Sedimentary Rocks

1
Chapter 6
Sedimentary RocksThe Archives of Earth History
2
History from Sedimentary Rocks

• How do we know whether sedimentary rocks were
  deposited on
• continentsriver floodplains or desert sand
  dunes?
• at the water's edge?
• in the sea?
• Sedimentary rocks
• preserve evidence of surface depositional
  processes
• also, many contain fossils
• These things give clues to the depositional
  environment
• Depositional environments are specific areas
• or environments where sediment is deposited

3
Beach Environment

• Sand deposition
• on a beach along the Pacific coast
• of the United States
• Many ancient sandstones
• possess features
• that indicate they were
• also deposited on beaches

4
Sedimentary rocks

• Sedimentary rocks may be
• detrital
• or chemical, including biochemical
• and all preserve evidence
• of the physical, chemical and biological
  processes
• that formed them
• Some sedimentary rocks are or contain resources
• phosphorous
• liquid petroleum
• natural gas

5
Phosphorous

• For instance, phosphorous
• from phosphorous-rich sedimentary rocks
• is used in
• metallurgy
• preserved foods
• ceramics
• matches
• chemical fertilizers
• animal-feed supplements

6
Investigating Sedimentary Rocks

• Observation and data gathering
• visit rock exposures (outcrops)
• carefully examine
• textures
• composition
• fossils (if present)
• thickness
• relationships to other rocks
• Preliminary interpretations in the field
• For example
• red rocks may have been deposited on land
• whereas greenish rocks are more typical of marine
  deposits
• (caution exceptions are numerous)

7
Investigating Sedimentary Rocks

• More careful study of the rocks
• microscopic examination
• chemical analyses
• fossil identification
• interpretation of vertical and lateral facies
  relationships
• compare with present-day sediments
• Make environmental interpretation

8
Composition of Detrital Rocks

• Very common minerals in detrital rocks
• quartz, feldspars, and clay minerals
• Only calcite is very common in limestones
• Detrital rock composition tells
• about source rocks,
• not transport and deposition
• Quartz sand may have been deposited
• in a river system
• on a beach or
• in sand dunes

9
Composition of Chemical Sedimentary Rocks

• Composition of chemical sedimentary rocks
• is more useful in revealing environmental
  information
• Limestone is deposited in warm, shallow seas
• although a small amount also originates in lakes
• Evaporites such as rock salt and rock gypsum
• indicate arid environments
• where evaporation rates were high
• Coal originates in swamps and bogs on land

10
Grain Size

• Detrital grain size gives some indication
• of the energy conditions
• during transport and deposition
• High-energy processes
• such as swift-flowing streams and waves
• are needed to transport gravel
• Conglomerate must have been deposited
• in areas where these processes prevail
• Sand transport also requires vigorous currents
• Silt and clay are transported
• by weak currents and accumulate
• only under low-energy conditions
• as in lakes and lagoons

11
Sorting and Rounding

• Sorting and rounding are two textural features
• of detrital sedimentary rocks
• that aid in determining depositional processes
• Sorting refers to the variation
• in size of particles
• making up sediment or sedimentary rocks
• It results from processes
• that selectively transport and deposit
• sediments of particular sizes

12
Sorting

• If the size range is not very great,
• the sediment or rock is well sorted
• If they have a wide range of sizes,
• they are poorly sorted
• Wind has a limited ability to transport sediment
• so dune sand tends to be well sorted
• Glaciers can carry any sized particles,
• because of their transport power,
• so glacier deposits are poorly sorted

13
Rounding

• Rounding is the degree to which
• detrital particles have their sharp corners and
  edges
• warn away by abrasion
• Gravel in transport is rounded very quickly
• as the particles collide with one another
• Sand becomes rounded
• with considerably more transport

14
Rounding and Sorting

• A deposit
• of well rounded
• and well sorted gravel

• Angular, poorly sorted gravel

15
Sedimentary Structures

• Sedimentary structures are
• features visible at the scale of an outcrop
• that formed at the time of deposition or shortly
  thereafter
• and are manifestations of the physical and
  biological processes
• that operated in depositional environments
• Structures
• seen in present-day environments
• or produced in experiments
• help provide information
• about depositional environments of rocks
• with similar structures

