When water is oversaturated with calcite , the mineral precipitates out around a nucleus, a sand grain or shell fragment, and forms little spheres called ooid Spheres of calcite that form in saline waters with slight wave agitation. As evaporation continues, the ooid Spheres of calcite that form in saline waters with slight wave agitation. Biochemical sedimentary rocks are not that different from chemical sedimentary rocks; they are also formed from ions dissolved in solution.
However, biochemical sedimentary rocks rely on biological processes to extract the dissolved materials out of the water. Most macroscopic marine organisms use dissolved minerals , primarily aragonite calcium carbonate , to build hard parts such as shells.
When organisms die the hard parts settle as sediment , which become buried, compacted and cemented into rock. This biochemical extraction and secretion is the main process for forming limestone , the most commonly occurring, non- clastic sedimentary rock. Solid calcite reacts with hydrochloric acid by effervescing or fizzing. Dolomite only reacts to hydrochloric acid when ground into a powder, which can be done by scratching the rock surface see Chapter 3, Minerals.
Limestone occurs in many forms, most of which originate from biological processes. Entire coral reef A topographic high found away from the beach in deeper water, but still on the continental shelf.
Typically, these are formed in tropical areas by organisms such as corals. Fossiliferous limestone contains many visible fossils. A type of limestone called coquina originates from beach sands made predominantly of shells that were then lithified. Coquina is composed of loosely-cemented shells and shell fragments. You can find beaches like this in modern tropical environments, such as the Bahamas. Chalk contains high concentrations of shells from a microorganism called a coccolithophore.
Micrite , also known as microscopic calcite mud, is a very fine-grained limestone containing microfossils that can only be seen using a microscope. Biogenetic chert forms on the deep ocean floor , created from biochemical sediment made of microscopic organic shells.
This sediment , called ooze, may be calcareous calcium carbonate based or siliceous silica-based depending on the type of shells deposited. For example, the shells of radiolarians zooplankton and diatoms phytoplankton are made of silica, so they produce siliceous ooze.
Under the right conditions, intact pieces of organic material or material derived from organic sources, is preserved in the geologic record. Although not derived from sediment , this lithified organic material is associated with sedimentary strata and created by similar processes—burial, compaction , and diagenesis.
C Deposits of these fuels develop in areas where organic material collects in large quantities. Lush swamplands can create conditions conducive to coal Former swamp-derived plant material that is part of the rock record. Shallow-water, organic material-rich marine sediment can become highly productive petroleum and natural gas deposits. See Chapter 16, Energy and Mineral Resources, for a more in-depth look at these fossil -derived energy sources. In contrast to detrital sediment , chemical, biochemical , and organic sedimentary rocks are classified based on mineral composition.
Most of these are monomineralic, composed of a single mineral , so the rock name is usually associated with the identifying mineral. Chemical sedimentary rocks consisting of halite are called rock salt. Rocks made of Limestone calcite is an exception, having elaborate subclassifications and even two competing classification methods: Folk Classification and Dunham Classification.
The Folk Classification deals with rock grains and usually requires a specialized, petrographic microscope. The Dunham Classification is based on rock texture , which is visible to the naked eye or using a hand lens and is easier for field applications.
Most carbonate geologists use the Dunham system. Sedimentary rock identification chart. What is the most likely cause of a detrital sediment with highly rounded grains? The general rule of thumb is: the longer the transport distance, the more the rounding. Shale is the fissile , very fine grained sedimentary rock and splits easily into thin layers. Clastic or detrital rocks are categorized based on their grain size i. Conglomerates are rounded, and breccias are angular.
All chemical rocks are names based on composition i. Which of the following is a biochemical sedimentary rock? Coquina , chalk , and fossiliferous limestone are forms of biochemical rocks since their components are precipitated by organisms. Shale and sandstone are detrital even if they include fossils , banded iron formation is chemical, and coal Former swamp-derived plant material that is part of the rock record. Sedimentary structures are visible textures or arrangements of sediments within a rock.
Geologists use these structures to interpret the processes that made the rock and the environment in which it formed. They use uniformitarianism to usually compare sedimentary structures formed in modern environments to lithified counterparts in ancient rocks. Below is a summary discussion of common sedimentary structures that are useful for interpretations in the rock record.
