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The Mesozoic

Terrestrial transformation (245-66 million years ago)

The last of the warm shallow marine seas invaded the Lake Mead area early during the Triassic Period Later the sea receded to the west and marine conditions gave way to continental forest, river, mud flat, flood plain, lake, and dune environments in the Lake Mead region. Gravel, sand, and silt deposited in these environments eventually hardened to form conglomerate, sandstone, and siltstone. Volcanic ash preserved in these sedimentary rocks provides evidence of explosive volcanic eruptions associated with plate collision continuing to the west. The region must have been covered by great forests during the time these sediments were deposited because certain layers contain petrified wood nearly everywhere they are found.

Jurassic Aztec Sandstone
Arc-shaped sand layers (cross-stratification) can be clearly seen in this brilliant red Aztec Sandstone outcrop in Lake Mead National Recreationa area.

A fascinating episode in Lake Mead's sedimentary history began in the last part of the Triassic and continued through the Jurassic Period, between 210-185 million years ago. A widespread sand sea grew to cover most of Nevada, eastern Utah, northeastern Arizona, western Colorado, parts of southern Wyoming, and parts of southeastern California. The large dunes are now preserved as the Aztec Sandstone. These brilliant red rocks still show large, sweeping arc-shaped sand layers (cross-stratification) that formed as dune sands marched across the landscape over 185 million years ago. The Aztec Sandstone accumulated well over 600 meters (2,000 feet) of is nearly pure sand in most areas, including Lake Mead. To find modern examples of sand seas you would have to travel to the deserts of Africa, Arabia, Asia, or Australia.

The intense red Jurassic Aztec Sandstone dune rocks are made up of very pale grains of mostly quartz sand. Where does the red color come from? Those pale grains are bonded together by small amounts of red or yellow iron-rich (rust) cement.

The reorganization of tectonic plates that began in the Late Paleozoic continued into the Mesozoic. Volcanic arcs, sediment scraped from the subducting plate, and possibly tiny plates called microplates, continued to add to the western edge of the growing North American continent. This process, known as accretion, eventually increased the width of North America by up to 300 kilometers, adding much of land mass now known as California, Oregon and Washington. High mountain ranges were uplifted throughout much of the west. Great sheets of rock were thrust to the east over rock that had been deposited in Early Paleozoic seas. Rocks caught between the massive colliding plates and thrust sheets were extensively deformed and metamorphosed. As plates shifted throughout the Mesozoic, huge slivers of continental material were shifted many kilometers northward along transform faults.

By the middle of the Mesozoic, in the Jurassic Period (about 175 million years ago), the supercontinent Pangaea, formed at the end of the Paleozoic Era, began to break up. North America broke away from Africa and Eurasia, forming the Atlantic ocean basin and the tectonically quiet, 'passive' margin that we see today. Meanwhile, on the west coast, plate collision continued along a subduction zone that extended the entire length of North and South America. Intense magmatic activity associated with subduction produced an arc of continental volcanoes. Molten rock that solidified beneath the surface formed huge masses of granitic rock that form the backbone of the Sierra Nevada now exposed at Yosemite National Park. Mountain ranges effectively cut the Lake Mead region off from ocean advances. The transformation from an occasional coastal environment to a landlocked terrestrial landscape was complete.

How did all of this effect the Lake Mead Region? Continued eastward thrusting of huge wedges of rock thickened, uplifted and deformed the region, cutting into ancient Precambrian rock that had remained undisturbed for hundreds of millions years. These great thrusts can be seen at Longwell ridge in Lake Mead NRA and at Red Rock Canyon State Park near Las Vegas.

Finally, near the end of the Mesozoic and into the Early Cenozoic, the intense tectonic activity that began in the Late Paleozoic waned. By 55 million years ago, much of western North America had been uplifted to form high plateaus with large lakes in many of the intervening valleys between them. The elevated terrain gradually eroded, shedding thick aprons of sediment into surrounding basins.

On to the Cenozoic...
Earth view 237 million years ago
Earth view 195 million years ago
Earth view 152 million years ago
Earth view 94 million years ago
Earth view 69 million years ago

The earth 237 million years ago

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