Early Paleozoic Earth History

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Title: Early Paleozoic Earth History

1
Chapter 10
Early Paleozoic Earth History
2
The First Geologic Map

William Smith,
a canal builder, published the first geologic map
on August 1, 1815

3
The First Geologic Map

Measuring more than eight feet high and six feet
wide,
Smith's hand-painted geologic map of England
represented more than 20 years
of detailed study of the rocks and fossils of
England
England is a country rich in geologic history
Five of the six geologic systems
were described and named
for rocks exposed in England
Cambrian, Ordovician, Silurian, Devonian, and
Carboniferous

4
Fuel for the Industrial Revolution Revolutionized
Geology

The Carboniferous coal beds of England
helped fuel the Industrial Revolution,
during the late 1700s and early 1800s
William Smith, first began noticing
how rocks and fossils repeated themselves
in a predicable fashion while mapping various
coal mines
Smith surveyed the English countryside
for the most efficient canal routes
to bring the coal to market

5
Understanding Geology Gave Smith an Advantage

Much of his success was based on the fact
he was able to predict what rocks
canal diggers would encounter
His observations of the geologic history
of England allowed William Smith
to make the first geologic map of an entire
county!
We will use the same basic geologic principles
William Smith used to interpret the geology
of the Paleozoic Era

6
Paleozoic History

The Paleozoic history of most continents
involves major mountain-building activity along
their margins
and numerous shallow-water marine
transgressions and regressions over their
interiors
These transgressions and regressions
were caused by global changes in sea level
that most probably were related
to plate activity and glaciation

7
Geologic History of North America

We will examine the geologic history of North
America
in terms of major transgressions and regressions
rather than a period-by-period chronology
and we will place those events in a global
context

8
Pangaea-Like Supercontinent

During the Precambrian
continental accretion
and orogenic activity
led to the formation of sizable continents
At least three large continents
existed during the Proterozoic
and these landmasses may have later collided
to form a single Pangaea-like supercontinent,
Pannotia

9
Cratons and Mobile Belts

This supercontinent began breaking apart
sometime during the latest Proterozoic
By the beginning of the Paleozoic Era,
six major continents were present
Each continent can be divided
into two major components
a craton
and one or more mobile belts

10
Continental Architecture

Cratons are the relatively stable
and immobile parts of continents
and form the foundation upon which
Phanerozoic sediments were deposited
Cratons typically consist of two parts
a shield
and a platform

11
Shields

Shields are the exposed portion of the
crystalline basement rocks of a continent
and are composed of
Precambrian metamorphic
and igneous rocks
that reveal a history of extensive orogenic
activity during the Precambrian
During the Phanerozoic, however,
shields were extremely stable
and formed the foundation of the continents

12
Paleozoic North America

The major cratonic structures
and mobile belts of North America that formed
during the Paleozoic Era

Shield
Mobile belts

13
Platforms

Extending outward from the shields are buried
Precambrian rocks
that constitute a platform,
another part of the craton,
the platform is buried by flat-lying or gently
dipping
Phanerozoic detrital sedimentary rocks
and chemical sedimentary rocks
The sediments were deposited
in widespread shallow seas
that transgressed and regressed over the craton

14
Paleozoic North America

Platform

15
Epeiric Seas

The transgressing and regressing shallow seas
called epeiric seas
were a common feature
of most Paleozoic cratonic histories
Continental glaciation
as well as plate movement
caused changes in sea level
and were responsible for the advance and retreat
of the seas in which the sediments were deposited

16
Mostly Flat Lying

Whereas most of the Paleozoic platform rocks
are still essentially flat lying
in some places they were gently folded into
regional arches, domes, and basins
In many cases some of the structures stood out
as low islands during the Paleozoic Era
and supplied sediments to the surrounding epeiric
seas

