chapter 5 the sedimentary archives “what sedimentary rocks tell us about the past”

Chapter 5The Sedimentary Archives

“What Sedimentary Rocks Tell Us About The Past”

Chapter 4 Content

• Clues that Sedimentary Rocks provide about Earth’s History.

• Factors that affect the formation of Sedimentary Rocks.

• Where Sedimentary Rocks form.• Small Scale evidence of Earth’s History.• Large Scale evidence of Earth’s History.• Interpreting Earth’s History

Clues That Sedimentary Rocks

Provide About Earth’s History

Clues that Sedimentary Rocks Provide about Earth’s History

• How the rock was formed.• Composition

– Minerals Present and Absent– Combinations of Minerals

• Grain Size• Grain Shape• Grain Sorting• Crystal Growth• Color

Factors That Affect the Formation of

Sedimentary Rocks

Factors That Affect the Formation of Sedimentary Rocks

• Tectonic setting. • Physical, chemical, and biological

processes in the depositional environment. • Method of sediment transport. • Source Rock of the Sedimentary Rock. • Climate (and its effect on weathering). • How the rock was lithified or turned into a

rock (cementation, compaction). • Time.

Tectonic Setting

Tectonic Setting

• Tectonics: The forces controlling deformation or structural behavior of a large area of the Earth's crust over a long period of time.

• An area may be: – Tectonically stable - like the midwestern US. – Subsiding (sinking) - like New Orleans or Mexico

City. – Rising gently - like New England and parts of

Canada after glacier retreat. – Rising actively to produce mountains and

plateaus - like parts of Oregon in the Cascade Mountains .

Tectonic Setting

• Tectonics influences the grain size and thickness of sedimentary deposits.

• Recent uplift of the source area leads to rapid erosion of coarse-grained sediment.

• Subsidence in the depositional basin leads to the accumulation of great thicknesses of sediment.

Principle Tectonic Elements of a Continent

• Craton - Stable interior of a continent, undisturbed by mountain-building events since the Precambrian.

• Shields - Large areas of exposed crystalline rocks. • Platforms - Ancient crystalline rocks are covered by

flat-lying or gently warped sedimentary rocks. • Orogenic belts - Elongated regions bordering the

craton which have been deformed by compression since the Precambrian Mountain belts.

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Environments Where Deposition Occurs


• Environment of Deposition = All of the physical, chemical, biological, and geographic conditions under which sediments are deposited.

• Sedimentary rocks may be: • Extrabasinal in origin - Sediments formed from the

weathering of pre-existing rocks outside the basin, and transported to the environment of deposition.

• Intrabasinal in origin - Sediments form inside the basin; includes chemical precipitates, most carbonate rocks, and coal.

• By comparing modern sedimentary deposits with ancient sedimentary rocks, the depositional conditions can be interpreted.


• There are three broad categories of depositional environments: 1. Marine environments (ocean).

2. Transitional environments (along contact between ocean and land).

3. Continental environments (on land).

Marine Depositional Environments • Continental shelf - The flooded edge of the continent.

– Exposed to waves, tides, and currents. – Covered by sand, silt, and clay. – Larger sedimentary grains are deposited closer to shore. – Coral reefs and carbonate sediments in tropical areas.

• Continental slope - The steeper slope at edge of the continent. – Deeper water. – Rapid sediment transport, muddy turbidity currents.

• Continental rise - At the base of the continental slope. – Water depths of 1400 to 3200 m. – Submarine fans form at mouth of submarine canyons. – Turbidity currents deposit thick accumulations of sediment

• Abyssal plain - Deep ocean floor. – Water depths of 3 to 5 km (2 - 3 miles), or more. – Covered by very fine-grained sediment and shells of microscopic

organisms.

Marine Depositional Environments

Transitional Depositional Environments• Transitional environments are those

environments at or near the transition between the land and the sea.

• Deltas – Fan-shaped accumulations of sediment – Formed where a river flows into a standing body of

water, such as a lake or the sea – Coarser sediment (sand) tends to be deposited near

the mouth of the river; finer sediment is carried seaward and deposited in deeper water.

– The delta builds seaward (or progrades) as sediment is deposited at the river mouth.

