona. Here erosion has created vertical channels giving the characteristic jagged shapes for which the mountain is named. (Montserrat literally means "jagged mountain.") The rock is strong enough to be used as a building material - see Montserrat abbey front at full resolution for detail of the rock structure.

Another spectacular example of conglomerate, the Crestone Conglomerate may be viewed in and near the town of Crestone, at the foot of the Sangre de Cristo Range in Colorado's San Luis Valley. The Crestone Conglomerate is a metamorphic rock stratum and consists of tiny to quite large rocks that appear to have been tumbled in an ancient river. Some of the rocks have hues of red and green.

Conglomerate may also be seen in the domed hills of Kata Tjuta, in Australia's Northern Territory.

Breccia

PROPERTIESDistinctive features:   Similar to conglomerate, but rock fragments are angular and set in fine-grained matrix. Distinguish from agglomerate (volcanic equivalent) by its sedimentary origin. 

Colour:  Variable, depending on the type of rock fragments. 

Texture and granularity:  Angular fragments of rock set in fine grained matrix. 

Composition:  Fragmented rocks of any kind can form breccia. The matrix is normally fine sand or silt, cemented by secondary silica or calcite.

Field associations: 

Derived from scress and fault zones. Often found near conglomerate, arkose and sandstone.

Varieties:Named according to rock type of which it is composed.

 

OCCURENCEWorldwide.

 

USESAggregate, ornamental when highly compacted.

 

Breccia ("BRET-cha") is a rock made of smaller rocks that are cemented together. It is like conglomerate that way. The difference between the two rock types is that breccia is made of sharp, broken clasts while conglomerate is made of smooth, eroded clasts.

This is a brecciated mudstone from a northern California beach. It started as a simple rock, but some time after it consolidated, something—probably motion along a fault—shattered it and cemented it together again. The matrix between the clasts appears to be the same substance as the clasts, although there are also a few small veins of silica from the process of brecciation.

There are many different ways to make breccia, and usually geologists add a word to signify the kind of breccia they're talking about. A sedimentary breccia arises from things like talus or landslide debris. A volcanic or igneous breccia forms during eruptive activities. A collapse breccia forms when rocks are partly dissolved, such as limestone or marble. The stone shown here is a fault breccia. And a new member of the family, first described from the Moon, is impact breccia. Breccias are a relatively common clastic sedimentary rock. They form in many different violent situations where host rocks are broken and not transported far from their source. These situations include any scenario in which rocks can be broken and re-accumulate to form the angular sediment.  Landslides, fault zones, cryptolithic explosion events and impact craters can produce breccias. Landslides or debris flows can occur on continental shelves, on the sides of mountains or in karst environments such as sink holes or collapsed caves.  In fault zones, where rocks or even continents slide past each other, breccia zones can be created that can vary from inches across to tens of meters across.  Cryptolithic explosions are subterranean explosions that can send rocks flying into the air and the debris that falls back to Earth forms brecciated deposits.  Meteorite impact craters can form breccias as the meteor impacts the Earth and the debris is strewn across the country side or back into the crater.  However breccias are formed, it usually is an exciting event!

The sediment from which it forms is composed of angular pebble to cobble sized fragments often dispersed in a finer matrix. The only difference between breccias and conglomerates is the roundness of the grains. In conglomerates, the grains are rounded and usually indicate that they have been transported or worked more than the angular grains found in breccias. Distinguishing between breccias and conglomerates is usually very easy as the grains are mostly large enough to see with the unaided eye. If the rock has a smaller grain size (< 2.0mm) which is almost too small to see, then the rock is sandstone.

Like sandstone and conglomerates, breccias are cemented by various minerals. Normal cementing agents include calcite, quartz (silica), clays and gypsum. When the sediment is first deposited there are lots of open spaces or pores. Cement can affect the amount of pore space that is left in a rock as it solidifies. Breccias usually have significant pore space and they are generally a good rock to act as a reservoir for ground water, natural gas and petroleum.

