Early Paleozoic Earth History

Chapter 10

• 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

The First Geologic Map

• 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

Fuel for the Industrial Revolution Revolutionized Geology

• 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

Understanding Geology Gave Smith an Advantage

• 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

Paleozoic History

• 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

Geologic History of North America

• During the Precambrian– continental accretion – and orogenic activity – led to the formation of sizable continents

• At least three large continents – existed during the Late Proterozoic– and these landmasses may have later collided – to form a single Pangaea-like supercontinent

Pangaea-Like Supercontinent

• 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

Cratons and Mobile Belts

• 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

Continental Architecture

• 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

Shields

• The major cratonic structures – and mobile belts

of North America

– that formed during the Paleozoic Era

Paleozoic North America

• Shield• Mobile

belts

• 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– thus forming the platform

Platforms

Paleozoic North America

• Platform

• 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

Epeiric Seas

• 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

Mostly Flat Lying

• 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

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

Four Mobile Belts

Paleozoic North America

• Mobil belts

• 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

Paleogeography

• 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

Paleogeographic Maps

• 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, evaporates,

and coals• as well as the distribution of plants and animals

Paleozoic paleogeography

• 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 – and island arcs associated with various microplates

Six Major Paleozoic Continents

• For the Late Cambrian Period

Paleogeography of the World

• For the Late Ordovician Period

Paleogeography of the World

• For the Middle Silurian Period

Paleogeography of the World

• 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

Early Paleozoic Global History

• 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 Laurentia’s passive margin in the Cambrian, – an active convergent plate boundary formed along

its eastern margin during the Ordovician – as indicated by the Late Ordovician Taconic

orogeny that occurred in New England

Ordovician and Silurian Periods

• 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, – created 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

Silurian Period

• 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

Early Paleozoic Evolution of North America

• White areas represent sequences of rocks

Cratonic Sequences of N. America

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

Cordilleran orogenies

Appa-lachian oro-genies

• 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

Cratonic Sequence

• 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

Global Transgressive and Regressive Cycles

• 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 relationships– and facies relationships

High-Resolution Stratigraphic Analysis

• 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

Sequence Stratigraphy

• Rocks of the Sauk Sequence– during the Late Proterozoic-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

The Sauk Sequence

• White areas = sequences of rocks

Cratonic Sequences of N. America

• Brown areas = large-scale uncon-formities

• 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

The Sauk Sequence

• By the Late Cambrian, – the Sauk Sea 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

Transcontinental Arch

• During this time North America straddled the equator

• Trans-continental Arch

Cambrian Paleogeography of North America

• 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

The Sauk Sediments

• 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

Sauk Carbonates

• 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

A Transgressive Facies Model

• 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 Late Proterozoic and Early Cambrian time,– most of the craton was above sea level– The Sauk Sea was still largely restricted to the

margins of the craton • on continental shelves and slopes

The Cambrian of the Grand Canyon Region

• 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

Transgression

• Cambrian strata exposed in the Grand Canyon

Cambrian Transgression

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

Arizona

• 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

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

Continued Transgression

• 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,

• thus illustrating the time-transgres-sive nature of formations and facies

Time Transgressive Formations

older shale

younger shale

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

formations

Cambrian Transgression

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

Arizona

• 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

Same Facies Relationship

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

Cambrian Facies

– showing 3 major Cambrian facies

– and the time transgressive nature of the units

– The carbonate facies developed progressively

– due to submergence of the detrital source areas by the advancing Sauk Sea

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

Upper Cambrian Sandstone

• 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

Regression and Unconformity

• Paleo-geography of North America– showing

change in the position of the the equator

• The continent – was rotating

counter-clockwise

Ordovician Period

• White areas = sequences of rocks

Cratonic Sequences of N. America

• brown areas = large-scale uncon-formities

• Regression• 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 mid-continent – and resulted from numerous cycles of weathering – and erosion of Proterozoic and Cambrian sandstones – deposited during the Sauk transgression

The Tippecanoe Sequence

• Resulted in the deposition of

• the St. Peter Sandstone – Middle

Ordovician

• over a large area of the craton

Transgression of the Tippecanoe Sea

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

St. Peter Sandstone

• 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

The Tippecanoe Sequence

• 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 we will discuss later

Dolostones and Shales

• 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

Tippecanoe Reefs and Evaporites

• 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

Modern Reef Requirements

• with reef-building organisms

Present-Day Reef Community

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

Reef Environments

• 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

Size and Shape of 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 along a fore-reef slope

