Ch10 Early Paleozoic Events Notes.docx

The Earth Through Time Chapter 10—Early Paleozoic Events CHAPTER OUTLINE FOR TEACHING I. Early Paleozoic Periods A. Cambrian: began 542 million years ago B. Ordovician C. Silurian: ended 416 million years ago II. Overview A. Larger Continents of Early Paleozoic 1. Laurentia 2. Baltica 3. Kazakhstania 4. Siberia 5. China 6. Gondwana B. Plate Tectonic Events (in order) 1. Breakup of Rodinia (proto-Pangaea) 2. Oceanic closing and orogeny to form Pangaea a. Taconic orogeny b. Acadian orogeny c. Alleghenian orogeny d. Caledonian orogeny e. Hercynian orogeny C. Paleogeographic Areas 1. Stable Interiors a. domes b. basins 2. Orogenic Belts III. Base of the Cambrian A. Former Marker: First appearance of shelly animals B. Current Marker: Lowest occurrence of trace fossil Trichophycus IV. Early Paleozoic Events A. Cratonic Sequences and Sea-Level Change 1. Sauk a. first major transgression b. transgressive sedimentary sequence 2. Tippecanoe a. second major transgression b. transgressive sequence terminated by evaporites 3. Kaskaskia 4. Absaroka B. Lithologic Evidence 1. Limestone (shallow marine, subtropical) 2. Evaporites (equatorial dry conditions) 3. Lithic sandstone and graywacke (mountain uplift) 4. Arkose (arid conditions) 5. Tillites (glacial deposits) 6. Quartz sandstone (shorelines, deserts) 7. Shales (deep marine; graptolite-bearing deposits) C. Orogenies of Early Paleozoic 1. Cordilleran orogeny begins 2. Rifting and displacement of Rodinia 3. Eastern U.S. orogeny begins a. Taconic orogeny b. vast bentonites from volcanic ash deposition 4. Ouachita-Marathon trough deformation 5. Caledonian orogenic belt a. Iapetus ocean expansion b. subsequent ocean contraction V. Early Paleozoic Climates A. Transgressions = mild climates, windswept low terrains B. Regressions and Orogenic Episodes = harsher more diverse climates; winds diverted by mountains C. Earth Rotation was Faster (days shorter, tidal effects greater) D. No Land Plants (Cambrian-Ordovician) 1. solar radiation reflected, not absorbed 2. severe temperature differences resulting E. End of Late Proterozoic Glacial Cycle: cool beginning for Early Paleozoic Answers to Discussion Questions 1. Book your vacation for Laurentia, Baltica, Kazakhstania, Siberia, China, and Gondwana. 2. Rock formations thin and (or) pinch out along the transcontinental arch indicating that it was a feature above sea level during most of Early Paleozoic. 3. A barred basin is a low area below sea level wherein sea water inflow and outflow is restricted by organic reefs, a raised sill, or a submerged bar. These kinds of basins are particularly effective in producing evaporite deposits because they generally restrict both inflow and outflow of water. Such a basin developed in the area of the lower peninsula of Michigan during Silurian. 4. A clastic wedge is a great mass of sedimentary rock representing sediments which have been shed off a rising mountain range during and after an orogenic event. The Queenston clastic wedge was shed off the Taconic Mountains during Early Paleozoic. 5. Sequence-bounding unconformities are the result of global changes in sea level as indicated by their development on all the world’s continents. If they were the result of an uplift of the North American craton, all other cratons would have had to move simultaneously for the same result. 6. The Sauk regression and Tippecanoe transgression were not instantaneous events but occurred over several million years thus an age difference across the unconformity surface, which separates the two sequences. 7. A dome is a radial symmetrical uplift of crust which was active during deposition. A basin is a similarly symmetrical feature which is negative or subsiding during deposition. In dome stratigraphy, strata thin toward the center or axis and unconformities become more numerous and notable toward the center or axis. In a basin, strata thicken toward the center and unconformities occur near the margins. On a geologic map, both may show a “bull’s eye” pattern, but the oldest layers will be in the center of a dome, the youngest at center of a basin. 8. Trace fossils are indirect evidence of life such as trails, tracks, burrows, borings, etc. Trace fossils may be more susceptible to destruction than other fossils under some conditions. Further, two or more organisms may make a very similar trace, thus confusing identification. 9. The paleoequator may be determined by paleomagnetic studies with precision, or by studies of rock and fossil distribution (e.g., reefs). Arid regions might be identified by deposits of evaporites and quartz sandstone with eolian cross-bedding. Glacial episodes would be identified by tillites, glacial lake deposits, glacial striations, moraines, and melt-water deposits. Mountain building due to plate collision produces folded and thrust-faulted strata, ophiolite belts, and great clastic wedges. Volcanics are also associated with these events. 