Chapter 7- Sedimentary Rocks.

Sorting:

Indicates the degree to which the clasts in a rock are all the same size or include a variety of sizes. Well-sorted sediment consists entirely of sediment of the same size, whereas poorly sorted sediment contains a mixture of more than one grain size. -If a sedimentary rock contains larger clasts surrounded by much smaller clasts (cobbles surrounded by sand), then the mass of smaller grains constitutes the "matrix" of the rock.

Conglomerate:

Later, a storm causes the fragments (clasts) to slide downslope into a river. In the water, clasts bang into each other and into the riverbed, a process that shatters them into still smaller pieces and breaks off their sharp edges. Angular clasts gradually become rounded clasts. When the river water slows, pebbles & cobbles stop moving and form a mound or bar of gravel. Burial & lithification of these rounded clasts produces conglomerate.

~Sedimentary Structures~

Layering of sedimentary rocks, for surface features on layers formed during deposition, and for the arrangement of grains within layers.

~Diagenesis

Lithification is an aspect of a broader phenomenon called diagenesis.

-Chert (biochemical)

Made from cryptocrystalline quartz, meaning quartz grains that are to small to be seen without the extreme magnification of an electron microscope. -The chert beneath the Golden Gate Bridge formed from the shells of silica-secreting plankton that accumulated on the sea floor. Gradually, after burial, the shells dissolved, forming a silica-rich gel. Chert then precipitated from this gel.

Next...

Meanwhile, silt and clay may accumulate in the flat areas bordering streams, regions called floodplains that become inundated only during floods. And some silt and mud settles in a wedge, called a delta, at the mouth of the river, or in lagoons or mudflats along the shore. The silt, when lithified, becomes siltstone, and the mud, when lithified, becomes shale OR mudstone.

Marine delta deposits:

Mouth of the river, where it empties into the sea. -Here, the river builds a delta of sediment out into the sea. River water stops flowing when it enters the sea, so sediment settles out. Large deltas are more complex here; include swamps, channels, floodplains, and submarine slopes. Sea-level changes may cause the position of the environments to change with time, so deposits of ocean-margin delta produce a great variety of sedimentary rock types.

3) Turbidity Currents & Graded Beds. Turbidity Current?

Moving submarine suspension of sediment. -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= a layer of sediment in which grain size varies from coarse at the bottom to fine at the top. -refer to a deposit from a turbidity current as a turbidite

Coastal beach sands:

Oceanic currents transport sand along the coastline. The sand washes back & forth in the surf, so it becomes well sorted and well rounded, and bc of the back and forth movement of ocean water over the sand, the sand surface may become rippled. well-sorted, medium-grained sandstone, perhaps with ripple marks, may be looking at remnants of a beach environment.

Where do sedimentary rocks occur?

Only in the upper part of the crust, and form a "cover" that buries the underlying "basement" of igneous and/or metamorphic rock.

2) Oil Shale:

Organic material, in the form of chemicals derived from fats and proteins that made up the flesh of plankton or algae, can mix with mud and be incorporated in shale. The organic material, which tends to color shale black, may gradually transform into oil consisting of chain-like organic molecules. -A fine-grained clastic rock containing a large amount of the organic precursor of oil is called oil shale.

Next...Sandstone

Over time, feldspar grains in sand continue to weather into clay so that gradually, during successive events that wash the sediment downstream, the sand loses feldspar and ends up being composed almost entirely of durable quartz grains. Some of the sand may make it to the sea, where wavs carry it to beaches, and some may end up in desert dunes. This sediment, when buried & lithified becomes quartz sandstone.

Deposition=

Process by which sediment settles out of the transporting medium. Sediment settles out of wind or moving water when these fluids slow, because as the velocity decreases, the fluid no longer has the ability to carry sediment. -Sediment is deposited by ice when the ice melts.

