chapter 7

Sedimentary Structures: Ripple Marks, Dunes, and Cross Bedding: Consequences of Deposition in a Current

Many clastic sediments accumulate in moving fluids (wind, rivers, or waves). Fascinating sedimentary structures develop at the interface between the sediment and the luid. hese structures, known as bedforms, relect factors such as the velocity of the low and the size of the clasts. -

diamicton

Diamicton is a very poorly sorted sediment that contains cobbles or boulders surrounded by a matrix of sand, silt, and clay; when lithified, diamicton becomes diamictite. Research suggests that diamictites can form from the lithification of debris flow or of glacial till,

Alluvial Fan Environments

- In arid regions, where there is not enough water for the stream to flow continuously, the stream deposits its load of sediment on the plain near the mountain front, producing a wedge-shaped apron of gravel and sand called an alluvial fan -Deposition takes place here because when the stream emerges from a canyon mouth, it spreads out over a broader area, so friction with the ground causes the water to slow down, and slowmoving water does not have the power to move pebbles, cobbles, or coarse sediment. Notably, the sedi- ment in the sand of an alluvial fan is derived from the erosion of granite => may still contain feldspar grains that have not yet weathered into clay. As a consequence, alluvial fan sediments become breccia, conglomerate, and arkose.

Coastal and Marine Depositional Environments

- Marine depositional environments start at the high-tide line and extend offshore to the deep seafloor. he type of sediment deposited at a marine location depends on the temperature, clarity, and depth of the water and on the supply of clasts. hat's because temperature and clarity determine the species of organisms that can live in the water, and the availability of clasts and the degree to which waves move water affect the size of the clasts that accumulate. -Marine Delta Deposits, Coastal Beach Sands, Shallow-Marine Clastic Deposits, Shallow-Marine Carbonate Environments, Deep-Marine Deposits

Origins : breccia

-1.large blocks or rock tumble off a cliff and slam into blocks at the bottom. => Impact shatters blocks => if these angular, sharp clasts cemented together = breccia.

Sedimentary structures; bedding and stratification

-A single layer of sediment or sedimentary rock with a recognizable top and bottom = a bed; -the boundary between two beds can be called a bedding plane. -Several beds together = constitute strata -and the overall division of sediment into a sequence of beds is bedding, or stratification. -Typically, contrasts in rock type distinguish one bed from adjacent beds.; if sequence of strata contains bed of sandstone overlain by bed of siltstone, the surface separating one rock type from another = bedding. Bedding may be represented by subtle changes in grain size, by surfaces developed as a consequence of interruptions in deposition, or by cracks that formed parallel to bedding planes.

Clastic Sedimentary Rocks: Origins

-Characteristics of a clastic sedimentary rock provide clues to the source of the sediment and to the environment of deposition.

Travertine (Chemical Limestone)

-Most limestone is biochemical, in that it forms from the shells of organisms. -travertine, consists of crystalline calcium carbonate (CaCO3) that precipitates directly from groundwater that has seeped out at the ground surface either in hot- or cold-water springs, or on the walls of caves. -Precipitation happens when the groundwater degasses, meaning that some of the carbon dioxide that had been dissolved in it bubbles out of solution, for removal of car- bon dioxide decreases the ability of the water to hold dissolved carbonate. Precipitation also occurs when water evaporates, thereby increasing the concentration of carbonate. -translucence (in thin slices) and from the colored growth bands it displays. he bands develop in response to changes in the composition of groundwater or in the environment into which the water drains

Bed-Surface Markings

-Mud cracks: If a mud layer dries up after deposition, it cracks into roughly hexagonal plates that typically curl up at their edges. -Scour marks: As currents low over a sediment surface, they may erode small troughs, called scour marks, parallel to the current low. -Fossils: Fossils are relicts of past life. Some fossils are shell imprints, footprints, or feeding traces on a bedding surface -Paleosols: If a soil that formed on the surface of a sedimentary bed becomes buried, it can be preserved as a paleosol. If lithified, a paleosol may delineate a bed surface because it has a different texture and, in some cases, a different color than does the internal portion of the bed

