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Chapter 14 Mesozoic Earth History Nevadan Orogeny and Gold • Approximately 150 to 210 million years after – the emplacement of massive plutons created the Sierra Nevada • Nevadan orogeny – gold was discovered at Sutter's Mill • on the South Fork of the American River at Coloma, California • On January 24, 1848, James Marshall, – a carpenter building a sawmill for John Sutter, – found bits of the glittering metal in the mill's tailrace Gold Rush • Soon, settlements throughout the state – were completely abandoned as word – of the chance for instant riches – spread throughout California • Within a year after – the news of the gold discovery reached the East Coast, – the Sutter's Mill area was swarming with more than 80,000 prospectors, – all hoping to make their fortune Gold Mining • By 1852, – mining operations were well underway – on the American River near Sacramento Prospecting Was Very Hard Work • At least 250,000 gold seekers – worked the Sutter's Mill area, • and although most were Americans, – they came from all over the world, • even as far away as China • Most of them thought – the gold was simply waiting to be taken, – and didn't realize that prospecting – was very hard work Shop Owners Made More Money • No one gave any thought – to the consequences of so many people converging on the Sutter's Mill area, – all intent on making easy money • In fact, life in the mining camps – was extremely hard and expensive • Frequently, the shop owners and traders – made more money than the prospectors Abandoning Their Dream • In reality, only a small percentage of prospectors – ever hit it big – or were even moderately successful • The rest barely eked out a living – until they eventually abandoned their dream and went home Placer Gold • Although many prospectors searched for the mother lode, – the gold they recovered was mostly in the form of placer deposits • deposits of sand and gravel containing gold particles • large enough to be recovered by panning • Placer deposits form – when gold-bearing igneous rocks weather – and stream transport mechanically separates minerals • by density Gold Panning • Panning is a common method for recovering placer deposits • In this method, – a shallow pan is dipped into a streambed, – the material is swirled around – and the lighter material is poured off • Gold, being about six times heavier – than most sand grains and rock chips, – concentrates on the bottom of the pan – and can then be picked out $200 million in gold • Although some prospectors – – – – – dug $30,000 worth of gold dust a week out of a single claim and some gold was found sitting on the surface most of this easy gold was recovered very early during the gold rush • Most prospectors made only a living wage working their claims • Nevertheless, during the five years • from 1848 to 1853 • that constituted the gold rush proper, – miners extracted more than $200 million in gold Mesozoic Era • The Mesozoic Era – 251 to 66 million years ago – was an important time in Earth history • The major geologic event – was the breakup of Pangaea, – which affected oceanic and climatic circulation patterns – and influenced the evolution of the terrestrial and marine biotas Other Mesozoic Events • Other important Mesozoic geologic events – resulting from plate movement • include – the origin of the Atlantic Ocean basin – and the Rocky Mountains – accumulation of vast salt deposits • that eventually formed salt domes • adjacent to which oil and natural gas were trapped – and the emplacement of huge batholiths • accounting for the origin of various mineral resources The Breakup of Pangaea • Just as the formation of Pangaea – influenced geologic and biologic events • during the Paleozoic, – the breakup of this supercontinent – profoundly affected geologic and biologic events • during the Mesozoic • The movement of continents – affected the global climatic and oceanic regimes – as well as the climates of the individual continents Effect of the Breakup • Populations became isolated – or were brought into contact – with other populations, – leading to evolutionary changes in the biota • So great was the effect of this breakup – on the world, – that it forms the central theme of the Mesozoic Progress of the Breakup • The breakup of Pangaea – began with rifting – between Laurasia and Gondwana during the Triassic • By the end of the Triassic, – the expanding Atlantic Ocean – separated North America from Africa • This change was followed – by the rifting of North America from South America • sometime during the Late Triassic and Early Jurassic Paleogeography of the World • During the Triassic Period Paleogeography of the World • During the Jurassic Period Paleogeography of the World • During the Late Cretaceous Period Oceans Responded to Continental Separation • Separation of the continents – allowed water from the Tethys Sea – to flow into the expanding central Atlantic Ocean, • while Pacific Ocean waters – flowed into the newly formed Gulf of Mexico, – which at that time was little more than a restricted bay • Thick evaporite deposits formed in these areas Early Mesozoic Evaporites • Evaporites accumulated in shallow basins – as Pangaea broke apart during the Early Mesozoic – Water from the Tethys Sea flowed into the Central Atlantic Ocean Early Mesozoic Evaporites • Water from the Pacific Ocean flowed into the the newly formed Gulf of Mexico • Marine water from the south flowed into the area that would eventually become the southern Atlantic Ocean Evaporite Deposits • During that time, these areas were located – – – – – in the low tropical latitudes where high temperatures and high rates of evaporation were ideal for the formation of thick evaporite deposits Further Breakup • During the Late Triassic and