Barrier Reefs

• Barrier Reef

Barrier Reef

• Fore-reef slope

• 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

The Lagoon

• 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

Ancient 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

Stromatoporoid-Coral Reefs

• 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 evaporates

within the basin

Michigan Basin Evaporites

• Paleogeography of North America during the Silurian Period

• Reefs developed in the Michigan, Ohio, and Indiana-Illinois-Kentucky areas

Silurian Period

• 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

Other Types of Reefs

• Northern Michigan Basin sediments during the Silurian Period

Northern Michigan Basin

• Stromato-poroid barrier-reef facies of the Michigan Basin

Stromatoporoid Reef Facies

• Evaporite facies

Evaporite

• Carbonate Facies

Carbonate Facies

• As the Tippecanoe Sea gradually regressed – from the craton during the Late Silurian, – precipitation of evaporate 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

Tippecanoe Regression and 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

Origin of Thick Evaporites

• Silled Basin Model for evaporite sedimentation by direct precipitation from seawater– Vertical scale

is greatly exaggerated

Silled Basin Model

• 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

Basin Brines

• 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 evaporate minerals

Replenishment of Salt

• 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

Order of Precipitation

• 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, due to – variations in the amount of seawater entering the

basin – and changing geologic conditions

Interfingering

• Thus, the periodic evaporation or seawater proposed by this model – could account for the observed vertical and lateral

distribution – of evaporates in the Michigan Basin

• However, associated with those evaporates– are pinnacle reefs, – and the organisms constructing those reefs – could not have lived in such a highly saline

environment

Problems with the Model

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

Reefs in a Highly Saline Environ-ment?

• How then, can such contradictory features be explained? – Numerous models have been proposed, ranging

from • cessation of reef growth followed by evaporate

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

No Model Is Perfect

• 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

The End of the Tippecanoe Sequence

• 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

Domes and Basins

– 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

The Appalachian Mobile Belt

• 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

Mountain Building

• 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

Iapetus Ocean

• Evolution of the Appalachian mobile belt• Late Proterozoic opening of Iapetus Ocean

Appalachian Mobile Belt

– with passive continen-tal margins

– and large carbon-ate plat-forms

• 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

The Taconic Orogeny

• 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, desiccation cracks, – and other sedimentary structures

Shallow-Water Deposition

• Carbonate deposition ceased along the East Coast – during the Middle Ordovician

• and was replaced by deep-water 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

Deep-Water Deposits

• 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

Eastern Sediment Source

• Middle Ordovician transition to convergence resulted in orogenic activity

Appalachian Mobile Belt

• 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

Evidence for Orogeny

• 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 between 440 to 480 million

years ago

• In addition, regional metamorphism – coincides with the radiometric dates

Orogeny Timing

• 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

Queenston Delta Clastic Wedge

• 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

• 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

A European 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 later because – it was intimately related to the Acadian orogeny

Caledonian Orogeny

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

Caledonian Orogeny

– Caledonian Orogeny

– was a mirror image of the Taconic Orogeny

• 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

Early Paleozoic Mineral Resources

• 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 provides permeable zones • for the oil or gas to migrate to the well

Silica Sand

• Thick deposits of Silurian evaporites, – mostly rock salt (NaCl) – and rock gypsum (CaSO4•H2O) 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

Salt and Oil

• 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

Lead and Zinc

• 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

Iron

Summary

• Six major continents existed – at the beginning of the Paleozoic Era– four of them were located near the paleo-

equator

• During the Early Paleozoic — Cambrian-Silurian– Laurentia was moving northward – and Gondwana moved to a south polar location, – as indicated by tillite deposits

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

• The geologic history of North America – can be divided into cratonic sequences – that reflect cratonwide transgressions and

regressions

Summary

• The Sauk Sea was the first major transgression onto the craton

• At its maximum, it covered the craton – except for parts of the Canadian Shield – and the Transcontinental Arch,

• a series of large northeast-southwest trending islands

• The Tippecanoe sequence began with – deposition of an extensive sandstone over – the exposed and eroded Sauk landscape

Summary

• During Tippecanoe time, – extensive carbonate deposition took place

• In addition, large barrier reefs – enclosed basins, – and resulted in evaporite deposition within

these basins

• The eastern edge of North America – was a stable carbonate platform during Sauk

time

Summary

• During Tippecanoe time – an oceanic-continental convergent plate

boundary formed, – resulting in the Taconic orogeny,

• the first of several orogenies to affect the Appalachian mobile belt

• The newly formed Taconic Highlands – shed sediments into the western epeiric sea – producing the Queenston Delta, a clastic wedge

Summary

• Early Paleozoic-age rocks contain a variety of mineral resources including – building stone, – limestone for cement, – silica sand, – hydrocarbons, – evaporites – and iron ores