Clast Composition:

Refers to the makeup of clasts in sedimentary rock. -Larger clasts (Pebbles or larger) typically consist of rock fragments meaning the clasts themselves are an aggregate of many mineral grains. -Small clasts (sand or smaller) typically consist of individual mineral grains. Lithic clasts: Chips of fine-grained rock may be mixed in with sand grains. -Some sedimentary rocks contain only clasts of one composition, but others contain several different kinds of clasts.

Lake environments:

Relatively quiet water can't move coast sediment; settles out at the stream's outlet. -Only fine clay makes it out into the center of the lake, where it settles to form mud on the lake bed. Lake sediments typically consist of finely laminated shale. Delta= A wedge of sediment that accumulates where moving water enters standing water. Gilbert showed deltas contain 3 components- topset beds composed of gravel forest beds of gravel and sand silty bottomset beds

River environments:

Rivers transport gravel, sand, silt, and mud. -The coarser sediments tumble along the bed in the river's channel and collect in cross-bedded rippled layers, while the finer sediments drift along, suspended in the water. This fine sediment settles out along the banks of the river, or on the floodplain; on the floodplain, mud layers dry out between floods, leading to the formation of mud cracks. -River sediments lithify to form sandstone, siltstone, & shale. -Geologists commonly refer to river deposits as fluvial sediments

What is a sedimentary rock?

Rock that forms at or near the surface of the Earth in one of several ways: by the cementing together of loose clasts (fragments or grains) that had been produced by physical or chemical weathering of pre-existing rock; by the growth of shell masses or the cementing together of shells & shell fragments; by the accumulation and subsequent alteration of organic matter from living organisms; or by the precipitation of minerals from water solutions.

~Transgression and Regression:

Sea-level changes, relative to the land surface, control the succession of sediments that we see in a sedimentary basin.

Clast Size:

Size refers to the diameter of fragments or grains making up a rock. Names used for size, ir order from coarsest to finest are: -Boulder -Cobble -Pebble -Sand -Silt -Clay "Gravel"= accumulation of pebbles + cobbles "Mud"= wet clay Clay: extremely small clasts- typically consist of clay minerals, but specs of quartz may be included.

-Limestone (biochemical)

Solid shells of calcium carbonate, occurs either as the minerals calcite or aragonite. When the organisms die, the shells remain and may accumulate. Rocks formed dominantly from this material are the biochemical version of Limestone. -Limestone type of carbonate rock.

Breccia:

Some large blocks of rock tumble off a cliff & slam into other blocks already at the bottom. The impact shatters the blocks, producing a pile of angular fragments with sharp edges. If these fragments were to be cemented together, the resulting rock would be breccia.

Sand-dune environments:

Strong winds can move dust and sand; the dust gets carried away, and the resulting well-sorted sand can accumulate in dunes. Thus, thick layers of well-sorted sandstone, in which we see large cross beds, are relicts of desert sand-dune environments.

Erosion=

The combination of processes that separate rock or regolith (surface debris) from its substrate. Erosion involves abrasion, plucking, scouring, and dissolution, and is caused by moving air, water, or ice.

Sediment maturity:

The degree to which a sediment has evolved from being just a crushed-up version of the original rock to a sediment that has lost its easily weathered components and has become well sorted and rounded. Immature sandstone= angular clasts, durable & easily weathered minerals, and is poorly sorted. Mature sandstone= contains only well-sorted grains of resistant minerals

Organic sedimentary rock has provided:

The fuel of modern industry & transportation, for the organic chemicals within burn to produce energy.

Alluvial-fan environments:

The mountain front, where the fast-moving stream empties onto a plain. -In arid regions, where there is not enough water for the stream to flow continuously, the stream deposits load of sediment near the mountain front, producing a wedge-shaped apron of gravel and sand called an alluvial fan. The sand here still contains feldspar grains, for they have not yet weathered into clay. Alluvial-fan sediments become arkose & conglomerate.