Clast size

-diameter of the grains making up a clastic sedimentary rock as the clast size or grain size -from coarsest to finest = boulder, cobble, pebble, sand, silt, and mud -gravel for an accumulation of pebbles and cobbles -mud for an accumulation of wet clay and very fine silt. Clay refers to all grains less than 0.004 mm in diameter—such grains consist mostly of clay minerals but also include tiny specks of quartz and other minerals.

How Do We Recognize Depositional Environments?

-examines grain size, clast composition, sorting, bed-surface marks, cross bed- ding, and fossils to identify a depositional environment. -Terrestrial (Nonmarine) Depositional Environments and Coastal and Marine Depositional Environments

Biochemical Sedimentary Rocks: Organic Sedimentary Rocks

-in some environments, such as quiet, oxygen-poor water in swamps, lagoons, or lakes, the debris settles along with other sediment and eventually gets buried and preserved. => at elevated temp and pressures => organic matter undergoes chemical reactions that slowly transform it into organic sedimentary rock, which is distinct from other sedimentary rock in that it contains a high proportion of organic chemicals.

Glacial Environments

-it's so cold that more snow collects in the winter than melts away -Solid ice can move sed- iment of any size. So as a glacier moves down a valley in the mountains, it carries along all the sediment that falls on its surface from adjacent cliffs or gets plucked from the ground at its base or sides. At the end of the glacier, where the ice inally melts away, it drops its sedimentary load and produces a pile of glacial till

Biochemical Sedimentary Rocks: Biochemical Limestone

-organisms make solid shells out of cal- cium carbonate (CaCO3), either as calcite or as its polymorph, aragonite. When the organisms die, the shells remain and may accumulate. Lithiication of this sediment yields biochemical limestone, a type of carbonate rock. -limestone can originate from reef builders (such as coral) that grew in place, from shell debris that was broken up and transported, or from carbonate mud consisting of plankton shells that settled like snow out of water. Because of this variety, we distinguish among fossiliferous limestone, containing visible fossil shells or shell fragments; micrite, consisting only of very fine carbonate mud; and chalk, consisting of plankton shells. -limestone occurs as a massive light-gray to dark- bluish-gray rock; because several processes take place that change the texture of the rock over time. Prior to lithification, organisms may burrow into recently formed or deposited shells and break them up. Later, water passing through the rock not only precipitates cement but also dissolves some carbonate grains and causes new ones to grow. As a result, new crystals of calcite replace the original crystals. Typically, all the aragonite that was originally in shells transforms into calcite, a more stable mineral, and larger crystals of calcite replace smaller ones.

Biochemical Sedimentary Rocks: Biochemical Chert

-rock consists of cryptocrystalline quartz -consisting of tiny quartz grains -forms from plankton, such as radiolaria and diatoms, that produce shells composed of SiO2 (silica), which accumulate along with clay to form a siliceous "ooze" on the seafloor. Gradually, the shells dissolve, forming silica solutions, which ill pores in the surrounding clay. Eventually, as tiny crystals grow from these solutions, the sediment lithifies to become rock. -chert containing traces of iron oxide tends to be red—such chert is also known as jasper.

Evaporites: Products of Saltwater Evaporation

-salt flats formed from the evaporation of an ancient salt lake. Under the heat of the Sun, the water turned to vapor and drifted up into the atmosphere, but the salt that had been dissolved in the water stayed behind. Such salt precipitation occurs wherever saltwater becomes saturated, as can happen in desert lakes with no outlet, or along the margins of restricted seas. -For thick deposits of salt to form, large volumes of water must evaporate. This may happen when plate tectonic movements temporarily cut of arms of the sea or during continental rifting, when sea water first begins to spill into the rift valley. -when 80% of the seawater trapped in a basin evaporates, gypsum forms, and when 90% of the water evaporates, halite precipitates. If seawater were to evaporate entirely, the resulting evaporite would consist of 80% halite, 13% gypsum, and other salts and carbonates would make up the remainder.