17
Mobile Belts

Mobile belts are elongated areas of mountain
building activity
They are located along the margins of continents
where sediments are deposited in the relatively
shallow waters of the continental shelf
and the deeper waters at the base of the
continental slope
During plate convergence along these margins,
the sediments are deformed
and intruded by magma
creating mountain ranges

18
Four Mobile Belts

Four mobile belts formed
around the margin
of the North American craton during the Paleozoic
Franklin mobile belt
Cordilleran mobile belt
Ouachita mobile belt
Appalachian mobile belt
Each was the site of mountain building
in response to compressional forces
along a convergent plate boundary
and formed such mountain ranges
as the Appalachians and Ouachitas

19
Paleozoic North America

Mobile belts

20
Paleogeography

Because of plate tectonics,
the present-day configuration of the continents
and ocean basins is merely a snapshot in time
As the plates move about, the location of
continents and ocean basins constantly changes
Historical geology provides past geologic and
paleogeographic reconstruction of the world
Paleogeographic maps show
the distribution of land and sea
possible climate regimes
and such geographic features as mountain ranges,
swamps, and glaciers

21
Paleogeographic Maps

Geologists use
paleoclimatic data
paleomagnetic data
paleontologic data
sedimentologic data
stratigraphic data
tectonic data
to construct paleogeographic maps
which are interpretations of the geography of an
area for a particular time in the geologic past

22
Paleozoic paleogeography

The paleogeographic history
of the Paleozoic Era is not as precisely known
as for the Mesozoic and Cenozoic eras
in part because the magnetic anomaly patterns
preserved in the oceanic crust
was subducted during the formation of Pangaea
Paleozoic paleogeographic reconstructions
are therefore based primarily on
structural relationships
climate-sensitive sediments such as red beds,
evaporites, and coals
as well as the distribution of plants and animals

23
Six Major Paleozoic Continents

At the beginning of the Paleozoic, six major
continents were present
Baltica - Russia west of the Ural Mountains and
the major part of northern Europe
China - a complex area consisting of at least
three Paleozoic continents that were not widely
separated and are here considered to include
China, Indochina, and the Malay Peninsula
Gondwana - Africa, Antarctica, Australia,
Florida, India, Madagascar, and parts of the
Middle East and southern Europe

24
Six Major Paleozoic Continents

Kazakhstan - a triangular continent centered on
Kazakhstan, but considered by some to be an
extension of the Paleozoic Siberian continent
Laurentia - most of present North America,
Greenland, northwestern Ireland, and Scotland
and Siberia - Russia east of the Ural Mountains
and Asia north of Kazakhstan and south Mongolia
Besides these large landmasses, geologists have
also identified
numerous small microcontinents
and island arcs associated with various
microplates

25
Paleogeography of the World

For the Late Cambrian Period

26
Paleogeography of the World

For the Late Ordovician Period

27
Paleogeography of the World

For the Middle Silurian Period

28
Early Paleozoic Global History

In contrast to today's global geography,
the Cambrian world consisted
of six major continents
dispersed around the globe at low tropical
latitudes
Water circulated freely among ocean basins,
and the polar regions were mostly ice free
By the Late Cambrian,
epeiric seas had covered areas of
Laurentia, Baltica, Siberia, Kazakhstania, China
while highlands were present in
northeastern Gondwana, eastern Siberia, and
central Kazakhstania

29
Ordovician and Silurian Periods

Plate movements played a major role
in the changing global geography
Gondwana moved southward during the Ordovician
and began to cross the South Pole
as indicated by Upper Ordovician tillites found
today in the Sahara Desert
In contrast to Laurentias passive margin in the
Cambrian,
an active convergent plate boundary existed along
its eastern margin during the Ordovician
as indicated by the Late Ordovician Taconic
orogeny that occurred in New England

30
Silurian Period

Baltica moved northwestward relative
to Laurentia and collided with it
to form the larger continent of Laurasia
This collision, which closed the northern Iapetus
Ocean,
is marked by the Caledonian orogeny
The southern part of the Iapetus Ocean
still remained open between Laurentia and
Gondwana
Siberia and Kazakhstania moved from
a southern equatorial position during the
Cambrian
to north temperate latitudes
by the end of the Silurian Period

