Chapter 2
Review of
Minerals and Rocks
Matter and Its Composition

Matter
anything that has mass and occupies space
 exists as solids, liquids, gases, and plasma
 consists of elements and atoms


Element
a chemical substance
 composed of tiny particles called atoms

Atoms

Atoms are the smallest units of matter


that retain the characteristics of the element
Atoms have

a compact nucleus containing
protons – particles with a positive electrical charge
 neutrons – electrically neutral particles


particles outside the nucleus

electrons – negatively charged particles
MODEL OF THE ATOM
Electron
Orbits
Nucleus
Protons – Red.
Positive Charge.
Neutrons – Green.
Neutral Charge.
Electrons – Tan.
Negative Charge.
Atoms

Atomic number
= the number of protons

Atomic mass number
= number of protons + number of neutrons

The number of neutrons in nucleus of an
element may vary
Isotopes

Isotopes
the different forms of an element’s atoms
 with varying numbers of neutrons


Different isotopes of the same element


have different atomic mass numbers
Isotopes are important in radiometric dating
Carbon Isotopes

Three isotopes of carbon (all with 6 protons)

6 neutrons = Carbon 12 (12C)
7 neutrons = Carbon 13 (13C)
 8 neutrons = Carbon 14 (14C)

Electrons and Shells


Electrons lie outside the nucleus in one or more shells
The outermost shells are involved



in chemical bonding
and contain up to 8 electrons
Noble gas configuration of 8 electrons




or 2 for helium
have complete outer shells
and are stable
Other atoms attain


a noble gas configuration
in the process of bonding
Fig. 2-2, p. 19
Bonding and Compounds

Bonding


the process whereby atoms join to other atoms
Compound
a substance resulting from the bonding
 of two or more elements



Oxygen gas (O2) is an element
Ice (H2O) is a compound


made up of hydrogen and oxygen atoms
Most minerals are compounds
Ionic Bonding

One way for atoms to attain the noble gas
configuration


is by transferring electrons, producing ions
Ion
an atom that has gained or lost one or more
electrons
 and thus has a negative or positive charge


Ionic bonding

attraction between two ions of opposite charge
Fig. 2-4a, p. 20
Covalent Bonding

Another way for atoms
to attain the noble gas configuration
 is by sharing electrons


Covalent bonding

results from
sharing electrons
shared electrons
Minerals

Geological definition of a mineral:
naturally occurring
 crystalline solid

crystalline means that minerals
 have atoms arranged in specific 3-dimensional
frameworks

minerals have a narrowly
defined chemical composition
 and characteristic physical
properties such as

density
 hardness
 color...

Minerals—The Building
Blocks of Rocks

A mineral’s composition is shown by a
chemical formula
a shorthand way of indicating how many atoms
of different kinds it contains
 Quartz molecules consist
Quartz: SiO2
of 1 silicon atom and 2
Ratio: 1: 2
oxygen atoms
 Orthoclase molecules
KAlSi3O8
consists of 1 potassium, 1
aluminum, 3 silicon, and 8
1: 1: 3: 8
oxygen atoms

Native Elements




A few minerals consist
of only one element.
They are not
compounds.
They are known as
native elements.
Examples:
Gold: Au
 Diamond: C

Crystalline Solids

By definition, minerals are crystalline solids


with atoms arranged in a specific 3D framework
If given enough room to grow freely,
minerals form perfect crystals with
 planar surfaces, called crystal faces
 sharp corners
 straight edges

Narrowly Defined
Chemical Composition

Some minerals have very specific
compositions


Examples: halite (NaCl), quartz (SiO2)
Other minerals have a range of compositions
because one element can substitute for another
 if the atoms of the two elements have

the same electrical charge
 and are about the same size


Example: olivine
(Mg,Fe)2SiO4
 iron and magnesium substitution in any proportion

Mineral Properties


Mineral properties are controlled by

Chemical composition

Crystalline structure
Mineral properties are particularly useful

for mineral identification and include:
color
 streak
 luster
 crystal form

cleavage
 fracture
 hardness
 specific gravity

How Many Minerals
Are There?