Terrestrial (Nonmarine) Depositional Environments

-those that develop inland, -settle on dry land or under and adjacent to fresh- water streams, glaciers, and lakes. In some settings, oxygen in surface water or groundwater reacts with the iron in terrestrial sediments to produce rust-like iron-oxide minerals, which give the sediment an overall reddish hue. Strata with this hue are informally called redbeds -Glacial Environments, Mountain Stream Environments, Alluvial Fan Environments, Desert Environments, Lake Environments, River Environments

Mountain Stream Environments

-when water flow slows, the largest clasts settle out to form gravel and boulder beds, while the water carries finer sediments, such as sand and mud, farther downstream - breccia and conglomerate is characteristic of this setting

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 of of pre-existing rocks 2. Biochemical sedimentary rock consists of shells grown by organisms. 3. Organic sedimentary rock consists of carbon-rich relicts of plants or other organisms. 4. Chemical sedimentary rock consists of minerals that precipitated directly from surface-water solutions.

Origins: arkose

2. storm causes the clasts to slide downslope into a turbulent river => in water clasts bang into one another => shatter into smaller pieces. Angular clasts become rounded, when deposited, pebbles and cobbles break apart into individual mineral grains ( disintegration of granite would yield a mixture of quartz, feldspar, and clay. Even slowly moving water can carry clay away, and Sandstone. Leave behind sand containing a mixture of quartz and some feldspar grains). This sediment, if buried and lithified, becomes arkose

Origins:

3. feldspar grains in sand Shale continue to weather into clay so that gradually, during successive events that wash the sediment downstream, the sand loses feldspar clasts and ends up being composed almost entirely of durable quartz sand grains. -sand back into sea or sand dunes. => when this sediment is buried and lithified => quartz and sandstone. -Meanwhile, silt and clay may accumulate in the flat areas bordering streams, regions called floodplains that become submerged only during floods. And some silt and mud settles in a wedge, called a delta, in lagoons or in mudflats. -The silt, when lithified, becomes siltstone, and the mud, when lithified, becomes shale or mudstone. (Layers of mud, exposed beneath marsh grass, lithify to form shale. Here the thin-bedded shale is interbedded with sandstone.)

Dunes

A dune looks like a ripple, only it's larger. Up to several metres high.

Deposition

A transporting medium does not carry sediment forever. Eventually, the sediment undergoes deposition, the process by which sediment settles out of the medium

Marine Delta Deposits

After following the river down-stream for a long distance, we reach its mouth, where it empties into the sea. Here the river water stops lowing, so water velocity slows, and sediment settles out to build a delta into the sea. -marine deltas host many different sedimentary environments, including swamps, channels, floodplains, and submarine slopes. Sea- level changes may cause the positions of the different environments to move inshore or offshore with time.

Turbidity Currents and Graded Beds

An earthquake or storm might disturb sediment on submarine slope and cause it to slip downslope, and mix with water to yield a murky, turbulent cloud. This cloud is denser than clear water, so it flows downslope like an underwater avalanche. Such a flow is known as a turbidity current. Downslope, the turbidity current slows, and the sediment that it carried starts to settle out. Larger grains sink faster through a fluid than do finer grains, so the coarsest sediment settles out first. Progressively finer grains accumulate on top, with the finest sediment (clay) settling out last. This process forms a graded bed

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 a clast has the same dimensions in all directions, so that it resembles the shape of a sphere

Bioturbation

Bedding is not always well preserved, however, for in some environments, burrowing organisms disrupt the layering. Speciically, worms, clams, and other creatures can churn sediment and leave behind burrows; this process is called bioturbation.

