Chapter 6- Sedimentary Rocks

What is a bedding plane

A bedding plane is a nearly flat surface of deposition separating two layers of rock.

What are the two main types of carbonates?

Carbonate rocks contain the CO3 ion as part of their chemical composition. The two main types of carbonates are limestone and dolomite.

What is more important in the lithification process-- compaction of cementation?

Cementation is the primary method lithification. Compaction by itself does not generally lithify sediment into sedimentary rocks. It does help consolidate clayey sediments by pressing the microscopic clay minerals so closely together that attraction forces at the atomic level can bind them together.

Why does coal change from brown to black?

Coal changes from brown to black as the amount of CARBON IN IT INCREASES. The more carbon in the coal, the more black the coal.

What is Dolomite

Dolomite is used both as a rock name and as a mineral name. To avoid confusion, many geologists refer to a rock that i made mostly of the mineral dolomite as dolostone.

Where have deposits of evaporites been found in the past and present?

In the past, deposits of rock salt and rock gypsum (the two most common evaportes) formed where shallow, continental seas existed in hot, arid climates. modern evaporite deposits are fond in the PERSIAN GULF AND IN THE RED SEA.

What does limestone consist of? What is limestone made of?

Limestone consists of calcite, formed either as a chemical precipitate in a reef, or, more commonly, by the cementation of shell and coral fragments or of ooids.

What is limestone composed mostly of?

Limestone is composed mostly of calcite (CaCo3).

What are limestones particularly susceptible to?

Limestones are particularly susceptible to recrystallization. Recrystallization is the process by which new crystals, often of the same mineral composition as the original grains, develop in a rock. Calcite grains recrystallize easily, particularly in the presence of water and under the weight of overlying sediment. The new crystals that form are typically large and can be easily seen in a rock as slight reflections off their broad, flat cleavage faces.

How do most conglomerate (detrital) sedimentary rocks form?

Most conglomerate (detrital) sedimentary rocks form from coarse, rounded sediment grains that often have been transported only a short distance by a river or waves.

How do most detrital sedimentary rocks form?

Most detrital sedimentary rocks form by compaction and cementation of grains. Matrix can partially fill the pore space of clastic rocks.

Why does most dolomite that we see in the rock record form?

Most dolomite that we see in the rock record probably forms as a replacement for existing carbonate material.

How are most fossils preserved

Most fossils are preserved by the rapid burial in sediment of bones, shells, or teeth which are the mineralized hard parts of animal most resistant to decay.

Where do mud cracks form?

Mud cracks form in drying mud.

Where do oil and natural gas originate from?

Oil and gas originate from organic matter in marine sediment. Microscopic organisms such as diatoms and other single-celled algae settle to the sea floor and accumulate in marine mud.

What is peat?

Peat is a brown, light weight, unconsolidated or semi consolidated deposit of plant remains that accumulate in wet bogs. Peat is transformed into coal largely by compaction after it has been buried by sediments.

What plant fossils are included in coal?

Plant fossils in coal beds include leaves, stems, tree trunks, and stumps with roots often extending into the underlying shales, so apparently most coal formed right at the place where the plants grew.

What does recrystallization do?

Recrystallization often destroys the original clastic texture and fossils of a rock, and replaces them with a new crystalline texture. Therefore, a rock that has been recrystallized is a rock who's geological history is very difficult to determine?

What does recrystallization do?

Recrystallization often destroys the original texture of chert (and some limestones).

What are the reactions of proto dolomite

The reactions for porto-dolomite are as follows: 1) evaporation in hot dry tidal flat environments produces salty brines there percolate through the pore spaces of lime mud 2) gypsum and other evaporate minerals begin to precipitate from the dense, salty brine. This removes calcium ions from the brine. 3) The brine becomes relatively enriched in Mg++ ions as more gypsum precipitates. When the ration of Mg and Ca becomes greater than 10/1 Dolomite becomes chemically stable and Mg++ starts to replace Ca++ on an ion by ion basis. 4) early, poorly crystalline proto-Dolomite crystals form by replacement as Mg++ ions substitute regularly for Ca++ ions in the crystal structure.