More than 3500 minerals are known
Only about 2 dozen are particularly common
Many others are important resources
Mineral groups:
minerals with the same negatively charged ion or
ion group
 belong to the same mineral group


Most minerals in the crust

belong to the group called silicates
Silicates

Silicates are minerals containing silica


These minerals make up perhaps 95% of
Earth’s crust


Si and O
and account for about 1/3 of all known minerals
The basic building block of silicates

is the silica tetrahedron
which consists of one silicon atom
 surrounded by four oxygen atoms

The silica
-4
(SiO4)
tetrahedron
Types of Silicates

Silica tetrahedra can be
isolated units bonded to other
elements
 arranged in chains (single or
double)
 arranged in sheets
 arranged in complex
3D networks

Types of Silicates

Ferromagnesian silicates


contain iron (Fe), magnesium (Mg), or both
Nonferromagnesian silicates

do not contain iron or magnesium
Ferromagnesian Silicates

Common ferromagnesian silicates include
Nonferromagnesian Silicates
Other Mineral Groups

Carbonates

minerals with carbonate ion (CO3)-2

calcite (CaCO3),


dolomite [CaMg(CO3)2],


constituent of limestone
constituent of dolostone
Other mineral groups are important,
but more as resources
 than as constituents of rocks

Table 2-1, p. 21
Rock-Forming Minerals

Most rocks are solid aggregates


of one or more minerals
Hundreds of minerals occur in rocks,
but only a few are common
 and called rock-forming minerals


Most rock-forming minerals are silicates,


but carbonates, halides, and sulfates are also important
Accessory minerals are present in small amounts

and are ignored in classifying rocks (example: pyrite)
ROCKS
Rock Cycle

The rock cycle is a pictorial representation
of events leading to
 the origin, destruction, change
 and reformation of rocks


Rocks belong to 3 major families
igneous
 sedimentary
 metamorphic


The rock cycle shows
how these rock families are interrelated
 and can be derived from one another

Rock
Cycle
Igneous Rocks

All igneous rocks
cool and crystallize from magma,
 solidify from lava,
 or consolidate from pyroclastic materials


Magma is molten material



below the surface
Lava is molten material on the surface
Pyroclastic materials

are particles such as volcanic ash
Igneous Part of the Rock Cycle
Pyroclastic
material
Lava
Categories of Igneous Rocks

Extrusive or volcanic rocks
formed at the surface
 from lava or pyroclastic materials


Intrusive or plutonic rocks
formed from magma injected into the crust
 or formed in place in the crust
 Plutons are intrusive bodies

Plutons
Igneous Rock Textures

Texture
is the size, shape, and arrangement
 of crystals, grains, and other constituents of a
rock


Igneous rocks have several textures

that relate to cooling rate of magma or lava
Cooling-Rate Textures

phaneritic,

with visible grains


aphanitic,

with grains too small to see without magnification


cooled quickly
porphyritic,

with larger grains (phenocrysts) surrounded by a
finer-grained groundmass


cooled slowly
cooled slowly intrusively, then expelled onto the surface
glassy,

with no grains

cooled too quickly for minerals to grow
Igneous Rock Textures

Other textures reveal further details


of the formation of the rock
Vesicular texture, with holes (vesicles),
indicates the rock formed
 as water vapor and other gases
 became trapped during cooling of lava


Pyroclastic or fragmental texture,
containing fragments,
 formed by consolidation of volcanic ash
 or other pyroclastic material

Igneous Rock Textures
Fine-grained igneous texture
Aphanitic Texture
Course-grained igneous texture
PhaneriticTexture
Porphyritic igneous texture
Porphyritic with aphanitic groundmass
Igneous Rock Textures
Classifying Igneous Rocks

Texture and composition are the criteria


used to classify most igneous rocks
Composition categories are based on mineral
composition
light colored, nonferromagnesian minerals
 intermediate composition
 dark colored, ferromagnesian minerals

Classification of igneous rocks
Classifying Igneous Rocks
Common Igneous Rocks
Basalt
Andesite
Gabbro
Diorite
Common Igneous Rocks
Rhyolite
Granite
Classifying Igneous Rocks with
Special Textures
Sedimentary Rocks

Sedimentary rocks form


by the lithification of sediment
In the rock cycle, sediment originates when:

mechanical and chemical weathering


Transport removes sediment


disintegrate and decompose rocks at the surface
from its source area and carries it elsewhere
Sediments accumulate in deposits,
or as minerals that precipitate from solution
 or that organisms extract from solution.