In what geologic settings do sedimentary basins form? Plate tectonics theory provides a key.

Categories of Basins in the Context of Plate Tectonics Theory, Transgression and Regression, and Diagenesis

Transgression and Regression

Changes in sea level, relative to the land surface, can control the succession of sediments that we see in a sedimentary basin. Global sea- level changes may be due to a number of factors—including climate change—that control the amount of ice stored in polar ice caps and cause changes in the volume of ocean basins. Sea level at a location may also be changed by the local uplift or sinking of the land surface. When relative sea level rises, the shoreline migrates inland. During this process, known as transgression, coastal sediments progressively bury terrestrial sediments, and deeper- water sediments bury coastal sediments. When relative sea level falls, the coast migrates sea- ward. We call this process regression. Typically, the record of a regression will not be well preserved because as sea level drops, areas that had been sites of deposition become exposed to erosion.

Classification

Clast size, Clast composition, angularity and sphericity, sorting, sedimentary maturity, character of cement. Note that grain size serves as the primary basis for the classiication of clastic rocks. Geologists further distinguish among kinds of sandstone (quartz sandstone, arkose, or wacke) based on clast composition or on sorting. We also distinguish between shale and mudstone based on the way in which the rock breaks (shale splits into thin sheets, whereas mudstone does not), and between breccia and conglomerate based on angularity.

Weathering:

Clasts from which clastic rocks form come from the disintegration of pre-existing bedrock into separate grains due to physical and chemical weathering. The dissolved ions that eventually precipitate as new minerals to hold the grains together in sedimentary rocks come from chemical weathering.

Organic Sedimentary Rocks: coal

Coal is a black, combustible rock containing between 40% and 90% carbon; remainder consists of clay and quartz => coal forms from plant remains that have been buried deeply. Under the pressure and temperature conditions found at depth, the organic material of the plants becomes tightly compacted, and volatile molecules such as hydrogen, water, carbon dioxide, and ammonia break free and escape. As this happens, the carbon atoms reorganize into macerals

Clast composition

Composition refers to the makeup of clasts in sedimentary rock. Larger clasts (pebbles or larger) typically include rock fragments, meaning that the clasts themselves are aggregates of many mineral grains, whereas smaller clasts (sand, silt, or clay) typically consist of individual minerals. In some cases, lithic clasts, chips of fine-grained rock, may be mixed in with sand grains.

Foreland basins

Foreland basins form on the continental side of a mountain belt because the forces produced during convergence or collision push large slices of rock up faults and onto the surface of the continent. The weight of these slices pushes down on the surface of the lithosphere, producing a wedge-shaped depression adjacent to the mountain range that fills with sediment eroded from the range. Fluvial and deltaic strata accumulate in foreland basins

Shallow-Marine Clastic Deposits

From the beach, we proceed offshore. In deeper water, where wave energy does not stir the seafloor, finer sediment can accumulate. Clastic sediments that accumulate in this environment tend to be fine-grained, well-sorted, and well-rounded silt. A great variety of organisms such as mollusks, gastropods, and worms live in or on this sediment. Therefore, if you see beds of siltstone and mudstone containing marine fossils, you may be looking at shallow-marine clastic deposits.

Categories of Basins in the Context of Plate Tectonics Theory

Geologists distinguish among different kinds of sedimentary basins on the basis of the region of a lithospheric plate in which the basin forms, as defined in the context of plate tectonics theory. A few examples= Rift basins, Passive margin basins, Intercontinental basins, Foreland basins

Lithification:

Geologists refer to the last stage of clastic sedimentary rock production, namely, the transformation of loose sediment into solid rock, as lithification. Lithiication of clastic sediment involves two steps. First, once the sediment has been buried, pressure generated by the weight of the overlying material squeezes out the water and air that had been trapped between clasts and presses the clasts together tightly, a process called compaction. Second, during and/or after compaction, cementation binds sediment in place to make coherent sedimentary rock. During cementation, minerals (commonly quartz or calcite) precipitate from groundwater and fill spaces between clasts. The resulting cement acts like glue in that it holds grains together.