3) ORGANIC Sedimentary Rocks:

The organic compounds (cellulose, fat, carbohydrate) of once living organisms gets eaten by other organism/decays at Earth's surface. In some places, such as oxygen poor places, the debris settles long with other sediment and eventually gets buried. -At the elevated T & P that exist at depth below the surface, organic matter undergoes chemical reactions that transform it into organic sedimentary rock, distinctive from other sedimentary rock in that it contains abundant organic chemicals.

Subsidence?

The process by which the surface of the lithosphere sinks, providing space in which sediment collects. Sedimentary basin: the sediment-filled depression.

1) Terrestrial (Non marine) Sedimentary Environments:

The sediments of terrestrial deposits do NOT accumulate under seawater-rather, they settle on dry land, or under and adjacent to freshwater 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 which have this hue are called redbeds.

Lithification=

The transformation of loose sediment into solid rock. 2 steps: 1) Compaction: When the sediment has been buried, pressure caused by the weight of overlying material squeezes out water & air that had been trapped between clasts and clasts press together tightly... compacted sediment may then be bound int place to make coherent sedimentary rock by the process of 2) Cementation: Cement consists of minerals (commonly quartz or calcite) that precept from groundwater and fill the spaces between clasts. Cement acts like glue & holds grains together.

2 & 3) Coastal and Marine Sedimentary Environments

The type of sediment deposited at a location depends on the environment, water depth, and whether or not clastic grains are available.

~Sedimentary Basins~

Thick accumulations of sediment form only in special regions where the surface of the Earth's lithosphere sinks, providing spice in which sediment collects.

Travertine (chemical limestone)

Travertine is a rock composed of crystalline calcium carbonate formed by chemical precipitation from groundwater that has seeped out at the ground surface either in hot-or cold water springs, or on the walls of caves.

Mountain Stream Environments:

Turbulent streams rush downslope in steep-sided valleys. This fast-moving water has the power to carry large clasts; during floods, boulders & cobbles can tumble down the stream bed. When the water slows, the largest clasts settle out to from gravel and boulder beds, while the stream carries finer sediments like sand and mud farther downstream.

Chert (replacement).

Unlike the deep-sea (biochemical) chert, the chert collected by the Onondaga formed when cryptocrystalline quartz gradually replaced calcite crystals within a body of limestone long after the limestone was deposited. -comes in many colors, depending on the impurities it contains -Petrified wood is chert that forms when silica-rich sediment, such as ash from a volcanic eruption, buries a forest. Dissolved silica precipitates at crypotocrystallize quartz within wood, gradually replacing the woods' cellulose. -Some chert, known as agate, precipitates in concentric rings inside hollows in a rock.

2) BIOCHEMICAL Sedimentary Rocks:

When organisms die, the solid material in their shells survives and settles to the floor of lakes, seas, or rivers. This material, when lithified, comprises biochemical sedimentary rock.

Transgression?

When relative sea level rises, the shoreline migrates inland. -During this process, terrestrial sediments are buried by coastal sediments, and coastal sediments are buried by deeper-water sediment. Thus as transgression occurs, an extensive layer of beach and sand eventually forms.

Regression?

When sea level falls, the coast migrates seaward. 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. -A succession of strata deposited during a cycle of transgression and regression is called a "depositional sequence."

What causes this precipitation?

When the groundwater degasses, meaning that some of the carbon dioxide that had been dissolved in the groundwater bubbles out of solution, for removal of carbon dioxide decreases the ability of the water to hold dissolved carbonate. -precipitation also occurs when water evaporates, thereby increasing the concentration of carbonate. -Various kinds of microbes live in the environments in which travertine accumulates, so biologic activity may contribute to the precipitation process. -terraces of travertine grew at Mammoth Hot Springs in Yellowstone park. in cave settings, travertine builds up beautiful and complex growth forms called speleothems.

