Mesozoic Geology
Learning Objectives
The Mesozoic is often called the "Age of Dinosaurs." During the Mesozoic Pangea will break apart and the Atlantic Ocean will open. Orogenic activity ceases in the East and begins with earnest in the West.
The Mesozoic is divided into three periods: Links to global reconstruction for each period
Paleogeographic maps of North America:
Triassic: 230 mya 245 mya
Jurassic: 150 mya 170 mya 200mya
Cretaceous: 70 mya 90 mya 120 mya
I) North American Craton:
Stable during Mesozoic. On the boundary with the Western region there is a transgression/regression sequence.
II) Eastern Margin:
The Appalachians exist as rugged mountains from the Allegheny Orogeny of the Paleozoic. Clastic material is eroding into lowlands on either side of the mountains.
Triassic: In Late Triassic the Atlantic Ocean begins to open as tensional forces produce fault block basins (Triassic Basins) from Nova Scotia to North Carolina. Clastic material is deposited in these basins-poorly sorted, immature sediments indicate a rapid burial in a semiarid environment. Evidence of frequent intrusions of igneous material.
Jurassic: widening of the Atlantic continues, ocean moves in and periodic evaporite sequences deposited. Early basins filled with marine sediments but through time as widening increases the fault block basins move inland (Richmond, Gettysburg, Danville) and fill with nonmarine clastics. Basins continue to fill with erosional material from Appalachians.
Cretaceous: Rifting continues. A transgression event occurs. Coastal plain and continental shelf forms by deposit of delta and marine sediments over the filled basins.
III) Southern Margin:
Triassic: South America moving away from the area during Triassic. Rifting forms shallow ocean finger.
Jurassic: periodic influx of marine deposits, thick evaporite sequences deposited. North America is slowly moving northward, this region is close to the equator and VERY arid. By late Jurassic full marine sedimentation begins to cover the evaporite deposits. The weight of this over-burden forms salt domes which eventually become oil traps.
Cretaceous: A regression of the sea followed by a transgression.
IV) Western Margin:
Westward movement of the North American Plate over the Pacific Plate causes compression. The Pacific Ocean Plate had a series of island arc and micro-continent features on it; as it is subducted under the North American Plate these are welded onto North America as allocthanous terranes. Over 50 of these terranes have been described from Alaska through California. Welding of allocthanous terranes onto North America caused growth of the continent along its western edge.
Triassic: In the western section of this margin continuation of the Sonoma orogeny that began in the Permian. Eastward moving island arc collides and thrusting occurs. In the eastward section of this margin (where it borders the craton) there is a large transgression/regression event. Sediments of Zion National Park, Petrified Forest and Painted Desert deposited.
Jurassic: a period of intense orogenic activity. Nevadian Orogeny: compression and deformation in the western section proceeds eastward eventually reaching the edge of the craton. Severe folding, faulting and metamorphism result. Crust is shortened due to change in direction of subduction zone. Sierra Nevada batholiths emplaced. During the Mid to Late Jurassic transgression of the Sundance Sea from the North (Arctic to central Utah.) Nevadian Orogeny erodes into this forming swampy floodplains: Morrison Formation (rich in dinosaur fossils).
Cretaceous: orogenic activity moves to the eastward side of the Western Margin. Sevier Orogeny: thrusting of older over younger. Large scale transgression from both north and south connects Arctic to Gulf region, dividing North America into two landmasses. Laramide Orogeny in the Late Cretaceous early Cenozoic forms the Rocky Mountains.
Other:
Europe undergoes a series of transgression/regression events. Africa starts to move northward. Australia and Antarctica rift off of Africa but remain attached to each other. India starts to move northward and South America rifts from Africa.
Climate: At the end of the Paleozoic climate is dry and fairly cool. Climate warms steadily during the Mesozoic, by Cretaceous time the global climate is much warmer than today (tropical conditions to 70 degrees latitude). At the end of the Cretaceous the climate cools and a decline in ocean temperature begins approximately 80 million years ago.