31
Early Paleozoic Evolution of North America

The geologic history of the North American craton
may be divide into two parts
the first dealing with the relatively stable
continental interior over which epeiric seas
transgressed and regressed,
and the other dealing with the mobile belts where
mountain building occurred
In 1963 American geologist Laurence Sloss
proposed
that the sedimentary-rock record of North America
could be subdivided into six cratonic sequences

32
Cratonic Sequences of N. America

White areas represent sequences of rocks

That are separated by large-scale uncon-formities
shown in brown

Appa-lachian oro-genies

Cordilleran orogenies

33
Cratonic Sequence

A cratonic sequence is
a large-scale lithostratigraphic unit
greater than supergroup
representing a major transgressive-regressive
cycle
bounded by cratonwide unconformities
The six unconformities
extend across the various sedimentary basins of
the North American craton
and into the mobile belts along the cratonic
margin

34
Global Transgressive and Regressive Cycles

Geologists have also recognized
major unconformity bounded sequences
in cratonic areas outside North America
Such global transgressive and regressive cycles
are caused by sea-level changes
and are thought to result
from major tectonic and glacial events

35
High-Resolution Stratigraphic Analysis

The subdivision and correlation of cratonic
sequences
provides the foundation for an important concept
in geology
sequence stratigraphy
that allows high-resolution analysis
within sedimentary rocks of
time and facies relationships

36
Sequence Stratigraphy

Sequence stratigraphy is the study of rock
relationships
within a time-stratigraphic framework of related
facies
bounded by erosional or nondepositional surfaces
The basic unit of sequence stratigraphy is the
sequence,
which is a succession of rocks bounded by
unconformities
and their equivalent conformable strata

37
Sequence Stratigraphy

Sequence boundaries form
as a result of a relative drop in sea level
Sequence stratigraphy is an important tool in
geology
because it allows geologists to subdivide
sedimentary rocks
into related units
that are bounded
by time-stratigraphically significant boundaries
Geologists use sequence stratigraphy
for high-resolution correlation and mapping,
as well as interpreting and predicting
depositional environments

38
The Sauk Sequence

Rocks of the Sauk Sequence
during the Neoproterozoic-Early Ordovician
record the first major transgression onto the
North American craton
Deposition of marine sediments
during the Late Proterozoic and Early Cambrian
was limited to the passive shelf areas of the
Appalachian and Cordilleran borders of the craton
The craton itself was above sea level
and experiencing extensive weathering and erosion

39
Cratonic Sequences of N. America

White areas sequences of rocks

Brown areas large-scale uncon-formities

Sauk sequence

40
The Sauk Sequence

Because North America was located
in a tropical climate at this time
and there is no evidence of any terrestrial
vegetation,
weathering and erosion of the exposed
Precambrian basement rocks must have proceeded
rapidly
During the Middle Cambrian,
the transgressive phase of the Sauk
began with epeiric seas encroaching over the
craton

41
Transcontinental Arch

By the Late Cambrian,
the epeiric seas had covered most of North
America,
leaving above sea level only
a portion of the Canadian Shield
and a few large islands
These islands,
collectively named the Transcontinental Arch,
extended from New Mexico
to Minnesota and the Lake Superior region

42
Cambrian Paleogeography of North America

During this time North America straddled the
equator
Trans-continental Arch

43
The Sauk Sediments

The sediments deposited
on both the craton
and along the shelf area of the craton margin
show abundant evidence of shallow-water
deposition
The only difference
between the shelf and craton deposits
is that the shelf deposits are thicker
In both areas,
the sands are generally clean and well sorted
and commonly contain ripple marks
and small-scale cross-bedding