Breccias have very unique angular textures and are prized as ornamental rocks for buildings, monuments, grave stones, tiles and many other ornamental uses.  They have been used by people for centuries for many ornamental uses and some breccias are even considered to be semi-precious and have found uses in jewelry.

Breccia is a term that has been applied to non-sedimentary rocks of igneous origin too.  At times there are situations in the formation of igneous rocks that produce angular fragments that solidify with a breccia like texture.  These rocks are sometimes referred to as breccia, but are not sedimentary and it is probably better to use the term as an adjective such as a brecciated gabbro for example instead of calling the rock a breccia. 

Breccia (pronounced /ˈbrɛtʃiə, ˈbrɛʃiə/, Italian: breach) is a rock composed of angular fragments of several minerals or rocks in a matrix, that is a cementing material, that may be similar or different in composition to the fragments. A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia, tectonic breccia, igneous breccia, impact breccia and hydrothermal breccia.

Nomenclature

Breccias can be classified by their constituents, mode of occurrence, constituent fragment size, the types of clasts and source of clasts. Several textural terms are used to describe the morphology and textural variations observed in breccias.

MillingBreccias which are formed by injection of a slurry (be it as a hydrofracture breccia or, more usually, a volcanic or intrusive breccia) often show evidence of rounding of the clasts. With a sedimentary rock this may be called a conglomerate, except when the

breccia is discordant with former lithology (clastic dike). For an intrusive breccia, erosion and transport in a watercourse cannot be invoked to explain rounding. Breccias of this type which are rounded are said to be milled, a process by which the breccia matrix grinds the larger clasts and rounds them off. This has been observed to have occurred in some hydrothermal breccias.

AutobrecciationAutobrecciation is the process by which a rock's mechanism of formation causes it to become broken and to include its broken fragments within itself. This is properly explained in the section on lava (Volcanic breccias).

Sedimentary

Sedimentary breccias are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They are formed by either submarine debris flows, avalanches, mud flow or mass flow in an aqueous medium. Technically, turbidites are a form of debris flow deposit and are a fine-grained peripheral deposit to a sedimentary breccia flow.

The other derivation of sedimentary breccia is as angular, poorly sorted, very immature fragments of rocks in a finer grained groundmass which are produced by mass wasting. These are, in essence, lithified colluvium. Thick sequences of sedimentary (colluvial) breccias are generally formed next to fault scarps in grabens.

In the field, it may at times be difficult to distinguish between a debris flow sedimentary breccia and a colluvial breccia, especially if one is working entirely from drilling information. Sedimentary breccias are an integral host rock for many SEDEX ore deposits.

Sedimentary breccias can be described as 'arenaceous', from the Latin word harena meaning 'sand', which are sandy or pebbly in nature.

A conglomerate by contrast is a sedimentary rock composed of rounded fragments or clasts of pre-existing rocks. Both breccias and conglomerates are composed of fragments averaging greater than 2 millimeters in size. The angular shape of the fragments indicate that the material has not been transported far from its source. Breccias indicate accumulation in a juvenile stream channel or accumulations because of gravity erosion. Talus slopes might become buried and the talus cemented in a similar manner.

Collapse

Collapse breccias form where there has been a collapse of rock, typically in a karst landscape. Collapse breccias form blankets in highly weathered regolith due to the removal of rock components by dissolution.

Tectonic

Tectonic breccias form where two tectonic plates create a crumbling of the interface, by their relative movements.

Fault

Fault breccias result from the grinding action of two fault blocks as they slide past each other. Subsequent cementation of these broken fragments may occur by means of mineral matter introduced by groundwater.