The most basic sedimentary structure is bedding planes , the planes that separate the layers or strata in sedimentary and some volcanic rocks. Visible in exposed outcroppings, each bedding plane indicates a change in sediment deposition conditions.
This change may be subtle. For example, if a section of underlying sediment firms up, this may be enough to create a form a layer that is dissimilar from the overlying sediment. Each layer is called a bed A specific layer of rock with identifiable properties. As would be expected, bed A specific layer of rock with identifiable properties. Technically, a bed A specific layer of rock with identifiable properties.
A layer thinner than 1 cm 0. Varves are bedding planes created when laminae and bed A specific layer of rock with identifiable properties. Varves are valuable geologic records of climatic histories, especially those found in lakes and glacial deposits. Graded bedding refers to a sequence of increasingly coarse- or fine-grained sediment layers. Graded bedding often develops when sediment deposition occurs in an environment of decreasing energy.
A Bouma sequence is graded bedding observed in clastic rock called turbidite. Bouma sequence bed A specific layer of rock with identifiable properties. These subsea density flows begin when sediment is stirred up by an energetic process and becomes a dense slurry of mixed grains.
The sediment flow courses downward through submarine channels and canyons due to gravity acting on the density difference between the denser slurry and less dense surrounding seawater. As the flow reaches deeper ocean basins it slows down, loses energy, and deposits sediment in a Bouma sequence of coarse grains first, followed by increasingly finer grains see figure.
In fluid systems, such as moving water or wind, sand is the most easily transported and deposited sediment grain. Smaller particles like silt and clay are less movable by fluid systems because the tiny grains are chemically attracted to each other and stick to the underlying sediment.
Under higher flow rates, the fine silt and clay sediment tends to stay in place and the larger sand grains get picked up and moved. Bedforms are sedimentary structures created by fluid systems working on sandy sediment. Grain size , flow velocity, and flow regime or pattern interact to produce bedforms having unique, identifiable physical characteristics.
Flow regimes are divided into upper and lower regimes, which are further divided into uppermost, upper, lower, and lowermost parts. The table below shows bedforms and their associated flow regimes. For example, the dune A large pile of sediment, deposited perpendicular to flow. Internal bedding in dunes dips toward flow direction i. Formed in the upper part of the lower flow regime.
Plane bed A specific layer of rock with identifiable properties. The flat, parallel layers form as sandy sediment piles and move on top of layers below. Even non-flowing fluid systems, such as lakes, can produce sediment plane bed A specific layer of rock with identifiable properties.
They may look identical to lower-flow-regime bed A specific layer of rock with identifiable properties. Ripples are known by several names: ripple marks, ripple cross bed A specific layer of rock with identifiable properties. The ridges or undulations in the bed A specific layer of rock with identifiable properties.
With the exception of dune A large pile of sediment, deposited perpendicular to flow. Occasionally, large flows like glacial lake outbursts, can produce ripples as tall as 20 m 66 ft. First scientifically described by Hertha Ayrton, ripple shapes are determined by flow type and can be straight-crested, sinuous, or complex. Asymmetrical ripples form in a unidirectional flow. Symmetrical ripples are the result of an oscillating back-and-forth flow typical of intertidal swash zones.
Climbing ripples are created from high sedimentation rates and appear as overlapping layers of ripple shapes see figure. Cross bedding happens when ripples or dune A large pile of sediment, deposited perpendicular to flow. Desert sand dune A large pile of sediment, deposited perpendicular to flow. British geologist Agnold considered only Barchan and linear Seif dune A large pile of sediment, deposited perpendicular to flow. Other workers have recognized transverse and star dunes as well as parabolic and linear dunes anchored by plants that are common in coastal areas as other types of dune A large pile of sediment, deposited perpendicular to flow.
The biggest difference between river dune A large pile of sediment, deposited perpendicular to flow. Some famous air-formed dune A large pile of sediment, deposited perpendicular to flow. As airflow moves sediment along, the grains accumulate on the dune A large pile of sediment, deposited perpendicular to flow.
The angle of the windward side is typically shallower than the leeward downwind side, which has grains falling down over it. This difference in slopes can be seen in a bed A specific layer of rock with identifiable properties. There are typically two styles of dune A large pile of sediment, deposited perpendicular to flow. In tidal locations with strong in-and-out flows, dune A large pile of sediment, deposited perpendicular to flow. This produces a feature called herringbone cross bedding.
