Chapter 10. Early Paleozoic Events

Transcription

1 Chapter 10 Early Paleozoic Events

2 The Phanerozoic Eon Consists of three eras (from oldest to youngest): Paleozoic = "ancient life" ( m.y. ago) Mesozoic = "middle life" ( m.y. ago) Cenozoic = "recent life" (65.5 m.y. ago - present)

3 Paleozoic Era Paleozoic can be divided into: Early Paleozoic = Cambrian, Ordovician and Silurian Late Paleozoic = Devonian, Mississippian, Pennsylvanian, and Permian

4 Paleozoic overview

5 Paleozoic orogenic belts

6 Orogenic Belts Orogenic belts are present along the edges of the continent. In the orogenic belts, strata are intensely deformed, with folding, faulting, metamorphism, and igneous intrusions. Deformation occurred as a result of continental collision.

7 Orogenies In the Appalachian region, there were three Paleozoic mountain-building events (or orogenies): Taconic orogeny Acadian orogeny Alleghanian orogeny

8 Paleozoic Rocks of the Platform Across the platform, in the continental interior, Paleozoic strata are relatively flatlying to gently dipping, and warped into basins, domes, arches, and broad synclines.

9 Paleozoic Paleogeography Paleogeography = "ancient geography." The ancient geographic arrangement of the continents. Reconstructing the paleogeography requires paleomagnetic, paleoclimatic, geochronologic, tectonic, sedimentologic, and biogeographic fossil data.

10 Paleozoic Paleoclimates Paleoclimatic evidence comes from environmentally-sensitive sedimentary rocks (glacial deposits, coal swamp deposits, reef carbonates, evaporites). Early Paleozoic climate was affected by several factors: The Earth spun faster and had shorter days. Tidal effects were stronger because the Moon was closer to Earth. No vascular plants were present on the land.

11 Neoproterozoic Paleogeography Just before Paleozoic began, the Precambrian supercontinent, Rodinia, had rifted apart to form six large continents and several smaller continents.

12 Early Paleozoic Continents 1. Laurentia (North America, Greenland, Ireland, and Scotland) 2. Baltica (Northern Europe and western Russia) 3. Kazakhstania (between the Caspian Sea and China) 4. Siberia (Russia east of the Ural Mtns and north of Mongolia) 5. China (China, Indochina, and the Malay Peninsula) 6. Gondwana (Africa, South America, India, Australia, Antarctica)

13 When continents are located on a pole, if conditions are right, glaciers will form. During glaciations, sea level is lowered worldwide because the water is tied up in the ice sheets. Shallow epicontinental seas are unlikely during glaciations.

14 By Late Cambrian, the continents moved off the pole. Some continents lie on the equator. Glaciers melted, sea levels rose, and shallow epicontinental seas flooded the continents.

15 Transgressions and Regressions Shallow epicontinental seas transgressed across the Laurentian (North American) craton during Early Paleozoic as the glaciers melted and sea level rose. The seas regressed as the glaciers enlarged and sea level dropped.

16 Transgressive-Regressive Sequences The transgression and regression of the seas deposited sequences of sedimentary rocks that reflect the deepening and shallowing of the waters. These are called transgressive-regressive sequences.

17 Epicontinental Seas Wave-washed sands, muds, and carbonates were deposited in the shallow epicontinental seas. The epicontinental seas were sites of major diversification of marine life.

18 Unconformities During regressions, the former seafloor was exposed to erosion, creating extensive unconformities that mark the boundaries between the transgressive-regressive sequences.

19 Cratonic Sequences The unconformities can be used to correlate particular sequences from one region to another. The unconformity-bounded sequences are sometimes called cratonic sequences. Two major transgressions occurred during Early Paleozoic in North America: Sauk sequence (older - primarily Cambrian) Tippecanoe sequence (Ordovician-Silurian)

20 North American cratonic sequences Green = sedimentary deposits Yellow = missing strata associated with unconformities

21 Worldwide Sea Level Change Similar transgressive-regressive sequences are found on other continents, suggesting that worldwide sea level change caused the transgressions and regressions. Worldwide sea level changes were probably related to glaciations and/or sea floor spreading. During times of rapid sea floor spreading, mid-ocean ridge volcanism displaces sea water onto the continents.

22 Cambrian Paleogeography Laurentia is nearly covered by shallow epicontinental seas. Laurentia lies on the equator, so water is warm. Deposition of sand & carbonate sediments Water deepens toward edges of continent, where shale is deposited

23 The Base of Cambrian The base of Cambrian was formerly identified by the first-occurrence of shell-bearing organisms such as trilobites. During the 1970s, small shelly fossils were found below the first trilobites, and dated at 544 m.y. The small shelly fauna includes sponge spicules, brachiopods, molluscs, and possibly annelids.

24 The Base of Cambrian The base of Cambrian is now placed at the oldest occurrence of feeding burrows of the trace fossil Trichophycus, and dated at 542 m.y.

25 Cambrian Sedimentary Deposits - The Sauk Sequence During Cambrian, there were no vascular plants on the land, so the landscape was barren. Erosion was active and severe without plant roots to hold the soil. After Neoproterozoic glaciation, the sea transgressed onto the craton. Shoreline (beach) deposition produced a vast apron of clean quartz sand. Carbonate deposition occurred farther from land.