Igneous

Igneous clastic rocks can be divided into two classes

Broken, fragmental rocks associated with volcanic eruptions, both of lava and pyroclastic type

Broken, fragmental rocks produced by intrusive processes, usually associated with plutons or porphyry stocks

Volcanic

Volcanic pyroclastic rocks are formed by explosive eruption of lava and any rocks which are entrained within the eruptive column. This may include rocks plucked off the wall of the magma conduit, or physically picked up by the ensuing pyroclastic surge. Lavas, especially rhyolite and dacite flows, tend to form clastic volcanic rocks by a process known as autobrecciation. This occurs when the thick, nearly solid lava breaks up into blocks and these blocks are then reincorporated into the lava flow again and mixed in with the remaining liquid magma. The resulting breccia is uniform in rock type and chemical composition.

Lavas may also pick up rock fragments, especially if flowing over unconsolidated rubble on the flanks of a volcano, and these form volcanic breccias, also called pillow breccias.

The volcanic breccia environment is transitional into the plutonic breccia environment in the volcanic conduits of explosive volcanoes, where lava tends to solidify and may be repeatedly shattered by ensuing eruptions. This is typical of volcanic caldera settings.

Intrusive

Clastic rocks are also commonly found in shallow subvolcanic intrusions such as porphyry stocks, granites and kimberlite pipes, where they are transitional with volcanic breccias.[1]

Intrusive rocks can become brecciated in appearance by multiple stages of intrusion, especially if fresh magma is intruded into partly consolidated or solidified magma. This may be seen in many granite intrusions where later aplite veins form a late-stage stockwork through earlier phases of the granite mass. When particularly intense, the rock may appear as a chaotic breccia.

Clastic rocks in mafic and ultramafic intrusions are known and form via several processes;

consumption and melt-mingling with wall rocks, where the felsic wall rocks are softened and gradually invaded by the hotter ultramafic intrusion (termed taxitic texture by Russian geologists)

Accumulation of rocks which fall through the magma chamber from the roof, forming chaotic remnants

Autobrecciation of partly consolidated cumulate by fresh magma injections or by violent disturbances within the magma chamber (e.g. postulated earthquakes)

Accumulation of xenoliths within a feeder conduit or vent conduit

Impact

Alamo bolide impact breccia (Late Devonian, Frasnian) near Hancock Summit, Pahranagat Range, Nevada.

Impact breccias are thought to be diagnostic of an impact event such as an asteroid or comet striking the Earth, and are usually found at impact craters. Impact breccia, a type of impactite, forms during the process of impact cratering when large meteorites or comets impact with the Earth or other rocky planets or asteroids. Breccia of this type may be present on or beneath the floor of the crater, in the rim, or in the ejecta expelled beyond the crater. Impact breccia may be identified by its occurrence in or around a known impact crater, and/or an association with other products of impact cratering such as shatter cones, impact glass, shocked minerals, and chemical and isotopic evidence of contamination with extraterrestrial material (e.g. iridium and osmium anomalies).

Hydrothermal

Hydrothermal breccias usually form at shallow crustal levels (<1 km) between 150 to 350oC, when seismic activity (an earthquake) causes a void to open along a fault deep underground. The void draws in hot water and as pressure in the cavity drops, the water violently boils - akin to an underground geyser. In addition, the sudden opening of a cavity causes rock at sides of the fault to destabilise and implode inwards, the broken rock gets caught up in a churning mixture of rock, steam and boiling water. Rock

fragments hit each other and sides of the fault, and attrition quickly rounds angular breccia fragments. Volatile gases are lost to the steam phase as boiling continues, in particular CO2. As a result, the chemistry of the fluids change and ore minerals rapidly precipitate.

Breccia-hosted ore deposits are ubiquitous.[2]

Main article: Ore_genesis#Hydrothermal_processes

The morphology of breccias associated with ore deposits varies from tabular sheeted veins and clastic dikes associated with overpressured sedimentary strata, to large-scale intrusive diatreme breccias, or even some synsedimentary diatremes formed solely by the overpressure of pore fluid within sedimentary basins. Hydrothermal breccias are usually formed by hydrofracturing of rocks by highly pressured hydrothermal fluids. They are typical of the epithermal ore environment and are intimately associated with intrusive-related ore deposits such as skarns, greisens and porphyry-related mineralisation. Epithermal deposits are mined for copper, silver and gold.

