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

Paleogeography 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)

Early Paleozoic Global History 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

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 Evolution of North America 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

Paleozoic History 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

Cratonic Sequence 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 Sequences of N. America

The Sauk Sequence 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

Transcontinental Arch 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

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

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

End of Cambrian Most of North America underwater Sauk Transgression Epeiric sea shallow inland sea (over continent) < 200 m deep

Cambrian Transgression Cambrian strata exposed in the Grand Canyon

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 Shark Bay, Australia http://www.mlssa.asn.au/journals/1999Journal.htm

Regression and Unconformity 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

Ordovician Period Paleogeography of North America showing change in the position of the the equator

What North American mountain range began to form in the Ordovician? Orogeny What North American mountain range began to form in the Ordovician? Appalachians

The Taconic Orogeny Named after present-day Taconic Mountains of eastern New York central Massachusetts and Vermont First of several orogenies to affect the Appalachian region

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

Appalachian Mobile Belt Middle Ordovician transition to convergence resulted in orogenic activity

Ordovician Plate Tectonics

Orogeny Timing 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

Clastic Wedge Debris eroded from mountains into adjacent basin Thick layer of sediment adjacent to source

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

Silurian Sedimentation