Where do the silt and clay deposits that lithify as shale accumulate?

The silt and clay deposits that lithify as shale accumulate on lake bottoms, at the end of rivers in deltas, on river floor plains, and on quiet parts to the deep-ocean floor.

What are the two major types of limestone?

The two major types of limestone are biochemical or inorganic limestone.

Where do microscopic organisms such as diatoms and other single-celled algae settle on the sea floor and accumulate in marine mud?

This mostly likely occurs on restricted basins with poor water circulations, particularly on CONTINENTAL SHELVES. The organic matter may partially decompose, using up the dissolved oxygen in the sediment. As soon as the oxygen is gone, decay stops and the remaining organic matter is preserved.

Name three chemical sedimentary rocks.

Three chemical sedimentary rocks include carbonates, chert, and evaporites.

What are tufa and travertine?

Tufa and travertine are inorganic limestones that form from FRESH WATER (as opposed to sea water). Tufa is precipitated from solution in the water of a continental spring or lake, or form percolating groundwater. Traversing may form in caves when carbonate rich water loses CO2 to the cave atmosphere. Tufa and travertine both have a crystalline texture; however, to is generally more porous, cellular, or open than travertine, which tends to be more dense.

What causes a turbid current

Turbidity currents can be caused by earthquakes, collapsing slopes, and other geological disturbances. Once set in motion, the turbid water rushes downward and can change the physical shape of the sea floor. These currents also deposit huge amounts of sediment wherever they flow, usually in a gradient or fan pattern, with the largest particles at the bottom and the smallest ones on top.

How are varieties of coal recognized?

Varieties of coal are recognized on the bassi of the type of original plant material and the degree of compaction.

What are ripple marks and how are they formed

a system of subparallel wavy ridges and furrows left on sand or mud by the action of water or wind, and sometimes fossilized.

are oil and gas less or more dense than water?

oil and gas are less dense than water so they generally tend to rise upward through water-saturated rock and sediment.

What is CHALK

Chalk is a light-colored, proud, very fine grained variety of bioplastic limestone that forms from the seafloor accumulation of microscopic marine organisms that drift near the sea surface.

What are chemical sedimentary rocks precipitated from?

Chemical sedimentary rocks are precipitated from a low temperature aqueous environment. They are precipitated either directly by inorganic processes or by the actions of organisms.

What is chert and what does it consist of?

Chert consists of silica and usually forms from the accumulation of microscopic marine organisms. Recrystallization can often destroy the original texture of chert.

What is chert?

Chert is a hard, compact, fine grained sedimentary rocks formed almost entirely of silica. It occurs in two principal forms- as irregular, limp nodules within other rocks and as layered deposits like other sedimentary rocks.

What is coal?

Coal is a consolidated plant material and a major fuel.

What is mudstone and what does it contain?

Mudstone contains both silt and clay, having the same grain size and smooth feel of shale but lacking shale's laminations and fissility. Mudstone is massive and blocky, while shale is visibly layers and fissile.

What represents the end point in petroleum maturation?

Natural gas represents the end point in petroleum maturation.

Where is sedimentary breccia found?

Sedimentary breccia is commonly a talus slope deposit that forms at the base of a steep rock cliff that is being mechanically weathered. Landslide deposits also might lithify into sedimentary breccia.

Where are sedimentary rocks usually found?

Sedimentary rocks are usually found in beds separated by bedding planes because the original sediments are deposited in horizontal layers.

How do sedimentary rocks form? By which processes do they form?

Sedimentary rocks form by lithification of sediment, by crystallization from solution, or by consolidation of remains of organisms. These are called detrital, chemical, or organic.

When do sedimentary structures form?

Sedimentary structures from after deposition but before lithification.

