Chapter 10 Early Paleozoic Earth History The First

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 Revolutionized Geology • The Carboniferous coal beds of England – helped fuel the Industrial Revolution, – during the late 1700 s and early 1800 s • 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 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 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 • 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-Illinois. Kentucky 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 barrier-reef 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, coarse -grained 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 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 (Na. Cl) – and rock gypsum (Ca. SO 4 • 2 H 2 O) altered to rock anhydrite (Ca. SO 4) – 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