In the mesothermal regime, at much greater depths, over-pressured fluids under lithostatic pressure can be released during seismic activity associated with mountain building. The pressurised fluids ascend towards shallower crustal levels that are under lower hydrostatic pressure. On their journey, high-pressure fluids crack rock by hydrofracturing, forming an angular jigsaw breccia. Rounding of rock fragments less common in the mesothermal regime, as the formational event is brief. If boiling occurs, methane and hydrogen sulfide may be lost to the steam phase and ore may precipitate. Mesothermal deposits are often mined for gold.

Ornamental uses

The striking visual appearance of breccias has for millennia made them a popular sculptural and architectural material. Breccia was employed for column bases in the Minoan palace of Knossos on Crete in about 1800 BC.[3] Breccia was used on a limited scale by the ancient Egyptians - one of the best-known examples is the statue of the goddess Tawaret in the British Museum). It was regarded by the Romans as an especially precious stone and was often used in high-profile public buildings. Many types of marble are brecciated, such as Breccia Oniciata or Breche Nouvelle.

It is most often used as an ornamental or facing material in walls and columns. A particularly striking example can be seen in the Pantheon in Rome, which features two gigantic columns of pavonazzetto, a breccia coming from Phrygia (in modern Turkey). Pavonazzetto obtains its name from its extremely colourful appearance, which is reminiscent of a peacock's feathers (pavone is "peacock" in Italian).

Absolute dating and relative dating

How do we determine the age of a rock?

1. Relative dating - Steno's Laws, etc."A is older than B"

2. Absolute datingQuantify the date in years. Radiometric Dating

Principles of Radiometric Dating

Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.

Radioactive parent elements decay to stable daughter elements.

Radioactivity was discovered in 1896 by Henri Becquerel. In 1905, Rutherford and Boltwood used the principle of radioactive decay to measure the age of rocks and minerals (using Uranium decaying to produce Helium. In 1907, Boltwood dated a sample of urnanite based on uranium/lead ratios. Amazingly, this was all done before isotopes were known, and before the decay rates were known accurately.

The invention of the MASS SPECTROMETER after World War I (post-1918) led to the discovery of more than 200 isotopes.

Many radioactive elemtns can be used as geologic clocks. Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.

In the above table, note that the number is the mass number (the total number of protons plus neutrons). Note that the mass number may vary for an element, because of a differing number of neutrons. Elements with various numbers of neutrons are called isotopes of that element.

Each radioactive isotope has its own unique half-life.A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.

Radioactive decay occurrs at a constant exponential or geometric rate. The rate of decay is proportional to the number of parent atoms present.

The proportion of parent to daughter tells us the number of half-lives, which we can use to find the age in years.For example, if there are equal amounts of parent and daughter, then one half-life has passed.If there is three times as much daughter as parent, then two half-lives have passed. (see graph, above)

Radioactive decay occurs by releasing particles and energy.

Uranium decays producing subatomic particles, energy, and lead.

As uranium-238 decays to lead, there are 13 intermediate radioactive daughter products formed (including radon, polonium, and other isotopes of uranium), and 8 alpha particles and 6 beta particles released. There are three types of subatomic particles involved:

1. Alpha particleslarge, easily stopped by papercharge = +2mass = 4

2. Beta particlespenetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays.charge = -1mass = negligible

3. neutronshighly penetratingno chargemass = 1

Gamma rays (high energy X-rays) are also produced. Highly penetrating electromagnetic radiation. Photons (light).No charge or mass.Can penetrate concrete. Lead shield can be used.

Minerals you can date

Most minerals which contain radioactive isotopes are in igneous rocks. The dates they give indicate the time the magma cooled.