Another dune A large pile of sediment, deposited perpendicular to flow. These bed A specific layer of rock with identifiable properties. Antidunes are so named because they share similar characteristics with dune A large pile of sediment, deposited perpendicular to flow. While dune A large pile of sediment, deposited perpendicular to flow. Antidunes form in phase with the flow; in rivers they are marked by rapids in the current. Antidunes are rarely preserved in the rock record because the high flow rates needed to produce the bed A specific layer of rock with identifiable properties.
Bioturbation is the result of organisms burrowing through soft sediment , which disrupts the bedding layers. These tunnels are backfilled and eventually preserved when the sediment becomes rock. Bioturbation happens most commonly in shallow, marine environments, and can be used to indicate water depth. Mudcracks occur in clay-rich sediment that is submerged underwater and later dries out. When this waterlogged sediment begins to dry out, the clay grains shrink.
The sediment layer forms deep polygonal cracks with tapered openings toward the surface, which can be seen in profile. The cracks fill with new sediment and become visible veins running through the lithified rock. These dried-out clay bed A specific layer of rock with identifiable properties. What makes this sedimentary structure so important to geologists, is they only form in certain depositional environments —such as tidal flats that form underwater and are later exposed to air.
Syneresis cracks are similar in appearance to mudcracks but much rarer; they are formed when subaqueous underwater clay sediment shrinks. Sole marks are small features typically found in river deposits. They form at the base of a bed A specific layer of rock with identifiable properties.
They can indicate several things about the deposition conditions, such as flow direction or stratigraphic up-direction see Geopetal Structures section.
Flute casts or scour marks are grooves carved out by the forces of fluid flow and sediment loads. The upstream part of the flow creates steep grooves and downstream the grooves are shallower.
The grooves subsequently become filled by overlying sediment , creating a cast Material filling in a cavity left by a organism that has dissolved away. Formed similarly to flute casts but with a more regular and aligned shape, groove casts are produced by larger clasts or debris carried along in the water that scrape across the sediment layer.
Tool marks come from objects like sticks carried in the fluid downstream or embossed into the sediment layer, leaving a depression that later fills with new sediment.
Load cast Material filling in a cavity left by a organism that has dissolved away. Like their name implies, raindrop impressions are small pits or bumps found in soft sediment.
While they are generally believed to be created by rainfall, they may be caused by other agents such as escaping gas bubbles. Imbrication is a stack of large and usually flat clasts—cobbles, gravels, mud chips , etc. The clasts may be stacked in rows, with their edges dipping down and flat surfaces aligned to face the flow see figure. Or their flat surfaces may be parallel to the layer and long axes aligned with flow.
Imbrications are useful for analyzing paleocurrents , or currents found in the geologic past, especially in alluvial deposits. Geopetal structures , also called up-direction indicators, are used to identify which way was up when the sedimentary rock layers were originally formed. This is especially important in places where the rock layers have been deformed, tilted, or overturned.
Well preserved mudcracks , sole marks , and raindrop impressions can be used to determine up direction. Other useful geopetal structures include:. Ripples are formed in the slowest flows of the features listed, with speeds right above sediments laid down in flat laminae.
Next fastest are cross bed A specific layer of rock with identifiable properties. Which of these can indicate a paleocurrent and show the direction water has flowed in the past? Asymmetrical ripple marks show a current flowed in the past and indicates the direction it flowed. When mud dries out, mudcracks can form. These only form in conditions where land can be covered by water, then uncovered and dried.
The ultimate goal of many stratigraphy studies is to understand the original depositional environment. Knowing where and how a particular sedimentary rock was formed can help geologists paint a picture of past environments—such as a mountain glacier , gentle floodplain , dry desert, or deep-sea ocean floor. The study of depositional environments is a complex endeavor; the table shows a simplified version of what to look for in the rock record.
Marine depositional environments are completely and constantly submerged in seawater. Their depositional characteristics are largely dependent on the depth of water with two notable exceptions, submarine fans and turbidites. Abyssal sedimentary rocks form on the abyssal plain. The plain encompasses relatively flat ocean floor with some minor topographical features, called abyssal hills. These small seafloor mounts range m to 20 km in diameter, and are possibly created by extension.
