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 in the 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 In all at least 250 000 gold seekers prospected the Sutter s Mill area and though 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 Shopowners Made More Money Of course 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 shopowners 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 The gold these prospectors sought was mostly in the form of placer deposits Placer deposits form when gold bearing igneous rocks weather Stream transport mechanically separates minerals by density Although many prospectors searched for the mother lode all of the gold recovered during the gold rush came from placers Gold Panning Panning is a common method of mining 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 gold was found practically 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 245 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 this chapter 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 Evaporites 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 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 western end of the Tethys Sea began closing toward the east 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