• 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

• Paleogeography of North America during the Permian Period

Permian Period

• Paleogeography 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 separated 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

DinosaurRemainsfound