1. Early Paleozoic Earth History

2. 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

3. Early Paleozoic Climate of NA
North America in tropical location
End of Snowball Earth
Warmer in Cambrian and Ordovician

4. 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)

5. 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

6. Paleogeography of the World
For the Late Cambrian Period

7. Paleogeography of the World
For the Late Ordovician Period

8. Paleogeography of the World
For the Middle Silurian Period

9. 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

10. 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

11. 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

12. Cratonic Sequences of N. America

13. 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

14. 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

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

16. Cambrian Facies

17. 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,

18. Where were they deposited?
Fossils suggest marine environment
No land animals at this time
How do we interpret environment?
Use sedimentary structures!

19. Cross-stratification
Formed by wind or water
Wind – dunes > 1 m high
Water – typically smaller

20. Cross-stratification
2 m
WIND!
and homepage.ntlworld.com/donald.mcintyre/menu_files/stones_files/crossbeds_files/peddie_230.jpg

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

22. Cambrian Transgression
Cambrian strata exposed in the Grand Canyon

23. 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

24. Stromatolites Constrain water depth
need sunlight for photosynthesis
Photic zone – m
All evidence suggests very shallow Sauk Sea
Shark Bay, Australia

25. 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

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

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

28. 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

29. 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

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

31. Ordovician Plate Tectonics

32. 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

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

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

35. Silurian Sedimentation