Early Paleozoic Earth History

2. Chapter 10 Early Paleozoic Earth History

3. The First Geologic Map • William Smith, • a canal builder, published the first geologic map • on August 1, 1815

4. Paleozoic History • The Paleozoic history of continents: • major mountain-building activity along continental margins • numerous shallow-water marine transgressions and regressions over their interiors • These transgressions and regressions • were caused by global changes in sea level • that most probably were related • to plate activity and glaciation

8. Continental Architecture • Cratons are the relatively stable • and immobile parts of continents • and form the foundation upon which • Phanerozoic sediments were deposited • Cratons typically consist of two parts • a shield • and a platform

9. Epeiric Seas • The transgressing and regressing shallow seas • called epeiric seas • were a common feature of Paleozoic • Continental glaciation • as well as plate movement • caused changes in sea level • and were responsible for the advance and retreat • of the seas in which the sediments were deposited

10. Mobile Belts • Mobile belts are elongated areas of mountain building activity – “orogenic activity” • along the margins of continents • where sediments are deposited in the relatively shallow waters of the continental shelf • and the deeper waters at the base of the continental slope • During plate convergence along these margins, • the sediments are deformed • and intruded by magma • creating mountain ranges

11. Paleogeographic Maps • Geologists use • paleoclimatic data • paleomagnetic data • paleontologic data • sedimentologic data • stratigraphic data • tectonic data

12. Six Major Paleozoic Continents • At the beginning of the Paleozoic, six major continents were present including: • Baltica- Russia west of the Ural Mountains and the major part of northern Europe • Gondwana - Africa, Antarctica, Australia, Florida, India, Madagascar, and parts of the Middle East and southern Europe • Laurentia - most of present North America, Greenland, northwestern Ireland, and Scotland

13. Six Major Paleozoic Continents (others) • China- a complex area consisting of at least three Paleozoic continents that were not widely separated and are here considered to include China, Indochina, and the Malay Peninsula • Kazakhstan - a triangular continent centered on Kazakhstan, but considered by some to be an extension of the Paleozoic Siberian continent • and Siberia - Russia east of the Ural Mountains and Asia north of Kazakhstan and south Mongolia • Besides these large landmasses, geologists have also identified • numerous small microcontinents • and island arcs associated with various microplates

14. Paleogeography of the World • For the Late Cambrian Period

15. Paleogeography of the World • For the Late Ordovician Period

16. Paleogeography of the World • For the Middle Silurian Period

17. Early Paleozoic Evolution of North America • The geologic history of the North American craton may be divide into two parts • stable continental interior over which epeiric seas transgressed and regressed • mobile belts where mountain building occurred

18. Cordilleraorogenies Cratonic Sequences of N. America • Appalachian oro-genies • White areas represent sequences of rocks That are separated by large-scale unconformities shown in brown

19. Cratonic Sequence • A cratonic sequence is • a large-scale lithostratigraphic unit • representing a major transgressive-regressive cycle • bounded by cratonwide unconformities

20. The Sauk Sequence • Rocks of the Sauk Sequence • during the Neoproterozoic-Early Ordovician • record the first major transgression onto the North American craton • Deposition of marine sediments • was limited to the passive shelf areas of the • Appalachian and Cordilleran borders of the craton • The craton itself was above sea level • and experiencing extensive weathering and erosion

21. White areas = sequences of rocks • Sauk sequence

22. The Sauk Sequence:Middle Cambrian Period • North America was located in a tropical climate at this time • there is no evidence of any terrestrial vegetation, NO plants! • Rapid weathering and erosion of the exposed Precambrian basement rocks • the transgressive phase of the Sauk • began with epeiric seas encroaching over the craton

23. Transcontinental Arch • By the Late Cambrian, • the epeiric seas had covered most of North America, • leaving above sea level only • a portion of the Canadian Shield • and a few large islands • These islands, • collectively named the Transcontinental Arch, • extended from New Mexico • to Minnesota and the Lake Superior region

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

25. Sauk Carbonates • Many of the Sauk carbonates (limestones) are • bioclastic • composed of fragments of organic remains • contain stromatolites, • or have oolitic textures • contain small, spherical calcium carbonate grains • Such sedimentary structures and textures • indicate shallow-water deposition

26. A Transgressive Facies Model • Sediments become finer away from land • coarse detrital sediments are typically deposited in the nearshore environment, • and finer-grained sediments are deposited in the offshore environment • Carbonates form farthest from land in the marine environment beyond the reach of detrital sediments

27. A Transgressive Facies Model • Recall that facies are sediments • that represent a particular environment • During a transgression, the coarse (sandstone), • fine (shale) and carbonate (limestone) facies • migrate in a landward direction

28. The Cambrian of the Grand Canyon Region • This region provides an excellent example • of sedimentation patterns of a transgressing sea • The region of the Grand Canyon occupied • the western margin of the craton during Sauk time, • a passive shelf • During Neoproterozoic and Early Cambrian time, • most of the craton was above sea level • deposition of marine sediments • was mainly restricted to the margins of the craton • on continental shelves and slopes

