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Early Paleozoic Earth History http://jan.ucc.nau.edu/~rcb7/510NAt.jpg North American Paleogeography • Broke away from supercontinent ~600 Ma • Configuration of supercontinent not wellunderstood • How do we know? – Evidence of rifting (divergent margin) – Basalt – Subsidence – cooling and thinning of lithosphere Early Paleozoic Climate of NA • North America in tropical location • End of Snowball Earth • Warmer in Cambrian and Ordovician Paleogeography • Location of continents and ocean basins constantly changes • Historical geology provides past geologic and paleogeographic reconstruction of the world • Paleogeographic maps show – distribution of land and sea – possible climate regimes – geographic features (mountain ranges, swamps, glaciers) Early Paleozoic Global History • Cambrian world consisted of six major continents at low tropical latitudes • Water circulated freely among ocean basins, and the polar regions were mostly ice free Paleogeography of the World • For the Late Cambrian Period Paleogeography of the World • For the Late Ordovician Period Paleogeography of the World • For the Middle Silurian Period Early Paleozoic Evolution of North America • The geologic history of the North American craton may be divided into two parts – relatively stable continental interior over which epeiric seas transgressed and regressed – mobile belts where mountain building occurred • Sedimentary-rock record of NA subdivided into six cratonic sequences Paleozoic History • The Paleozoic history of most continents – major mountain-building activity along margins – numerous shallow-water marine transgressions and regressions • These transgressions and regressions – caused by global changes in sea level – related to plate activity and glaciation Cratonic Sequence • A cratonic sequence is – large-scale lithostratigraphic unit – represents a major transgressive-regressive cycle – bounded by unconformities • The six unconformities extend across the North American craton Cratonic Sequences of N. America The Sauk Sequence • Rocks of the Sauk Sequence – Late Proterozoic-Early Ordovician – record the first major transgression onto the North American craton • Deposition of marine sediments limited to the shelf areas of the craton • The craton itself was above sea level Transcontinental Arch • By the Late Cambrian, Sauk Sea had covered most of North America, leaving above sea level only – a portion of the Canadian Shield – and a few large islands, the Transcontinental Arch, • extended from New Mexico to Minnesota and the Lake Superior region Cambrian Paleogeography of North America • During this time North America straddled the equator • Transcontinental Arch Cambrian Facies Cambrian Facies • Sediment derived from craton • Sandy facies occur closest to craton • Quartz-rich sand derived from craton – craton surface weathered and eroded for half a billion years! – no land plants yet – erosion by wind more common – Mature sandstone: well-rounded, well-sorted, 99% Qtz, Where were they deposited? • Fossils suggest marine environment • No land animals at this time • How do we interpret environment? Use sedimentary structures! Cross-stratification • Formed by wind or water • Wind – dunes > 1 m high • Water – typically smaller Cross-stratification 2m WIND! www.env.duke.edu/eos/geo41/win2.htm and homepage.ntlworld.com/donald.mcintyre/menu_files/stones_files/crossbeds_files/peddie_230.jpg End of Cambrian • Most of North America underwater • Sauk Transgression • Epeiric sea – shallow inland sea (over continent) – < 200 m deep Cambrian Transgression • Cambrian strata exposed in the Grand Canyon Beginning of Ordovician • Very little land remaining above SL • Little terrigenous clastic sedimentation • Shift to carbonate deposition – limestones formed from shell debris – clear, shallow water Stromatolites • Constrain water depth – need sunlight for photosynthesis – Photic zone – 150-200 m • All evidence suggests very shallow Sauk Sea Shark Bay, Australia http://www.mlssa.asn.au/journals/1999Journal.htm Regression and Unconformity • As the Sauk Sea regressed during the Early Ordovician, it revealed a landscape of low relief • The rocks exposed were predominately limestones • The resulting craton-wide unconformity marks the boundary between the Sauk and Tippecanoe sequences Ordovician Period • Paleogeography of North America showing change in the position of the the equator Orogeny What North American mountain range began to form in the Ordovician? Appalachians The Taconic Orogeny • Named after present-day Taconic Mountains of – eastern New York – central Massachusetts – and Vermont • First of several orogenies to affect the Appalachian region Appalachian Area • East coast was passive margin in Early Ordovician – like modern Gulf of Mexico • Changed to active margin in Middle Ordovician • Collision with microcontinent or volcanic arc Appalachian Mobile Belt • Middle Ordovician transition to convergence resulted in orogenic activity Ordovician Plate Tectonics Orogeny Timing • Volcanic rocks from present-day Georgia to Newfoundland • Clustering of radiometric ages between 440 to 480 million years ago • In addition, regional metamorphism coincides with the radiometric dates Clastic Wedge Debris eroded from mountains into adjacent basin Thick layer of sediment adjacent to source Silurian Period • Reefs developed in the Michigan, Ohio, and Indiana-IllinoisKentucky areas Silurian Sedimentation