Transitional Depositional Environments• Beaches and Barrier Islands

– Shoreline deposits – Exposed to wave energy – Dominated by sand – Marine fauna – A few km or less in width but may be more than 100

km long – Separated from the mainland by a lagoon (or salt

marsh) – Commonly associated with tidal flat deposits

Transitional Depositional Environments

• Lagoons – Bodies of water on the landward side of

barrier islands – Protected from the pounding of the ocean

waves by barrier islands – Contain finer sediment than the beaches

(usually silt and clay) – Lagoons are also present behind reefs, or in

the center of atolls.


• Tidal flats – Nearly flat, low relief areas that border lagoons,

shorelines, and estuaries – Periodically flooded and exposed by tides (usually

twice each day) – May be cut by meandering tidal channels – May be marshy, muddy, sandy or mixed sediment

types (either terrigenous or carbonate) – Laminations and ripples are common – Sediments are intensely burrowed


• Estuaries – Mouth of a river drowned by the sea – Brackish water (mixture of fresh and salt water) – May trap large volumes of sediment – Sand, silt, and clay may be deposited depending on

energy level – Many estuaries formed due to sea level rise as

glaciers melted at end of last Ice Age – Some formed due to tectonic subsidence, allowing

sea water to migrate upstream

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Continental Environments

Continental environments are those environments which are present on the continents. – Rivers or fluvial environments – Alluvial fans – Lakes (or lacustrine environments) – Glacial environments – Eolian environments


• Rivers or fluvial environments – Include river and stream systems – Channel deposits consist of coarse, rounded

gravel, and sand. – Bars are made of sand or gravel. – Levees are made of fine sand or silt. – Floodplains are covered by silt and clay.


• Alluvial fans – Fan-shaped deposits formed at the base of

mountains. – Most common in arid and semi-arid regions where

rainfall is infrequent but torrential, and erosion is rapid.

– Sediment is typically coarse, poorly- sorted gravel and sand.


• Lakes (or lacustrine environments) – May be large or small. – May be shallow or deep. – May be filled with terrigenous (eroded

particles), carbonate, or evaporitic sediments. – Sediments are typically fine grained but may

be coarse near the edges.


• Glacial environments – Sediment is eroded, transported, and

deposited by ice (glaciers). – Glacial deposits called till contain large

volumes of unsorted mixtures of boulders, gravel, sand, and clay.

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• Eolian environments – Wind is the agent of sediment transport and

deposition. – Dominated by sand and silt. – Common in many desert regions.

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Small Scale Evidence of Earth’s History

Small Scale Evidence of Earth’s History

• Color• Texture

– Grain Size– Grain Shape– Grain Sorting– Grain Arrangement

Rock Color

Color of Sedimentary Rocks

• Color of sedimentary rocks provides useful clues to the depositional environment. – Black– Red– Green and Gray

Color of Sedimentary Rocks• Black and dark gray

– Indicates the presence of organic carbon and/or iron.

– Organic carbon in sedimentary requires anoxic environmental conditions (lacking free oxygen): quiet water marine, deep lakes, and estuaries.

– In these environments, iron combines with sulfur to form the mineral pyrite (FeS2), which can also contribute to the black color.

– Black, organic-rich sediments may also form in environments where the accumulation of organic matter exceeds the capacity of the environment to oxidize it.

Color of Sedimentary Rocks• Red, Brown, Purple, or Orange

– indicates the presence of iron oxides. – In well-oxygenated continental sedimentary

environments, the iron in the sediments is oxidized to form hematite or ferric iron oxide (Fe2O3), which colors the sediment red, brown, or purple.

– Typically deposited in continental (or transitional) sedimentary environments such as flood plains, alluvial fans, and deltas.

– Also formed in marine environments (due to oxidation of the iron in the sediment after deposition), or to erosion of red sediment from the land.

Color of Sedimentary Rocks

• Green and gray – also indicates the presence of iron, but in a

reduced (rather than an oxidized) state. – Ferrous iron (Fe+2) generally occurs in

oxygen-deficient environments.

Rock Texture

Rock Texture

• Texture refers to the size, shape, sorting, and arrangement of grains in a sedimentary rock.

• There are three "textural components" to most clastic sedimentary rocks: – Clasts - the larger grains in the rock (gravel, sand,

silt). – Matrix - the fine-grained material surrounding

clasts (often clay). – Cement - the "glue" that holds the rocks together.

• Silica, Calcite, Iron oxide, & Other Minerals

Rock Texture

• The texture of a sedimentary rock can provide clues to the depositional environment. – Fine-grained textures typically indicate

deposition in quiet water. – In general, it takes higher energy (higher

water velocity) to transport larger grains.