Potassium 40 is found in: o potassium feldspar (orthoclase) o muscovite o amphibole o glauconite (greensand; found in some sedimentary rocks; rare)

Uranium may be found in: o zircon o urananite o monazite o apatite o sphene

Note that some elements have both radioactive and non-radioactive isotopes. Examples: carbon, potassium.

As seen in the tables above, there are three isotopes of uranium. Of these, U-238 is by far the most abundant (99.2739%).

Radioactive elements tend to become concentrated in the residual melt that forms during the crystallization of igneous rocks. More common in SIALIC rocks (granite, granite pegmatite) and continental crust.

Radioactive isotopes don't tell much about the age of sedimentary rocks (or fossils). The radioactive minerals in sedimentary rocks are derived from the weathering of igneous rocks. If the sedimentary rock were dated, the age date would be the time of cooling of the magma that formed the igneous rock. The date would not tell anything about when the sedimentary rock formed.

To date a sedimentary rock, it is necessary to isolate a few unusual minerals (if present) which formed on the seafloor as the rock was cemented. Glauconite is a good example. Glauconite contains potassium, so it can be dated using the potassium-argon technique.

How does Carbon-14 dating work?

1. Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide. 2. Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle. 3. All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion). 4. At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14. 5. The change in the Carbon 14 to Carbon 12 ratio is the basis for dating. 6. The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages). 7. Assumes that the rate of Carbon 14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years).

Fission Track Dating

Charged particles from radioactive decay pass through mineral's crystal lattice and leave trails of damage called FISSION TRACKS. These trails are due to the spontaneous fission of uranium.

Procedure to study:

o Enlarge tracks by etching in acid (so that they may be visible with light microscope)

o See readily with electron microscope o Count the etched tracks (or note track density in an area)

Useful in dating:

o Micas (up to 50,000 tracks per cm squared) o Tektites o Natural and synthetic (manmade) glass

Reheating "anneals" or heals the tracks.

The number of tracks per unit area is a function of age and uranium concentration.

What is absolute dating?

Absolute dating is used by geologists to determine the actual age of a material. It can be achieved through the use of historical records and through the analysis of biological and geological patterns. Although development of radiometric methods led to the first breakthroughs in establishing an absolute time scale, other absolute methods have limited applications. Chief among these are dendochronology, varve analysis, hydration dating, and TL dating.

Absolute dating

Absolute dating is the process of determining a specific date for an archaeological or palaeontological site or artifact. Some archaeologists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies a certainty and precision that is rarely possible in archaeology. Absolute dating is usually based on the physical or chemical properties of the materials of artifacts, buildings, or other items that have been modified by humans. Absolute dates do not necessarily tell us when a particular cultural event happened, but when taken as part of the overall archaeological record they are invaluable in constructing a more specific sequence of events.

Absolute dating contrasts with the relative dating techniques employed, such as stratigraphy. Absolute dating provides a numerical age for the material tested, while relative dating can only provide a sequence of age.

Radiocarbon dating

Main article: Radiocarbon dating

One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains. This is a radiometric technique since it measures radioactive decay. Carbon-14 is an unstable isotope of normal carbon, carbon-12. Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they absorb carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the absorption of carbon-14 stops. This unstable isotope starts to break down into nitrogen-14. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the proportion of carbon-14 in organic material, scientists can determine an organic artifact's date of death.

Disadvantages

Because the half-life of carbon-14 is short, the older a specimen is, the greater the margin of error becomes. Carbon dating is only reliable within the past 40,000 years. Radiocarbon is also less useful for historic sites or recent sites. The standard margin of error is plus or minus 50 years. Because of this, the technique usually cannot pinpoint the date of a site better than historic records and previous knowledge of the site.

A further issue is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as dead trees to remain in their natural state for hundreds of years before people use them as firewood, after which they become part of the archaeological record. Dating when that particular tree died does not necessarily indicate when the fire burned. This is also true of the heartwood of a tree, which will appear younger than the outer rings of the same tree because it has had less time to incorporate carbon-14 into its makeup. For this reason, many archaeologists prefer to use samples from short-lived plants (such as weeds or crops) for radiocarbon dates. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be derived from a very small sample, has been very useful in this regard.