Most abyssal plains do not experience significant fluid movement, so sedimentary rock formed there are very fine grained. There are three categories of abyssal sediment. Calcareous oozes consist of calcite -rich plankton shells that have fallen to the ocean floor. An example of this type of sediment is chalk. Siliceous oozes are also made of plankton debris, but these organisms build their shells using silica or hydrated silica. In some cases such as with diatomaceous earth, sediment is deposited below the calcite compensation depth , a depth where calcite solubility increases.
Any calcite -based shells are dissolved , leaving only silica-based shells. Chert is another common rock formed from these types of sediment. These two types of abyssal sediment are also classified as biochemical in origin. The third sediment type is pelagic clay. Very fine-grained clay particles, typically brown or red, descend through the water column very slowly. Pelagic clay deposition occurs in areas of remote open ocean, where there is little plankton accumulation.
Two notable exceptions to the fine-grained nature of abyssal sediment are submarine fan and turbidite deposits. Submarine fans occur offshore at the base of large river systems. They are initiated during times of low sea level, as strong river currents carve submarine canyons into the continental shelf.
When sea levels rise, sediment accumulates on the shelf typically forming large, fan-shaped floodplains called deltas. Periodically, the sediment is disturbed creating dense slurries that flush down the underwater canyons in large gravity-induced events called turbidites.
The submarine fan is formed by a network of turbidites that deposit their sediment loads as the slope decreases, much like what happens above-water at alluvial fans and deltas. This sudden flushing transports coarser sediment to the ocean floor where they are otherwise uncommon. Turbidites are also the typical origin of graded Bouma sequences. Continental slope deposits are not common in the rock record.
The most notable type of continental slope deposits are contourites. Contourites form on the slope between the continental shelf and deep ocean floor. Deep-water ocean currents deposit sediment into smooth drifts of various architectures, sometimes interwoven with turbidites. The lower shoreface lies below the normal depth of wave agitation, so the sediment is not subject to daily winnowing and deposition. These sediment layers are typically finely laminated, and may contain hummocky cross-stratification.
Lower shoreface bed A specific layer of rock with identifiable properties. The upper shoreface contains sediments within the zone of normal wave action, but still submerged below the beach environment.
These sediments usually consist of very well sorted, fine sand. The main sedimentary structure is planar bedding consistent with the lower part of the upper flow regime , but it can also contain cross bedding created by longshore currents. Transitional environments, more often called shoreline or coastline environments , are zones of complex interactions caused by ocean water hitting land. The sediment preservation potential is very high in these environments because deposition often occurs on the continental shelf and underwater.
Shoreline environments are an important source of hydrocarbon deposits petroleum , natural gas. The study of shoreline depositional environments is called sequence stratigraphy. Sequence stratigraphy examines depositional changes and 3D architectures associated with rising and falling sea levels, which is the main force at work in shoreline deposits. These sea-level fluctuations come from the daily tides, as well as climate changes and plate tectonics. A steady rise in sea level relative to the shoreline is called transgression.
Regression is the opposite, a relative drop in sea level. Some common components of shoreline environments are littoral zones, tidal flats , reef A topographic high found away from the beach in deeper water, but still on the continental shelf. The battering force of ocean waves also erodes seaside cliff s. The action of erosion can create an array of coastal landscape features. For example, erosion can bore holes that form cave s. When water breaks through the back of the cave, it can create an arch.
The continual pounding of waves can cause the top of the arch to fall, leaving nothing but rock columns called sea stack s.
The seven remaining sea stacks of Twelve Apostles Marine National Park, in Victoria, Australia, are among the most dramatic and well-known of these features of coastal erosion. Wind is a powerful agent of erosion. Aeolian wind-driven processes constantly transport dust, sand, and ash from one place to another.
Wind can sometimes blow sand into towering dune s. Some sand dune s in the Badain Jaran section of the Gobi Desert in China, for example, reach more than meters 1, feet high.
In dry areas, windblown sand can blast against a rock with tremendous force, slowly wearing away the soft rock.
Wind can also erode material until little remains at all. Ventifact s are rocks that have been sculpted by wind erosion. The enormous chalk formations in the White Desert of Egypt are ventifacts carved by thousands of years of wind roaring through the flat landscape.