44
Sauk Carbonates

Many of the carbonates are
bioclastic
composed of fragments of organic remains
contain stromatolites,
or have oolitic textures
contain small, spherical calcium carbonate grains
Such sedimentary structures and textures
indicate shallow-water deposition

45
A Transgressive Facies Model

Sediments become increasingly finer
the farther away from land one goes
Where sea level remains the same, in a stable
environment
coarse detrital sediments are typically deposited
in the nearshore environment,
and finer-grained sediments are deposited in the
offshore environment
Carbonates form farthest from land in the area
beyond the reach of detrital sediments

46
A Transgressive Facies Model

Recall that facies are sediments
that represent a particular environment
During a transgression, the coarse (sandstone),
fine (shale) and carbonate (limestone) facies
migrate in a landward direction

47
The Cambrian of the Grand Canyon Region

This region provides an excellent example
of sedimentation patterns of a transgressing sea
The region of the Grand Canyon occupied
the western margin of the craton during Sauk
time,
a passive shelf
During Neoproterozoic and Early Cambrian time,
most of the craton was above sea level
deposition of marine sediments
was mainly restricted to the margins of the
craton
on continental shelves and slopes

48
Transgression

A transgression covered
the Grand Canyon region.
The Tapeats Sandstone represents
the basal transgressive shoreline deposits
that accumulated as marine waters
transgressed across the shelf
and just onto the western margin
of the craton during the Early Cambrian

49
Cambrian Transgression

Cambrian strata exposed in the Grand Canyon

The three formations exposed
along the Bright Angel Trail, Grand Canyon Arizona

50
Transgression

The Tapeats sediments
are clean, well-sorted sands
of the type one would find on a beach today
As the transgression continued into the Middle
Cambrian,
muds of the Bright Angle Shale
were deposited over the Tapeats Sandstone

51
Continued Transgression

The Sauk Sea had transgressed so far onto the
craton
by the Late Cambrian that
in the Grand Canyon region
carbonates of the Muav Limestone were being
deposited over the Bright Angel Shale
This vertical succession of
sandstone (Tapeats)
shale (Bright Angel)
and limestone (Muav)
forms a typical transgressive sequence
and represents a progressive migration
of offshore facies toward the craton through time

52
Time Transgressive Formations

Cambrian rocks of the Grand Canyon region
also illustrate how many formations are time
transgressive
that is, their age is not the same every place
they are found
Mapping and correlations based on faunal evidence
indicate that deposition of the Mauv Limestone
had already started on the shelf
before deposition of the Tapeats Sandstone
was completed on the craton

53
Time Transgressive Formations

Faunal analysis of the Bright Angel Shale
indicates
that it is Early Cambrian in age in California
and Middle Cambrian in age in the Grand Canyon
region,
thus illustrating the time-
transgressive nature of formations and
facies

younger shale
older shale
54
Cambrian Transgression

Cambrian strata exposed in the Grand Canyon
Observe the time transgressive nature of the
three formations

The three formations exposed
along the Bright Angel Trail, Grand Canyon Arizona

55
Same Facies Relationship

This same facies relationship also occurred
elsewhere on the craton
as the seas encroached from the Appalachian and
Ouachita mobile belts onto the craton interior
Carbonate deposition dominated on the craton as
the Sauk transgression continued
during the early Ordovician,
and the islands of the Transcontinental Arch were
soon covered by the advancing Sauk Sea
By the end of Sauk time, much of the craton
was submerged beneath a warm, equatorial epeiric
sea

56
Cambrian Facies

Block diagram from the craton interior to the
Appalachian mobile belt margin

showing 3 major Cambrian facies
and the time transgressive nature of the units
The carbonate facies developed progressively
because of submergence of the detrital source
areas by the advancing Sauk Sea