Jurassic periods, – Antarctica and Australia, • which remained sutured together, – began separating from South America and Africa • Also during this time, – India began rifting from the Gondwana continent – and moved nothward • During the Jurassic, – South America and Africa began separating Paleogeography of the World • During the Jurassic Period Thick Evaporites from the Southern Ocean • The subsequent separation of South America and Africa – – – – formed a narrow basin where thick evaporite deposits accumulated from the evaporation of southern ocean waters Thick Southern Ocean Evaporites • Marine water flowed into the southern Atlantic Ocean from the south Tethys Sea • During this time, the eastern end of the Tethys Sea – began closing – as a result of the clockwise rotation – of Laurasia and the northward movement of Africa • This narrow Late Jurassic and Cretaceous seaway – between Africa and Europe – was the forerunner – of the present Mediterranean Sea End of the Cretaceous • By the end of the Cretaceous, – – – – Australia and Antarctica had separated, India was nearly to the equator, South America and Africa were widely separated, and Greenland was essentially an independent landmass Paleogeography of the World • During the Late Cretaceous Period Higher Heat Flow Caused Sea Level Rise • A global rise in sea level – during the Cretaceous – resulted in worldwide transgressions – onto the continents • These transgressions were caused – by higher heat flow along the oceanic ridges – caused by increased rifting – and rapid expansion of oceanic ridges Middle Cretaceous Sea Level Was High • By the Middle Cretaceous, – sea level was probably as high – as at any time since the Ordovician, – and approximately one-third of the present land area – was inundated by epeiric seas Paleogeography of the World • During the Late Cretaceous Period Final Stage in Pangaea's Breakup • The final stage in Pangaea's breakup – occurred during the Cenozoic • During this time, – Australia continued moving northward, • and Greenland completely separated – from Europe and North America – and formed a separate landmass The Effects on Global Climates and Ocean Circulation Patterns • By the end of the Permian Period, – Pangaea extended from pole to pole, – covered about one-fourth of Earth's surface, – and was surrounded by Panthalassa, • a global ocean that encompassed about 300 degrees of longitude • Such a configuration exerted tremendous influence – on the world's climate – and resulted in generally arid conditions – over large parts of Pangaea's interior Paleogeography of the World • For the Late Permian Period Ocean Currents and Continents • The world's climates result from the complex interaction between – wind and ocean currents – and the location and topography of the continents • In general, dry climates occur – – – – on large landmasses in areas remote from sources of moisture and where barriers to moist air exist, such as mountain ranges • Wet climates occur – near large bodies of water – or where winds can carry moist air over land Climate-Sensitive Deposits • Past climatic conditions can be inferred from – the distribution of climate-sensitive deposits • Evaporites are deposited – where evaporation exceeds precipitation • While dunes and red beds – may form locally in humid regions, – they are characteristic of arid regions • Coal forms in both warm and cool humid climates – Vegetation that is eventually converted into coal – requires at least a good seasonal water supply • Thus, coal deposits are indicative of humid conditions Evaporites, Red Beds, Dunes, Coal • Widespread Triassic evaporites, red beds, and desert dunes – in the low and middle latitudes – of North and South America, Europe, and Africa – indicate dry climates in those regions, • while coal deposits – are found mainly in the high latitudes, – indicating humid conditions • These high-latitude coals are analogous to – today's Scottish peat bog – or Canadian muskeg Bordering the Tethys Sea • The lands bordering the Tethys Sea – were probably dominated by seasonal monsoon rains – resulting from the warm, moist winds – and warm oceanic currents – impinging against the east-facing coast of Pangaea Faster Circulation • The temperature gradient – between the tropics and the poles – also affects oceanic and atmospheric circulation • The greater the temperature difference – between the tropics and the poles, – the steeper the temperature gradient – and the faster the circulation of the oceans and atmosphere Areas Dominated by Seas are Warmer • Oceans absorb about 90% of the solar radiation they receive, – while continents absorb only about 50%, – even less if they are snow covered • The rest of the solar radiation is reflected back into space • Therefore, areas dominated by seas are warmer – than those dominated by continents Oceans Still Quite Warm • By knowing the distribution of continents and ocean basins, – – – – geologists can generally estimate the average annual temperature for any region on Earth, as well as determining a temperature gradient • The breakup of Pangaea – – – – – during the Late Triassic caused the global temperature gradient to increase because the Northern Hemisphere continents moved farther northward, displacing higher-latitude ocean waters Global Temperature Gradient • Decrease in temperature in the high latitudes – and the changing positions of the continents, – caused the steeper global temperature gradient • Thus, oceanic and atmospheric circulation patterns – greatly accelerated during the Mesozoic • Though the temperature gradient and seasonality on land – were increasing during the Jurassic and Cretaceous, – the middle- and higher-latitude oceans – were still quite warm Equable Worldwide Climate • Higher-latitude oceans remained warm – because warm