Such salt precipitation occurs:

Wherever saturated saltwater develops- along desert lakes with no outlet and along margins of restricted seas. -for thick deposits of salt to form, large volumes of water must evaporate. This may happen when plate tectonic moments temporarily cut off arms of the sea or during continental rifting, when seawater first begins to spill into the rift valley.

*Though no single characteristic serves a compete basis for classifying clastic rocks:

grain size is the most important one.

Glacial Environments:

high in the mountains; glaciers-rivers or sheets of ice- develop & slowly flow. Because ice is a solid, it can move sediment of any size. -At the end of a glacier, where the ice finally melts away, it drops its sedimentary load and makes a pile of "glacial till". Till is unsorted & unstratified- it contains clasts ranging from clay size to boulder size all mix together- and thus becomes a type of diamiction.

~Rock Names for Clastic Rocks~

look at chart (188)

Character of cement:

not all clastic rocks have the same kind of cement. -some are predominantly quartz, where some is entirely calcite.

~Classifying Clastic Sedimentary Rocks~

-Clast size -Clast composition -Angularity & Sphericity -Sorting -Character of cement

Others:

-Diamictite: forms either from debris flow both on land & underwater or in glacial settings where ice deposits clasts of all sizes. -Wacke: typically forms from the deposits of submarine avalanches. grayish color.

What are the four major classes of sedimentary rocks? -based on their mode of origin

1) Clastic sedimentary rock: Consists of cemented-together class, solid fragments, & grains broken off of pre-existing rocks. 2) Biochemical sedimentary rock: Consists of shells. 3) Organic sedimentary rock: Consists of carbon-rich relicts of plants or other organisms 4) Chemical sedimentary rock: Made up of minerals that precipitated directly from water solutions. -70 to 85% of all the sedimentary rocks on Earth are siliceous or argillaceous clastic rocks, whereas 15 to 25% are carbonate biochemical or chemical rocks.

2 types of Organic sedimentary rock:

1) Coal- Black, combustible rock consisting of over 50 to 90% carbon. The remainder consists of O, N, H, S, Si, etc. -typically, the carbon in coal occurs in large, complex organic molecules made of many rings *DOES NOT occur in Calcium Carbonate. Coal forms when plant remains have been buried deeply enough and long enough for the material to become compacted and to lose significant amounts of volatiles (hydrogen, water, CO2, and ammonia); as the volatiles seep away, a concentration of carbon remains.

2 types of bedforms? ~the growth of both produces cross bedding, a special type of lamination within beds.

1) Ripple Marks: Relatively small, elongated ridges that form on a bed surface at right angles to the direction of the current flow. -If the current always flows in the same direction, the ripple marks are asymmetric, with a steeper slope on the downstream side. -Along the shore, where water flows back & forth due to wave action, ripples tend to be symmetric. -can find ripples on modern beaches and preserved on bedding planes of ancient rocks

What are the 3 basic categories of depositional environment?

1- Terrestrial 2- Coastal 3- Marine

The loose grains of sediment transform into clastic sedimentary rock by the following 5 steps:

1- Weathering 2- Erosion 3- Transportation 4- Deposition 5- Lithification

Geologic map?

A map that portrays the distribution of stratigraphic formations.

4) Bed-Surface Markings.

A number of features appear on the surface of a bed as a consequence of events that happen during deposition or soon after, while the sediment layer remains soft. They include:

Stratigraphic formation? or formation.

A sequence of strata that is distinctive enough to be traced as a package across a fairly large region. -formations are often named after the locality where they were first found & studied.

Diagenesis?

All the physical, chemical, & biological processes that transform sediment into sedimentary rock & that alter characteristics of sedimentary rock after the rock has formed. -can cause chemical reactions in the rock that produce new minerals and can also cause cement to dissolve or precipitate. *The transition between diagenesis and metamorphism in sedimentary rocks is gradational & occurs between temperatures of 150 degrees C and 300 degrees C.