Shallow-Marine Carbonate Environments

In shallow- marine settings where relatively little sand and mud enter the water, the warm, clear, nutrient-rich water can host an abundance of organisms with carbonate shells, which become carbonate sediment. Beaches collect sand composed of shell fragments; lagoons are sites where carbonate mud accumulates; and reefs consist of coral and coral debris. Farther offshore from a reef, we can find a sloping apron of reef fragments. Shallow-marine carbonate sediments transform into various kinds of limestone.

Lake Environments

In temperate climates, where water remains at the surface throughout the year, lakes form. In lakes, the relatively quiet water can't move coarse sediment; any coarse sediment brought into the lake by a stream settles out at the stream's outlet. Only ine clay makes it out into the center of the lake, where it settles to form mud on the lake bed. Not surprisingly, lake sediments typically consist of finely laminated shale. At the mouths of streams that empty into lakes, small deltas may form. A delta is a wedge of sediment that accumulates where moving water enters standing water. Small deltas contain three components; topset beds of gravel, foreset beds of gravel and sand, and bottomset beds of silt.

Desert Environments

In very dry climates, few plants can grow, so the ground surface lies exposed. Strong winds can pick up dust and sand from the surface. In places, deserts contain low areas in which water collects during loods but dries up between loods. Salts precipitate in such settings and can build into beds of evaporites.

Intracontinental basins:

Intracontinental basins develop in the interiors of continents, initially because of subsidence over a rift. Illinois and Michigan are each underlain by an intracontinental basin—the Illinois basin and the Michigan basin, respectively—in which up to 7 km of fluvial, deltaic, or shallow-marine sediment accumulated. At times, extensive swamps formed along the shorelines of these basins. The plant matter of these swamps transformed into coal when it was later buried deeply.

Dolostone

Not all carbonate rock consists of pure calcite. A variety of carbonate rock called dolostone difers from limestone in that it contains the mineral dolomite (Ca Mg[CO3]2) -Magnesium in dolomite comes from; Dolomite is formed by a chemical reaction between solid calcite and magnesium-bearing groundwater. This change may take place beneath lagoons along a shore soon after the limestone forms or, a long time later, after the limestone has been buried deeply.

Chemically Precipitated Chert: Agate

Not all chemically precipitated chert replaces other minerals; agate, precipitates in concentric rings inside open hollows in a rock. The successive rings of chert have different colors, giving the rock a striped appearance. These colors are caused by variations in the type and concentration of impurities.

character of cement

Not all clastic sedimentary rocks contain the same kind of cement. In some, the cement consists predominantly of quartz, whereas in others it consists predominantly of calcite. Other kinds of mineral cements do occur, but they are rarer.

Coastal Beach Sands

Now we leave the delta and wander along the coast. Ocean currents transport sand along the coastline. The sand washes back and forth in the surf, so it becomes well sorted and well rounded, and because of the back-and-forth movement of ocean water, the sand surface may become rip- pled. So, if you ind well-sorted, medium-grained sandstone, perhaps with ripple marks, you may be looking at the remnants of a beach environment.

Erosion

Once formed, clasts produced by weathering do not stay in place forever. Gravity may cause them to fall of an outcrop, or they may be removed from the outcrop by erosion—the process by which moving water, ice, or air separates clasts from their substrate and carries them away.

Transportation

Once produced by weathering, and removed by erosion, clasts and dissolved ions can be carried away in a transporting medium.

stratigraphic formation

Over geologic time, long-term changes in a depositional environment may take place, so a given sequence of strata may difer markedly from sequences of strata above or below. A sequence of strata distinctive enough to be traced across a fairly large region is called a stratigraphic formation, or simply a formation. -For example, a region may contain a succession of alternating sandstone and shale beds deposited by rivers, overlain by beds of marine limestone deposited later when the region was submerged by the sea.