29. Transgression • A transgression covered • the Grand Canyon region. • The Tapeats Sandstone represents the shoreline depositsare clean, well-sorted sands • of the type one would find on a beach today • As the transgression continued into the Middle Cambrian, • muds of the Bright Angle Shale • were deposited over the Tapeats Sandstone

30. Continued Transgression • The Sauk Sea had transgressed so far onto the craton • by the Late Cambrian that • in the Grand Canyon region • carbonates of the Muav Limestone were being deposited over the Bright Angel Shale • This vertical succession of • sandstone (Tapeats) • shale (Bright Angel) • and limestone (Muav) • forms a typical transgressive sequence

31. Cambrian Transgression • Cambrian strata exposed in the Grand Canyon • The three formations exposed • along the Bright Angel Trail, Grand Canyon Arizona

32. Cambrian Transgression • Cambrian strata exposed in the Grand Canyon • Observe the time transgressive nature of the three formations • The three formations exposed • along the Bright Angel Trail, Grand Canyon Arizona

33. Upper Cambrian Sandstone • Outcrop of cross-bedded Upper Cambrian sandstone in the Dells area of Wisconsin

34. Regression and Unconformity • As the Sauk Sea regressed • from the craton during the Early Ordovician, • it revealed a landscape of low relief • The rocks exposed were predominately • limestones and dolostones • that experienced deep and extensive erosion • because North America was still located in a tropical environment • The resulting cratonwide unconformity • marks the boundary between the Sauk • and Tippecanoe sequences

35. Cratonic Sequences of N. America • brown areas = large-scale uncon-formities • White areas = sequences of rocks • Regression • Tippecanoe sequence

36. The Tippecanoe Sequence • A transgressing sea deposited the Tippecanoe sequence over most of the craton • Middle Ordovician-Early Devonian • Like the Sauk sequence, this major transgression deposited clean, well-sorted quartz sands • The Tippecanoe basal rock is the St. Peter Sandstone, • an almost pure quartz sandstone used in manufacturing glass

37. Ordovician Period • Paleo-geography of North America • showing change in the position of the the equator • The continent • was rotating counter-clockwise

38. Transgression of the Tippecanoe Sea • Resulted in deposition of • the St. Peter Sandstone • Middle Ordovician • over a large area of the craton

39. St. Peter Sandstone • Outcrop of St. Peter Sandstone in Governor Dodge State Park, Wisconsin

40. The Tippecanoe Sequence • The Tippecanoe basal sandstones were followed by widespread carbonate deposition • The limestones were generally the result of deposition • by calcium carbonate-secreting organisms such as • corals, • brachiopods, • stromatoporoids, • and bryozoans

41. Tippecanoe Reefs and Evaporites • Organic reefs are limestone structures • constructed by living organisms, • some of which contribute skeletal materials to the reef framework • Today, corals, and calcareous algae • are the most prominent reef builders, • but in the geologic past other organisms • played a major role in reef building • Reefs appear to have occupied • the same ecological niche in the geological past as today

42. Modern Reef Requirements present-day reefs are confined • To between 30 degrees north and south of the equator • Corals, • the major reef-building organisms today, • require warm, clear, shallow water • of normal salinity for optimal growth

43. Present-Day Reef Community • with reef-building organisms

44. Reef Environments • Block diagram of a reef showing the various environments within the reef complex

45. Michigan Basin Evaporites • The Middle Silurian rocks of the present-day Great Lakes region Tippecanoe sequence are reef and evaporite deposits • The most significant structure in the region • the Michigan Basin • is a broad, circular basin surrounded by large barrier reefs • These reefs contributed to increasingly restricted circulation • and the precipitation of Upper Silurian evaporites within the basin

46. Silurian Period • Paleogeography of North America during the Silurian Period • Reefs developed in the Michigan, Ohio, and Indiana-Illinois-Kentucky areas

47. Tippecanoe Regression and Evaporites • As the Tippecanoe Sea gradually regressed • from the craton during the Late Silurian, • precipitation of evaporite minerals occurred in the Michigan Basin • approximately 1500 m of sediments were deposited, • nearly half of which are halite and anhydrite (gypsum)

48. Origin of Thick Evaporites(Gypsum, Halite) • How did such thick sequences of evaporites accumulate? 1. When sea level dropped, the tops of the barrier reefs were as high as or above sea level, • thus preventing the influx of new seawater into the basin • Evaporation of the basinal seawater would result in the precipitation of salts 2. Alternatively, the reefs grew upward so close to sea level • that they formed a sill or barrier that eliminated interior circulation

49. Silled Basin Model • Silled Basin Model for evaporite sedimentation by direct precipitation from seawater • Vertical scale is greatly exaggerated

50. Basin Brines • Because North America was still near the equator during the Silurian Period, • temperatures were probably high