Early Paleozoic Earth History

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

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 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

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 seas in which the sediments were deposited

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 • Mobil 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 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

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, evaporates, 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 • 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 • 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

Silurian Period • Baltica moved northwestward relative • to Laurentia and collided with it • to form the larger continent of Laurasia • This collision, which closed the northern Iapetus Ocean, • 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

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 • That are separated by large-scale uncon-formities shown in brown • White areas represent sequences of rocks Appa-lachian oro-genies Cordilleran orogenies

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

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 relationships • 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 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

Cratonic Sequences of N. America • Brown areas = large-scale uncon-formities • White areas = sequences of rocks • 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 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

Cambrian Paleogeography of North America • During this time North America straddled the equator • Trans-continental 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 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

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 • The three formations exposed • along the Bright Angel Trail, Grand Canyon Arizona • Cambrian strata exposed in the Grand Canyon

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

Early Paleozoic Earth History