Sedimentary Part of the
Rock Cycle
Lithification

Lithification means


converting sediment into sedimentary rock
Lithification occurs by

compaction
Pressure exerted by overlying sediments
 reduction of the amount of pore space between particles


cementation
precipitation of minerals within pores
 effectively binds sediment together




calcium carbonate (CaCO3) cement is common
silica (SiO2) cement is common
iron oxide or iron hydroxide (Ex: Fe2O3) cement is less common
Categories of Sedimentary
Rocks

Detrital sedimentary rocks
consist of solid particles
 derived from preexisting rocks (detritus)


Chemical sedimentary rocks
consist of minerals derived from materials in
solution and
 extracted by either

inorganic chemical processes
 or by the activities of organisms


subcategory biochemical sedimentary rocks, in
which

the activities of organisms are important
Detrital Sedimentary Rocks

are composed of fragments or particles



known as clasts = Clastic texture
These rocks are defined primarily by size of
clasts
conglomerate
composed of gravel (>2mm)
 with rounded clasts


sedimentary breccia
also composed of gravel (>2mm)
 but clasts are angular


sandstone

composed of sand
Classification of
sedimentary rocks
Detrital Sedimentary Rocks

Mudrocks consist of particles < 1/16 mm

mudstone
composed of particles less than 1/16 mm particles
 consists of both silt- and clay-size particles


siltstone


claystone


composed of silt-sized particles 1/16-1/256 mm
composed of clay-sized particles <1/256 mm
shale
mudstone or claystone that
 breaks along closely spaced parallel planes (fissile)

Shale with plant fossils
Chemical Sedimentary Rocks

Recall that these rocks result
when inorganic chemical processes
 or organisms extract minerals from solution


This can result in different textures

Crystalline texture


has an interlocking mosaic of mineral crystals
Clastic texture

has an accumulation of broken pieces of shells
Fossiliferous limestone
Chemical Sedimentary Rocks

Limestone – carbonate rock made of calcite precipitated
chemically or (most commonly) by organisms
 Dolostone – carbonate rock made of dolomite usually
altered from limestone

Evaporites formed by
inorganic chemical precipitation during evaporation
 Rock salt and rock gypsum – evaporite made of gypsum
 Chert – compact, hard, fine grained silica, formed by chemical or

biological precipitation
 Coal – made of partially altered, compressed remains of land
plants accumulated in swamps
Common Sedimentary Rocks
Conglomerate
Quartz sandstone
Sedimentary breccia
Shale
Common Sedimentary Rocks
Rock gypsum
Fossiliferous limestone
Rock salt
Chert
Coal
Sedimentary rocks

Features of sedimentary rocks
•
•
•
Strata, or beds (most characteristic)
Bedding planes separate strata
Fossils
•
•
•
•
•
Traces or remains of prehistoric life
Are the most important inclusions
Help determine past environments
Used as time indicators
Used for matching rocks from different places
Metamorphic Rocks

Metamorphic rocks
result from transformation of other rocks
 in the solid state, without melting


Changes from metamorphism include

compositional


textural


new minerals form
minerals become aligned
or both
Metamorphic Part of the
Rock Cycle
Agents of Metamorphism

Heat
Increases the rate of chemical reactions
 Yields different minerals from parent rock


Pressure

Lithostatic pressure
Weight of overlying rocks
 Forms smaller, denser minerals


Differential pressure


exerts force more intensely from one direction
Fluid activity is an important metamorphic agent as
well
Types of Metamorphism

Contact metamorphism
heat and chemical fluids
 from an igneous body
 alter adjacent rocks


Regional metamorphism
most common
 large, elongated areas
 tremendous pressure, elevated temperatures, and
fluid activity
 occurs at convergent and divergent plate
boundaries

Metamorphic Textures

Foliated texture
platy and elongate minerals aligned parallel to one
another
 caused by differential pressure


Nonfoliated texture

mosaic of roughly equidimensional minerals
Formation of Foliation

When rocks are subjected to differential pressure
the minerals typically rearrange in a parallel
fashion
Formation of Foliation