57
Upper Cambrian Sandstone

Outcrop of cross-bedded Upper Cambrian sandstone
in the Dells area of Wisconsin

58
Regression and Unconformity

As the Sauk Sea regressed
from the craton during the Early Ordovician,
it revealed a landscape of low relief
The rocks exposed were predominately
limestones and dolostones
that experienced deep and extensive erosion
because North America was still located in a
tropical environment
The resulting cratonwide unconformity
marks the boundary between the Sauk
and Tippecanoe sequences

59
Ordovician Period

Paleo-geography of North America
showing change in the position of the the equator
The continent
was rotating counter-clockwise

60
Cratonic Sequences of N. America

White areas sequences of rocks

brown areas large-scale uncon-formities

Regression

Tippecanoe sequence

61
The Tippecanoe Sequence

A transgressing sea deposited the Tippecanoe
sequence over most of the craton
Middle Ordovician-Early Devonian
Like the Sauk sequence, this major transgression
deposited clean, well-sorted quartz sands
The Tippecanoe basal rock is the St. Peter
Sandstone,
an almost pure quartz sandstone used in
manufacturing glass
that occurs throughout much of the midcontinent
and resulted from numerous cycles of weathering
and erosion of Proterozoic and Cambrian
sandstones
deposited during the Sauk transgression

62
Transgression of the Tippecanoe Sea

Resulted in deposition of
the St. Peter Sandstone
Middle Ordovician
over a large area of the craton

63
St. Peter Sandstone

Outcrop of St. Peter Sandstone in Governor Dodge
State Park, Wisconsin

64
The Tippecanoe Sequence

The Tippecanoe basal sandstones were followed by
widespread carbonate deposition
The limestones were generally the result of
deposition
by calcium carbonate-secreting organisms such
as
corals,
brachiopods,
stromatoporoids,
and bryozoans

65
Dolostones and Shales

Besides the limestones, there were also many
dolostones
Most of the dolostones formed as a result of
magnesium replacing calcium in calcite,
thus converting limestones into dolostones
In the eastern portion of the craton, the
carbonates grade laterally into shales
These shales mark the farthest extent
of detrital sediments derived from
weathering and erosion of the Taconic Highlands
a tectonic event in the Appalachian mobile belt

66
Tippecanoe Reefs and Evaporites

Organic reefs are limestone structures
constructed by living organisms,
some of which contribute skeletal materials to
the reef framework
Today, corals, and calcareous algae
are the most prominent reef builders,
but in the geologic past other organisms
played a major role in reef building
Reefs appear to have occupied
the same ecological niche in the geological past
that they do today regardless of the organisms
involved

67
Modern Reef Requirements

Because of the ecological requirements
of reef-building organisms,
present-day reefs are confined
to a narrow latitudinal belt
between 30 degrees north and south of the equator
Corals,
the major reef-building organisms today,
require warm, clear, shallow water
of normal salinity for optimal growth

68
Present-Day Reef Community

with reef-building organisms

69
Reef Environments

Block diagram of a reef showing the various
environments within the reef complex

70
Size and Shape of Reefs

The size and shape of a reef
are largely the result of the interaction between
the reef-building organisms,
the bottom topography,
wind and wave action,
and subsidence of the seafloor
Reefs also alter the area around them
by forming barriers to water circulation
or wave action

71
Barrier Reefs

Reefs typically are long,
linear masses forming a barrier between
a shallow platform on one side
and a comparatively deep marine basin
on the other side
Such reefs are known as barrier reefs
Reefs create and maintain a steep seaward front
that absorbs incoming wave energy
As skeletal material breaks off
from the reef front,
it accumulates as talus along a fore-reef slope

72
Barrier Reef

Barrier Reef

Fore-reef slope

73
The Lagoon

The reef barrier itself is porous
and composed of reef-building organisms
The lagoon area is a low-energy,
quiet water zone where fragile,
sediment-trapping organisms thrive
The lagoon area can also become the site
of evaporitic deposits
when circulation to the open sea is cut off
Modern examples of barrier reefs
are the Florida Keys, Bahama Islands,
and Great Barrier Reef of Australia