Deep-marine environments:

Along the transition between coastal regions and the deep ocean, turbidity currents deposit graded beds. -In the deep-ocean realm, only fine clay & plankton provide a source for sediment. -The clay eventually settles out onto the deep-sea floor, forming deposits of finely laminated mudstones and plankton shells settle to form chalk or chert. So, deposits of mudstone, chalk, or bedded chert indicate a deep-marine origin.

Angularity and Sphericity:

Angularity indicates the degree to which clasts have smooth or angular corners & edges. Sphericity refers to the degree to which a cat is equidimensional, or resembles a sphere.

Dolostone.

Another carbonate rock, dolostone, differs from limestone in that it contains the mineral dolomite. -most dolostone forms by a chemical reactions between solid calcite & magnesium-bearing groundwater -This change takes place beneath lagoons along a shore soon after the limestone formed, or a long time later, after the limestone has been buried deeply.

Scour Marks:

As currents flow over a sediment surface, they may erode small troughs, called scour marks, parallel to the current flow. These indentations can be buried and preserved.

~Introduction~

At first, drilling brings up soft mud and loose sand, silt, pebbles, and shell fragments. As the hole goes deeper, the material coming up holds together in soft but coherent clumps. -finds that the composition of fragments changes with depth.

Evaporites:

Because salt deposits form as a consequence of evaporation, geologists refer to them as evaporites. -the specific type of salt minerals comprising an evaporite depends on the amount of evaporation when 80% of water evaporates: gypsum forms. when 90% of water evaporates: halite precipitates.

1) Bedding & Stratification

Bed: A single layer of sediment or sedimentary rock with a recognizable top and bottom. Boundary between 2 beds= bedding plane Several beds together= strata & the overall arrangement of sediment into a sequence of beds is bedding, or stratification. -beds appear as bands or stripes across a cliff face -Typically, a contrast in rock type distinguishes one bed from adjacent beds.

2) Ripple Marks, Dunes, & Cross Bedding: Consequences of Deposition in a Current

Bedforms: sedimentary structures that develop at the interface between the sediment and the fluid. Bedforms that develop at a given location reflect factors such as the velocity of the flow, and the size of the clasts.

Evaporites: The products of saltwater evaporation.

Bonneville Salt Flats of Utah= The salt flats formed by 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.

4) CHEMICAL Sedimentary Rocks:

Consists of rock formed primarily by the precipitation of minerals directly out of water solutions. -typically have a crystalline texture, partly formed during their original precipitation and partly when, at a later time, new crystals grow at the expense of old ones through a process called recrystallization.

~How do we recognize Depositional Environments~

Depositional environment: the conditions in which sediment was deposited. -beach, glacial, & river environments.

Weathering=

Detritus forms by disintegration of bedrock into separate grains due to physical & chemical weathering.

3) Intracontinental basins:

Develop in the interiors of continents, initially because of subsidence over a rift. -Illinois/Michigan basin (7 km of sediment has accumulated) -Most of this sediment is fluvial, deltaic, or shallow marine. -At times, extensive swamps formed along the shoreline in these basins. The plant matter of these swamps was buried to form coal.

2) Dunes

Dunes look like ripples, only they are larger. Ex: Dunes on the bed of a stream may be tens of cm high, & wind-formed dunes formed in desserts may be tens to over 100 meters high.

1) CLASTIC sedimentary rocks: What is sandstone?

Example of detrital, or clastic sedimentary rock. -It consists of detritus (loose clasts) that have been stuck together to form a solid mass. -The grains can consist of individual minerals (grains of quartz/flakes of clay) or of fragments of rock (pebbles of granite).

Limestone comes in a variety of textures because the material that forms it accumulates in a variety of ways.

For example, limestone can originate from reef builders (coral) that grew in place, from shell debris that was broken up and transported, or from carbonate mud that settled like snow out of water.