Microscopic
view of a
metamorphic
rock with
foliation showing
the parallel
arrangement of
minerals
Foliated Metamorphic Rocks

Slate


Phyllite


fine-grained (coarser than slate but grains are still
too small to see without magnification)
Schist


very fine-grained, low-grade metamorphism
clearly visible platy and/or elongate minerals
Gneiss

alternating dark and light bands of minerals
Nonfoliated Metamorphic
Rocks

Marble


Quartzite


Green, altered mafic igneous rock
Hornfels


Composed of quartz metamorphosed from quartz
sandstone
Greenstone


Composed of calcite or dolomite metamorphosed from
limestone or dolostone
Clay-rich, results from contact metamorphism
Anthracite

Black, lustrous, hard coal
Common Metamorphic Rocks
Slate
Gneiss
Schist
Marble
Quartzite
Index Minerals



Certain minerals have a limited P-T range.
These “index minerals” record metamorphic
grade.
Index mineral maps.
Define metamorphic zones.
 Grade boundaries called
isograds.

Plate Tectonics
and the Rock Cycle

The atmosphere, hydrosphere and biosphere



Earth’s internal heat


act on earth materials
and cause weathering, erosion, and deposition
aids melting and metamorphism
Plate tectonics recycles Earth materials

heat and pressure at convergent plate boundaries


lead to metamorphism and igneous activity
Some rocks in subducted plate are deformed and incorporated
into an evolving mountain system

that in turn weather and erode to form sediment
Plate Tectonics and the Rock Cycle
Earth Materials and
Historical Geology

Our record of Earth’s history


Sedimentary rocks are especially useful


in deciphering Earth and life history
Igneous rocks reveal much


is preserved in rocks
about the history of plate activity
Metamorphic rocks provide information

about processes deep in the crust
CHAPTER 6
Sedimentary Rocks—
The Archives of Earth
History
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
 and many contain fossils
 These things give clues to the depositional
environment


Depositional environments are specific areas

or environments where sediment is deposited
Sedimentary rocks

Sedimentary rocks




preserve evidence
of the physical, chemical and biological processes
that formed them
Some sedimentary rocks are resources, or contain
resources






phosphorous
liquid petroleum
natural gas
bituminous coal
rock salt
rock gypsum
Investigating Sedimentary
Rocks

Observation and data gathering
by visiting rock exposures (outcrops)
 and carefully examining

textures
 composition
 fossils (if present)
 thickness
 relationships to other rocks


Preliminary interpretations in the field may be
made

For example:
red rocks may have been deposited on land
 whereas greenish rocks are more typical of marine deposits
 (Caution: Exceptions are numerous!)

Investigating Sedimentary
Rocks

More careful study of the rocks
microscopic examination
 chemical analyses
 fossil identification
 interpretation of vertical and lateral facies
relationships
 comparison with present-day sediments


When data have been analyzed, geologists make
an environmental interpretation
Composition of 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

Composition of
Chemical Sedimentary Rocks

Composition of chemical sedimentary rocks


Limestone is deposited in warm, shallow seas


is more useful in revealing environmental
information
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
Grain Size

Detrital grain size gives some indication



High-energy processes




such as swift-flowing streams and waves
are needed to transport gravel
Conglomerate must have been deposited


of the energy conditions
during transport and deposition
in areas where these processes prevail
Sand transport also requires high-energy transport
Silt and clay are transported



by weak currents and accumulate
only under low-energy conditions
as in lakes, lagoons, offshore marine in deeper water
Sorting and Rounding


Texture refers to the size, shape, and arrangement of
clasts
Sorting and rounding are two textural features



Sorting refers to the variation



of detrital sedimentary rocks
that aid in determining depositional processes
in size of particles
making up sediment or sedimentary rocks
It results from processes


that selectively transport and deposit
sediments of particular sizes
Sorting

If the size range is not very great,


If they have a wide range of sizes,


they are poorly sorted
Wind has a limited ability to transport sediment


the sediment or rock is well sorted
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

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
Rounding and Sorting

A deposit
of well rounded
 and well sorted gravel


Versus a deposit
of angular
 poorly sorted
gravel

Sedimentary Structures

Sedimentary structures are
features 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

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
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