waters from the Tethys Sea – were circulating to the higher latitudes • The result was a relatively equable worldwide climate – through the end of the Cretaceous Oceanic Circulation Evolved • From a simple pattern in a single ocean (Panthalassa) with a single continent (Pangaea) Oceanic Circulation Evolved • to a more complex pattern in the newly formed oceans of the Cretaceous Period The Mesozoic History of North America • In North America, the beginning of the Mesozoic Era – was essentially the same in terms of tectonism and sedimentation – as the preceding Permian Period • Terrestrial sedimentation continued over much of the craton, – while block faulting and igneous activity – began in the Appalachian region – as North America and Africa began separating Permian Period • Paleogeography of North America during the Permian Period Triassic Period • Paleogeography of North America during the Triassic Period Gulf of Mexico • The newly forming Gulf of Mexico – experienced extensive evaporite deposition – during the Late Triassic and Jurassic – as North America separated from South America Jurassic Period • Paleogeography of North America during the Jurassic Period Global Sea-Level Rise • A global rise in sea level – during the Cretaceous – resulted in worldwide transgressions – onto the continents such that marine deposition – was continuous over much of the North American Cordilleran Volcanic Island Arc at the Western Edge of the Craton • A volcanic island arc system – that formed off the western edge of the craton – during the Permian • was sutured to North America – sometime during the Permian or Triassic • This event is referred to as the Sonoma orogeny Cordilleran Area • During the Jurassic, – – – – – – the entire Cordilleran area was involved in a series of major mountain-building episodes that result in the formation of the Sierra Nevada, the Rocky Mountains, and other lesser mountain ranges • Although each orogenic episode – has its own name, – the entire mountain-building event – is simply called the Cordilleran orogeny Next, Specific Regions • Keeping in mind this simplified overview – of the Mesozoic history of North America, – we will now examine the specific regions of the continent Continental Interior • Recall that the history of the North American craton – can be divided into unconformity-bound sequences – reflecting advances and retreats of epeiric seas – over the craton • Although these transgressions and regressions – played a major role in the Paleozoic geologic history of the continent, – they were not as important during the Mesozoic Cratonic Sequences of North America • White areas represent sequences of rocks • that are separated by largescale unconformities • shown in brown Continental Interior With Inundation • Cratonic sequences are less important because – – – – most of the continental interior during the Mesozoic was well above sea level and did not experience epeiric sea inundation • As we examine the Mesozoic history – of the continental margin regions of North America • we will combine the two cratonic sequences, – the Absaroka Sequence • Late Mississippian to Early Jurassic – and Zuni Sequence • Early Jurassic to Early Paleocene Cratonic Sequences of North America • Absaroka sequence • Zuni sequence Eastern Coastal Region • During the Early and Middle Triassic, – coarse detrital sediments derived from the erosion of the recently uplifted Appalachians • Alleghenian Orogeny – filled the various intermontane basins – and spread over the surrounding areas • As weathering and erosion continued during the Mesozoic, – this once lofty mountain system was reduced to a low-lying plain Fault-block Basins • During the Late Triassic, – the first stage in the breakup of Pangaea began – with North America separating from Africa • Fault-block basins developed – – – – in response to upwelling magma beneath Pangaea in a zone stretching from present-day Nova Scotia to North Carolina Triassic Fault Basins • Areas where Triassic faultblock basin deposits – crop out in eastern North America Fault-Block Basins • After the Appalachians were eroded to a lowlying plain – by the Middle Triassic, • fault-block basins formed – as a result of Late Triassic rifting – between North America and Africa Newark Group • Erosion of the adjacent fault-block mountains – filled these basins with great quantities • up to 6000 m – of poorly sorted red nonmarine detrital sediments – known as the Newark Group Down-dropped valleys accumulated sediments • Down-dropped valleys accumulated tremendous thickness of sediments – and were themselves broken – by a complex of normal faults during rifting Reptile Footprints • Reptiles roamed along the margins – – – – of the various lakes and streams that formed in these basins, leaving their footprints and trackways in the soft sediments • Although the Newark Group rocks contain numerous dinosaur footprints, – they are almost completely devoid of dinosaur bones! • The Newark Group is mostly Late Triassic, – but in some areas deposition began in the Early Jurassic Reptile Tracks • Reptile tracks in the Triassic Newark Group – were uncovered during the excavation – for a new state building in Hartford, Connecticut • Because the tracks were so spectacular, – the building side was moved – and the excavation was designated as a state park Reptile Tracks Igneous Activity • Concurrent with sedimentation – – – – in the fault-block basins were extensive lava flows that blanketed the basin floors as well as intrusions of numerous dikes and sills • The most famous intrusion – is the prominent Palisades sill – along the Hudson River – in the New York-New Jersey area Palisades Sill of the Hudson River • This sill was one of many that were intruded into the Newark sediments – during the Late Triassic rifting – that marked the separation – of North America from Africa Passive Continental Margin • As the Atlantic Ocean grew, – – – – rifting ceased along the eastern margin of North America, and this once active plate margin became a passive, trailing continental margin • The fault-block mountains – that were produced by this rifting – continued eroding • during the Jurassic and Early Cretaceous – until all that was left was a large low-relief area Eastern Continental Shelf • The sediments produced – by this erosion – contributed to the growing eastern continental shelf • During the Cretaceous Period, – – – – – – the Appalachian region was re-elevated and once again shed sediments onto the continental shelf, forming a gently dipping, seaward-thickening wedge of rocks up to 3000 m thick Seaward-Thickening Wedge • The seaward-thickening wedge of rocks – – – – is currently exposed in a belt extending from Long Island, New York, to Georgia Gulf Coastal Region • Paleogeographic Map of North America during the Triassic Period • The Gulf Coastal region was above sea level until the Late Triassic - Evaporites in Gulf of Mexico • As North America separated from South America – during the Late Triassic and Early Jurassic, – the Gulf of Mexico began to form • With oceanic waters flowing into – this newly formed, shallow, restricted basin, – conditions were ideal for evaporite formation • More than 1000 m of evaporites were precipitated, and – these Jurassic evaporites are thought to be the source – for the Paleogene salt domes – found today in the Gulf of Mexico and southern Louisiana Jurassic Period • Paleogeographic reconstruction for the Jurassic Period • The Gulf of Mexico began to form – with the precipitation of evaporites Paleogene Salt Domes Evaporite Deposition Ended • By the Late Jurassic, – circulation in the Gulf of Mexico – was less restricted, – and evaporite deposition ended Normal Marine Conditions • Normal marine conditions – returned to the area – with alternating transgressing and regressing seas • The resulting sediments were – covered and buried by thousands of meters – of Cretaceous and Cenozoic sediments • During the Cretaceous, – the Gulf Coastal region, – like the rest of the continental margin, – was flooded by northward-transgressing seas Cretaceous Period • Paleogeography of North America during the Cretaceous Period – with its northwardtransgressing seas Transgressions and Regression • As a result of the transgression, – nearshore sandstones – are overlain by finer sediments – characteristic of deeper waters • Following an extensive regression – – – – at the end of the Early Cretaceous, a major transgression began during which a wide seaway extended from the Arctic Ocean to the Gulf of Mexico • Sediments that were deposited in the Gulf Coastal region – formed a seaward-thickening wedge Cretaceous Period • Paleogeography of North America during the Cretaceous Period • Cretaceous Interior Seaway Cretaceous Bivalve Reefs • Reefs were also widespread – in the Gulf Coastal region during the Cretaceous • Bivalves called rudists – were the main constituent – of many of these reefs • Because of their high porosity and permeability, – rudistoid reefs make excellent petroleum reservoirs – A good example of a Cretaceous reef complex occurs in Texas Gulf Shelf-Margin Facies • Early Cretaceous shelf-margin facies around the Gulf of Mexico Basin • The reef trend shows as a black line Reef Environments • Depositional environment and facies changes across the Stuart City reef trend, South Texas Rudist Reef Facies Patterns • Here the reef trend – had a strong influence – on the carbonate platform deposition of the region • The facies patterns of these carbonate rocks – are as complex as those found – in the major barrier-reef systems – of the Paleozoic Era Western Region— Mesozoic Tectonics • The Mesozoic geologic history – – – – – of the North American Cordilleran mobile belt is very complex, involving the eastward subduction of the oceanic Farallon plate under the continental North American plate • Activity along this oceanic-continental convergent plate boundary, – resulted in an eastward movement of deformation Cordilleran Orogenic Activity • This orogenic activity – progressively affected – the trench and continental slope, the continental shelf, and the cratonic margin, – causing a thickening of the continental crust • The accretion of terranes and microplates – played a significant role in this area Sonoma Orogeny • Except for the Late Devonian-Early Mississippian Antler orogeny, – the Cordilleran region of North America experienced little tectonism during the Paleozoic • During the Permian, however, an island arc and ocean basin formed – off the western North American craton – followed by subduction of an oceanic plate • beneath the island arc – and the thrusting of oceanic and island arc rocks – eastward against the craton margin Sonoma Orogeny • This event, known as the Sonoma orogeny, – occurred at or near the Permian-Triassic boundary • Like the Antler orogeny, – it resulted in the suturing of island-arc terranes – along the western edge of North America. Triassic Period • Paleogeography of North America during the Triassic Period – with a volcanic island arc in the west Sonoma Orogeny • Tectonic activity that culminated – in the Permian-Triassic Sonoma orogeny • in western North America – was the result of a collision – between the southwestern margin of North America – and an island arc system Oceanic-Continental Convergent Plate Boundary • Following the Late Paleozoic-Early Mesozoic – – – – destruction of the volcanic island arc during the Sonoma orogeny, the western margin of North America became an oceanic-continental convergent plate boundary Steeply Dipping Subduction Zone • During the Late Triassic, – – – – a steeply