16
Bedding

• Sedimentary rocks generally have bedding or
  stratification

• Individual layers less than 1 cm thick are
  laminations
• common in mudrocks

• Beds are thicker than 1 cm
• common in rocks with coarser grains

17
Graded Bedding

• Some beds show an upward gradual decrease
• in grain size, known as graded bedding

• Graded bedding is common in turbidity current
  deposits
• which form when sediment-water mixtures flow
  along the seafloor

• As they slow,
• the largest particles settle out
• then smaller ones

18
Cross-Bedding

• Cross-bedding forms when layers come to rest
• at an angle to the surface
• upon which they accumulate
• as on the downwind side of a sand dune
• Cross-beds result from transport
• by either water or wind
• The beds are inclined or dip downward
• in the direction of the prevailing current
• They indicate ancient current directions,
• or paleocurrents
• They are useful for relative dating
• of deformed sedimentary rocks

19
Cross-Bedding

• Tabular cross-bedding forms by deposition on sand
  waves

• Tabular cross-bedding in the Upper Cretaceous Two
  Medicine Formation in Montana

20
Cross-Bedding

• Trough cross-bedding formed by migrating dunes

• Trough cross-beds in the Pliocene Six Mile Creek
  Formation, Montana

21
Ripple Marks

• Small-scale alternating ridges and troughs
• known as ripple marks are common
• on bedding planes, especially in sandstone
• Current ripple marks
• form in response to water or wind currents
• flowing in one direction
• and have asymmetric profiles allowing geologists
• to determine paleocurrent directions
• Wave-formed ripple marks
• result from the to-and-fro motion of waves
• tend to be symmetrical
• Useful for relative dating of deformed
  sedimentary rocks

22
Current Ripple Marks

• Ripples with an asymmetrical shape
• In the close-up of one ripple,
• the internal structure
• shows small-scale cross-bedding
• The photo shows current ripples
• that formed in a small stream channel
• with flow from right to left

23
Wave-Formed Ripples

• As the waves wash back and forth,
• symmetrical ripples form
• The photo shows wave-formed ripple marks
• in shallow seawater

24
Mud Cracks

• When clay-rich sediments dry, they shrink
• and crack into polygonal patterns
• bounded by fractures called mud cracks
• Mud cracks require wetting and drying to form,

• as along a lakeshore
• or a river flood plain
• or where mud is exposed at low tide along a
  seashore

25
Ancient Mud Cracks

• Mud cracks in ancient rocks
• in Glacier National Park, Montana
• Mud cracks typically fill in
• with sediment
• when they are preserved
• as seen here

26
Biogenic Sedimentary Structures

• Biogenic sedimentary structures include
• tracks
• burrows
• trails
• called trace fossils
• Extensive burrowing by organisms
• is called bioturbation
• It may alter sediments so thoroughly
• that other structures are disrupted or destroyed

27
Bioturbation

• U-shaped burrows

• Vertical burrows

28
Bioturbation

• Vertical, dark-colored areas in this rock are
  sediment-filled burrows
• Could you use burrows such as these to relatively
  date layers in deformed sedimentary rocks?

29
No Single Structure Is Unique

• Sedimentary structures are important
• in environmental analyses
• but no single structure is unique to a specific
  environment
• Example
• Current ripples are found
• in stream channels
• in tidal channels
• on the sea floor
• Environmental determinations
• are usually successful with
• associations of a groups of sedimentary
  structures
• taken along with other sedimentary rock properties

30
Geometry of Sedimentary Rocks

• The three-dimensional shape or geometry
• of a sedimentary rock body
• may be helpful in environmental analyses
• but it must be used with caution
• because the same geometry may be found
• in more than one environment
• can be modified by sediment compaction
• during lithification
• and by erosion and deformation
• Nevertheless, it is useful in conjunction
• with other features

31
Blanket or Sheet Geometry

• Some of the most extensive sedimentary rocks
• in the geologic record result from
• marine transgressions and regressions
• The rocks commonly cover
• hundreds or thousands of square kilometers
• but are perhaps only
• a few tens to hundreds of meters thick
• Their thickness is small compared
• to their length and width
• Thus, they are said to have
• blanket or sheet geometry

32
Elongate or Shoestring Geometry

• Some sand deposits have an elongate or shoestring
  geometry
• especially those deposited in
• stream channels
• or barrier islands