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


The beds are inclined or dip downward


by either water or wind
in the direction of the prevailing current
They indicate ancient current directions,

or paleocurrents
Cross-Bedding


Cross-bedding in
sandstone at Natural
Bridges National
Monument, Utah
Individual beds are
deposited at an angle
(left). Example of a crossbedded, coarse fluvial
channel sandstone of the Morrison Formation in
Emery County, Utah. Notice the relatively sharp
contact between the mudstone below (where
hammer is located) and the crossbedded
sandstone above. The mudstone below the
sandstone is a floodplain deposit. (photo by E.L.
Crisp, 1999).
(right). Crossbedding in a coarse fluvial
sandstone of the Morrison Formation,
Emery County, Utah. (photo by
E.L. Crisp, 1999).
Large scale crossbeds in the Middle Jurassic Navajo Sandstone near
Escalante, Utah. These large crossbeds were formed by migrating sand
dunes in a desert environment that covered Utah and bordering states during
a portion of Middle Jurassic time. (photo by E.L. Crisp, May 2002)
Ripple Marks

Small-scale alternating ridges and troughs



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



known as ripple marks are common
on bedding planes, especially in sandstone
result from the to-and-fro motion of waves
and tend to be symmetrical
Useful features for relative dating of deformed
sedimentary rocks
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

Wave-Formed Ripples

As the waves
wash back
and forth,


symmetrical
ripples form
The photo
shows waveformed ripple
marks

in shallow
seawater
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

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

Biogenic Sedimentary
Structures

Biogenic sedimentary structures include
tracks
 burrows
 trails



called trace fossils
Extensive burrowing by organisms
is called bioturbation
 and may alter sediments so thoroughly
 that other structures are disrupted or destroyed

Bioturbation

U-shaped burrows

Vertical burrows
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?
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 groups of sedimentary structures
 taken along with other sedimentary rock properties

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.
Geometry can be modified by sediment compaction
during lithification
and by erosion and deformation
Nevertheless, it is useful in conjunction

with other features
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
Elongate or Shoestring
Geometry

Some sand deposits have an elongate or
shoestring geometry

especially those deposited in
stream channels
 or barrier islands

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

Fossils—The Biological
Content of Sedimentary Rocks

Fossils





Many limestones are composed


are the remains or traces of prehistoric organisms
can be used in stratigraphy for relative dating and correlation
are important constituents of rocks, sometimes making up the
entire rock
and provide evidence for depositional environments
in part or entirely of shells or shell fragments
Much of the sediment on the deep-seafloor

consists of microscopic shells of organisms
Fossils Are Constituents of
Sedimentary Rocks

This variety of
limestone,
known as
coquina,
 is made entirely
of shell
fragments

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
Environmental Analyses


What kind of habitat did the organisms originally
occupy?
Studies of a fossil’s structure



For example: clams with heavy, thick shells




and its living relatives, if any,
help environmental analysis
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
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

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
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


Depositional Environments
Continental environments
Transitional environments
Marine
environments
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

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

Braided Stream

The deposits of braided streams are mostly

gravel and cross-bedded sand with subordinate mud
Platte River, Nebraska, sandy braided stream- Tops of bars are
exposed at relatively low flow. Vegetation stablizes one of the bars.
(From:
http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/Sedstrat4/slide
show_4_7.htm)
Platte River, eastern Nebraska
Ground view of the Platte River, a classic braided stream. Note the river is
wide and shallow, with many sand bars. (From:
http://www.ship.edu/~cjwolt/geology/slides/str-sum.htm)
Braided Stream Deposits

Braided stream
deposits consist of
gravel
 cross-bedded sand
 but mud is rare or
absent

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
geometries
 point-bar deposits consisting of a sand body
 overlying an erosion surface
 that developed on the convex side of a meander
loop

Meandering Stream

Meandering
stream
deposits
are mostly fine-grained floodplain
 sediments with subordinate sand bodies

Side-looking radar (SLAR) image of the
flood plain between the Rio Japurá and Rio
Solimoes (Amazon River basin), taken in
1971/1972.
Río Socopo flowing off eastern slope of Venezuelan Andes.
Both images from:
http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/Sedstrat4/sedlect_4.ht
m
Meandering Stream Deposits

