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

2. Sedimentary Rocks
Are formed at or near the surface at relatively low temperatures.
Are formed from sediments which include boulders, cobbles, gravels, sands, silts, and clay particles.
Also included as sediments are particles which are suspended and dissolved in water.
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. How do we know whether sedimentary rocks were deposited on
Depositional environments are specific areas or environments where sediment is deposited
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?

4. Beach Environment Sand deposition
Sand-size particles are deposited on a beach along the Pacific coast of the United States
After many years and layers of deposition, sand is compacted, and eventually cemented to form sandstone.
Many ancient sandstones
possess features that indicate they were also deposited on beaches
Present day despositional environments are used as models to help decode the rocks of the past. (uniformitarianism)

5. 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 a natural resource for
phosphorous
liquid petroleum
natural gas

6. 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)

7. Detrital Rocks Very common minerals in detrital rocks:
quartz, feldspars, and clay minerals
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
Quartz is durable and can withstand water and wind transport

8. 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

9. 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 or caves
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. Sorting and Rounding Sorting and rounding are two textural features
Sorting refers to the variation in size of particles
“well sorted” – particles are similarly sized
“poorly sorted” – particles vary in size within a rock
Well-sorted material implies transport by water or wind.
Poorly sorted material implies transport by ice, or debris flow
Some sediments are not transported; they remain where deposited until lithified

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

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

13. 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
help provide information about depositional environments of rocks with similar structures

14. 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

15. 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

16. 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

17. Cross-Bedding Tabular cross-bedding forms by deposition on sand waves
Tabular cross-bedding in the Upper Cretaceous Two Medicine Formation in Montana

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

19. 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

20. 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

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

22. 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

23. 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

24. 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

25. Bioturbation
U-shaped burrows
Vertical burrows

26. 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?

27. 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

28. Geometries
Some Sediments are extensive “sheets” deposited during marine transgressions or regressions.
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

29. Fossils—The Biological Content of Sedimentary Rocks
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

30. Fossils Are Constituents of Sedimentary Rocks
This variety of limestone,
known as coquina,
is made entirely of shell fragments

31. 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

32. 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

33. 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

34. Depositional Environments
Continental environments
Transitional environments
Marine environments

35. 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

36. 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

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

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

39. 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

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

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

42. 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

43. 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

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

45. 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

46. 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

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

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

49. 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

50. Transitional Environments

51. 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

52. 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

53. Stream/River-Dominated Deltas
have long distributary channels
extending far seaward
Mississippi River delta

54. Wave-Dominated Deltas
such as the Nile Delta of Egypt
also have distributary channels
but their seaward margin
is modified by wave action

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

56. 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

57. Barrier Island Complex
Subenvironments of a barrier island complex

58. 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

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

60. 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

61. Marine Environments
Marine environments

62. 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

63. 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

64. 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

65. 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

66. 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

67. 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

68. 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

69. 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

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

71. 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

72. 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

73. 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

74. 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

75. 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

76. 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

77. 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

78. 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

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

80. Paleogeography Paleogeography deals with Using interpretations
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

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

82. Paleogeography Detailed studies of various rocks A broad coastal plain
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

83. Paleogeography Later, vast lakes, Interpretations the geologic record
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