10. Graptolites were attached to floating masses that allowed them to float widely through Cambrian and Ordovician seas. Graptolites evolved rather quickly and were dispersed widely in a short time. 11. d 12. a Chapter Activities Student activities for in-depth learning. 1. The current marker for the bas of the Cambrian System is the elongate trace fossil, Trichophycus. Take a look at the discussion and images available at this German university web page (in English) and look for images of this fossil: http://www.palaeontologie.uni-wuerzburg.de/Stuff/casu8.htm. Write a short summary of the global significance of this trace fossil both as a basal marker for the Cambrian and also as a fossil of evolutionary significance. There are other pages featuring images and discussion of Trichophycus, so take a look at those as well. Make a sketch of this trace fossil and the type of organism that you think might have made this trace. What is the significance of Fortune Head, Newfoundland? Why are such places so important in paleontology and stratigraphy? 2. Using the various links at this climate page (http://www.scotese.com/climate.htm), which is maintained by the PaleoMap Project, describe the climates that persisted over the Early Paleozoic (Cambrian-Silurian). What is the basis for the climatic interpretation (see links on the left side of the page)? Make a diagram showing how global average temperature is thought to have fluctuated over the Cambrian-Silurian time interval. Chapter 10—Early Paleozoic Events CHAPTER OVERVIEW Phanerozoic rocks are more accessible, less altered, and more fossiliferous than rocks of the earlier eons and therefore they give us a more complete picture of Earth’s history. The eon we call Phanerozoic is divided into Paleozoic (“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”). This chapter looks specifically at Early Paleozoic and specifically the oldest three geologic periods: Cambrian, Ordovician, and Silurian. These three periods together lasted about 126 million years. The geologic history of the Paleozoic is characterized by long periods of sedimentation, and punctuated by intervals of mountain building. In North America, the mountain-building events are called Taconic, Acadian, and Allegheny. During Late Neoproterozoic, the supercontinent Rodina broke up into six large continents and several smaller microcontinents. As these continents drifted, ocean tracts opened and closed and subduction zones developed that led to multiple thrust faults and orogenies. There was relatively little activity on the interior of the continents with the site of intense deformation, igneous activity, and earthquakes at the orogenic belts. Fossils of carbonate secreting marine organisms indicate that shallow seas were common throughout much of the Earth during this time interval. The chapter discusses the transgression of major seas that led to a variety of deposits. These sequences of transgression and regression of the seas are named Sauk, Tippecanoe, Kaskaskia, and Absaroka. Each of these is described in detail. With no evidence of compressional deformation, it is clear that western North America was a passive margin during the early Paleozoic. Orogenic activity was more or less continuous at one place or another during Silurian and Devonian. While generally warm conditions dominated Laurentia during Early Paleozoic, there were some time intervals of aridity. Extensive glacial deposits suggest frigid conditions in the region of today’s Sahara Desert. Other factors that affected the climate included shorter days, stronger tidal effects, and the absence of vascular plants. LEARNING OBJECTIVES By reading and completing information within this chapter, you should gain an understanding of the following concepts: Describe the breakup of the supercontinent Rodina, including the resulting land masses and their locations. Discuss the continental framework of Early Paleozoic using the events of the more stable interiors. Consider the terms craton, platform, dome, basin, and orogenic belt. Explain the approximate duration of the Sauk, Tippecanoe, Kaskaskia, and Absaroka sequences and how they correspond to Paleozoic orogenic events. Describe the events in the Cordillera including any deformational activities. Discuss dynamic events in eastern North America including deposition and the development of the Taconic Orogeny. Discuss aspects of Early Paleozoic climates including indicators such as glacial deposits, evaporites, fossiliferous limestones, extensive reefs, massive limestone deposits, etc. CHAPTER OUTLINE Dance of the Continents Some Regions Tranquil, Others Active The