Why are sedimentary structures useful to geologists

sedimentary structures are useful to geologists because they can revealed to geologists the orientation of rock deposits which help them unravel the geometry of rocks that have been folded and faulted in tectonically active regions.

What are fossils

Fossils are the remains of organisms preserved in sedimentary rock.

What are fossils?

Fossils are the traces of an organisms hard parts or tracks preserved in rock.

How do geologists try to determine a source area of a sedimentary rock?

Geologists try to determine the source area of a sedimentary rock by studying its grain size, composition, and sedimentary structures. The source area's rock type and location are important to determine.

Where do graded beds form and why?

Graded beds form as coarse particles fall from suspension before fine particles due to decreasing water flow velocity in a turbidity current.

How are sediments formed generally?

In general, sediment forms by the weathering and erosion of preexisting rocks and sometimes they form by chemical precipitation, and sometimes by organisms.

Where does an arkose form?

Mounts of granite in a desert could be a source of arkose because feldspar grains (25%) are preserved in the rock. If the feldspar had undergone chemical weathering, it would have been destroyed. It was probably from a desert because rapid erosion would allow feldspar to be mechanically weathered before it could be chemically weather, and thus destroyed (the arid climate in a desert slows chemically weathering)

What are mud cracks

Mud cracks are a polygonal pattern of cracks formed in very fine-grained sediment as it dries. Mud cracks form only in sediment EXPOSED ABOVE WATER. They may form in lake bottom sediment as the lake dries up, in flood-deposited sediment as a river level drops, or in marine sediment exposed to the air.

What is a "dirty sandstone"

A "dirty sandstone" is a matrix-rich sandstone that is poorly sorted and often dark in color. A matrix is fine-grained silt and clay (mud) in the space between larger sand grains.

What are some differences and similarities between conglomerates and breccia?

A breccia has angular fragments and probably didn't travel far from its source. It is often the deposit of a talus slope that formed at the base of a steep rock cliff and was mechanically weathered. Or, was lithified by a landslide deposit. A breccia is not a common type of rock. A conglomerate is similar to a breccia but its particles are rounded and not angular. Although they too may have not traveled very far, some transport was necessary to rock the particles. They could have been angular fragments that fell from a cliff and then were carried a few kilometers by a river or pounded by waves crashing in the surf.

What is a claystone and what is it composed of?

A claystome is a rock composed predominately of clay-zed particles but lacks the fissility of shale.

What is an Oolitic limestone

An Oolitic limestone is a distinctive variety of inorganic limestone formed by the cementation of sand sized odious, small spheres of calcite inorganically precipitated in warm, shallow, seawater. Strong tidal currently roll the oolites back and forth, allowing them to maintain a nearly spherical shape as they grow. Wave action may also contribute to their shape. Oolitic limestones have a clastic texture.

What is a sedimentary conglomerate?

A conglomerate is a coarse-grained sedimentary rock formed by the cementation of ROUNDED gravel. It can be distinguished from breccia by the definite roundness of its particles. Because the conglomerates are coarse-grained, they particles may not have traveled far; but some transport was necessary to round the particles. Angular fragments that fall from a cliff and then are carried a few kilometers by a river or pounded by waves crashing in the surf along a beach are quickly rounded. Gravel that is transported down steep submarine canyons or carried by glacial ice, can be transported tens and hundreds of kilometers before deposition.

What is a formation?

A formation is a convenient rock unit for mapping and describing rock. Formation are lithologically distinguishable from adjacent rocks; their boundaries are contacts.

What is a graded bed

A graded bed is a layer with a vertical change in particle size, usually from coarse grains at the bottom of the bed to progressively finger grains toward the top.

Where are graded beds commonly deposited?

A graded bed may be deposited by a turbidity current.

What is a graywacke sandstone? Where do they form? What does it consist of?

A graywacke sandstone is a sandstone in which more than 15% of the rock's volume consists of fine-grained matrix. They are hard and dense, and they are generally dark gray or green. The sand grains may be so coated with matrix that they are difficult to see, but they typically consist of QUARTZ, FELDSPAR, AND SAND SIZED fragments of other fine grained sedimentary, volcanic, and metamorphic rocks. Most graywackes probably formed from sediment-laden turbidity currents that are deposited in deep water.