dipping subduction zone developed along the western margin of North America in response to the westward movement of North America over the Farallon plate • This newly created oceanic-continental plate boundary – – – – – controlled Cordilleran tectonics for the rest of the Mesozoic Era and for most of the Cenozoic Era This subduction zone marks the beginning of the modern circum-Pacific orogenic system Cordilleran Orogeny • The general term Cordilleran orogeny – is applied to the mountain-building activity – that began during the Jurassic – and continued into the Cenozoic • The Cordilleran orogeny – consisted of a series – of individual named, but interrelated, mountainbuilding events – that occurred in different regions at different times – but overlapped to some extent Cordilleran Mobile Belt • Mesozoic orogenies – occurring in the Cordilleran mobile belt Cordilleran Orogeny • Most of this Cordilleran orogenic activity – is related to the continued westward movement of the North American plate – as it overrode the Farallon plate – and its history is highly complex Nevadan Orogeny • The first phase of the Cordilleran orogeny, – the Nevadan orogeny, – began during the Mid to Late Jurassic – and continued into the Cretaceous • During the Middle to Late Jurassic, – two subduction zones, dipping in opposite directions, – formed at the western margin of North America. • As the North American plate moved westward, – – – – as a result of the opening of the Atlantic Ocean, it soon overrode the westerly subduction zone leaving only the easterly dipping subduction zone along its western periphery Cordilleran Mobile Belt • Mesozoic orogenies – occurring in the Cordilleran mobile belt Nevadan Orogeny • As the easterly dipping ocean crust – – – – continued to be subducted, large volumes of granitic magma were generated at depth beneath the western edge of North America • These granitic masses – ascended as huge batholiths – that are now recognized as – the Sierra Nevada, Southern California, Idaho, and Coast Range batholiths • At this time, the Franciscan Complex and Great Valley Group were deposited and deformed Batholiths • Location of Jurassic and Cretaceous batholiths – in western North America Franciscan Complex • The Franciscan Complex, – – – – which is up to 7000 m thick, is an unusual rock unit consisting of a chaotic mixture of rocks that accumulated during the Late Jurassic and Cretaceous • The various rock types include – graywacke, volcanic breccia, siltstone, black shale, – chert, pillow basalt, and blueschist metamorphic rocks Franciscan Complex • The rock types suggest – that continental shelf, slope, and deep-sea environments – were brought together – in a submarine trench – when North America overrode the subducting Farallon plate Franciscan Complex • Map showing the location of the Franciscan Complex Depositional Environment • Reconstruction of the depositional environment – of the Franciscan Complex – during the Late Jurassic and Cretaceous periods Franciscan Complex • Bedded chert exposed in Marin County, California • Most of the layers are about 5 cm thick. Great Valley Group • The Franciscan Complex and Great Valley Group – that lies east of it – were both squeezed against the edge of the North American craton – as a result of subduction of the Farallon plate – beneath the North America plate. • The Franciscan Complex and the Great Valley Group – are currently separated – by a major thrust fault Great Valley Group • The Great Valley Group consists of – more than 16,000 m – of conglomerates, sandstones, siltstones, and shales • These sediments were deposited – on the continental shelf and slope – at the same time the Franciscan deposits – were accumulating in the submarine trench Great Valley Group Environment • Environments of the Great Valley Group – in relation to the Franciscan Complex Plutonic Activity Migrated Eastward • By the Late Cretaceous, – most of the volcanic and plutonic activity – had migrated eastward into Nevada and Idaho • This migration was probably caused – by a change from high-angle to low-angle subduction, – resulting in the subducting oceanic plate – reaching its melting depth farther east Eastward Migrating • A possible cause – for the eastward migration – of Cordilleran igneous activity – during the Cretaceous – was a change from high angle subduction to Lower-Angle Subduction • to low-angle subduction • As the subducting plate – moved downward – at a lower angle, – its melting depth – moved farther to the east Sevier Orogeny • Thrusting occurred progressively farther east – so that by the Late Cretaceous, – it extended all the way – to the Idaho-Washington border • The second phase of the Cordilleran orogeny, – – – – – the Sevier orogeny, was mostly a Cretaceous event although it began in the Late Jurassic and is associated with the tectonic activity of the Nevadan orogeny Cordilleran Mobile Belt • Mesozoic orogenies – occurring in the Cordilleran mobile belt Thrust Faults • Subduction of the Farallon plate – beneath the North American plate during this time – resulted in numerous overlapping, – low-angle thrust faults • As compressional forces generated in the subduction zone – – – – were transmitted eastward, numerous blocks of older Paleozoic strata were thrust eastward on top of younger strata • This deformation resulted in crustal shortening – and produced generally north-south-trending mountain ranges Sevier Orogeny • Associated tectonic features – of the Late Cretaceous Sevier orogeny – caused by subduction of the Farallon plate – under the North American plate Keystone Thrust Fault Keystone Thrust Fault • The Keystone thrust fault is a major fault in the Sevier overthrust belt – It is exposed west of Las Vegas, Nevada • The sharp boundary – between the light-colored Mesozoic rocks – and the overlying dark-colored Paleozoic rocks • marks the trace of the Keystone thrust fault Keystone Thrust Fault Laramide orogeny • During the Late Cretaceous to Early Cenozoic, – the final pulse of the Cordilleran orogeny occurred • The Laramide orogeny – developed east of the Sevier orogenic belt – in the present-day Rocky Mountain areas – of New Mexico, Colorado, and Wyoming Cordilleran Mobile Belt • Mesozoic orogenies – occurring in the Cordilleran mobile belt Present-Day Rocky Mountains • Most of the features – of the present-day Rocky Mountains – resulted from the Cenozoic phase – of the Laramide orogeny Mesozoic Sedimentation • Concurrent with the tectonism – in the Cordilleran mobile belt, • Early Triassic sedimentation – on the western continental shelf – consisted of shallow-water marine – sandstones, shales, and limestones • During the Middle and Late Triassic, – the western shallow seas – regressed farther west, – exposing large areas of former seafloor to erosion Marine and Nonmarine Triassic Rocks • Marginal marine and nonmarine Triassic rocks, – particularly red beds, – contribute to the spectacular – and colorful scenery of the region • The Lower Triassic Moenkopi Formation – of the southwestern United States – consists of a succession of brick-red – and chocolate-colored mudstones Triassic and Jurassic Formations • Triassic and Jurassic formations in the western United States Sedimentary Structures • Such sedimentary structures – as desiccation cracks and ripple marks, • as well as fossil amphibians and reptiles and their tracks, – indicate deposition in a variety of continental environments, • including stream channels, floodplains, and fresh and brackish water ponds • Thin tongues of marine limestones – indicate brief incursions of the sea, – while local beds with gypsum and halite crystal casts – attest to a rather arid climate Shinarump and Chinle • Unconformably overlying the Moenkopi – is the Upper Triassic Shinarump Conglomerate, – a widespread unit generally less than 50 m thick • Above the Shinarump – are the multicolored shales, siltstones, and sandstones – of the Upper Triassic Chinle Formation • This formation is widely exposed – throughout the Colorado Plateau – and is probably most famous for its petrified wood, – spectacularly exposed in Petrified Forest National Park, Arizona Triassic and Jurassic Formations • Triassic and Jurassic formations in the western United States Petrified Wood and Plants Fossils • Whereas fossil ferns are found here, – – – – the park is best known for its abundant and beautifully preserved logs of gymnosperms, especially conifers and plants called cycads • Fossilization resulted from the silicification of the plant tissues • Weathering of volcanic ash beds – interbedded with fluvial and deltaic Chinle sediments – provided most of the silica for silicification Cycads Fossilization • Some trees were preserved in place, – but most were transported during floods – and deposited on sandbars – and on floodplains, – where fossilization took place • After burial, silica-rich groundwater – percolated through the sediments – and silicified the wood Other Fossils • Although best known for its petrified wood, the Chinle Formation has also yielded fossils of – labyrinthodont amphibians, – phytosaurs, – and small dinosaurs • Palynologic studies show similar assemblages of pollen – from the Chinle and Lower Newark Group – indicating that they are the same age Upward in the Stratigraphy • Early Jurassic-age deposits in large part of the western region – consist mostly of clean, cross-bedded sandstones – indicative of windblown deposits • The lowermost unit is the Wingate Sandstone, – a desert dune deposit, – which if overlain by the Kayenta Formation, – a stream and lake deposit, • These two formations are well exposed – in southwestern Utah Triassic and Jurassic Formations • Triassic and Jurassic formations in the western United States Early Jurassic Sandstones • The thickest and most prominent of the Jurassic cross-bedded sandstones – is the Navajo Sandstone, • a widespread formation – that accumulated in a coastal dune environment – along the southwestern margin of the craton Navajo Sandstone, Zion Canyon Navajo Sandstone, Zion Canyon • View of East Entrance of Zion Canyon, Zion National Park, Utah • The light-colored massive rocks – are the Jurassic Navajo Sandstone • while the slope-forming rocks below the Navajo – are the Lower Jurassic Kayenta Formation Navajo Sandstone, Zion Canyon Navajo Sandstone's Large-Scale Cross-Beds • The Navajo Sandstone's most distinguishing feature – is its large-scale cross-beds, – some of which are more than 25 m high Navajo Sandstone • Large cross-beds of the Jurassic Navajo Sandstone in Zion National Park, Utah Sundance Sea • The upper part of the Navajo – contains smaller cross-beds – as well as dinosaur and crocodilian fossils • Marine conditions returned to the region – during the Middle Jurassic – when a seaway called the Sundance Sea – twice flooded the interior of western North America Sundance Sea • The resulting deposits, – the Sundance Formation, – were produced from erosion – of tectonic highlands to the west – that paralleled the shoreline Sundance Sea Retreated Northward • These highlands – resulted from intrusive igneous activity – and associated volcanism – that began during the Triassic • During the Late Jurassic, – – – – a mountain chain formed in Nevada, Utah, and Idaho as a result of the deformation produced by the Nevadan orogeny • As the mountain chain grew – and shed sediments eastward, – the Sundance Sea began retreating northward Morrison Formation • A large part of the area – formerly occupied by the Sundance Sea – was then covered – by multicolored sandstones, mudstones, shales, and occasional lenses of conglomerates – that comprise the world-famous Morrison Formation • The Morrison Formation – contains the world's richest assemblage – of Jurassic dinosaur remains Morrison Formation • View of the Jurassic Morrison Formation – from the Visitors’ center – at Dinosaur National Monument, Utah Skeletons Deposited on Sandbars • Although most of the dinosaur skeletons – are broken up, – as many as 50 individuals – have been found together in a small area • Such a concentration indicates – that the skeletons were brought together – during times of flooding and deposited on sandbars • in stream channels • Soils in the Morrison indicate – that the climate was seasonably dry Dinosaur National Monument • Although most major museums have either – complete dinosaur skeletons – or at least bones from the Morrison Formation, – the best place to see the bones still embedded in the rocks – is the visitors' center at Dinosaur National Monument near Vernal, Utah • The north wall of the visitors’ center – shows dinosaur bones in bas relief – just as they were deposited 140 million years ago North Wall Mid-Cretaceous Transgressions • Shortly before the end of the Early Cretaceous, – Arctic waters spread southward – over the craton, forming a large inland sea – in the Cordilleran foreland basin area • Mid-Cretaceous transgressions – – – – – also occurred on other continents, and all were part of the global mid-Cretaceous rise in sea level that resulted from accelerated seafloor spreading as Pangaea continued to fragment Black Shale Deposition • Middle Cretaceous transgressions are marked – by widespread black shale deposition – within oceanic areas, – the shallow sea shelf areas, – and the continental regions • that were inundated by the transgressions. Cretaceous Interior Seaway • By the beginning of the Late Cretaceous, – this incursion – joined the northward-transgressing waters from the Gulf area – to create an enormous Cretaceous Interior Seaway – that occupied the area east of the Sevier orogenic belt Cretaceous Interior Seaway • Extending from the Gulf of Mexico – to the Arctic Ocean – and more than 1500 km wide at its maximum extent, • this seaway – effectively divided North America – into two large landmasses – until just before the end of the Late Cretaceous Cretaceous Interior Seaway • Paleogeography of North America during the Cretaceous Period • Cretaceous Interior Seaway Cretaceous Deposits • Cretaceous deposits – less than 100 m thick indicate – that the eastern margin of the Cretaceous Interior Seaway – subsided slowly – and received little sediment – from the emergent, low-relief craton to the east • The western shoreline, however, – – – – shifted back and forth, primarily in response to fluctuations in the supply of sediment from the Cordilleran Sevier orogenic belt to the west Facies Relationships • The facies relationships – show lateral changes – from conglomerate and coarse sandstone adjacent to the mountain belt – through finer sandstones, siltstones, shales, – and even limestones and chalks in the east • During times of particularly active mountain building, – these coarse clastic wedges of gravel and sand – prograded even further east Cretaceous Facies Related to Sevier • This restored west-east cross section – of Cretaceous facies of the western Cretaceous Interior Seaway – shows the facies relationship to the Sevier orogenic belt Cretaceous Interior Seaway • As the Mesozoic Era ended, • the Cretaceous Interior Seaway – withdrew from the craton. • During the regression, – – – – marine waters retreated to the north and south, and marginal marine and continental deposition formed widespread coal-bearing deposits on the coastal plain. Accretion of Terranes • Orogenies along convergent plate boundaries – resulted in continental accretion • Much of the material accreted to continents – during such events is simply eroded older continental crust, • but a significant amount of new material – is added to continents – such as igneous rocks that formed as a consequence – of subduction and partial melting Accretion of Terranes • Although subduction – is the predominant influence – on the tectonic history – in many regions of orogenesis, • other processes are also involved – in mountain building – and continental accretion, – especially the accretion of terranes Terranes • Geologists now know that portions of many mountain systems – are composed of small accreted lithospheric blocks – that are clearly of foreign origin • These terranes – – – – – – differ completely in their fossil content, stratigraphy, structural trends, and paleomagnetic properties from the rocks of the surrounding mountain system and adjacent craton Accretion of Terranes • In fact, terranes are so different from adjacent rocks – – – – – that most geologists think they formed elsewhere and were carried great distances as parts of other plates until they collided with other terranes or continents • Geologic evidence indicates – – – – that more than 25% of the entire Pacific Coast from Alaska to Baja California consists of accreted terranes Accretion of Terranes • The accreting terranes – – – – – – are composed of volcanic island arcs, oceanic ridges, seamounts, volcanic plateaus, hot spot tracks, and small fragments of continents • that were scraped off and accreted – to the continent's margin – as the oceanic plate with which they were carried – was subducted under the continent More Than 100 Terranes • It is estimated that more than 100 differentsized terranes – have been added to the western margin – of North America – during the last 200 million years • Good examples of this – are the Wrangellian terranes – which have been accreted – to North America's western margin Terranes of Western North America • Some of the accreted lithospheric blocks – called terranes – that