74
Ancient Reefs

Reefs have been common features since the
Cambrian
and have been built by a variety of organisms
The first skeletal builders of reeflike
structures
were archaeocyathids
These conical-shaped organisms lived
during the Cambrian and had double,
perforated, calcareous shell walls
Archaeocyathids built small mounds
that have been found on all continents
except South America

75
Stromatoporoid-Coral Reefs

Beginning in the Middle Ordovician,
stromatoporoid coral reefs
became common in the low latitudes,
and similar reefs remained so throughout the rest
of the Phanerozoic Eon
The burst of reef building seen in the Late
Ordovician through Devonian
probably occurred in response to evolutionary
changes
triggered by the appearance
of extensive carbonate seafloors and platforms
beyond the influence of detrital sediments

76
Michigan Basin Evaporites

The Middle Silurian rocks of the present-day
Great Lakes region
Tippecanoe sequence
are famous for their reef and evaporite deposits
The most significant structure in the region
the Michigan Basin
is a broad, circular basin surrounded by large
barrier reefs
These reefs contributed to increasingly
restricted circulation
and the precipitation of Upper Silurian
evaporites within the basin

77
Silurian Period

Paleogeography of North America during the
Silurian Period
Reefs developed in the Michigan, Ohio, and
Indiana-Illinois-Kentucky areas

78
Other Types of Reefs

Within the rapidly subsiding interior
of the basin, other types of reefs are found
Pinnacle reefs are tall,
spindly structures up to 100 m high
They reflect the rapid upward growth
needed to maintain themselves near sea level
during subsidence of the basin
Besides the pinnacle reefs,
bedded carbonates and thick sequences of salt
and anhydrite are also found in the Michigan Basin

79
Northern Michigan Basin

Northern Michigan Basin sediments during the
Silurian Period

80
Stromatoporoid Reef Facies

Stromato-poroid barrier-reef facies of the
Michigan Basin

81
Evaporite

Evaporite facies

82
Carbonate Facies

Carbonate Facies

83
Tippecanoe Regression and Evaporites

As the Tippecanoe Sea gradually regressed
from the craton during the Late Silurian,
precipitation of evaporite minerals occurred in
the
Appalachian Basin,
Ohio Basin,
and Michigan Basin
In the Michigan Basin alone,
approximately 1500 m of sediments were deposited,
nearly half of which are halite and anhydrite

84
Origin of Thick Evaporites

How did such thick sequences of evaporites
accumulate?
1. When sea level dropped, the tops of the
barrier reefs were as high as or above sea level,
thus preventing the influx of new seawater into
the basin
Evaporation of the basinal seawater would result
in the precipitation of salts
2. Alternatively, the reefs grew upward so close
to sea level
that they formed a sill or barrier that
eliminated interior circulation

85
Silled Basin Model

Silled Basin Model for evaporite sedimentation by
direct precipitation from seawater
Vertical scale is greatly exaggerated

86
Basin Brines

Because North America was still near the equator
during the Silurian Period,
temperatures were probably high

87
Basin Brines

As circulation to the Michigan Basin was
restricted,
seawater within the basin evaporated,
forming a brine
Because the brine was heavy,
it concentrated near the bottom,
and minerals precipitated on the basin floor

88
Replenishment of Salt

Some seawater flowed in over the sill
and through channels cut in the barrier reefs,
but this only added new seawater that later
became concentrated as brine
In this way, the brine in the basin became
increasingly concentrated
until the salts could no longer stay in solution,
thus precipitating to form evaporite minerals

89
Order of Precipitation

The order and type of salts precipitating from
seawater depends on
their solubility,
the original concentration of seawater,
and local conditions of the basin
Salts generally precipitate in order beginning
with the least soluble
and ending with the most soluble
Therefore, the order of precipitation is
calcium carbonate first,
followed by gypsum
and lastly halite

