CAMBRIAN - BEACHES, LAGOONS, AND MUD BANKS
These are the earliest sedimentary rocks known regionally. Mostly they are clastic in nature, which means they are made from fragments derived from the older rocks which were transported to their eventual site of deposition. They are primarily beds of sandstone, siltstone, and shale that were deposited in the coastal zone in beach, lagoonal, deltaic, and offshore mud bank environments. These deposits were laid down during an event that is known as a marine transgression. During the transgression a shallow sea slowly invaded from the west, flooding the older landscape that had developed on the metamorphic rocks. This happened because the ancient metamorphic basement rocks had become beveled to a nearly flat and featureless surface, known as a peneplain, by a protracted period of erosion and because the ancient surface also subsided very slowly below sea level. The marine transgression occurred during early and middle Cambrian times about 530 to 560 million years ago in the Lake Mead area. The transgression started prior to that time in the Death Valley area, where a large rift in the ancient continent apparently formed, and it eventually extended far inland with shallow seas covering all of the area between Lake Mead and the state of Colorado. These beds are about 230 m (750 feet) thick in the Lake Mead area.
These rocks are formally named in most places. The basal beds, which are sandstone in most places, are called the Tapeats Sandstone in the Grand Canyon and Lake Mead regions and they are known as the Prospect Mountain Quartzite in other nearby regions. Shale and limestone beds overlie the Tapeats. These are locally referred to, from oldest to youngest, as the Pioche Shale, Lyndon Limestone, and Chisholm Shale. There is no limestone in the Grand Canyon and there the shale sequence is called the Bright Angel Shale.
Cambrian clastic rocks are exposed on the west side of Frenchman Mountain, just east of Iceberg Canyon, at several localities in the southern Virgin Mountains, along Azure Ridge, along Bunkerville Ridge, and up the Colorado River in the Lower Granite Gorge.
CAMBRIAN TO PERMIAN -- A TIME OF CARBONATE SHELVES AND BANKS
For a long time after the marine transgression and its associated regional flooding, the land continued to subside slowly, facilitating the deposition of much more sediment above the initial clastic deposits. Warm, clear, and shallow seas covered large parts of the western US, including the Lake Mead area, for millions of years from Late Cambrian times, about 520 million years ago, to Early Permian times, about 370 million years ago. Layer upon layer of carbonate rocks, chiefly limestone and dolomite, accumulated throughout this time period. These rocks formed primarily from the remains of the countless plants and animals that lived on this broad, submerged continental shelf. The modern Bahaman Banks might be what much of the region looked like during this time period. The slow subsidence kept pace with deposition, allowing the sea to remain shallow while the rock column beneath the sea bed got thicker and thicker. Approximately 2,000 meters, which is a little over 6,000 feet, of these reef and carbonate bank deposits accumulated in the Lake Mead area. The same age of deposits to the east in the Grand Canyon are only about 425 meters, or about 1,300 feet, thick and there many of the individual beds contain more sandy material than they do in the Lake Mead region. To the west near the site of the ancient rift in the continent, all of the equivalent units are much thicker. For example the same section in Death Valley region totals over 4,000 meters, or a little over 12,500 feet, in thickness. Some of the thick western part of the section is presently preserved in the Muddy Mountains, north of Lake Mead where they were shoved to the east over the thinner Lake Mead section on a large horizontal thrust fault during the Cretaceous Period.
Different names are used for these rocks in each region. At Frenchman Mountain, in the Virgin Mountains, and in the Grand Canyon, the Cambrian formations are mostly called the Muave Formation, but in the Muddy Mountains they are the Bonanza King Formation. The Devonian and Mississippian formations are called the Muddy Peak Limestone and Monte Cristo Limestone, respectively, around Lake Mead, but the same units are known as the Temple Butte Limestone and Redwall Limestone in the Grand Canyon. Pennsylvanian and Permian limestone units are known as the Bird Spring and Pakoon Formations around Lake Mead and they are mostly red sandstone, known as the Supai Formation, in the Grand Canyon.
The rocks of the Paleozoic carbonate shelf are preserved at Frenchman Mountain and in the ranges to its north, in the Muddy Mountains, in the south Virgin Mountains, at Virgin Peak and in the range to its northeast, at Azure Ridge, and along the Grand Wash Cliffs near east of Meadview.
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