Stable Interior Orogenic Belts Identifying the Base of the Cambrian Early Paleozoic Events Cratonic Sequences: the Seas Come In, the Seas Go Out The Sauk and Tippecanoe Sequences The First Major Transgression Islands in the Inland Sea Cambrian Rocks in the Southwest: A Transgressive Succession The Second Transgression Evaporites Mark the Sea’s “Last Stand” Way Out West: Events in the Cordillera In the Beginning, a Passive Margin Early Paleozoic Rocks Orogeny Begins Deposition in the Far North Dynamic Events in the East Taconic Orogeny Northern Appalachian Region Southern Appalachian Region The Caledonian Orogenic Belt Ocean Expansion Ocean Contraction Aspects of Early Paleozoic Climate Key Terms (pages given in parentheses) Absaroka Sequence (283): A sequence of Permian-Pennsylvanian sediments bounded both above and below by a regional unconformity and providing a record an episode of marine transgression over an eroded surface, full flood level of inundation, and regression from the craton. Acadian Orogeny (277): An episode of mountain building in the northern Appalachians during the Devonian Period. Alleghenian orogenic belt (277): The belt of rocks formed by mountain building tectonics of the Allegheny Orogeny in North America. Allegheny Orogeny (277): Late Paleozoic episodes of mountain building along the present trend of the Appalachian Mountains. Likely caused by the collision, occurring during Late Carboniferous, of northwest Africa and the southern part of the Appalachian belt. The third of the orogenies that built the Appalachian Mountains of North America. barred basin (287): A basin which had its opening to the sea restricted by a raided sill (or possibly also by a submerged bar). bentonite (293): Bentonite is composed of clay minerals that are formed by the alteration of volcanic ash. Cretaceous rocks of Wyoming and Colorado include extensive beds of this soft, plastic, light-colored clayey rock. Caledonian orogenic belt (296): An orogenic belt extending from Ireland and Scotland northwestward through Scandinavia. Created by the Caledonian Orogeny during Early Paleozoic episode of mountain building in Europe. Caledonian Orogeny (277, 297): A major Early Paleozoic episode of mountain building affecting Europe, which created an orogenic belt, the Caledonides, extending from Ireland and Scotland northwestward through Scandinavia. Cordillera (287): The region of North America that consists of the entire system of more-or-less parallel mountain ranges that extend westward from the Rocky Mountains to the Pacific coast. It includes the Rocky Mountains, Sierra Nevada, and Coast Ranges. cratonic sequences (283): Repeated advances (transgressions) and retreats (regressions) of epicontinental seas. The regressions exposed old seafloors to erosion, creating extensive unconformities that mark the boundaries of each transgressive-regressive cycle of deposition. graptolites (287): Extinct colonial marine invertebrates considered to be protochordates. Graptolites range from the Late Cambrian to the Mississippian. Hercynian orogeny (277): The belt of rocks formed by mountain building tectonics of the Hercynian Orogeny in Europe. Iapetus ocean (277): During Early Cambrian, rifts were forming between once firmly-adjoined land masses. The ocean flooded into the broadening rift zones, one of which separated North America and what is now northern Europe. The Paleozoic ocean is know as Iapetus. Kaskaskia sequence (283): A sequence of Devonian-Mississippian sediments, bounded above and below by regional unconformities and recording an episode of transgression followed by full flooding of a large part of the craton and by subsequent regression. orogenic belt (280): Belt of crustal compression, mountain building, and metamorphism. Ouachita-Marathon trough (296): This trough extends across the southern margin of the North American craton. Although over 1500 kilometers long, only about 300 kilometers of its folded strata are exposed. Ouachita orogenic belt (277): The belt of rocks formed by mountain building tectonics of the Ouachita Orogeny in North America. Queenston clastic wedge (286, 294): A clastic wedge of red beds shed westward from highlands elevated in the course of the Taconic Orogeny. Rheic Ocean (277): An ocean that began to form as the Iapetus began closing during Ordovician and Silurian. Located north of Iapetus, between Laurentia and Africa. Sauk sequence (283): A sequence of Upper Precambrian to Ordovician sediments bounded both above and below by a regional unconformity and recording an episode of marine transgression, followed by full flooding of a large part of the craton, and ending with a regression from the craton. Taconic Orogeny (277, 291): A major episode of orogeny that affected the Appalachian region in Ordovician time. The