What is a quartz sandstone, and how does it form?

A quartz sandstone is a sandstone in which more than 90% of the grains are quartz. Because quartz is resistant to chemical weathering, it tends to concentrate in sand deposits as the less-resistant minerals (such as feldspar) are weathered away. The quartz grain in quartz sandstone are typically well-sorted and well-rounded because they have been transported for great distances and for a long time (they can't chemically weather). Most quartz sandstone gets deposited at the beach as beach sand or in a sand dune.

When is a sediment considered mature?

A sediment is considered mature when it has been well-rounded, well-sorted, and contains a high level of quartz.

What is a siltstone and what are its characteristsics

A siltstone is a rock consisting mostly of silt grains. It is somewhat coarser-grained that most shales, and silstones lack the fissility (ability to split into layers) and laminations (layers) of shale.

What is a turbidity current

A turbidity current is a rapid, downhill flow of water caused by increased density due to high amounts of sediment.

What is a turbidity current

A turbidity current is a turbulently flowing mass of sediment-laden water that is heavier than clear water and therefore flows downslope along the bottom of sea of a lake. Turbidity currents are underwater avalanches and are typically triggered by earthquakes or submarine landslides.

Have geologists observed significant quantities of dolomite forming in modern environments?

Although dolomite is relatively common in the ancient rock record, geologists have not observed significant quantities of dolomite forming in modern environments.

What is an arkose? How does an arkose form? Where is an arkose typically found?

An arkose is a sandstone in which 25% of the grains consist of feldspar. Because feldspar grains are preserved in the rock, the arkose obviously did not undergo severe chemical weathering or the feldspar would have been destroyed. Mountains of granite in a desert could be a sources for such a sediment because the rapid erosion (associated with the rugged terrain) would allow feldspar to be mechanically weathered and eroded before it can be chemically weathered (dry climate slows chemical weathering). Most arkose contain coarse, angular grains, this means that transportation distances were probably short. An arkose is often deposited within an alluvial fan (a large, fan shaped pile of sediment that usually forms where a stream emerges from a narrow canyon onto a flat plan that the foot of a mountain)

How is an inorganic limestone precipitated?

An inorganic limestone is precipitated directly as the result of inorganic processes. Oolitic limestone is a distinctive variety of inorganic limestone formed by the cementation of sand sized odious, small spheres of calcite inorganically precipitated in warm, shallow, sea water.

What is bedding

Bedding is a series of visible layers within rock. Most bedding is horizontal because the sediments from which the sedimentary rocks formed were originally deposited as horizontal layers.

What can create bedding planes

Bedding planes can be created by a change in grain size of composition of the particles being deposited or a pause during deposition.

How are biochemical limestones precipitated?

Biochemical limestones are precipitate through the actions of organisms. Most biochemical limestone are formed on continental shelves in warm, shallow seawater. Biochemical limestone may be precipitated directly in the core of a reef by coral, encrusting algae, or other shell-forming organisms. Such a rock would contain the fossil remains of organisms preserved in growth position. Biochemical limestone may also form from wave-broken fragments of algae, corals, and shells. The fragments may be of any size (gravel, sand, silt, and clay) and are commonly sorted and rounded as they are transported by waves and currents across the sea floor. The action of these waves and currents and subsequent cementation of these fragments into rock give these limestone a CLASTIC TETURE. These bioplastic limestone (or skeletal) have a variety of appearances. They may be relatively coarse grained with recognizable fossils or uniformly fine grained and dense from the accumulation of microscopic fragments of calcareous algae.

What is coal

Coal is an ORGANIC SEDIMENTARY ROCK that forms from the compaction of plant material that has NOT completely decayed. Rapid plant growth and deposition in water with a low oxygen content are needed, so shallow swamps or bogs in a temperate or tropical climate as likely environments of deposition.