In meandering stream
deposits,
fine-grained floodplain
sediment is common
 with subordinate sand
bodies

These rocks of the Brushy Basin Member of the Morrison Formation in the San
Rafael Swell region of Emery County, Utah were formed by stream deposition of
channel sands and floodplain muds about 150 million years ago during the Late
Jurassic Period. The Morrison Formation is a graveyard of dinosaur bones and
the remains of most of the popular Jurassic dinosaurs were found in the fluvial
deposits of the Morrison Formation.
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

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

Associations in Desert Basin

Huge alluvial fans formed
at the base of the Panamint
Mountains, Death Valley

Sand dunes also are present
in Death Valley
Dune Cross-Beds

Large-scale cross-beds
in a Permian-aged
 wind-blown dune
deposit in Arizona

Glacial Environments

All sediments deposited in


Till is poorly sorted, nonstratified drift



deposited directly by glacial ice
mostly in ridge-like deposits called moraines
Outwash is sand and gravel deposited


glacial environments are collectively called drift
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
Moraines and Till

Moraines and poorly sorted till
Glacial Varves

Glacial lake deposits show


Each dark-light couplet is a varve,




alternating dark and light laminations
representing one year’s accumulation of sediment
light layers accumulate in spring and summer
dark layers in winter
Dropstones
liberated from
icebergs
 may also be present

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
 barrier islands and lagoons
 tidal flats

Transitional Environments
Transitional environments
Simple Deltas

The simplest deltas are those in lakes. They
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

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


Strong wave action


a stream/river-dominated delta results
produces a wave dominated delta
Tidal influences

result in tide-dominated deltas
Stream/River-Dominated
Deltas

Stream/riverdominated
deltas
have long
distributary
channels
 extending far
seaward
 Mississippi
River delta

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

Tide-Dominated Deltas

Tide-Dominated Deltas,

such as the Ganges-Brahmaputra delta
have tidal
sand
bodies
 along the
direction
of tidal
flow

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
Barrier Island Complex

Subenvironments of a barrier island complex
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 herringbone cross-bedding
 or sets of cross-beds that dip in opposite directions

Tidal Flats

Tidal-flat deposits showing a prograding
shoreline
Notice the distinctive cross-beds
 that dip in opposite directions

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

Marine Environments
Marine
environments
Detrital Marine Environments

The gently sloping area adjacent to a continent


It consists of a high-energy inner part that is


shaped into large cross-bedded dunes
Bedding planes are commonly marked


periodically stirred up by waves and tidal currents
Its sediment is mostly sand,


is a continental shelf
by wave-formed ripple marks
Marine fossils and bioturbation are typical
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

Slope and Rise

Once sediment passes the outer margin
of the self, the shelf-slope break,
 turbidity currents transport it



So sands with graded bedding are common
as well as mud that settled from seawater
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
Deep Sea

Beyond the continental rise, the seafloor is

nearly completely covered by fine-grained deposits


with no sediment at all



pelagic clay and ooze
near mid-ocean ridges
sand and gravel are notably absent
The main sources of sediment are
windblown dust from continents or oceanic islands
 volcanic ash
 shells of microorganisms that dwelled in surface
waters of the ocean

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
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,
instead of being transported there
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 ooids


which make up the grains in oolitic limestone
Carbonate Shelf

The
carbonate
shelf is
attached to
a continent

Examples
occur in
southern
Florida
and the
Persian
Gulf
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

Carbonate Subenvironments

Reef rock tends to be



Carbonate banks are made up of




structureless
composed of skeletons of corals, mollusks, sponges and other
organisms
layers with horizontal beds
cross-beds
wave-formed ripple marks
Lagoons tend to have



micrite
with marine fossils
bioturbation
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

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

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

Navajo Sandstone
Checkerboard Mesa,
Zion National Park,
Utah



Vertical
fractures
intersect cross
beds of desert
dunes
making the
checker-board
pattern
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

Paleogeography

and from Late Precambrian to Middle Cambrian

the shoreline migrated inland from east and west

during a marine transgression
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
Paleogeography

Later, vast lakes,
river floodplains, alluvial fans
 covered much of this area
 and the sea had withdrawn
from the continent


Interpretations of the
geologic record
will be based on similar
 amounts of supporting
evidence
