Transcript No Slide Title

Chapter 10
Early Paleozoic Earth History
The First
Geologic
Map
• William
Smith,
– a canal
builder,
published the
first geologic
map
– on August 1,
1815
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
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
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
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
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
Pangaea-Like Supercontinent
• During the Precambrian
– continental accretion
– and orogenic activity
– led to the formation of sizable continents
• Movement of these continents
– resulted in the formation of
– a single Pangaea-like supercontinent, Pannotia
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
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
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
Paleozoic North America
• The major
cratonic
structures
– and mobile belts
of North
America that
formed during
the Paleozoic Era
• Shield
• Mobile
belts
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 and chemical sedimentary
rocks
• The sediments were deposited
– in widespread shallow seas
– that transgressed and regressed over the craton
Paleozoic North America
• Platform
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 epeiric seas
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
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
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
Paleozoic North America
• Mobile belts
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
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
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
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
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
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
• such as Avalonia (Belgium, northern France, England, Wales,
Ireland, and the Maritime provinces and Newfoundland of Canada)
– and island arcs associated with various microplates
Paleogeography of the World
• For the Late Cambrian Period
Paleogeography of the World
• For the Late Ordovician Period
Paleogeography of the World
• For the Middle Silurian Period
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
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
– Avalonia separated from Gondwana and collided with
Baltica
• In contrast to Laurentia’s 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
Silurian Period
• Baltica, with attached Avalonia, 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
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
Cratonic Sequences of N. America
• White areas represent sequences of rocks
• That are
separated
by largescale
Cordilleran
unconorogenies
formities
shown in
brown
Appalachian
orogenies
Cratonic Sequence
• A cratonic sequence is
– a large-scale lithostratigraphic unit
• greater than supergroup
– representing a major transgressive-regressive cycle
– bounded by craton-wide unconformities
• The six unconformities
– extend across the various sedimentary basins of the
North American craton
– and into the mobile belts along the cratonic margin
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
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
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
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
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
Cratonic Sequences of N. America
• White areas = sequences of rocks
• Brown
areas =
largescale
unconformities
• Sauk
sequence
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
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
Cambrian Paleogeography of
North America
• During this
time North
America
straddled the
equator
• Transcontinental
Arch
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
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
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
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
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
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
Cambrian Transgression
• Cambrian strata exposed in the Grand Canyon
• The three formations exposed
– along the Bright Angel Trail, Grand Canyon
Arizona
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
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
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
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,
younger
• thus illustrating the timeshale
transgressive nature of formations and facies
older shale
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
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
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
Upper Cambrian Sandstone
• Outcrop of cross-bedded Upper Cambrian
sandstone
in the
Dells area
of
Wisconsin
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 craton-wide unconformity
– marks the boundary between the Sauk
– and Tippecanoe sequences
Ordovician Period
• Paleogeography of
North America
– showing
change in the
position of the
the equator
• The continent
– was rotating
counterclockwise
Cratonic Sequences of N. America
• White areas = sequences of rocks
• brown
areas =
largescale
unconformities
• Regression
• Tippecanoe
sequence
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
Transgression of the
Tippecanoe Sea
• Resulted in
deposition of
• the St. Peter
Sandstone
– Middle
Ordovician
• over a large
area of the
craton
St. Peter Sandstone
• Outcrop of St. Peter Sandstone in Governor
Dodge State Park, Wisconsin
The Tippecanoe Sequence
• The Tippecanoe basal sandstones were
followed by widespread carbonate deposition
• The limestones were generally the result of
deposition
– by
calcium carbonatesecreting organisms
such as
•
•
•
•
corals,
brachiopods,
stromatoporoids,
and bryozoans
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
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
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
Present-Day Reef Community
• with reef-building organisms
Reef Environments
• Block diagram of a reef showing the various
environments within the reef complex
Size and Shape of Reefs
• The size and shape of a reef
–
–
–
–
–
are mostly 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
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
Barrier Reef
• Barrier Reef
• Fore-reef slope
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
Ancient Reefs
• Reefs have been common features since the
Cambrian
– and have been built by a variety of organisms
• The first skeletal builders of reef-like 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
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
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
Silurian Period
• Paleogeography
of North
America during
the Silurian
Period
• Reefs developed
in the Michigan,
Ohio, and
Indiana-IllinoisKentucky areas
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
Northern Michigan Basin
• Northern
Michigan
Basin
sediments
during the
Silurian
Period
Stromatoporoid Reef Facies
• Stromatoporoid
barrierreef facies
of the
Michigan
Basin
Evaporite
• Evaporite facies
Carbonate Facies
• Carbonate
Facies
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
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
Silled Basin Model
• Silled Basin
Model for
evaporite
sedimentation
by direct
precipitation
from seawater
– Vertical scale
is greatly
exaggerated
Basin Brines
• Because North America was still near the
equator during the Silurian Period,
– temperatures were probably high
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
Replenishment of Salt
• Some seawater flowed in over the sill
–
–
–
–
–
and through channels cut in the barrier reefs,
this replenishment added new seawater
allowing the process of brine formation
and precipitation of evaporites
to repeat itself
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
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
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
Reefs in a Highly Saline Environment?
