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Chapter 10 Early Paleozoic Events The Phanerozoic Eon • Phanerozoic = "visible life". • 542 million years ago to the present • 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 The Paleozoic Era can be divided into: • Early Paleozoic = Cambrian, Ordovician and Silurian • Late Paleozoic = Devonian, Mississippian, Pennsylvanian, and Permian Paleozoic overview The Paleozoic Era The Paleozoic is characterized by long periods of sedimentation punctuated by mountain building. • Paleozoic rocks of the platform are relatively flat-lying to gently dipping. • Paleozoic rocks in the Ouachita-Appalachian orogenic belt are folded, faulted, metamorphosed, and intruded by granitic rocks. Paleozoic Rocks on 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. 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 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 Paleogeography • Paleomagnetic evidence provides information on the latitude at which the rocks formed. • The orientation of the continent can be determined from the direction to the paleomagnetic pole, as recorded by bits of iron in the rock. • Longitudes, however, cannot be determined (which accounts for some of the differences in the paleogeographic reconstructions). Paleozoic Paleoclimates • Paleoclimatic evidence comes from environmentally-sensitive sedimentary rocks (glacial deposits, coal swamp deposits, reef carbonates, evaporites). • The 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. Neoproterozoic (Late Precambrian) Paleogeography Just before the Paleozoic began, the Precambrian supercontinent, Rodinia, had rifted apart to form six large continents and several smaller continents. The 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) 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. By the Cambrian Period, the continents moved off the pole. Some continents lie on the equator. Glaciers melted, sea levels rose, and shallow epicontinental seas flooded the continents. Transgressions and Regressions Shallow epicontinental seas transgressed across the Laurentian (North American) craton during the Early Paleozoic as the glaciers melted and sea level rose. The seas regressed 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. 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. Unconformities During regressions, the former seafloor was exposed to erosion, creating extensive unconformities that mark the boundaries between the transgressiveregressive 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 the Early Paleozoic in North America: – Sauk sequence (older - primarily Cambrian) – Tippecanoe sequence (Ordovician-Silurian) North American cratonic sequences Green = sedimentary deposits Yellow = unconformities Unconformities • Unconformities cover a greater time range near the center of the craton. • Unconformities near the edge of the craton span less time, if they are present at all. • This is because the edges of the craton are most likely to remain flooded. • The center of the craton is flooded only during times of major sea level high stands or transgressions. 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, midocean ridge volcanism displaces sea water onto the continents. Vail Curves • Cratonic sequences correspond to Vail curves of global sea level change. • Vail curves are derived from seismic stratigraphic profiles, which permit tracing of unconformities across the craton and into thick continental margin sedimentary rocks. Vail Curves showing global sea level change Cambrian Paleogeography • No continents at poles. Continents are on equator. • Shallow seas cover many of the continents. • Evaporites within 30o N and S of equator - the latitude at which deserts are present today. • Iapetus Ocean (or Proto-Atlantic) formed as Laurentia drifted away from South America. 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 The Base of the Cambrian • The base of the Cambrian was formerly identified by the first-occurrence of shell-bearing organisms such as trilobites. • In the 1970's, 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. The Base of the Cambrian • The base of the Cambrian is now placed at the oldest occurrence of feeding burrows of the trace fossil Phycodes pedum, and dated radiometrically at 542 m.y. using uranium-lead isotope dates from rocks in Oman coinciding with a chemical anomaly known as the "negative carbon-isotope excursion”. Cambrian Sedimentary Deposits The Sauk Sequence • During the 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 the 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. 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. 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. 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 Early Cambrian in the west, and Middle Cambrian in the east. Cambrian Deposits of the Grand Canyon Region Near the end of the Early Ordovician, the seas regressed (due to glaciation). The Muav Limestone was exposed to subaerial erosion and a widespread unconformity developed. Comparison of Cambrian and Ordovician Paleogeography LEFT = Global paleogeography during the Cambrian Period RIGHT = Global paleogeography during the Ordovician Period Ouachita Terrane The Ouachita Terrane or "Ouachita embayment microcontinent" has broken off from Laurentia/North America, and is headed for a collision with South America in the Andes region. This is the missing part of the Appalachian Mountain chain between Alabama and Arkansas. Ordovician Paleogeography The Taconic Orogenic Belt lies between Laurentia (North America) and Baltica (Europe and western Russia) in the Ordovician. Ordovician Paleogeography • Global sea levels were high. Shallow seas cover large areas of some of the continents, particularly North America (Sauk epicontinental sea) and Siberia. Ordovician Carbonate Rocks • In the Appalachian area, shallow water carbonate rocks were deposited during the Cambrian and early Ordovician. • Shallow water deposition is indicated by the presence of mudcracks and stromatolites. End of Carbonate Deposition • The depositional setting changed dramatically during the Middle Ordovician. • Carbonate sedimentation ended. • The carbonate platform in eastern North America collapsed or was downwarped. • This was caused by the partial closure of the Iapetus Ocean along a subduction zone. 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. 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). Ordovician Glaciation • By Middle Ordovician, Gondwanaland moved toward the South Pole, leading to glaciation in Africa at the end of the Ordovician. • Glacial deposits are present in NW Africa (Sahara desert region), indicating that this region was located in the South Pole region. Ordovician Glaciation Sea levels fluctuated during the Ordovician, and dropped sharply at the end of the Ordovician, coinciding with the glaciation. Plate tectonic cross-section showing forces that caused the Taconic Orogeny. A - Eastern North America in the 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. • The area in eastern North America that had been deep water shales during the Cambrian was deformed and uplifted to form the Taconic mountain belt. • The shales were altered to metamorphic and igneous rocks by the high temperatures and pressures associated with mountain building (orogeny). 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. 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. 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. 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, in the Late Silurian to early Devonian. The Caledonian event is recognized in the Canadian Maritime Provinces, northeastern Greenland, northwestern Great Britain, and Norway. Comparison of Ordovician and Silurian Paleogeography • Laurentia (North America) still sits on the equator • The Iapetus Ocean is beginning to close as Laurentia and Baltica converge. • Gondwanaland moves toward the South Pole. • Silurian sea levels were high worldwide. • In Laurentia (North America), much of the craton was flooded, indicating melting of the Late Ordovician glaciers. • This was the second major transgression of the Paleozoic, which deposited the Tippecanoe Sequence. 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. 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. • The 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. Silurian Michigan Basin Evaporites • Near the end of the Tippecanoe sequence, reeffringed 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 750 m thick in the Michigan Basin. • Accumulation of thick evaporites requires continual addition and evaporation of sea water, indicating that the basin was connected to the sea. • This restricted basin is called a barred basin because it has a bar or sill between it and the sea. • Sea water flows into the basin over the bar. • Evaporation produces dense brines, which sink to the bottom. When the brine becomes sufficiently concentrated, evaporite minerals are precipitated. Silurian Iron Ore • Economically important sedimentary iron ore deposits accumulated during the 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. • In the Middle Silurian, shallow seas appear to have covered more of the continents than at any other time. • The epicontinental seas withdrew (regressed) toward the end of the Silurian. Silurian Orogenic Activity • Orogenic activity (mountain building) was more or less continuous at one place or another during the 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.