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 Nevadan Orogeny and Gold

By 1852, – mining operations were well underway – on the American River near Sacramento Gold Mining

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 Mesozoic Era

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 Other Mesozoic Events

Separation of the continents – allowed water 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 Oceans Responded to Continental Separation

Evaporites accumulated in shallow basins – as Pangaea broke apart during the Early Mesozoic – Water flowed into the Central Atlantic Ocean Early Mesozoic Evaporites

Water from the Pacific Ocean flowed into the the newly formed Gulf of Mexico Early Mesozoic Evaporites Marine water from the south flowed into the area that would eventually become the southern Atlantic Ocean

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 Evaporite Deposits

During the Jurassic Period Paleogeography of the World

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 End of the Cretaceous

During the Late Cretaceous Period Paleogeography of the World

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 the consequent expansion of oceanic crust Higher Heat Flow Caused Sea Level Rise

During the Late Cretaceous Period Paleogeography of the World

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 Ocean Currents and Continents

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 Faster Circulation

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 Oceanic Circulation Evolved

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 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 Oceans Still Quite Warm

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 Equable Worldwide Climate

Paleogeograph y of North America during the Permian Period Permian Period

Paleogeograph y of North America during the Triassic Period Triassic Period

Paleogeography of North America during the Jurassic Period Jurassic Period

White areas represent sequences of rocks Cratonic Sequences of North America that are separate d by large- scale uncon- formities shown in brown

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 Fault-block Basins

Areas where Triassic fault- block basin deposits – crop out in eastern North America Triassic Fault Basins

Down-dropped valleys accumulated tremendous thickness of sediments – and were themselves broken – by a complex of normal faults during rifting Down-dropped valleys accumulated sediments

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 Footprints

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

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 Igneous Activity

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 Palisades Sill of the Hudson River

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 Passive Continental Margin

Paleogene Salt Domes

Paleogeography of North America during the Cretaceous Period Cretaceous Interior Seaway Cretaceous Period Dinosaur Remains found