• Organisms
constructing
reefs could
not have
lived in
such a
highly
saline
environment
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
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
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
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
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
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
Appalachian Mobile Belt
• Evolution of the Appalachian mobile belt
• Neoproterozoic opening of Iapetus Ocean
– with
passive
continental
margins
– and large
carbonate
platforms
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
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
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
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
Appalachian Mobile Belt
• Middle Ordovician transition to convergence
resulted in orogenic activity
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
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
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
• 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
Queenston Delta Clastic Wedge
• Queenston Delta clastic wedge
– consists of
thick,
coarsegrained
detrital
sediments
nearest the
highlands
– and thins
laterally into
finer-grained
sediments
on the
craton
• Taconic
Highlands
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
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
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
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
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
passageways
• for the oil or gas to migrate to the well
Salt and Oil
• Thick deposits of Silurian evaporites,
– mostly rock salt (NaCl)
– and rock gypsum (CaSO4•2H2O) 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
Lead and Zinc
• The host rocks for deposits of lead and zinc
– in southeast Missouri are Cambrian dolostones,
– although some Ordovician rocks contain these
metals as well
• These deposits have been mined since 1720
– but have been largely depleted
• Now most lead and zinc mined in Missouri
– come from Mississippian-age sedimentary rocks
Iron
• The Silurian Clinton Formation crops out
– from Alabama north to New York,
– and equivalent rocks are found in Newfoundland
• This formation has been mined for iron in
many places
• In the United States, the richest ores
– and most extensive mining occurred near
Birmingham, Alabama,
– but only a small amount of ore is currently
produced in that area
Summary
• Most continents consisted of two major
components
– a relatively stable craton over which epeiric seas
transgressed and regressed,
– surrounded by mobile belts in which mountain building
took place
• Six major continents and numerous
microcontinents existed
– at the beginning of the Paleozoic Era
– and these were dispersed at low latitudes around the
globe
– during the Cambrian
Summary
• During the Ordovician and Silurian
– plate movement resulted in a changing global geography
• Gondwana moved southward and began to cross the
South Pole
– as indicated by Upper Ordovician tillite deposits
– The microcontinent Avalonia separated from Gondwana
during the Early Ordovician
• and collided with Baltica during the Late Ordovician-Early
Silurianwana During the Early Paleozoic (Cambrian-Silurian)
• Baltica and Avalonia moved northwestward relative
to Laurentia
– and collided to form Laurasia
– during the Silurian
Summary
• Geologists divide the geologic history of North
America
– into cratonic sequences
– that reflect craton-wide transgressions and regressions
• The first major marine transgression resulted in
deposition of the Sauk Sequence
• At its maximum, the Sauk Sea covered the craton
– except for parts of the Canadian Shield
– and the Transcontinental Arch,
• a series of large northeast-southwest trending islands
Summary
• The Tippecanoe Sequence began with
– deposition of an extensive sandstone over
– the exposed and eroded Sauk landscape
• During Tippecanoe time,
– extensive carbonate deposition took place
• In addition, large barrier reefs
– enclosed basins,
– and resulted in evaporite deposition within
these basins
Summary
• The eastern edge of North America
– was a stable carbonate platform during Sauk
time
• During Tippecanoe time
– an oceanic-continental convergent plate
boundary formed,
– resulting in the Taconic orogeny,
• the first of three major orogenies to affect the
Appalachian mobile belt
Summary
• The newly formed Taconic Highlands
– shed sediments into the western epeiric sea
– producing the Queenston Delta, a clastic wedge
• Early Paleozoic-age rocks contain a variety
of mineral resources including
–
–
–
–
–
–
building stone,
limestone for cement,
silica sand,
hydrocarbons,
evaporites,
and iron ores