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

2. Sedimentary Rocks
formed at or near the surface at relatively low temperatures.
from sediments which include boulders, cobbles, gravels, sands, silts, and clay particles.
OR 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: continent, marine, transitional
environments where sediment is deposited such as beach, desert, stream, lake, marine

3. 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)

4. Sedimentary rocks Sedimentary rocks may be Detrital
chemical, including biochemical
Each rock preserves evidence of how it was formed

5. Investigating Sedimentary Rocks
Observation and data gathering
carefully examine
textures
composition
fossils (if present)
Thickness of rock bed
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. Grain Size Detrital grain size High-energy processes
energy conditions
transport and deposition
High-energy processes
streams and waves
transport gravel
Rock type: Conglomerate
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

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

8. Bedding Sedimentary rocks generally have bedding or stratification
Individual layers less than 1 cm thick are laminations
common in mudrocks
Sedimentary Structures
Beds are thicker than 1 cm
common in rocks with coarser grains

9. 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

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

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

12. 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

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

14. 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

15. 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

16. 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

17. Bioturbation
U-shaped burrows
Vertical burrows

18. 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?

19. No Single Structure Is Unique to any one environment
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

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

21. Depositional Environments
Continental environments
Transitional environments
Marine environments

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

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

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

25. 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

26. 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

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

28. 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

29. Glacial Environments Till is poorly sorted, nonstratified drift
Glacial deposits and 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
Indicates that glaciers were involved

30. Moraines and Till
Origin of glacial drift

31. Glacial Dropstone in 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

32. Moraines are made of poorly sorted till

33. Transitional Environments

34. 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

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

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

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

40. Barrier Island Complex
Subenvironments of a barrier island complex

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

42. Marine Environments
Marine environments

43. 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

44. 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

45. 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

46. 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

48. 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

49. 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

50. 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

51. 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

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

53. 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

54. 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

55. 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

56. 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

57. 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

58. 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

59. 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

60. 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

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

62. 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

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

64. 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

65. 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