Passive-margin basins:

Passive-margin basins form along edges of continents that are not plate boundaries. hey are underlain by stretched lithosphere, the remnants of a rift whose evolution successfully led to the formation of a mid-ocean ridge. These basins ill both with sediment carried to the sea by rivers, and with carbonate sediment formed in coastal reefs.

Rift basins

Rift basins form in continental rifts, regions where the lithosphere has been stretched. During rifting, the surface of the Earth subsides simply because the crust becomes thinner as it stretches. As the rift grows, slip on faults sends blocks of crust downward, producing low troughs bordered by narrow mountain ranges. Alluvial fan deposits form along the bases of the mountains, and salt lats or lakes develop in the interior portions of the troughs. During rifting, warm asthenosphere rises beneath the rift and heats up the thin lithosphere. When rifting ceases, the rifted lithosphere cools, thickens, and becomes denser. his heavier lithosphere sinks, causing more subsidence,

Ripple marks

Ripple marks are relatively small (generally no more than a few centimeters high) elongated ridges that form on a bed surface at right angles to the direction of current low. If the current always lows in the same direction, the rip- ple marks tend to be asymmetric, with a steeper slope on the downstream (lee) side. Along the shore, where water lows back and forth due to wave action, ripples tend to be symmetric. he crest (the high ridge) of a symmetric ripple delineates a sharp ridge, whereas the trough between adjacent ridges tends to be a smooth, concave-up curve

River Environments

Rivers transport gravel, sand, silt, and mud. The coarser sediment tumbles along the bed in the river's channel and collects in cross-bedded, rippled layers, while the finer sediment drifts along, suspended in the water. This fine sediment settles out along the floodplain. Mud layers on floodplain dry out between floods, leading to the formation of mud cracks. River sediments lithify to form sandstone, siltstone and shale. Typically, channels of coarser sediment are sur- rounded by layers of ine-grained loodplain deposits

Clastic Sedimentary Rocks: formation

Sandstone serves as an example of a clastic sedimentary rock (also known as a detrital sedimentary rock) because it consists of loose clasts (detritus) that have been stuck together to form a solid mass. he clasts, or grains, can consist of individual mineral fragments, such as grains of quartz or lakes of clay minerals, or of chunks of rock, such as pebbles of granite. By weathering, Erosion, Transportation, Deposition and Lithification

Chemical Sedimentary Rocks

They all consist of rock formed primarily by the precipitation of minerals directly out of water solutions. We call such rocks chemical sedimentary rocks. They typically have a crystalline texture. In some chemical sedimentary rocks, crystals are coarse, whereas in others, they are too small to see. -many types of chemical sedimentary rocks, primarily on the basis of composition; Evaporites, Travertine, Dolostone, Chemically precipitated chert

sedimentary basins

Thick accumulations of sediment form only in special regions where the surface of the Earth's lithosphere sinks, providing space in which sediment can collect. Geologists use the term subsidence to refer to the process by which this sinking takes place and sedimentary basin = sediment filled depression

Chemically Precipitated Chert

Unlike the biochemical chert we described earlier, the chert collected by the Onondaga didn't form from layers of siliceous plankton shells. Rather, the chert nodules grew when microscopic quartz crystals gradually precipitated and replaced calcite crystals within a bed of limestone, long after the limestone was originally deposited. (replacement chert or nodular chert) -Replacement chert doesn't form only in limestone. Some grows in buried silica-rich volcanic ash beds when the silica in groundwater precipitates as microcrystalline quartz within wood that has been buried by the ash, forming petrified wood. The quartz gradually replaces the interiors of the wood's cells.