northern and Newfoundland Appalachians were the most severely deformed during this orogeny. Tippecanoe sequence (283): A sequence of Ordovician to Lower Devonian sediments bounded above and below by regional unconformities and recording an episode of marine transgression, followed by full flooding of a large region of the craton and subsequent regression. CHAPTER 10 Early Paleozoic Events THE PHANEROZOIC EON ?Consists of three eras (from oldest to youngest): ?Paleozoic = "ancient life" (542-251 m.y. ago) ?Mesozoic = "middle life" (251-65.5 m.y. ago) ?Cenozoic = "recent life" (65.5 m.y. ago - present) PALEOZOIC ERA Paleozoic periods can be divided into: ?Early Paleozoic = Cambrian, Ordovician and Silurian ?Late Paleozoic = Devonian, Mississippian, Pennsylvanian, and Permian PALEOZOIC OVERVIEW FIGURE 10-1 Major events of the Paleozoic Era. Harold Levin PALEOZOIC OROGENIC BELTS Orogenic belts are the result of the rebuilding of a supercontinent during the Paleozoic. FIGURE 10-7 Cratons and orogenic belts of North America and Europe. 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. OROGENIES In the Appalachian region, there were three Paleozoic mountain-building events (or orogenies): ?Taconic orogeny ?Acadian orogeny ?Alleghanian orogeny PALEOZOIC ROCKS OF THE PLATFORM Across the platform, in the continental interior, Paleozoic strata are relatively flat-lying to gently dipping, and warped into basins, domes, arches, and broad synclines. FIGURE 10-5 Central platform of the United States showing major basins and domes. 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. 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. ?????????????????????????????????????????????? NEOPROTEROZOIC PALEOGEOGRAPHY Just before Paleozoic began, the Precambrian supercontinent, Rodinia, had rifted apart to form six large continents and several smaller continents. 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. FIGURE 10-2 Landmasses during the Neoproterozoic, about 750 million years ago. 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) EARLY PALEOZOIC CONTINENTS FIGURE 10-2 Landmasses during the Neoproterozoic, about 750 million years ago. 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. EARLY PALEOZOIC CONTINENTS (C. R. Scotese, 2001, Atlas of Earth History, Vol., 1, Paleogeography, PALEOMAP Project.) 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. TRANSGRESSIONS & REGRESSIONS Shallow epicontinental seas transgressed (onlap or advance onto the continent) across the Laurentian (North American) craton during Early Paleozoic as the glaciers melted and sea level rose. The seas regressed (offlap or retreat off the continent) as the glaciers enlarged and sea level dropped. 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. UNCONFORMITIES During regressions, the former seafloor was exposed to erosion, creating extensive unconformities that mark the boundaries between the transgressive-regressive sequences. 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) Green = sedimentary deposits Yellow = missing strata associated with unconformities NORTH AMERICAN CRATONIC SEQUENCES 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. ?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 CAMBRIAN PALEOGEOGRAPHY FIGURE 10-4 North America during the Cambrian Period. 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 base of Cambrian is now placed at the oldest occurrence of feeding burrows of the trace fossil Trichophycus, and dated at 542 m.y. FIGURE 10-8 The trace fossil Trichophycus. 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. 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. FIGURE 10-12 East-west section of Cambrian strata exposed in the Grand Canyon. 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. FIGURE 10-12 East-west section of Cambrian strata exposed in the Grand Canyon. 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. FIGURE 10-12 East-west section of Cambrian strata exposed in the Grand Canyon. 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. FIGURE 10-12 East-west section of Cambrian strata exposed in the Grand Canyon. 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. FIGURE 10-12 East-west section of Cambrian strata exposed in the Grand Canyon. COMPARISON OF CAMBRIAN AND ORDOVICIAN PALEOGEOGRAPHY (C. R. Scotese, 2001, Atlas of Earth History, Vol., 1, Paleogeography, PALEOMAP Project.) 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). FIGURE 10-27 Paleogeography of Ordovician North America. ORDOVICIAN OROGENIES The Taconic Orogenic Belt lay between Laurentia (North America) and Baltica (Europe and western Russia) during Ordovician. (C. R. Scotese, 2001, Atlas of Earth History, Vol., 1, Paleogeography, PALEOMAP Project.) Plate tectonic cross-section showing forces that caused the Taconic Orogeny. ORDOVICIAN OROGENIES FIGURE 10-23 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. ORDOVICIAN OROGENIES FIGURE 10-23 Plate tectonic forces that caused the Taconic orogeny. 