Grain Size

• Sedimentary grains are categorized according to size using the Wentworth Scale. – Gravel > 2 mm– Sand 1/16 - 2 mm– Silt 1/256 - 1/16 mm– Clay <1/256 mm

• The grain sizes in a sediment or sedimentary rock can provide clues to help interpret the depositional environment.

• Stronger currents are required to move larger particles than to move smaller particles.

SortingSorting refers to the distribution of grain sizes in a

rock. – If all of the grains are the same size, the rock is "well

sorted."If there is a mixture of grain sizes, the rock is "poorly sorted."

– The range of grain sizes in a sediment or sedimentary rock can provide clues to help interpret the depositional environment.

– Windblown sediments are better sorted than wave-washed sediments.

– Well-sorted sands have higher porosity and permeability than poorly-sorted sands (if they are not tightly cemented).

– Poor sorting is the result of rapid deposition of sediment without sorting by currents.

Grain Shape• Grain shape is described in terms of rounding

of grain edges and sphericity (equal dimensions, or how close it is to a sphere).

• Rounding results from abrasion against other particles and grain impact during transport. – Very well-rounded sand grains suggest that a

sand may have been recycled from older sandstones.

• Shape of clasts is important in naming the coarser-grained sedimentary rocks (those with gravel-sized clasts).

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Orientation of Grains

• A study of grain orientation or arrangement may indicate whether the grains are clustered into zones or mixed up. – This relates to the method of transport and

deposition of the grains.

• Grain orientation may also be used to interpret ancient current or wind directions. – The long axis of the grain becomes oriented

parallel to the flow direction.

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Large Scale Evidence of Earth’s History

Sedimentary Structures

• Sedimentary structures are larger features which form during (or shortly after) deposition of the sediment, but before lithification.

• Some sedimentary structures are created by the water or wind which moves the sediment. Other sedimentary structures form after deposition — such as footprints, worm trails, or mudcracks.

• Sedimentary structures can provide information about the environmental conditions under which the sediment was deposited; some structures form in quiet water under low energy conditions, whereas others form in moving water or high energy conditions.


• Types of Sedimentary Structures:– Stratification – Graded bedding – Cross-bedding or cross-stratification – Ripple marks – Mud cracks – Scour marks


• Stratification (= layering or bedding) is the most obvious feature of sedimentary rocks. – The layers (or beds or strata) are visible because of

differences in the color or texture of adjacent beds.– Strata thicker than 1 cm are commonly referred to as

beds. – Thinner layers are called laminations or laminae. – The upper and lower surfaces of these layers are

called bedding planes.


• Graded bedding results when a sediment-laden current (such as a turbidity current) begins to slow down. – The grain size within a graded bed ranges

from coarser at the bottom to finer at the top. Hence, graded beds may be used as "up indicators."


• Cross-bedding or Cross-stratification is an arrangement of beds or laminations in which one set of layers is inclined relative to the others. – The layering is inclined at an angle to the

horizontal, dipping downward in the downcurrent direction.

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Sedimentary Structures• Ripple marks are undulations of the sediment surface

produced as wind or water moves across sand. – Symmetric ripple marks are produced by waves or

oscillating water. – Asymmetric ripples form in unidirectional currents

(such as in streams or rivers). • Asymmetric ripples have a steep slope on the

downstream side, and a gentle slope on the upstream side.

• Because of this unique geometry, asymmetrical ripples in the rock record may be used to determine ancient current directions or paleocurrent directions.

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• Mud cracks are a polygonal pattern of cracks produced on the surface of mud as it dries. – The mud polygons between the cracks may

be broken up later by water movement, and redeposited as intraclasts (particularly in lime muds).

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• Scour marks are depressions or erosional features formed as a current flows across a bed of sand. – Sediment may be deposited over the scoured

layer, filling the depressions. – When the overlying sediment becomes

consolidated, you can see positive-relief casts on the base of the overlying bed.

• These casts are termed "sole marks," because they appear on the bottom (or sole) of a bed of sediment.


• Determining "up direction"– Sedimentary structures can be used to determine "up

direction."– Sedimentary structures such as graded beds, cross

beds, mudcracks, flute marks, symmetrical (but not asymmetrical) ripples, burrows, and tracks can be used to establish the original orientation of the beds.

– Features which can be used to determine "up direction" are called geopetal structures.

– Fossils can also be used to establish up direction, if they are present in the rock in life position.