Deep-Marine Deposits

along transition between coastal regions and the deep ocean, turbidity currents deposit graded beds. In deep ocean realm only fine clay and plankton provide sources of sediment. The clay eventually settles out onto deep seafloor, forming deposits of finely laminated mudstone, and plankton shells settle to form chalk or chert. Deposits of mudstone, chalk or bedded chert = deep marine origin

Sedimentary maturity:

degree to which a sediment has evolved from being just a crushed-up version of its source rock into a well-sorted and well-rounded collection of clasts consisting only of the minerals that are most resistant to weathering

sorting

degree to which the clasts in a rock all have the same size as sorting. Well-sorted sediment consists entirely of clasts of the same size, whereas poorly sorted sediment contains a mixture of more than one grain size. If a sedimentary rock contains larger clasts surrounded by much smaller clasts (e.g., cobbles surrounded by sand), then the mass of smaller grains constitutes the matrix of the rock.

Diagenesis

diagenesis = for all the physical, chemical, and biological processes that transform sediment into sedimentary rock and that alter characteristics of sedimentary rock after the rock has formed. In sedimentary basins, sedimentary rocks may become buried very deeply. As a result, the rocks endure high pressures and temperatures and come in contact with warm groundwa- ter. Diagenesis, under such conditions, can cause chemical reactions in the rocks that produce new minerals and can also cause cement to dissolve or precipitate. At sufficiently high temperatures and pressures, metamorphism begins: a new assemblage of minerals forms, and mineral grains may become aligned parallel to one another.

cross bedding

f you examine a vertical slice cut into a ripple or dune, you will ind distinct internal laminations inclined at an angle to the boundary of the main sedimentary layer. Such laminations are known as cross beds. -The current erodes and picks up clasts from the upwind or upstream part of the bedform and deposits them on the leeward part. Sediment builds up on the leeward side until gravity causes it to slip down. With time, the leeward side of the bedform builds in the downstream direction. The surface of the slip face establishes the shape of the cross beds. Eventually, a new cross-bedded layer may build out over a pre-existing one. -boundary between two successive layers as the main bedding, and the internal curving surfaces within the layer as cross bedding

sedimentary rock

is rock that forms at or near the surface of the Earth in one of several ways: by the cement- ing together of loose clasts (fragments or grains) that were produced by physical or chemical weathering of pre-existing rock, by the growth of shell masses or the cementing together of shells and shell fragments, by the accumulation and sub- sequent alteration of organic matter derived from living organisms, or by the precipitation of minerals directly from surface-water solutions.

Organic Sedimentary Rocks: Oil shale

like all shales, contains not only clay, but also between 15% and 30% organic material in a form called kerogen. The kerogen in oil shale comes from the fats and proteins that made up the living parts of plankton or algae. -When the mud gets buried and lithified, the organic material undergoes chemical reactions that transform it into kerogen. he presence of organic material colors oil shale black.

use various adjectives to characterize sedimentary-rock composition:

siliceous rocks contain mostly quartz, argillaceous rocks contain mostly clay minerals, and carbonate rocks contain mostly calcite and/or dolomite. By some estimates, 70% to 85% of all the sedimentary rocks on the Earth are siliceous or argillaceous clastic rocks, and 15% to 25% are carbonate biochemical or chemical rocks.

Why Study Sedimentary Structures?

the presence of ripple marks and cross bedding indicates that layers were deposited in a current; the presence of mud cracks indicates that the sediment layer was exposed to the air and dried out on occasion; and graded beds indicate deposition by turbidity currents. Fossil types can tell us whether sediment was deposited along a river or in the deep sea, for diferent species of organisms live in these different environments.

Why does bedding form

think about how sediment accumulates. Changes in climate, water depth, current velocity, or the sediment source control the type of sediment deposited at a location at a given time. On a normal day, a slow-moving river may carry only silt, which collects on the riverbed. During a flood, the river flows faster and carries sand and pebbles, so a layer of sandy gravel forms over the silt layer. Then, when the flooding stops, more silt buries the gravel. If this succession of sediments becomes lithiied and exposed for you to see, the layers appear as alternating beds of siltstone and sandy conglomerate