Early Paleozoic Earth History

1. Early Paleozoic Earth History http://jan.ucc.nau.edu/~rcb7/510NAt.jpg

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! www.env.duke.edu/eos/geo41/win2.htm 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 – 150-200 m • All evidence suggests very shallow Sauk Sea Shark Bay, Australia http://www.mlssa.asn.au/journals/1999Journal.htm

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. 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