Compositional Evidence Provided by Sedimentary

Rocks

Sands and Sandstones• Sandstones are classified on the basis of the composition of their

grains. • Three components are generally considered:

– Quartz grains. – Feldspar grains. – Rock fragment grains.

• The particular minerals present provide information on the amount of weathering and transport experienced by the sand grains.

• Intense weathering and long transport tend to destroy the feldspars and ferromagnesian minerals because they are less stable, and produce a sandstone dominated by quartz. Such sandstones are referred to as compositionally mature.

• Sandstones with abundant feldspars, and ferromagnesian minerals, on the other hand, indicate relatively little weathering and transport. These sandstones are compositionally immature.

Sands and Sandstones

• Major types of sandstone described by Levin: – Quartz sandstone (also called quartz arenite) -

dominated by quartz grains. – Arkose - contain 25% or more feldspar, with quartz. – Graywacke - contains about 30% dark fine-grained

matrix (clay, silt, chlorite, micas) along with quartz, feldspar, and rock fragments.

– Lithic sandstone (or subgraywacke) - dominated by quartz, muscovite, chert, and rock fragments with matrix less than 15%. Feldspars scarce.

Sandstone Environmental Interpretation

• Each type of sandstone implies something about depositional history and environment:

• Quartz sandstone implies a long time of formation. Deposition typically in shallow-water environments. – Common sedimentary structures are ripple marks and cross-bedding.

• Arkose implies a short time of formation(because feldspar typically weathers quickly to clay). Also implies rapid erosion, arid climate, tectonic activity, steep slopes. – Commonly deposited in fault troughs or low areas along granitic

mountains. Often has a pinkish color due to oxidized iron, suggesting continental deposition.

• Graywacke implies a tectonically active source area and rapid erosion. Graded bedding is common. – Associated with volcanic rocks, shales, and cherts of deep water origin.

• Lithic sandstone found in deltaic coastal plains, and may be deposited in nearshore marine environments, swamps, or marshes. – Associated with coal and micaceous shales.

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Carbonate Rocks and Sediments• Carbonate rocks consist of limestone and

dolostone. Limestones are the most abundant carbonate rocks.

• Carbonate rocks are chemical or biochemical in origin.

• The minerals present in carbonate rocks are: • Limestone

– Calcite – Aragonite

• Dolostone – Dolomite

Carbonate Rocks and SedimentsDepositional Conditions• Shallow marine environment, lakes, caves and hot springs. • Direct or indirect result of biologic activity. • May contain shells or the remains of other marine

organisms • May precipitate from seawater as a result of biologic activity• Characteristics of most marine carbonate environments:

– Warm water – Shallow water (less than 200 m deep) – Tropical climate (30 ° N - 30 ° S of equator) – Clear water (low to no terrigenous input) – Sunlight required for photosynthesis by algae

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Carbonate Rocks and Sediments• Origin of carbonate sediments

– Much lime mud forms from the disintegration of calcareous algae.

• When the calcareous algae die, their skeletons break down and disintegrate producing aragonite needle muds.

• These lime muds lithify to form fine-grained limestone.

– Blue-green algae are involved in the formation of oolites (or ooids).

• Oolites form in warm shallow seas with constant wave agitation.

– Precipitation of calcium carbonate from seawater as a result of biologic activity

– Abrasion of shells • Coquina, • Fossiliferous Limestone,

– Accumulation of fecal pellets produced by burrowing organisms.

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Carbonate Rocks and Sediments

• Dolomite– Dolomite is a calcium-magnesium

carbonate mineral (CaMg(CO3)2) that comprises the sedimentary rock dolostone. (Sometimes the rock is also called dolomite.)

– Dolomite is interpreted to form when magnesium that has been concentrated in sea water replaces calcium in calcium carbonate in a previously deposited limestone.

Clays and Shales• The word “Clay" has two definitions.

– It is both a grain size term, and – a term referring to a layered silicate mineral which

behaves plastically when wet and hardens upon drying or firing.

• Shale is a very fine-grained rock composed of clay, mud, and silt. – Shale is fissile; this means that it splits readily into

thin, flat layers. – There are quartz shales, feldspathic shales, chloritic

shales, and micaceous shales, based on the composition of the silt-sized grains.

– The environmental interpretations of these shales are similar to those of the various types of sandstones.

