Early Paleozoic Earth History

http://jan.ucc.nau.edu/~rcb7/510NAt.jpg

North American Paleogeography

• Broke away from supercontinent ~600 Ma

• Configuration of supercontinent not well-understood

• How do we know?– Evidence of rifting (divergent margin)– Basalt– Subsidence – cooling and thinning of lithosphere

Early Paleozoic Climate of NA

• North America in tropical location

• End of Snowball Earth

• Warmer in Cambrian and Ordovician

• Location of continents and ocean basins constantly changes

• Historical geology provides past geologic and paleogeographic reconstruction of the world

• Paleogeographic maps show – distribution of land and sea– possible climate regimes– geographic features (mountain ranges,

swamps, glaciers)

Paleogeography

• Cambrian world consisted of six major continents at low tropical latitudes

• Water circulated freely among ocean basins, and the polar regions were mostly ice free

Early Paleozoic Global History

• 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 divided into two parts– relatively stable continental interior over

which epeiric seas transgressed and regressed

– mobile belts where mountain building occurred

• Sedimentary-rock record of NA subdivided into six cratonic sequences

Early Paleozoic Evolution of North America

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

margins

– numerous shallow-water marine transgressions and regressions

• These transgressions and regressions – caused by global changes in sea level

– related to plate activity and glaciation

Paleozoic History

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

regressive cycle – bounded by unconformities

• The six unconformities extend across the North American craton

Cratonic Sequence

Cratonic Sequences of N. America

• Rocks of the Sauk Sequence– Late Proterozoic-Early Ordovician– record the first major transgression onto

the North American craton

• Deposition of marine sediments limited to the shelf areas of the craton

• The craton itself was above sea level

The Sauk Sequence

• By the Late Cambrian, Sauk Sea had covered most of North America, leaving above sea level only – a portion of the Canadian Shield – and a few large islands, 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

Cambrian Facies

Cambrian Facies• Sediment derived from craton

• Sandy facies occur closest to craton

• Quartz-rich sand derived from craton– craton surface weathered and eroded for

half a billion years!– no land plants yet – erosion by wind more

common– Mature sandstone: well-rounded, well-

sorted, 99% Qtz,

Where were they deposited?

• Fossils suggest marine environment

• No land animals at this time

• How do we interpret environment?

Use sedimentary structures!

Cross-stratification

• Formed by wind or water

• Wind – dunes > 1 m high

• Water – typically smaller

Cross-stratification

www.env.duke.edu/eos/geo41/win2.htm and homepage.ntlworld.com/donald.mcintyre/menu_files/stones_files/crossbeds_files/peddie_230.jpg

2 m

WIND!

End of Cambrian• Most of North America underwater

• Sauk Transgression

• Epeiric sea– shallow inland sea (over continent)– < 200 m deep

• Cambrian strata exposed in the Grand Canyon

Cambrian Transgression

Beginning of Ordovician• Very little land remaining above SL

• Little terrigenous clastic sedimentation

• Shift to carbonate deposition– limestones formed from shell debris– clear, shallow water

Stromatolites• Constrain water depth

– need sunlight for photosynthesis– Photic zone – 150-200 m

• All evidence suggests very shallow Sauk Sea

http://www.mlssa.asn.au/journals/1999Journal.htm

Shark Bay, Australia

• As the Sauk Sea regressed during the Early Ordovician, it revealed a landscape of low relief

• The rocks exposed were predominately limestones

• The resulting craton-wide unconformity marks the boundary between the Sauk and Tippecanoe sequences

Regression and Unconformity

• Paleogeography of North America showing change in the position of the the equator

Ordovician Period

What North American mountain range began to form

in the Ordovician?

Appalachians

Orogeny

• Named after present-day Taconic Mountains of– eastern New York– central Massachusetts– and Vermont

• First of several orogenies to affect the Appalachian region

The Taconic Orogeny

Appalachian Area

• East coast was passive margin in Early Ordovician – like modern Gulf of Mexico

• Changed to active margin in Middle Ordovician

• Collision with microcontinent or volcanic arc

• Middle Ordovician transition to convergence resulted in orogenic activity

Appalachian Mobile Belt

Ordovician Plate Tectonics

• Volcanic rocks from present-day Georgia to Newfoundland

• Clustering of radiometric ages between 440 to 480 million years ago

• In addition, regional metamorphism coincides with the radiometric dates

Orogeny Timing

Clastic Wedge

Debris eroded from mountains into adjacent basinThick layer of sediment adjacent to source

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

Silurian Period

Silurian Sedimentation