33
Other Geometries

• Delta deposits tend to be lens shaped
• when viewed in cross profile or long profile
• but lobate when observed from above
• Buried reefs are irregular
• but many are long and narrow
• or rather circular

34
FossilsThe Biological Content of Sedimentary
Rocks

• Fossils
• are the remains or traces of prehistoric
  organisms
• can be used in stratigraphy for relative dating
  and correlation
• are constituents of rocks, sometimes making up
  the entire rock
• and provide evidence of depositional environments
• Many limestones are composed
• in part or entirely of shells or shell fragments
• Much of the sediment on the deep-seafloor
• consists of microscopic shells of organisms

35
Fossils Are Constituents of Sedimentary Rocks

• This variety of limestone,
• known as coquina,
• is made entirely of shell fragments

36
Fossils in Environmental Analyses

• Did the organisms in question live where they
  were buried?
• Or where their remains or fossils transported
  there?
• Example
• Fossil dinosaurs usually indicate deposition
• in a land environment such as a river floodplain
• But if their bones are found in rocks with
• clams, corals and sea lilies,
• we assume a carcass was washed out to sea

37
Environmental Analyses

• What kind of habitat did the organisms originally
  occupy?
• Studies of a fossils structure
• and its living relatives, if any,
• help environmental analysis
• For example clams with heavy, thick shells
• typically live in shallow turbulent water
• whereas those with thin shells
• are found in low-energy environments
• Most corals live in warm, clear,
• shallow marine environments where
• symbiotic bacteria can carry out photosynthesis

38
Microfossils

• Microfossils are particularly useful
• because many individuals can be recovered
• from small rock samples
• In oil-drilling operations, small rock chips
• called well cuttings are brought to the surface
• These cuttings rarely
• contain complete fossils of large organisms,
• but they might have thousands of microfossils
• that aid in relative dating and environmental
  analyses

39
Trace Fossils In Place

• Trace fossils, too, may be characteristic of
  particular environments
• Trace fossils, of course, are not transported
  from their original place of origin

40
Depositional Environments

• A depositional environment
• is anywhere sediment accumulates
• especially a particular area
• where a distinctive kind of deposit originates
• from physical, chemical, and biological processes
• Three broad areas of deposition include
• continental
• transitional
• marine
• each of which has several specific environments

41
Depositional Environments

• Continental environments

Transitional environments
Marine environments
42
Continental Environments

• Deposition on continents (on land) might take
  place in
• fluvial systems rivers and streams
• deserts
• areas covered by and adjacent to glaciers
• Deposits in each of these environments
• possess combinations of features
• that allow us to differentiate among them

43
Fluvial

• Fluvial refers to river and stream activity
• and to their deposits
• Fluvial deposits accumulate in either of two
  types of systems
• One is a braided stream system
• with multiple broad, shallow channels
• in which mostly sheets of gravel
• and cross-bedded sand are deposited
• mud is nearly absent

44
Braided Stream

• The deposits of braided streams are mostly
• gravel and cross-bedded sand with subordinate mud

45
Braided Stream Deposits

• Braided stream deposits consist of
• conglomerate
• cross-bedded sandstone
• but mudstone is rare or absent

46
Fluvial Systems

• The other type of system is a meandering stream
• with winding channels
• mostly fine-grained sediments on floodplains
• cross-bedded sand bodies with shoestring geometry
• point-bar deposits consisting of a sand body
• overlying an erosion surface
• that developed on the convex side of a meander
  loop

47
Meandering Stream

• Meandering stream deposits

• are mostly fine-grained floodplain
• sediments with subordinate sand bodies

48
Meandering Stream Deposits

• In meandering stream deposits,
• mudstone deposited in a floodplain is common
• sandstones are point bar deposits
• channel conglomerate is minor

49
Desert Environments

• Desert environments contain an association of
  features found in
• sand dune deposits,
• alluvial fan deposits,
• and playa lake deposits
• Windblown dunes are typically composed
• of well-sorted, well-rounded sand
• with cross-beds meters to tens of meters high
• land-dwelling plants and animals make up any
  fossils

50
Associations in Desert Basin

• A desert basin showing the association
• of alluvial fan,
• sand dune,
• and playa lake deposits
• In the photo,
• the light colored area in the distance
• is a playa lake deposit in Utah

