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Title: -two types: clastic and chemical:

1
Chapter 6
Sedimentary RocksThe Archives of Earth History

-two types clastic and chemical
a. clastic sandstone, shale, etc
b. chemical limestone, dolomite, evaporites
-distinguish by grain size, sorting, rounding,
sedimentary structures
-sedimentary rocks contain fossils- telling
relative age of rocks
-occur in many environments terrestrial,
transitional, marine
Fluvial (streams) braided, meandering
Deltas 3 types bird foot, tidal, wave dominated
Deep marine oozes, clays, turbidites
Paleogeography employ all these tools to
determine what environment
was like when these sediments were deposited

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
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)

4
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
Derived from weathering of existing Igneous,
Metamorphic or Sedimentary rocks
2 types of Sedimentary rocks detrital and
chemical

5
1. Composition of Detrital Rocks

Detrital particles eroded from existing 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

6
2. Composition of Chemical Sedimentary Rocks

Chemical precipitate from seawater or in swamps
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

7
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

8
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

9
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

10
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

11
Rounding and Sorting

A deposit
of well rounded
and well sorted gravel

Angular, poorly sorted gravel

12
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

13
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

14
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

15
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 scale differences
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

16
Cross-Bedding

Tabular cross-bedding forms by deposition on sand
waves

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

A few cms
17
Cross-Bedding

Trough cross-bedding formed by migrating dunes

10s to 100s cm

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

18
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

19
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

20
Wave-Formed Ripples

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

21
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

22
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

23
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

24
Bioturbation

U-shaped burrows

Vertical burrows

25
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?

26
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

27
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

28
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

29
Elongate or Shoestring Geometry

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

30
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

31
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

32
Fossils Are Constituents of Sedimentary Rocks

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

33
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

34
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

35
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/terrestrial
Transitional
Marine
each of which has several specific environments

36
Depositional Environments

Continental environments

Transitional environments
Marine environments
37
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

38
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

39
Braided Stream

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

40
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

41
Meandering Stream

Meandering stream deposits

are mostly fine-grained floodplain
sediments with subordinate sand bodies

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
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

45
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

46
Moraines and Till

Origin of glacial drift

Moraines and poorly sorted till

47
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

48
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

49
Transitional Environments
Transitional environments
50
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

51
Stream/River-Dominated Deltas

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

52
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

53
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

54
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

55
Barrier Island Complex

Subenvironments of a barrier island complex

56
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

57
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

58
Marine Environments
Marine environments
59
Sea Floor Topography

60
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
61
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

62
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

63
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

64
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

65
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

66
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

67
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

68
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

69
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

70
Navajo Sandstone
Checkerboard Mesa, Zion National Park, Utah

Vertical fractures
intersect cross beds of desert dunes
making the checker-board pattern
100s m thick

71
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

72
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

73
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

74
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

75
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

76
Summary