2) Passive-Margin basins:

Form along the edges of continents that are not plate boundaries. -Form because thermal subsidence of stretched lithosphere continues long after rifting ceases & sea-floor spreading begins. -they fill with sediment carried to the sea by rivers & with carbonate rocks formed in coastal reefs. -can reach thickness of 15 to 20 km deep

1) Rift Basins:

Form in continental rifts, regions where the lithosphere has been stretched. -In early stages, surface of the earth subsides simply bc crust becomes thinner as it stretches. As the rift grows, slip on faults drops blocks of crust down, producing low areas bordered by narrow mountain ridges. Alluvial-fan deposits form along the base of the mountains, & salt flats or lakes develop in the low areas between the mountains. Thermal subsidence: Sinking due to cooling of the lithosphere.

4) Foreland basins:

Form on the continent side of a mountain belt because the forces produced during convergence or collision push large slices of rock up faults & 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 here

Types of limestone?

Fossiliferous Limesone: consists of visible fossil shells or shell fragments Micrite: Consists of very fine carbonate mud Chalk: Consists of plankton shells.

Fossils:

Fossils are relicts of past life. Some fossils are shell imprints or footprints on a bedding surface.

Categories of Basins in the Context of the Plate Tectonics Theory:

Geologists distinguish amount different kinds of sedimentary basins on the basis of the region of a lithosphere plate in which the basin formed as defined in the context of plate tectonics theory:

Why is this information helpful?

Geologists may be able to determine the nature of the basin in which ancient sedimentary deposits accumulated by looking at the character & thickness of the deposits.

Transportation=

Gravity, wind, water, or ice carry sediment. The ability of a medium to carry sediment depends on its viscosity & velocity. Solid Ice: Can transport sediment of any size, regardless of how slowly the ice moves. Very fast moving, turbulent water can transport coarse fragments (cobbles & boulders) Moderately fast moving water can carry only sand and gravel Slowly moving gravel carries silt & mud Strong winds can move sand & dust, but gentle breezes carry only dust.

Mud Cracks:

If a mud layer dries up after deposition, it cracks into roughly hexagonal plates that typically curl up at their edges. We refer to the openings between the plates as mud cracks. Later, these fill with sediment & can be preserved.

Arkose:

If the gravel stays put for a long time, it undergoes chemical weathering. As a consequence, cobbles & pebbles break apart into individual mineral grains, eventually producing a mixture of quartz, feldspar, and clay. Clay is so fine that it can be carried far downstream, leaving sand containing a mixture of quartz and some feldspar grains- this sediment, if buried and lithified, become arkose.

Cross beds?

If you examine a vertical slice cut into a ripple or dune, you will find distinct internal laminations that are inclined at an angel to the boundary of the main sedimentary layer. -internal curving surfaces within the layer constitute the cross bedding

Shallow-marine clastic deposits:

In deeper water, where wave energy does not stir the sea floor, finer sediment can accumulate. water here is only a few meters to a few tens of meters deep, so shallow environment. Clastic sediments that accumulate in this environment tend to be fine-grained, well-sorted, well-rounded silt, and they are inhabited by a great variety of organisms such as mollusks & worms. -if you see beds of siltstone & mudstone containing marine fossils, you may be looking at the shallow-marine clastic deposits.

Shallow-water carbonate environments:

In shallow-marine settings in places where relatively little clastic sediment (sand and mud) enters the water, warm, clear, nutrient-rich water hosts an abundance of organisms. -Their shells make up most of the sediment that accumulates -The nature of carbonate sediment depends on the water depth. Beaches= collect sand composed of shell fragments Lagoons= carbonate mud accumulates Reefs= coral + coral debris. *shallow-water carbonate environments transform into various kinds of limestone.

What is bioturbation?

In some environments, burrowing organisms disrupt the layering. Worms, clams, and other creatures churn sediment and may leave behind burrows; called bioturbation.