Transcript No Slide Title

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 wellunderstood
• 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
• Transcontinental
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
2m
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
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-IllinoisKentucky areas
Silurian Sedimentation