51
Dune Cross-Beds

• Large-scale cross-beds
• in a Permian-aged
• wind-blown dune deposit in Arizona

52
Alluvial Fans and Playa Lakes

• Alluvial fans form best along the margins of
  desert basins
• where streams and debris flows
• discharge from mountains onto a valley floor
• They form a triangular (fan-shaped) deposit
• of sand and gravel
• The more central part of a desert basin
• might be the site of a temporary lake, a playa
  lake,
• in which laminated mud and evaporites accumulate

53
Glacial Environments

• All sediments deposited in
• glacial environments are collectively called
  drift
• Till is poorly sorted, nonstratified drift
• deposited directly by glacial ice
• mostly in ridge-like deposits called moraines
• Outwash is sand and gravel deposited
• by braided streams issuing from melting glaciers
• The association of these deposits along with
• scratched (striated) and polished bedrock
• is generally sufficient to conclude
• that glaciers were involved

54
Moraines and Till

• Origin of glacial drift

• Moraines and poorly sorted till

55
Glacial Varves

• Glacial lake deposits show
• alternating dark and light laminations
• Each dark-light couplet is a varve,
• representing one years accumulation of sediment
• light layers accumulate in summer
• dark in winter

• Dropstones
• liberated from icebergs
• may also be present
• Varves with a dropstone

56
Transitional Environments

• Transitional environments include those
• with both marine and continental processes
• Example
• Deposition where a river or stream (fluvial
  system)
• enters the sea
• yields a body of sediment called a delta
• with deposits modified by marine processes,
  especially waves and tides
• Transitional environments include
• deltas
• beaches
• barrier islands and lagoons
• tidal flats

57
Transitional Environments
Transitional environments
58
Simple Deltas

• The simplest deltas are those in lakes and
  consist of

• topset beds
• foreset beds
• bottomset beds

• As the delta builds outward it progrades

• and forms a vertical sequence of rocks
• that becomes coarser-grained from the bottom to
  top
• The bottomset beds may contain marine (or lake)
  fossils,
• whereas the topset beds contain land fossils

59
Marine Deltas

• Marine deltas rarely conform precisely
• to this simple threefold division because
• they are strongly influenced
• by one or more modifying processes
• When fluvial processes prevail
• a stream/river-dominated delta results
• Strong wave action
• produces a wave dominated delta
• Tidal influences
• result in tide-dominated deltas

60
Stream/River-Dominated Deltas

• Stream/river-dominated deltas
• have long distributary channels
• extending far seaward
• Mississippi River delta

61
Wave-Dominated Deltas

• Wave-dominated deltas
• such as the Nile Delta of Egypt
• also have distributary channels
• but their seaward margin
• is modified by wave action

62
Tide-Dominated Deltas

• Tide-Dominated Deltas,
• such as the Ganges-Brahmaputra delta

• of Ban-gladesh
• have tidal sand bodies
• along the direction of tidal flow

63
Barrier Islands

• On broad continental margins
• with abundant sand, long barrier islands lie
  offshore
• separated from the mainland by a lagoon
• Barrier islands are common along the Gulf
• and Atlantic Coasts of the United States
• Many ancient deposits formed in this environment
• Subenvironments of a barrier island complex
• beach sand grading offshore into finer deposits
• dune sands contain shell fragments
• not found in desert dunes
• fine-grained lagoon deposits
• with marine fossils and bioturbation

64
Barrier Island Complex

• Subenvironments of a barrier island complex

65
Tidal Flats

• Tidal flats are present
• where part of the shoreline is periodically
  covered
• by seawater at high tide and then exposed at low
  tide
• Many tidal flats build or prograde seaward
• and yield a sequence of rocks grading upward
• from sand to mud
• One of their most distinctive features
• is sets of cross-beds that dip in opposite
  directions

66
Tidal Flats

• Tidal-flat deposits showing a prograding
  shoreline
• Notice the distinctive cross-beds
• that dip in opposite directions
• How could this happen?