Ice, usually in the form of glaciers, can erode the earth and create dramatic landforms. In frigid areas and on some mountaintops, glaciers move slowly downhill and across the land.
As they move, they transport everything in their path, from tiny grains of sand to huge boulders. Rocks carried by glaciers scrape against the ground below, eroding both the ground and the rocks. In this way, glaciers grind up rocks and scrape away the soil. Moving glaciers gouge out basin s and form steep-sided mountain valleys. Eroded sediment called moraine is often visible on and around glaciers. These glacial periods are known as ice age s. Ice Age glaciers carved much of the modern northern North American and European landscape.
Ice Age glaciers scoured the ground to form what are now the Finger Lakes in the U. They carved fjord s, deep inlets along the coast of Scandinavia.
The snout of a glacier eroded Cape Cod Bay, Massachusetts, and formed the recognizable fishhook shape of Cape Cod itself. Today, in places such as Greenland and Antarctica, glaciers continue to erode the earth. Ice sheet s there can be more than a mile thick, making it difficult for scientists to measure the speed and patterns of erosion.
However, ice sheets do erode remarkably quickly—as much as half a centimeter. Thermal erosion describes the erosion of permafrost along a river or coastline. Warm temperature s can cause ice-rich permafrost to break off coastlines in huge chunks, often carrying valuable topsoil and vegetation with them. Mass wasting describes the downward movement of rocks, soil, and vegetation.
Mass wasting incidents include landslides, rockslides, and avalanche s. Mass wasting can erode and transport millions of tons of earth, reshaping hills and mountains and, often, devastating communities in its path.
Some of the natural factors impacting erosion in a landscape include climate, topography, vegetation, and tectonic activity. Climate is perhaps the most influential force impacting the effect of erosion on a landscape. Climate includes precipitation and wind. Climate also includes seasonal variability, which influences the likelihood of weathered sediments being transported during a weather event such as a snowmelt, breeze, or hurricane.
Topography , the shape of surface features of an area, can contribute to how erosion impacts that area. The earthen floodplains of river valleys are much more prone to erosion than rocky flood channels, which may take centuries to erode. Soft rock like chalk will erode more quickly than hard rocks like granite. Vegetation can slow the impact of erosion. Plant roots adhere to soil and rock particles, preventing their transport during rainfall or wind events.
Trees, shrub s, and other plants can even limit the impact of mass wasting events such as landslides and other natural hazards such as hurricanes. Deserts, which generally lack thick vegetation, are often the most eroded landscapes on the planet. Finally, tectonic activity shapes the landscape itself, and thus influences the way erosion impacts an area. Tectonic uplift , for example, causes one part of the landscape to rise higher than others.
In a span of about 5 million years, tectonic uplift caused the Colorado River to cut deeper and deeper into the Colorado Plateau, land in what is now the U. When the water freezes it expands and the cracks are opened a little wider. Over time pieces of rock can split off a rock face and big boulders are broken into smaller rocks and gravel. Chemical weathering decomposes or decays rocks and minerals.
An example of chemical weathering is water dissolving limestone. When ice melts or wind and water slow down they can't carry as much sediment. Such moving water is among the most powerful of nature's landscape-altering tools. Weathering and erosion slowly chisel, polish, and buff Earth's rock into ever evolving works of art—and then wash the remains into the sea.
The processes are definitively independent, but not exclusive. Weathering is the mechanical and chemical hammer that breaks down and sculpts the rocks.
Erosion transports the fragments away. Working together they create and reveal marvels of nature from tumbling boulders high in the mountains to sandstone arches in the parched desert to polished cliffs braced against violent seas. Water is nature's most versatile tool. For example, take rain on a frigid day. The water pools in cracks and crevices. Then, at night, the temperature drops and the water expands as it turns to ice, splitting the rock like a sledgehammer to a wedge.
The next day, under the beating sun, the ice melts and trickles the cracked fragments away. Repeated swings in temperature can also weaken and eventually fragment rock, which expands when hot and shrinks when cold. Such pulsing slowly turns stones in the arid desert to sand. Likewise, constant cycles from wet to dry will crumble clay. Bits of sand are picked up and carried off by the wind, which can then blast the sides of nearby rocks, buffing and polishing them smooth.
On the seashore, the action of waves chips away at cliffs and rakes the fragments back and forth into fine sand.
0コメント