Name common evaporites.

Common evaporites that form are 1) rock salt (composed of halite) and 2) rock gypsum (composed of gypsum). Less common evaporites include the borates, potassium salts, and magnesium salts.

What is Coquina Limestone?

Coquina limestone forms from the cementation of shells and shell fragments that accumulate on the shallow sea floor near shore. The coquina limestone has a clastic texture and is usually coarse grained with easy recognizable shells and shell fragments in it.

What can cracked mud lithify to

Cracked mud can lithify to form shale, preserving the cracks.

Where do cross-beds and ripples marks develop and form?

Cross-beds and ripples marks develop as moving sediment forms ripples and dunes during transport by wind, underwater currents, and waves.

What are cross-beds

Cross-beds are the groups of inclined layers, and the inclined layers are known as cross strata. Cross bedding forms during deposition on the inclined surfaces of bedforms such as ripples and dunes, and indicates that the depositional environment contained a flowing medium (typically water or wind).

When else does Dolomite form?

Dolomite can also form when pressure and temperature increase as a result of basin subsidence and deep burial of carbonate sediment. Higher temperas and pressies increase the stability of dolomite over calcite or aragonite, so carbonate rocks that have been buried often change to dolostones. This process can be enhanced by hot, salty pore fluids that migrate through the carbonate material. These deep burial dolostone are also a replacement for the original carbonate material, and may contain large, clear dolomite cement crystals. Deep burial dolostones tend to be thick and widespread, and they often have increased porosity from the conversion of calcite or aragonite to dolomite. Deep burial dolomite forms slowly in geological terms and this mechanism does not explain the shallow interbedded limestones and dolostone that are common in the rock record.

Does Dolomite precipitate directly from sea water in modern environments?

Dolomite does not precipitate directly from sea water in modern environments.

Why is Dolomite formation important?

Dolomite formation is important from an economic perspective because the porosity of carbonate rocks often increases as dolomite forms. Many important oil reservoirs are in dolostone layers that have enhanced porosity from dolomite formation.

Does Where is Dolomite common (even though it does not precipitate directly from sea water)

Dolomite is common in the PERSIAN GULF. In this arid region, dolomite forms in broad, FLAT tidal zones where the sediment is made of fine, highly reactive lime mud. The lime mud is composed of very small particles of calcite or aragonite. Replacement reactions start when seawater evaporates from the tidal flat, leaving behind a dense salty brine that percolates through the pore spaces of the lime mud. As the brine grows more concentrated, gypsum starts to precipitate, removing CA IONS FROM THE PORE WATER. THE REMAIN PORE WATER BECOMES RELATIVELY ENRICHED IN MG, AND SMALL, POORLY FORMED DOLOMITE CRYSTALS BEGIN TO REPLACE THE ORIGINAL CARBONATE MATERIAL. THIS IS called "proto"- dolomite, and it forms are an early replacement for limed in shallow tidel flat environments.

What does Dolomite typically form from? What does Dolomite consist of?

Dolomite usually forms from the alteration of limestone by magnesium-rich solutions.

What are evaporites?

Evaporates are rocks formed from crystals the precipitate during evaporation of water. They form from the evaporation of seawater of a saline lake, such as Great Sale Lake in Utah.

What are some evaporites and how do they form?

Evaporites, such as rock salt and gypsum, form as water evaporates.

What role does compaction play in the formation of shale?

Fine-grained rocks such as shale typically undergo pronounced compaction as they lithify. Before compaction, as much as 80% of the volume of the wet mud may have been pore space filled with water. The flake-like clay minerals were randomly arranged within the wet mud. Then, pressure from overlying material packed the sediment grains together and reduced the overall volume by squeezing water out of the pores. They clay minerals are then reoriented perpendicular to the pressure, becoming parallel to one another like a deck of cards. The fissility (layers) of the shale is DUE TO WEAKNESSES between these parallel clay flakes.