67
Marine Environments

• Marine environments include
• continental shelf
• continental slope
• continental rise
• deep-seafloor
• Much of the detritus eroded from continents
• is eventually deposited in marine environments
• but sediments derived from chemical
• and organic activity are found here as well, such
  as
• limestone
• evaporites
• both deposited in shallow marine environments

68
Marine Environments
Marine environments
69
Detrital Marine Environments

• The gently sloping area adjacent to a continent
• is a continental shelf
• It consists of a high-energy inner part that is
• periodically stirred up by waves and tidal
  currents
• Its sediment is mostly sand,
• shaped into large cross-bedded dunes
• Bedding planes are commonly marked
• by wave-formed ripple marks
• Marine fossils and bioturbation are typical

70
Slope and Rise

• The low-energy part of the shelf
• has mostly mud with marine fossils,
• and interfingers with inner-shelf sand
• Much sediment derived from the continents
• crosses the continental shelf
• and is funneled into deeper water
• through submarine canyons
• It eventually comes to rest
• on the continental slope and continental rise
• as a series of overlapping submarine fans

71
Slope and Rise

• Once sediment passes the outer margin
• of the self, the shelf-slope break,
• turbidity currents transport it
• So sand with graded bedding is common
• Also common is mud that settled from seawater

72
Detrital Marine Environments

• Shelf, slope and rise environments
• The main avenues of sediment transport
• across the shelf are submarine canyons

Turbidity currents carry sediment to the
submarine fans
Sand with graded bedding and mud settled from
seawater
73
Deep Sea

• Beyond the continental rise, the seafloor is
• nearly completely covered by fine-grained
  deposits
• no sand and gravel
• or no sediment at all
• near mid-ocean ridges
• The main sources of sediment are
• windblown dust from continents or oceanic islands
• volcanic ash
• shells of microorganisms dwelling
• in surface waters of the ocean

74
Deep Sea

• Types of sediment are
• pelagic clay,
• which covers most of the deeper parts
• of the seafloor
• calcareous (CaCO3) and siliceous (SiO2) oozes
• made up of microscopic shells

75
Carbonate Environments

• Carbonate rocks are
• limestone, which is composed of calcite
• dolostone, which is composed of dolomite
• most dolostone is altered limestone
• Limestone is similar to detrital rock in some
  ways
• Many limestones are made up of
• gravel-sized grains
• sand-sized grains
• microcrystalline carbonate mud called micrite
• but the grains are all calcite
• and are formed in the environment of deposition,
• not transported there

76
Limestone Environments

• Some limestone form in lakes,
• but most limestone by is deposited
• in warm shallow seas
• on carbonate shelves and
• on carbonate platforms rising from oceanic depths
• Deposition occurs where
• little detrital sediment, especially mud, is
  present
• Carbonate barriers form in high-energy areas and
  may be
• reefs
• banks of skeletal particles
• accumulations of spherical carbonate grains known
  as oolites
• which make up the grains in oolitic limestone

77
Carbonate Shelf

• The carbonate shelf is attached to a continent
• Examples occur in southern Florida and the
  Persian Gulf

78
Carbonate Platform

• Carbonates may be deposited on a platform
• rising from oceanic depths
• This example shows a cross-section
• of the present-day Great Bahama Bank
• in the Atlantic Ocean southeast of Florida

79
Carbonate Subenvironments

• Reef rock tends to be
• structureless
• composed of skeletons of corals, mollusks,
  sponges and other organisms
• Carbonate banks are made up of
• layers with horizontal beds
• cross-beds
• wave-formed ripple marks
• Lagoons tend to have
• micrite
• with marine fossils
• bioturbation

80
Evaporite Environments

• Evaporites consist of
• rock salt
• rock gypsum
• They are found in environments such as
• playa lakes
• saline lakes
• but most of the extensive deposits formed in the
  ocean
• Evaporites are not nearly as common
• as sandstone, mudrocks and limestone,
• but can be abundant locally

81
Evaporites

• Large evaporite deposits
• lie beneath the Mediterranean Seafloor
• more than 2 km thick
• in western Canada, Michigan, Ohio, New York,
• and several Gulf Coast states
• How some of these deposits originated
• is controversial, but geologists agree
• that high evaporation rates of seawater
• caused minerals to precipitate from solution
• Coastal environments in arid regions
• such as the present-day Persian Gulf
• meet the requirements

82
Evaporites

• Evaporites could form
• in an environment similar to this
• if the area were in an arid region,

