Early Paleozoic Earth History

Chapter 10

• William Smith, – a canal

builder, published the first geologic map

– on August 1, 1815

The First Geologic

Map

• The Paleozoic history of continents:– major mountain-building activity along

continental margins

– 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

• 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

• The transgressing and regressing shallow seas– called epeiric seas– were a common feature of Paleozoic

• 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

• Mobile belts are elongated areas of mountain building activity – “orogenic activity”

• 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

• Geologists use – paleoclimatic data – paleomagnetic data – paleontologic data – sedimentologic data – stratigraphic data – tectonic data

Paleogeographic Maps

• At the beginning of the Paleozoic, six major continents were present including:– Baltica - Russia west of the Ural Mountains

and the major part of northern Europe– Gondwana - Africa, Antarctica, Australia,

Florida, India, Madagascar, and parts of the Middle East and southern Europe

– Laurentia - most of present North America, Greenland, northwestern Ireland, and Scotland

Six Major Paleozoic Continents

– 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

– Kazakhstan - a triangular continent centered on Kazakhstan, but considered by some to be an extension of the Paleozoic Siberian continent

– 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 (others)

• 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

• The geologic history of the North American craton may be divide into two parts

– stable continental interior over which epeiric seas transgressed and regressed

– mobile belts where mountain building occurred

Early Paleozoic Evolution of North America

• White areas represent sequences of rocks That are separated by large-scale unconformities shown in brown

Cratonic Sequences of N. America Cordilleraorogenies Appalachian oro-

genies

• A cratonic sequence is – a large-scale lithostratigraphic unit – representing a major transgressive-

regressive cycle – bounded by cratonwide unconformities

Cratonic 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

– 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

• Sauk sequence

• North America was located in a tropical climate at this time – there is no evidence of any terrestrial vegetation, NO

plants!– Rapid weathering and erosion of the exposed

Precambrian basement rocks– the transgressive phase of the Sauk – began with epeiric seas encroaching over the craton

The Sauk Sequence:Middle Cambrian Period

• 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

Transcontinental Arch

• During this time North America straddled the equator

• Trans-continental Arch

Cambrian Paleogeography of North America

• Many of the Sauk carbonates (limestones) 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 finer away from land– 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

marine environment 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 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

The Cambrian of the Grand Canyon Region

• A transgression covered – the Grand Canyon region. – The Tapeats Sandstone represents the shoreline

depositsare 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

Continued Transgression

• Cambrian strata exposed in the Grand Canyon

Cambrian Transgression

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

Arizona

• 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

• 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

• 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

The Tippecanoe Sequence

• Paleo-geography of North America– showing

change in the position of the the equator

• The continent – was rotating

counter-clockwise

Ordovician Period

• Resulted in 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

• 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 as today

Tippecanoe Reefs and Evaporites

present-day reefs are confined – To 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 Middle Silurian rocks of the present-day Great Lakes region Tippecanoe sequenceare 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

Michigan Basin Evaporites

• Paleogeography of North America during the Silurian Period

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

Silurian Period

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

in the Michigan Basin – approximately 1500 m of sediments were

deposited, – nearly half of which are halite and anhydrite

(gypsum)

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(Gypsum, Halite)

• 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

• 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 evaporite

deposition, • to alternation of reef growth and evaporite

deposition

– no model yet proposed completely explains various reef, carbonate, and evaporite facies

No Model Is Perfect

– where the first Phanerozoic orogeny – began during the Middle Ordovician– How would a mountain range influence – the climate and sedimentary history of the

craton?

The Appalachian Mobile Belt

• 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– Sandstone – shale - carbonate

Mountain Building

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-continent convergent

plate boundary

• the Appalachian mobile belt was born

Iapetus Ocean

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

Appalachian Mobile Belt

– with passive continental margins

– and large carbonate platforms

• 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 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

• 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 – coarse-

grained detrital sediments near the highlands

– 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 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

– Caledonian Orogeny

– was a mirror image of the Taconic Orogeny