Where do ripple marks and cross beds form

Ripple marks and cross beds form in conglomerates, sandstones, siltstones, and limestones, and in environments such as deserts, river channels, river deltas, and shorelines.

When will ripple marks be symmetrical and when will ripple marks be asymmetrical

Ripple marks will be asymmetrical is the current is flowing in one direction (e.g. rivers). Ripple marks will be symmetrical if the water moves back and forth (e.g. waves in an ocean).

What are rocks consisting are fine-grained silt and clay called?

Rocks are consisting of fine-grained silt and clay are called shale, silt,stone, claystone, and mudstone.

What is sandstone, how does it form, and where is it found?

Sandstone is formed by the cementation of sand grains. Any deposit of sand can be lithified into sandstone. Rivers deposit sand in their channels and wind piles up sand into dunes. Waves deposit sand on the beaches and in shallow water. Deep-sea currents spread sand over the sea floor. Because any deposit of sand can be lithified into sandstone, there is a great deal of variation in mineral composition, degree of sorting, and degree of rounding.

Where does sandstone form?

Sandstone mostly forms from sand deposited by rivers, wind, waves, or turbidity currents.

How are sediment particles defined by grain size?

Sediment particles are defined by grain size through GRAVEL, SAND, SILT, AND CLAY.

What is a sedimentary breccia and how is it formed and where is it found?

Sedimentary breccia is NOT particularly common. It is a coarse-grained sedimentary rock formed by the cementation of coarse, ANGULAR fragments of rubble. It is unlikely that the angular fragments within the breccia moved very far from their source (more rounding would have taken place). Sedimentary breccia is commonly a talus slope deposited that forms at the base of a steep rock cliff that is being mechanically weathered. Landslide deposits also might lithify into sedimentary breccia.

What does shale consist of?

Shale consists of both silt and clay (2/3 clay sized minerals and 1/3 silt-sized quartz). They are so fine-grained that the surface of the rock feels very smooth. The silt and clay deposits that lithify as shale accumulate on lake bottoms, at the ends of rivers in deltas, on river flood plains, and on quiet parts of the deep-ocean floor.

Where does shale form?

Shale forms from rivers, lakes, or ocean mud.

What is shale and what is it known for?

Shale is a fine-grained sedimentary rocks that is known for its ability to split into layers (called fissility). Splitting of the layers takes places alone the surfaces of very thing layers called laminations within the shape.

How does shale split into layers?

Shale is known for its ability to split into layers (called fissility). The splitting takes places along the surfaces of very thing layers called laminations.

What is the Dolomite Problem?

Significant quantities of dolomite do not seem to be forming in modern environments even though they were quite common in the past.

What is the composition of sediment governed by?

The composition of sediment is governed by the rates of chemical weathering, mechanical weather, and erosion. During transportation, grains can become rounded and sorted.

How is the environment of deposition of a sedimentary rock determined?

The environment of deposition of a sedimentary rock is determined by studying bed sequence, grain composition and rounding, and sedimentary structures. Typical environments include alluvial fans, river channels, floor plains, lakes, dunes, deltas, beaches, shallow marine shelves, reefs, and deep-sea floor.

What can the filling of mud cracks by sand do

The filling of mud cracks by sand can form casts of the cracks in an overlying sandstone.

How do the two principal forms of Chert form?

The nodules often found in limestone probably formed from inorganic precipitation as underground water replaced part of the original rock with silica. The layered deposits typically form from the accumulation of delicate, glass-like shells of microscopic marine organisms on the sea floor. Microscopic fossils composed of silica are abundant in SOME CHERTS. But because chert is susceptible to recrystallization, the original fossils are easily destroyed, and the origin of many cherts remain unknown.

What is the principle of original horizontality

The principle of original horizontality states that most water laid sediment is deposited in horizontal or near horizontal layers that essentially parallel to Earth's surface. This is also true for sediments deposited by ice or wind.

Turbidity currents deposit what

turbidity deposit graded bedding into a fan pattern.