form the western margin – of the North American Craton • The dark brown blocks – probably originated as terranes – and were accreted to North America Terranes of Western North America • The light green blocks – are possibly displaced parts of North America • Dark green – represents the North American craton Growth along Active Margins • The basic plate tectonic reconstruction – of orogenies and continental accretion – remains unchanged, • but the details of such reconstructions – are decidedly different – in view of terrane tectonics • For example, growth along active continental margins – is faster than along passive continental margins – because of the accretion of terranes New Additions • Furthermore, these accreted microplates – are often new additions to a continent, – rather than reworked older continental material • So far, most terranes – – – – have been identified in mountains of the North American Pacific Coast region, but a number of such plates are suspected to be present in other mountain systems as well • They are more difficult to recognize in older mountain systems, – such as the Appalachians, however, – because of greater deformation and erosion Terranes • Thus, terranes – – – – provide another way of viewing Earth and gaining a better understanding of the geologic history of the continents Mesozoic Mineral Resources • Although much of the coal in North America – is Pennsylvanian or Paleogene in age, – important Mesozoic coals – occur in the Rocky Mountains states • These are mostly lignite and bituminous coals, – but some local anthracites are present as well • Particularly widespread in western North American – are coals of Cretaceous age • Mesozoic coals are also known – from Australia, Russia, and China Petroleum in Gulfs • Large concentrations of petroleum – occur in many areas of the world, – but more than 50% of all proven reserves – are in the Persian Gulf region • During the Mesozoic Era, – what is now the Gulf region – was a broad passive continental margin – conducive for the formation of petroleum • Similar conditions existed in what is now the Gulf Coast region – of the United States and Central America Gulf Coast Region • Here, petroleum and natural gas – also formed on a broad shelf – over which transgressions and regressions occurred • In this region, the hydrocarbons – – – – are largely in reservoir rocks that were deposited as distributary channels on deltas and as barrier-island and beach sands • Some of these hydrocarbons are associated – with structures formed adjacent to rising salt domes Louann Salt • The salt, called the Louann Salt, – initially formed in a long, narrow sea – when North America separated from Europe and North Africa – during the fragmentation of Pangaea Salt • Salt deposits in the Gulf of Mexico • formed during the initial opening of the Atlantic Uranium Ores • The richest uranium ores in the United States – are widespread in Mesozoic rocks – of the Colorado Plateau area of Colorado – and adjoining parts of Wyoming, Utah, Arizona, and New Mexico • These ores, consisting of fairly pure masses – of a complex potassium-, uranium-, vanadiumbearing mineral • called carnotite, – are associated with plant remains in sandstones – that were deposited in ancient stream channels Mesozoic Iron Ores • Proterozoic banded iron formations – are the main sources of iron ores • Exceptions exist such as – the Jurassic-age "Minette" iron ores of Western Europe, – which are composed of oolitic limonite and hematite, – and are important ores in France, Germany, Belgium, and Luxembourg • In Great Britain, low-grade Jurassic iron ores – consist of oolitic siderite, which is an iron carbonate • In Spain, Cretaceous rocks are the host rocks for iron minerals Kimberlite Pipes • South Africa, – the world's leading producer of gem-quality diamonds – and among the leaders in industrial diamond production, – mines these minerals from conical igneous intrusions • called kimberlite pipes – Kimberlite pipes • are composed of dark gray or blue igneous rock known as kimberlite Cretaceous Kimberlite Pipes • Diamonds, • which form at great depth where pressure and temperature are high, – are brought to the surface • during the explosive volcanism • that forms kimberlite pipes • Although kimberlite pipes have formed throughout geologic time, – – – – the most intense episode of such activity in South Africa and adjacent countries was during the Cretaceous Period Mother Lode • Emplacement of Triassic and Jurassic – diamond-bearing kimberlites – also occurred in Siberia • The mother lode • or source for the placer deposits mined during the California gold rush – is in Jurassic-age intrusive rocks of the Sierra Nevada • Gold placers are also known in Cretaceous-age conglomerates – of the Klamath Mountains of California and Oregon Porphyry Copper • Porphyry copper was originally named – for copper deposits in the western United States – mined from porphyritic granodiorite, – but the term now applies to large, low-grade copper deposits – disseminated in a variety of rocks • These porphyry copper deposits – are an excellent example of the relationship – between convergent plate boundaries – and the distribution, concentration, and exploitation of valuable metallic ores Origin of Porphyry Copper • Magma generated by partial melting – of a subducting plate – rises toward the surface, – and as it cools, it precipitates and concentrates various metallic ores • The world's largest copper deposits – were formed during the Mesozoic and Cenozoic – in a belt along the western margins – of North and South America Plate Tectonics and the Distribution of Natural Resources • Magma generated by subduction can create this activity – Bingham Mine in Utah is a – Example: copper huge open-pit copper mine deposits in western Americas