• with restricted inflow of normal seawater
• into the lagoon
• leading to increased salinity and salt depositions

83
Environmental Interpretations and Historical
Geology

• Present-day gravel deposits
• by a swiftly-flowing stream
• Most transport and deposition takes place when
  the stream is higher

• Nearby gravel deposit probably less than a few
  thousand years old

84
Environmental Interpretations and Historical
Geology

• Conglomerate more than 1 billion years old
• shows similar features

• We infer that it too was deposited
• by a braided stream in a fluvial system
• Why not deposition by glaciers or along a
  seashore?
• Because evidence is lacking for either
• glacial activity or transitional environment

85
Interpretation

• Jurassic-aged Navajo Sandstone
• of the Southwestern United states
• has all the features of wind-blown sand dunes
• the sandstone is mostly well-sorted, well-rounded
  quartz
• measuring 0.2 to 0.5 mm in diameter
• tracks of land-dwelling animals,
• including dinosaurs, are present
• cross-beds up to 30 m high have current ripple
  marks
• like those produced on large dunes by wind today
• cross-beds dip generally southwest
• indicating a northeast prevailing wind

86
Navajo Sandstone
Checkerboard Mesa, Zion National Park, Utah

• Vertical fractures
• intersect cross beds of desert dunes
• making the checker-board pattern

87
Paleogeography

• Paleogeography deals with
• Earths geography of the past
• Using interpretations
• of depositional environment
• such as the ones just discussed
• we can attempt to reconstruct
• what Earths geography was like
• at these locations at various times in the past
• For example,
• the Navajo Sandstone shows that a vast desert
• was present in what is now the southwest
• during the Jurassic Period

88
Paleogeography

• and from Late Precambrian to Middle Cambrian
• the shoreline migrated inland from east and west
• during a marine transgression

89
Paleogeography

• Detailed studies of various rocks
• in several western states
• allow us to determine
• with some accuracy
• how the area appeared
• during the Late Cretaceous
• A broad coastal plain
• sloped gently eastward
• from a mountainous region
• to the sea

90
Paleogeography

• Later, vast lakes,
• river floodplains, alluvial fans
• covered much of this area
• and the sea had withdrawn from the continent
• Interpretations the geologic record
• we examine later
• will be based on similar
• amounts of supporting evidence

91
Summary

• The physical and biological features
• of sedimentary rocks reveal something about
• the depositional processes that form them
• Environmental analysis
• of sedimentary rocks uses
• mainly sedimentary structures and fossils
• but also textures, rock body geometry
• and even composition
• Geologists recognize
• three primary depositional areas
• continental, transitional, and marine
• each with several specific environments

92
Summary

• Fluvial systems might be braided or meandering
• Braided streams deposit mostly sand and gravel,
• whereas deposits of meandering streams are mostly
  mud and subordinate sand bodies with shoestring
  geometry
• An association of alluvial fan, sand dune,
• and playa lake deposits
• is typical of desert depositional environments
• Glacial deposits consist mostly of till
• in moraines and outwash

93
Summary

• The simplest deltas, those in lakes,
• consist of a three-part sequence of rocks
• grading from finest at the base,
• upward to coarser-grained rocks
• Marine deltas dominated by
• fluvial processes, waves, or tides
• are much larger and more complex
• A barrier island system includes beach,
• dune, and lagoon subenvironments,
• each characterized a unique association
• of rocks, sedimentary structures, and fossils

94
Summary

• Inner shelf deposits are mostly sand,
• whereas those of the outer shelf are mostly mud
• both have marine fossils and bioturbation
• Much of the sediment from land
• crosses the shelves and is deposited
• on the continental slope and rise as submarine
  fans
• Either pelagic clay or oozes
• derived from the shells of
• microscopic floating organisms cover
• most of the deep seafloor

95
Summary

• Most limestone originates in shallow,
• warm seas where little detrital mud is present
• Carbonate rocks (just as detrital rocks)
• may possess cross-beds, ripple marks,
• mud cracks, and fossils
• that provide information
• about depositional processes
• Evaporites form in several environments,
• but the most extensive ones were deposited
• in marine environments
• In all cases, though, they formed
• in arid regions with high evaporation rates

96
Summary

• With information from sedimentary rocks,
• as well as other rocks,
• geologists determine the past distribution
• of Earth's surface features