26 Cambrian Deposits of the Grand Canyon Region In the Grand Canyon region, the Lower Cambrian Tapeats Sandstone is an example of the sandy beach deposits unconformably overlying Precambrian rocks.

27 Cambrian Deposits of the Grand Canyon Region Tapeats Sandstone is overlain by Bright Angel Shale, an offshore deposit. Bright Angel Shale is overlain by Muav Limestone, deposited farther from land. These rocks form a transgressive sequence.

28 Cambrian Deposits of the Grand Canyon Region These sedimentary units are diachronous (i.e., they cut across time lines). In each case, the sedimentary units are older in the west than in the east. The red lines are trilobite zones, which approximate time lines.

29 Cambrian Deposits of the Grand Canyon Region The three facies (sandstone, shale, and limestone) coexisted and migrated laterally as sea level rose. The Bright Angel Shale is Lower Cambrian in the west, and Middle Cambrian in the east.

30 Cambrian Deposits of the Grand Canyon Region Near the end of Early Ordovician, the seas regressed (due to glaciation). The Muav Limestone was exposed to subaerial erosion and a widespread unconformity developed.

31 Comparison of Cambrian and Ordovician Paleogeography

32 Ordovician Paleogeography Note the mountains and volcanoes in the Appalachian region. Volcanic ash deposits are found in Ordovician rocks throughout the eastern U.S. (Now altered to a clay called bentonite).

33 Ordovician orogenies The Taconic Orogenic Belt lay between Laurentia (North America) and Baltica (Europe and western Russia) during Ordovician.

34 Plate tectonic cross-section showing forces that caused the Taconic Orogeny.

35 A - Eastern North America during Cambrian and Early Ordovician, following the breakup of Rodinia. B - Large volcanic island arc nears eastern North America. C - Volcanic island arc collides with eastern North America causing Taconic orogeny.

36 Volcanic Island Arc Collides with Eastern North America As the Iapetus Ocean narrowed, a volcanic island arc approached and collided with the North American craton, causing folding, faulting, metamorphism, and mountain building. This mountain-building event in the Appalachian region is called the Taconic orogeny ( m.y. ago).

37 Upper Ordovician Sedimentary Deposits As the Taconic mountain belt eroded, Upper Ordovician to Lower Silurian red sandstones and shales were deposited to the west in huge delta systems.

38 Upper Ordovician Sedimentary Deposits These sediments formed a wedge-shaped deposit known as the Queenston clastic wedge, or the Queenston delta. Red deltaic sediments coarsen and thicken to the east (toward the mountainous source area), and become thinner and finer grained to the west.

39 Upper Ordovician Sedimentary Deposits The size of the clastic wedge suggests that the mountains may have been more than 4000 m (13,100 ft) high. There were two main highland areas; the higher of the two was in the northern Appalachians.

40 Caledonian Orogenic Belt The Caledonian orogenic belt (which extends along the northwestern edge of Europe) is part of the same trend as the Taconic orogenic belt. The Caledonian orogeny reached its climax slightly later, during Late Silurian to Early Devonian. The Caledonian event is recognized in the Canadian Maritime Provinces, northeastern Greenland, northwestern Great Britain, and Norway.

41 Comparison of Ordovician and Silurian Paleogeography

42 Silurian global conditions Silurian sea levels were high worldwide. In Laurentia (North America), much of the craton was flooded, indicating melting of Late Ordovician glaciers. This was the second major transgression during Paleozoic, which deposited the Tippecanoe Sequence.

43 Silurian Paleogeography Mountains in eastern N. America are eroding. Sandstone & conglomerat deposits. Widespread carbonate deposition. Deep marine deposits in NW and SE U.S. Reefs and evaporites.

44 Silurian Sedimentary Deposits As the Tippecanoe Sea flooded North America, deposition began with nearshore sands. These include the famous St. Peter Sandstone, an unusually pure, well sorted, well rounded quartz sandstone. Silurian Tuscarora Sandstone was deposited in the central Appalachian region.

45 Silurian Sedimentary Deposits Sandstone is overlain by extensive limestone deposits, locally replaced by dolomite. In eastern U.S., limestones are overlain by and interbedded with shales along the periphery of the Queenston delta. Niagara Falls is a classic locality where these rocks are exposed.

46 Silurian Michigan Basin Evaporites Near the end of the Tippecanoe sequence, reef-fringed basins developed, such as the Michigan Basin. Evaporation led to the precipitation of immense quantities of rock salt and gypsum within the basin, indicating an arid paleoclimate. Evaporite minerals total over 2500 ft (750 m) thick in the Michigan Basin.

47 Silurian Iron Ore Economically important sedimentary iron ore deposits accumulated during Silurian in the southern Appalachians, particularly around Birmingham, Alabama. Steel was produced for many years in Birmingham from this iron ore. Fuel was supplied by nearby Late Paleozoic coal deposits. Limestone, also found nearby, was used as flux in the blast furnace.

48 Silurian Orogenic Activity Orogenic activity (mountain building) was more or less continuous at one place or another during Silurian and Devonian. The Caledonian orogeny was most intense in Norway, as the Iapetus Ocean closed. The folded rocks of the Caledonians end in Ireland, but can be traced to NE Greenland, Newfoundland, and Nova Scotia, Canada.

49 Paleozoic review