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I. Lesson Concepts:

• cratonic sequenques
• facies maps and their interpretation
• isopach maps and their interpretation
• paleogeographic map constructions and limits on conjecture

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II. Introduction

• The Cambrian and Ordovician were periods of relatively stability for North America.
• For much of the time, N. America had passive margins which experienced a major transgression, regression, and transgression again.
• The first of these transgressions deposited the Sauk cratonic sequence (M. Cambrian-E. Ordovician) and the second the Tippecanoe sequence (M. Ordovician-Late Silurian).
• During the Ordovician, the eastern passive margin turned to an active margin, as the Iapetus Ocean closed, causing the Taconic orogeny.

IV. Climate and Paleogeography

A. Paleogeography

• Cambrian: Poles in oceanic areas, no land north or south of 60 degrees
• Ordovician: portions of Gondwana move to polar postion
1. Isolated continents in well conected sea
2. Iapetus Ocean separates N. America from Baltica and Gondwanaland (African portion)
3. Baltica begins to moved rapidly toward Laurentia closing the Iapetus Ocean and leading to the first Paleozoic tectonic cycle (Taconic Orogeny)
4. North America and Laurentia astride equator
5. Gonwanaland moved more southward leading to glaciation in its southern extent

B. Climate

• continents without vegetation (absorb more of sun's radiation)
• transgressions further warm and moisten continent interiors- why?
• Early Cambrian stll warming from cold Proterozoic conditions
• By Late Cambrian most continents temperate, except portions of Gondwana
• Cooling in Late Ordovician causing extinctions
• glacial deposits in N. Africa
• sea level drop

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V. Cambrian

• Supercontinent break-up and rise of sea
• Sauk transgression (continues through early Ordovician)
• examination of passive margin sedimentation
• Grand Canyon sequence
• interpretation of isopach and sediment facies maps
• Transcontinetal Arch
• Transcontinental Arch
• mature quartz sandstones-how do they form?
• modern vs. Cambrian paleoenvironments

VI. Ordovician

• Terminal Ordovician Extinction
1. 100 families
2. crinoids-stromatoporoid-tabulate coral-rugose
3. more tropical groups extinct, likely response to cooling
4. extinctions compressed biogeographic zones toward equator
• what does this mean?
• Mid-Ordovician unconformity overlain by St. Peter Sandstone
• base of Tippecanoe Sequence
• Late Ordovician transgression, major inundation
1. second major transgression
2. St. Peter Sandstone covers 20,000 sq. km. (basal transgressive sand)
• pure quartz sandstone-discussion of Early Paleozoic weathering conditions
• recycled older sandstones
3. shelly facies (e.g. Ohio)
4. somewhat deeper than the Sauk
• Transcontinental Arch
• Middle to Late Ordovician of eastern margin
• prior passive margin conditions cease on the carbonate platform (shelf) and adjacent continental rise
• carbonate sedimentation halts, platform subsides to form foreland basin
1. black shales (graptolite facies) and immature sands spread westward over former carbonate platform
• graptolitic shale-graywacke (flysch)
• volcanics-bentonites
2. later flysch deposits covered by clastic red beds (molasse) as the foreland basin changed from marine to non-marine
• together the westward thinning basin of clastic sediments is called a clastic wedge, called the Queenston delta or clastic wedge (600,000 km³ of eroded rock).
• thrust faulting (Logan's Line) of continental rise sediments across 48 km of shelf and shield
• Tectonics:
1. mid to late Ordovician (south to north) thrust loading of the margin
2. foreland basin development
3. volcanic arc collision as Iapetus Ocean continues to close
4. orogeny called Taconic Orogeny after Taconic Mts. of New York
• Development of a highly successful type of reef community that would dominate reefs through much of the Paleozoic: tabulate and rugose corals, stromatoporoids

1. cratonic sequence

2. Sauk sequence

3. Tippecanoe sequence

4. Iapetus Ocean

5. flysch

6. bentonites

7. Transcontinental Arch

8. isopach

9. sediment facies

10. passive margin

11. active margin

12. foreland basin and thrust loading

13. molasse

14. clastic wedge

15. Taconic orogeny

16. ophiolite

17. Tippecanoe Sequence

18. Queenston delta

19. Laurentia

20. Baltica

21. Gondwanaland

22. St. Peter Sandstone