90
Interfingering

Gypsum is the common sulfate precipitated from
seawater,
but when deeply buried,
gypsum loses its water and is converted to
anhydrite
Many lateral shifts and interfingering
of the limestone, anhydrite, and halite facies
may occur, however, because of
variations in the amount of seawater entering the
basin
and changing geologic conditions

91
Problems with the Model

Thus, the periodic evaporation or seawater
proposed by this model
could account for the observed vertical and
lateral distribution
of evaporites in the Michigan Basin
However, associated with those evaporites
are pinnacle reefs,
and the organisms constructing those reefs
could not have lived in such a highly saline
environment

92
Reefs in a Highly Saline Environ-ment?

Organisms constructing reefs could not have lived
in such a highly saline environ-ment

93
No Model Is Perfect

How then, can such contradictory features be
explained?
Numerous models have been proposed, ranging from
cessation of reef growth followed by evaporite
deposition,
to alternation of reef growth and evaporite
deposition
Although the Michigan Basin has been studied
extensively for years,
no model yet proposed completely explains
the genesis and relationship of its various reef,
carbonate, and evaporite facies

94
The End of the Tippecanoe Sequence

By the Early Devonian,
the regressing Tippecanoe Sea
had retreated to the craton margin
exposing an extensive lowland topography
During this regression,
marine deposition was initially restricted to
a few interconnected cratonic basins and
by the end of the Tippecanoe
to only the mobile belts surrounding the craton

95
Domes and Basins

As the Tippecanoe Sea regressed
during the Early Devonian,
the craton experienced mild deformation
resulting in the formation of many domes, arches,
and basins
These structures were mostly eroded
during the time the craton was exposed
so that they were eventually covered by deposits
from the encroaching Kaskaskia Sea

96
The Appalachian Mobile Belt

Having examined the Sauk and Tippecanoe geologic
history of the craton,
we turn our attention to the Appalachian mobile
belt,
where the first Phanerozoic orogeny
began during the Middle Ordovician
The mountain building occurring
during the Paleozoic Era
had a profound influence on
the climate
and sedimentary history of the craton

97
Mountain Building

Additionally, it was part of the global tectonic
regime
that sutured the continents together,
forming Pangaea by the end of the Paleozoic
The Appalachian region
throughout Sauk time,
was a broad, passive, continental margin
Sedimentation was closely balanced by subsidence
as thick, shallow marine sands were succeeded
by extensive carbonate deposits

98
Iapetus Ocean

During this time,
the Iapetus Ocean was widening
as a result of movement
along a divergent plate boundary
Beginning with the subduction of the Iapetus
plate beneath Laurentia
which was an oceanic-continental convergent plate
boundary
the Appalachian mobile belt was born

99
Appalachian Mobile Belt

Evolution of the Appalachian mobile belt
Neoproterozoic opening of Iapetus Ocean

with passive continental margins
and large carbonate platforms

100
The Taconic Orogeny

The resulting Taconic orogeny,
named after present-day Taconic Mountains of
eastern New York,
central Massachusetts,
and Vermont
was the first of several orogenies
to affect the Appalachian region

101
Shallow-Water Deposition

The Appalachian mobile belt
can be divided into two depositional environments
The first is the extensive,
shallow-water carbonate platform
that formed the broad eastern continental shelf
and stretched from Newfoundland to Alabama
It formed during the Sauk Sea transgression
onto the craton when carbonates
were deposited in a vast shallow sea
The shallow water depth on the platform
is indicated by stromatolites, mud cracks,
and other sedimentary structures and fossils

102
Deep-Water Deposits

Carbonate deposition ceased along the East Coast
during the Middle Ordovician
and was replaced by deepwater deposits
characterized by
thinly bedded black shales,
graded beds,
coarse sandstones,
graywackes,
and associated volcanics
This suite of sediments marks the onset
of mountain building, the Taconic orogeny

