1. Sedimentary Rocks— The Archives of Earth History
Chapter 6
Sedimentary Rocks— The Archives of Earth History

2. History from Sedimentary Rocks
How do we know whether sedimentary rocks were deposited on
continents—river floodplains or desert sand dunes?
at the water's edge?
in the sea?
Sedimentary rocks
preserve evidence of surface depositional processes
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
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 (made through life processes)
all preserve evidence of the physical, chemical and biological processes that formed them

5. Investigating Sedimentary Rocks
Observation and data gathering
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)

6. 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

7. 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

8. 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

9. 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 and therefore only accumulate under low-energy conditions such as in lakes and lagoons

10. 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

11. 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
For example
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

12. 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

13. Rounding and Sorting A deposit of well rounded and well sorted gravel
Angular, poorly sorted gravel

14. Sedimentary Structures
Sedimentary structures are visible features that formed at the time of deposition or shortly thereafter
Represent 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

15. 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

16. Graded Bedding
Some beds show an upward gradual decrease in grain size, known as graded bedding
Graded bedding is common in turbidity current deposits
form when sediment-water mixtures flow along the seafloor
As they slow, the largest particles settle out then smaller ones

17. 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
useful for relative dating of deformed sedimentary rocks

18. Cross-Bedding Tabular cross-bedding forms by deposition on sand waves
Typically produce where streams enter large bodies of water (AKA Deltas)
Tabular cross-bedding in the Upper Cretaceous

19. Cross-Bedding Trough cross-bedding formed by migrating dunes
Trough cross-beds in the Pliocene Six Mile Creek Formation, Montana

20. 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
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

21. 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

22. Wave-Formed Ripples
As the waves wash back and forth, symmetrical ripples form
The photo shows wave-formed ripple marks in shallow seawater

23. 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,
lakeshore
river flood plain
or where mud is exposed at low tide along a seashore

24. 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

25. Biogenic Sedimentary Structures
Biogenic sedimentary structures include
tracks
burrows
trails
Also called trace fossils
Extensive burrowing by organisms is called bioturbation
may alter sediments so thoroughly that other structures are disrupted or destroyed

26. Bioturbation
U-shaped burrows
Vertical burrows

27. 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?

28. 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

29. 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
Nevertheless, it is useful in conjunction
with other features

30. Blanket or Sheet Geometry
Some of the most extensive sedimentary rocks in the geologic record result from marine transgressions and regressions
These 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 widthThus, they are said to have
blanket or sheet geometry

31. Elongate or Shoestring Geometry
Some sand deposits have an elongate or shoestring geometry
especially those deposited in
stream channels
or barrier islands

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

33. Fossils—The Biological Content of Sedimentary Rocks
Fossils are the remains or traces of prehistoric organisms
can be used in stratigraphy for relative dating and correlation
constituents of rocks, sometimes making up the entire rock
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

34. Fossils Are Constituents of Sedimentary Rocks
coquina is made entirely of shell fragments

35. Fossils in Environmental Analyses
Did the organisms in question live where they were buried?
Were 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

36. Environmental Analyses
What kind of habitat did the organisms originally occupy?
Studies of a fossil’s 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

37. 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

38. 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

39. 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

40. Depositional Environments
Continental environments
Transitional environments
Marine environments

41. 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

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

43. Braided Stream
The deposits of braided streams are mostly gravel and cross-bedded sand with subordinate mud

44. Braided Stream Deposits
Braided stream deposits consist of:
conglomerate
cross-bedded sandstone
mudstone is rare or absent

45. Fluvial Systems The other type of system is a meandering stream
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

46. Meandering Stream Meandering stream deposits
mostly fine-grained floodplain sediments with subordinate sand bodies

47. Meandering Stream Deposits
In meandering stream deposits,
mudstone deposited in a floodplain is common
sandstones are point bar deposits
channel conglomerate is minor

48. 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

49. Associations in Desert Basin
A desert basin showing the association
alluvial fan,
sand dune,
playa lake deposits
In the photo,
the light colored area in the distance is a playa lake deposit in Utah

50. Dune Cross-Beds
Large-scale cross-beds in a Permian-aged wind-blown dune deposit in Arizona

51. Alluvial Fans and Playa Lakes
Alluvial fans form best along the margins of desert basins where stream 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

52. 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

53. Moraines and Till Origin of glacial drift
Moraines and poorly sorted till

54. Glacial Varves
Glacial lake deposits show alternating dark and light laminations
Each dark-light couplet is a varve,
represents one year’s accumulation of sediment
light layers accumulate in summer
Dark in winter
Dropstones liberated from icebergs may also be present

55. 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

56. Transitional Environments

57. 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

58. 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

59. Stream/River-Dominated Deltas
Stream/river-dominated deltas have long distributary channels extending far seaward
Mississippi River delta

60. Wave-Dominated Deltas
Wave-dominated deltas such as the Nile Delta of Egyptalso have distributary channelsbut their seaward margin is modified by wave action

61. Tide-Dominated Deltas
such as the Ganges-Brahmaputra delta
have tidal sand bodies along the direction of tidal flow

62. 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

63. Barrier Island Complex
Subenvironments of a barrier island complex

64. 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

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

66. 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

67. Marine Environments

68. 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

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

70. 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

71. 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

72. 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

73. 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

74. 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
the grains are all calcite and are formed in the environment of deposition,
not transported there

75. Limestone Environments
Some limestone form in lakes, but most limestone 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

76. Carbonate Shelf The carbonate shelf is attached to a continent
Examples occur in southern Florida and the Persian Gulf

77. 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

78. 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

79. 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

80. Evaporites
Large evaporite depositslie 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

81. 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

82. 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

83. 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?

84. 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

85. Navajo Sandstone Checkerboard Mesa, Zion National Park, Utah
Vertical fractures intersect cross beds of desert dunes making the checker-board pattern

86. Paleogeography Paleogeography deals with Earth’s geography of the past
Using interpretations of depositional environment such as the ones just discussed we can attempt to reconstruct what Earth’s 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

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

88. 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

89. Paleogeography
Later, vast lakes, river floodplains, and alluvial fans covered much of this area
and the sea had withdrawn from the continent