Download Continental Environments

Chapter 5
The 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.
ch 5 class 3 - GY112 LECTURE SPRING
2
• Platforms2015
- 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.
f05_01_pg79
Environments Where
Deposition Occurs
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.
Environments Where Deposition Occurs
• 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).
Environments Where Deposition Occurs
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.
Transitional Depositional Environments
• 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
Transitional Depositional Environments
• 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
f05_09_pg85
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
Continental 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.
Continental Environments
• 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.
Continental Environments
• 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.
Continental Environments
• 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.
f05_07a_pg84
Continental Environments
• Eolian environments
– Wind is the agent of sediment transport and
deposition.
– Dominated by sand and silt.
– Common in many desert regions.
f05_08_pg84
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.
Sorting
Sorting 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 wavewashed 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).
f05_14_pg90
f05_15_pg90
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.
f05_16_pg91
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.
Sedimentary Structures
• Types of Sedimentary Structures:
– Stratification
– Graded bedding
– Cross-bedding or cross-stratification
– Ripple marks
– Mud cracks
– Scour marks
Sedimentary Structures
• 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.
Sedimentary Structures
• 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."
Sedimentary Structures
• 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.
f05_18a_pg92
f05_18b_pg92
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.
f05_21a_pg93
f05_21b_pg93
f05_22_pg94
Sedimentary Structures
• 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).
f05_17a_pg91
Sedimentary Structures
• 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.
Sedimentary Structures
• 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.
f05_24_pg95
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 Sediments
Depositional 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
f05_28_pg98
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.
f05_29_pg98
f05_30a_pg99
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 coarsergrained facies overlying finer-grained facies (coarsening-upward).
This is sometimes called an offlap sequence.
f05_32_pg100
f05_33_pg101
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
f05_38_pg105
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. Disconformity
2. Nonconformity
3. Angular Unconformity
f05_40_pg106
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