Potassium-argon dating

Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium-argon dating (K-Ar dating). Potassium-40 is a radioactive isotope of potassium that breaks down into argon-40, a gas. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14. With this method, the older the specimen, the more reliable the dating. Furthermore, whereas carbon-14 dating can be done only on organic remains, K-Ar dating can be used only for inorganic substances: rocks and minerals. As potassium-40 in rocks gradually breaks down into argon-40, the gas is trapped in the rock until the rock is heated intensely (as with volcanic activity), at which point it may escape. When the rock cools, the breakdown of potassium into argon resumes. Dating is done by reheating the rock and measuring the escaping gas. The date received from this test is for the last time that the object was heated. Common dates tested are the firing of ceramics (archaeology), and the setting of rocks (geology).

Thermoluminescence

Thermoluminesence testing also dates items to the last time they were heated. This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item. Heating an item to 350 degrees Celsius or higher releases the trapped electrons, producing light. This light can be measured to determine the last time the item was heated.

Disadvantages

Radiation levels do not remain constant over time. Fluctuating levels can skew results - for example, if an item went through several high radiation eras, thermoluminesence will return an older date for the item. Many factors can spoil the sample before testing as well,

exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger. Because of these and other factors, Thermoluminesence is at the most about 15% accurate. It cannot be used to accurately date a site on its own. However, it can be used to authenticate an item as antiquity.

Relative dating

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Before the advent of absolute dating in the 20th century, archaeologists and geologists were largely limited to the use of relative dating techniques. It estimates the order of prehistoric and geological events were determined by using basic stratigraphic rules, and by observing where fossil organisms lay in the geological record, stratified bands of rocks present throughout the world.

Though relative dating can determine the order in which a series of events occurred, not when they occurred, it is in no way inferior to radiometric dating; in fact, relative dating by biostratigraphy is the preferred method in paleontology, and is in some respects more accurate (Stanley, 167-9).

Biostratigraphy

Biostratigraphic methods are usually used in tandem with structural ones. For instance, the principle of faunal succession was probably the most important factor behind the elaboration of the geologic time scale, which was more or less complete long before an absolute time scale was available. Beds with a particular fauna can be correlated with others that share it (often globally), and also distinguished from upper and lower beds without them.

Rock units that contain a distinct assemblage of fossils are biostratigraphic units, and are based on the "range", or vertical interval in which a taxon is found. A zone, or biozone is the most basic biostratigraphic unit, one bound on its upper and lower boundaries by the ranges of given species; these can be zones where certain species coexist, or which are defined by the earliest appearance or latest disappearance of taxa in neighboring zones.

Index fossils (also guide fossils) are invaluable for biostratigraphy. The best index fossils are:

Abundant. Distinct from other flora/fauna. Geographically widespread. Found in many kinds of rocks. Narrow in stratigraphic range.

Unfortunately, few taxa fit all these criteria (Stanley, 157-8).

Unconformity is a place in a rock column where rock is missing.

Relative dating in archaeology

Relative dating methods in archaeology are similar to some of those applied in geology. Analysing the stratification of a site was developed from principles first discovered in geology. But note that in general the stratigraphic relationships in archaeology form a partially ordered set so that the true chronological sequence cannot be reconstructed by stratigraphic reasoning alone. The principles of typology can be compared to the biostratigraphic approach described above.

Planetological use

Relative dating is used to determine the order of events on objects other than Earth; for decades, planetary scientists have used it to decipher the evolution of bodies in the Solar System, particularly in the vast majority of cases in which we have no surface samples. Many of the same principles are used; for instance, if a valley on Mars cuts across a crater, the valley must be younger than the crater.

Craters themselves are highly useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known.(Hartmann, 258)