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 (480 - 460 m.y. ago). 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. FIGURE 10-26 Great wedges of clastic sediments spread westward. 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. FIGURE 10-26 Great wedges of clastic sediments spread westward. 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. FIGURE 10-26 Great wedges of clastic sediments spread westward. 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. FIGURE 10-7 Cratons and orogenic belts of North America and Europe. COMPARISON OF ORDOVICIAN AND SILURIAN PALEOGEOGRAPHY (C. R. Scotese, 2001, Atlas of Earth History, Vol., 1, Paleogeography, PALEOMAP Project.) ?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. SILURIAN GLOBAL CONDITIONS SILURIAN PALEOGEOGRAPHY ?Mountains in eastern N. America are eroding. ?Sandstone & conglomerate deposits. ?Widespread carbonate deposition. ?Deep marine deposits in NW and SE U.S. ?Reefs and evaporites. FIGURE 10-31 Silurian North America Paleogeography. SILURIAN SEDIMENTARY DEPOSITS ?As the Tippecanoe Sea flooded North America in the late Ordovician to Silurian, deposition began with nearshore sands. ?These include the famous St. Peter Sandstone, an unusually pure, well sorted, well rounded quartz sandstone. ?The purity of the St. Peter Sandstone is the result of reworking of older sedimentary rocks. ?Silurian Tuscarora Sandstone was deposited in the central Appalachian region. 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. FIGURE 10-15 Niagara Falls 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. ?Thick accumulation of salt it is likely the result of a restricted basin where a structure such as a organic reef would periodically allow communication with the ocean. FIGURE 10-17 Isopach map shows thickness of late Silurian evaporite basins. FIGURE 10-18 Cross section shows a deposition model for evaporites in a barred basin. 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. 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. EARLY PALEOZOIC CORDILLERA ?Passive margin during early Paleozoic ?Tentional basins deposits – Belt Supergroup (Montana-Idaho-British Columbia, Uinta (Utah), Pahrump (California) ?Starting in the Ordovician and Silurian the passive margin gave way to an active margin. Subduction zone ?Thick sequences of graywackes and volcanic rocks deposited in a trench FIGURE 10-20 Interpretive cross section of conditions across the Cordilleran region during early Paleozoic time. PALEOZOIC REVIEW FIGURE 10-1 Major events of the Paleozoic Era. • FIGURE 10-1 Major events of the Paleozoic Era. Source: Courtesy R. F. Dymek. • FIGURE 10-7 Cratons and orogenic belts of North America and Europe. Source: Harold Levin. • FIGURE 10-5 Central platform of the United States showing major basins and domes. Source: Harold Levin. • FIGURE 10-2 Landmasses during the Neoproterozoic, about 750 million years ago. Source: Harold Levin. • TABLE 10-1 Cratonic Sequences of North America. Source: Sloss, L., 1965, Bulletin of the Geological Society of America 74:93-114. • FIGURE 10-8 The trace fossil Trichophycus. Source: Jensen, S., The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives, Integrative and Comparative Biology: Oxford University Press. • FIGURE 10-12 East-west section of Cambrian strata exposed in the Grand Canyon. Source: After E. D. Mckee, 1945, Cambrian Stratigraphy of the Grand Canyon Region, Washington, Carnegie Institute, Publication 563. • FIGURE 10-27 Paleogeography of Ordovician North America.Source: Harold Levin. • FIGURE 10-23 Plate tectonic forces that caused the Taconic orogeny. Source: Harold Levin. • FIGURE 10-26 Great wedges of clastic sediments spread westward. Source: Harold Levin. • FIGURE 10-7 Cratons and orogenic belts of North America and Europe. Source: Harold Levin. • FIGURE 10-31 Silurian North America Paleogeography. Source: Harold Levin. • FIGURE 10-15 Niagara Falls (A) stratigraphic section and (B) block diagram. Source: Harold Levin. • FIGURE 10-17 Isopach map shows thickness of late Silurian evaporite basins. Source: After Alling, H. and Biggs, L., AAPG (c) 1961, 45:515–547. Reprinted by permission of AAPG, whose permission is required for further use. • FIGURE 10-18 Cross section shows a deposition model for evaporites in a barred basin. Source: Einsele, G., 2000, Sedimentary Basins: Evolution, Facies, and Sediment. New York, p. 261. With kind permission of Springer Science+Business Media. • FIGURE 10-20 Interpretive cross section of conditions across the Cordilleran region during early Paleozoic time. Source: Harold Levin.