Transcript Ch 10 ppt
Chapter 10 Early Paleozoic Earth History The First Geologic Map • William Smith, – a canal builder, published the first geologic map – on August 1, 1815 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 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 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 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 Paleogeographic Maps • Geologists use – paleoclimatic data – paleomagnetic data – paleontologic data – sedimentologic data – stratigraphic data – tectonic data 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 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 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 divide into two parts – stable continental interior over which epeiric seas transgressed and regressed – mobile belts where mountain building occurred Cordillera orogenies Cratonic Sequences of N. America Appalachian orogenies • White areas represent sequences of rocks That are separated by large-scale unconformities shown in brown Cratonic Sequence • A cratonic sequence is – a large-scale lithostratigraphic unit – representing a major transgressiveregressive cycle – bounded by cratonwide unconformities 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 • White areas = sequences of rocks • Sauk sequenc e 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 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 Cambrian Paleogeography of North America • During this time North America straddled the equator • Transcontinental Arch 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 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 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 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 Transgression • A transgression covered – the Grand Canyon region. – The Tapeats Sandstone represents the shoreline deposits are 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 – 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 Cambrian Transgression • Cambrian strata exposed in the Grand Canyon • The three formations exposed – along the Bright Angel Trail, Grand Canyon Arizona 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 Upper Cambrian Sandstone • Outcrop of cross-bedded Upper Cambrian sandstone in the Dells area of Wisconsin 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 Cratonic Sequences of N. America • White areas = sequences of rocks • brown areas = largescale unconformities • Regression • Tippecanoe sequence 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 Ordovician Period • Paleogeography of North America – showing change in the position of the the equator • The continent – was rotating counterclockwise Transgression of the Tippecanoe Sea • Resulted in deposition of • the St. Peter Sandstone – Middle Ordovician • over a large area of the craton St. Peter Sandstone • Outcrop of St. Peter Sandstone in Governor Dodge State Park, Wisconsin The Tippecanoe Sequence • The Tippecanoe basal sandstones were followed by widespread carbonate deposition • The limestones were generally the result of deposition – by calcium carbonatesecreting organisms such as • • • • corals, brachiopods, stromatoporoids, and bryozoans 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 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 Present-Day Reef Community • with reef-building organisms Reef Environments • Block diagram of a reef showing the various environments within the reef complex 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 Silurian Period • Paleogeography of North America during the Silurian Period • Reefs developed in the Michigan, Ohio, and Indiana-IllinoisKentucky areas 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) 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 Silled Basin Model • Silled Basin Model for evaporite sedimentatio n by direct precipitation from seawater – Vertical scale is greatly exaggerate d Basin Brines • Because North America was still near the equator during the Silurian Period, – temperatures were probably high Reefs in a Highly Saline Environ-ment? • Organisms constructing reefs could not have lived in such a highly saline environment No Model Is Perfect • How then, can such contradictory features be explained? – Numerous models have been proposed, ranging from • cessation of reef growth followed by evaporite deposition, • to alternation of reef growth and evaporite deposition – no model yet proposed completely explains various reef, carbonate, and evaporite facies The Appalachian Mobile Belt – where the first Phanerozoic orogeny – began during the Middle Ordovician – How would a mountain range influence – the climate and sedimentary history of the craton? Mountain Building • Global tectonic regime – that sutured the continents together, forming Pangaea by the end of the Paleozoic • The Appalachian region – throughout Sauk time, – was a broad, passive, continental margin – Sandstone – shale - carbonate Iapetus Ocean Iapetus Ocean was widening – as a result of movement – along a divergent plate boundary • Beginning with the subduction of the Iapetus plate beneath Laurentia – which was an oceanic-continent convergent plate boundary • the Appalachian mobile belt was born Appalachian Mobile Belt • Evolution of the Appalachian mobile belt • opening of Iapetus Ocean – with passive continental margins – and large carbonate platforms The Taconic Orogeny • The resulting Taconic orogeny, – named after present-day Taconic Mountains of • eastern New York, • central Massachusetts, • and Vermont – was the first of several orogenies – to affect the Appalachian region Eastern Sediment Source • The subduction of the Iapetus plate beneath Laurentia – resulted in volcanism – and downwarping of the carbonate platform • Throughout the Appalachian mobile belt, – indications that these deposits were derived from the east, come from • facies patterns, • paleocurrents, • and sedimentary structures • The sediment originated where – the Taconic Highlands – and associated volcanoes were rising Appalachian Mobile Belt • Middle Ordovician transition to convergence resulted in orogenic activity Queenston Delta Clastic Wedge • The final piece of evidence – for the Taconic orogeny is – the development of a large clastic wedge, • an extensive accumulation of mostly detrital sediments • were deposited adjacent to an uplifted area • and become thinner and finer grained away from the source area, • eventually grading into the carbonate cratonic facies • The clastic wedge resulting from the erosion – of the Taconic Highlands is referred – to as the Queenston Delta Queenston Delta Clastic Wedge • Queenston Delta clastic wedge – coarsegrained detrital sediments near the highlands – thins laterally into finergrained sediments on the craton • Taconic Highland s A European Orogeny • The Taconic orogeny – marked the first pulse of mountain building in the Appalachian mobile belt – and was a response to the subduction taking place beneath the east coast of Laurentia • As the Iapetus Ocean narrowed and closed, – another orogeny occurred in Europe during the Silurian Caledonian Orogeny • The Caledonian orogeny was essentially a mirror image of – the Taconic orogeny and the Acadian orogeny – and was part of the global mountain-building episode – that occurred during the Paleozoic Era • Even though the Caledonian orogeny – occurred during Tippecanoe time, – we will discuss it with the Acadian orogeny – because the two are intimately related Caledonian Orogeny • The transition to convergence resulted in orogenic activity in North America and Europe – Caledonian Orogeny – was a mirror image of the Taconic Orogeny