Clays and Shales

• Claystone is a very fine-grained rock composed of tiny (less than 1/256 mm) clay minerals, mica, and quartz grains. – The individual grains are too small to see with the

naked eye or a hand lens, and the rock feels smooth to the touch (not gritty).

– Claystone is not fissile, and breaks irregularly.

• Mud is defined as a mixture of silt and clay. – Rocks with both silt and clay are referred to as

mudstones or mudshales, depending on whether or not they are fissile.

Clays and Shales

• Deposition of clays– Because of its fine grain size, clay tends to

remain suspended in the water column. It will settle out of still, quiet water, given enough time.

– Clays and shales typically indicate low energy environments, sheltered from waves and currents. They are commonly found in lacustrine, lagoon, and deeper water marine deposits.

Classifying Rock Layers

Rock Facies

Rock Facies

• Rock Facies: a distinctive rock unit that forms under certain conditions of sedimentation

• It is used to define a particular process or environment.• Examples:

– Sand and Silt facies of a beach environment, – Shale facies in deeper, quieter water, – Carbonate facies, far from shore in warm shallow

seas. – Coal facies found in a swamp area on a delta,

• When a depositional environment grades laterally into other environments it is referred to as a facies change.

• The different fossil assemblages in a uniform rock unit may be referred to as biofacies.

Facies Changes Indicate Changes in Sea Level

Transgression.• A sea level rise is called a transgression. • A sea level rise will produce a vertical sequence of facies

representing progressively deeper water environments. As a result, a transgressive sequence will have finer-grained facies overlying coarser-grained facies. This is sometimes referred to as an onlap sequence.

Regression.• A sea level drop is called a regression. • A regression will produce a sequence of facies representing

progressively shallower water environments (shallowing-upward sequence). As a result, a regressive sequence will have coarser-grained facies overlying finer-grained facies (coarsening-upward). This is sometimes called an offlap sequence.

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Walther's Law (or Principle)

Walther's Law (or Principle)• Sedimentary environments that started out side-by-side

will end up overlapping one another over time due to sea level change (transgressions and regressions).

• The result is a vertical sequence of beds. The vertical sequence of facies mirrors the original lateral distribution of sedimentary environments.

Facies Changes Indicate Changes in Sea Level

• Fluctuations in sea level are caused by: – Plate Tectonic Changes– Glaciation

Correlating Rock Units

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Correlation

• The branch of geology that deals with the correlation of rock units from one area to another is known as stratigraphy.

• Three main types of correlation: – Lithostratigraphic correlation - Matching up rock units

on the basis of their lithology (composition, texture, color, etc.) and stratigraphic position.

– Biostratigraphic correlation - Matching up rock units on the basis of the fossils they contain.

– Chronostratigraphic correlation - Matching up rock units on the basis of age equivalence, as determined by radioactive dating methods or fossils.

– Geologists can trace beds from one exposure to another. This is called lithostratigraphic correlation.

Unconformities

Unconformities

• Indicates that something has changed• Three types:

1. Disconformity2. Nonconformity3. Angular Unconformity

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Unconformities

• Types of unconformities: – Angular unconformities - An erosional surface which truncates

folded or dipping (tilted) strata. – Nonconformities - An erosional surface which truncates

igneous or metamorphic rocks. – Disconformities - An irregular erosional surface which

truncates flat-lying sedimentary rocks.

• A fourth type of unconformity is the paraconformity, separating two parallel units of sedimentary rock. – There is no obvious evidence of erosion. – A paraconformity is virtually indistinguishable from a sharp

conformable contact. The fossils show that there is a considerable time gap represented by a paraconformity.

Depicting the Past

Depicting the Past• Methods:

– Geologic Columns Columnar sections show the vertical succession of rock units at a given location.

• They are used in correlation and construction of cross-sections. – Stratigraphic cross-sections tie together several geologic columns from

different locations. • The purpose is to show how rock units change in thickness, lithology,

and fossil content across a given area. – Structural cross-sections show the timing of tilting, folding, and faulting

of rock units. – Geologic Maps show the distribution of various layers and types of rocks

in an area. – Paleogeographic Maps are interpretive maps which depict the

geography of an area at some time in the past.– Isopach Maps show the thickness of formations or other units in an area. – Lithofacies Maps show the distribution of lithofacies that existed at a

given time over an area, or show the percentage of some lithologic component (such as clay), or show the ratio of one rock type to another within the unit.

THE END

chapter 5 the sedimentary archives “what sedimentary rocks tell us about the past”

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