103
Eastern Sediment Source

The subduction of the Iapetus plate beneath
Laurentia
resulted in volcanism
and downwarping of the carbonate platform
Throughout the Appalachian mobile belt,
indications that these deposits were derived from
the east, come from
facies patterns,
paleocurrents,
and sedimentary structures
The sediment originated where
the Taconic Highlands
and associated volcanoes were rising

104
Appalachian Mobile Belt

Middle Ordovician transition to convergence
resulted in orogenic activity

105
Evidence for Orogeny

Evidence for the timing and origin of this
orogeny comes from
additional structural,
stratigraphic,
petrologic,
and sedimentologic information
For example,
at many locations within the Taconic belt,
pronounced angular unconformities occur
where steeply dipping Lower Ordovician rocks
are overlain by gently dipping or horizontal
Silurian and younger rocks

106
Orogeny Timing

Other evidence in the area from
present-day Georgia to Newfoundland includes
volcanic activity in the form of deep-sea lava
flows,
volcanic ash layers,
and intrusive bodies
These igneous rocks show a clustering
of radiometric ages corresponding to Middle to
Late Ordovician
In addition, regional metamorphism
coincides with the radiometric dates

107
Queenston Delta Clastic Wedge

The final piece of evidence
for the Taconic orogeny is
the development of a large clastic wedge,
an extensive accumulation of mostly detrital
sediments
were deposited adjacent to an uplifted area
and become thinner and finer grained away from
the source area,
eventually grading into the carbonate cratonic
facies
The clastic wedge resulting from the erosion
of the Taconic Highlands is referred
to as the Queenston Delta

108
Queenston Delta Clastic Wedge

Queenston Delta clastic wedge

Taconic Highlands

consists of thick, coarse-grained detrital
sediments nearest the highlands
and thins laterally into finer-grained sediments
on the craton

109
A European Orogeny

The Taconic orogeny
marked the first pulse of mountain building in
the Appalachian mobile belt
and was a response to the subduction taking place
beneath the east coast of Laurentia
As the Iapetus Ocean narrowed and closed,
another orogeny occurred in Europe during the
Silurian

110
Caledonian Orogeny

The Caledonian orogeny was essentially a mirror
image of
the Taconic orogeny and the Acadian orogeny
and was part of the global mountain-building
episode
that occurred during the Paleozoic Era
Even though the Caledonian orogeny
occurred during Tippecanoe time,
we will discuss it with the Acadian orogeny
because the two are intimately related

111
Caledonian Orogeny

The transition to convergence resulted in
orogenic activity in North America and Europe

Caledonian Orogeny
was a mirror image of the Taconic Orogeny

112
Early Paleozoic Mineral Resources

Early Paleozoic-age rocks contain a variety
of important mineral resources, including
sand and gravel for construction,
building stone,
and limestone used in the manufacture of cement
Important sources of industrial or silica sand
are
the Upper Cambrian Jordan Sandstone of Minnesota
and Wisconsin,
the Lower Silurian Tuscarora Sandstone in
Pennsylvania and Virginia,
and the Middle Ordovician St. Peter Sandstone

113
Silica Sand

The St. Peter Sandstone,
the basal sandstone of the Tippecanoe sequence,
occurs in several states,
but the best-known area of production
is in La Salle County, Illinois
Silica sand has a variety of uses including
the manufacture of glass,
molds for casting iron, aluminum, and copper
alloys
and refractory bricks for blast furnaces
It is also pumped into oil and gas wells
to fracture the source rocks and provide
permeable zones
for the oil or gas to migrate to the well

114
Salt and Oil

Thick deposits of Silurian evaporites,
mostly rock salt (NaCl)
and rock gypsum (CaSO42H2O) altered to rock
anhydrite (CaSO4)
underlie parts of Michigan, Ohio, New York, and
adjacent areas in Ontario, Canada
and are important sources of various salts
In addition, barrier and pinnacle reefs
in carbonate rocks
associated with these evaporites
are the reservoirs for oil and gas in Michigan
and Ohio

115
Lead and Zinc