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HISTORICAL GEOLOGY LECTURE 11. EARLY MESOZOIC GEOLOGY. . TRIASSIC-JURASSIC (251-145 MYBP)

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HISTORICAL GEOLOGY LECTURE 11. EARLY MESOZOIC GEOLOGY.


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HISTORICAL GEOLOGY

LECTURE 11. EARLY MESOZOIC GEOLOGY.

TRIASSIC-JURASSIC (251-145 MYBP)

Introduction: The Mesozoic lasted about 186 million years. The first two of the three periods that make up the Mesozoic - the Triassic and the Jurassic lasted about 116 million years, or about the first half of the Mesozoic Era.

Harry Williams, Historical Geology

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The Triassic begun just after the suturing of N. america and Gondwanaland, in the Allegheny and Oauchita orogenies. Mountains now formed a continuous belt along these coastlines.

Harry Williams, Historical Geology

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The Triassic was also a time of major marine regression - the ABSAROKA REGRESSION, which exposed a land area over about the eastern 3/4 of the Craton. Much of the craton become land. Only on the west coast was there dominantly marine conditions.

Harry Williams, Historical Geology

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A major difference in geography between the Paleozoic and Mesozoic is that no sooner had Pangaea become assembled in the Paleozoic, it promptly begun to break up again in the Mesozoic.

Note: Tensional faulting along Appalachians; continued growth of Cordilleran mountain belt.

Harry Williams, Historical Geology

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2. Tectonics and Sedimentation

Major tectonic activity was related to the breakup of Pangaea, which begun in the Triassic. In N. America there were 2 main effects:

1. the east coast became a divergent margin, characterised by tensional forces.

2. subduction on the west coast was enhanced, causing increased orogenic activity in the Cordillera.

Harry Williams, Historical Geology

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The East Coast

In the Late Triassic (ca. 200 mybp), Gondwanaland began to separate from North America, opening a new ocean basin along the east coast - part of the modern Atlantic Ocean. This divergence caused fault-bounded tensional troughs to form throughout the Appalachian region.

Harry Williams, Historical Geology

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These troughs filled with arkosic red sandstones, from the erosion of the adjacent granitic uplands, forming the NEWARK GROUP of the east coast. In addition to the ARKOSE formation (which is characteristic of inland fault basins), the rifting also released numerous lava flows into the same area.

Arkose sandstone

Harry Williams, Historical Geology

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An example is the famous PALISADES of the Hudson River, a basaltic sill, dated at about 190 mybp.

Harry Williams, Historical Geology

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The rest of the Triassic/Jurassic consists of erosion of the fault-block mountains to a much more subdued surface and the development of the Atlantic Coastal Plain formed from the eroded mountains (this continued into the Cretaceous). Much of this wedge of clastic sediments is now offshore and forms the modern CONTINENTALSHELF.

Harry Williams, Historical Geology

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Cenozoic

Harry Williams, Historical Geology

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The South Coast

The early Mesozoic saw the erosion of much of the Ouachita Mountains. A new depositional basin - THE GULF OF MEXICO - begun to form at this time. The Gulf probably formed a shallow embayment between the separating North and South Americas. In the Triassic, sedimentation consisted mainly of evaporites - particularly SALTS (shallow seas, restricted circulations in Bay). In the Jurassic, the seas deepened with the Zuni transgression and carbonates started to dominate.

Harry Williams, Historical Geology

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Mesozoic and Cenozoic sedimentation in the Gulf of Mexico and the relationship of stratigraphy to oil deposits.

Harry Williams, Historical Geology

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The West Coast

During the early Mesozoic, tectonic activity was most active on the west coast. As the North American Craton was now moving away from Gondwanaland, it was actively "pushing" into the Pacific Plate, greatly increasing subduction and compression in the west. 3 major tectonic events occurred in the early Mesozoic:

Harry Williams, Historical Geology

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1. Late in the Permian, this subduction had formed an offshore island arc, surrounded by great thicknesses of greywackes and volcanics, and the thrust belt mountains of the Sonoma orogeny further inland. In the Mesozoic, this island arc and the surrounding marine sediments became progressively pushed up against the edge of the continent, becoming part of it.

Harry Williams, Historical Geology

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This Late Jurassic deformation took place in the west, resulting in the NEVADAN OROGENY - the results of which (mainly metamorphosed greywackes - Fig. 12.18) can be seen in California and Nevada.

Harry Williams, Historical Geology

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2. The intensified subduction also generated vast pockets of magma, which begun to be emplaced as massive GRANITIC BATHOLITHS, starting in the Jurassic and continuing through the Cretaceous. Examples include the Sierra Nevada (mainly California), Baja, Idaho and Coast Range (Washington and B.C.) Batholiths (these batholiths are a notable feature of the Nevadan Orogeny).

Harry Williams, Historical Geology

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Granite batholith exposed in the Sierra Nevadas.

Harry Williams, Historical Geology

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Many red beds in Utah, Colorado and Arizona are Triassic-Jurassic terrestrial sandstones and shales, formed in deserts and river floodplains.

Harry Williams, Historical Geology

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3. The other major tectonic event in the west was the accretion of ALLOCTHONOUS TERRANES - microcontinents, island arcs and submarine basaltic plateaus were all scraped off the subducting Pacific Plate and thrust up onto the continent. 50 of these exotic terranes have been recognized, making up over 70% of the Cordilleran Region (especially in B.C. and Alaska).

Harry Williams, Historical Geology