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USGS Geology in the Parks

PERMIAN - RIVERS, DUNES, AND CARBONATE COASTAL PLAINS

Paleozoic sedimentary rock layers

During the early part of the Permian Period, about 280 to 260 million years ago, the depositional patterns of the Lake Mead region changed considerably.

Initially during this period, the warm shallow sea waters receded to the west and the region dried to become an arid to semi-arid coastal plain. Large volumes of clastic material, chiefly composed of small sand-and-silt-sized quartz fragments, were shed across the landscape. The clastic sediment was eroded from far to the east in the continental interior where basement rocks had earlier become exposed in high mountain ranges, known as the ancestral Rocky Mountains. The sandy and silty sediment was borne to its ultimate site of deposition in the Lake Mead region by a large system of river drainages. Most of the sand and silt that accumulated in the region is still preserved in some type of ancient river deposit, but some of the sandy material was redistributed by wind action which caused it to accumulate into large dune fields. The light red rocks in the lower part of the photo are part of this red bed sequence.

During a later stage in the Early Permian, the shallow seas transgressed and regressed back and forth across the region. The alternate wetting and drying of the region resulted in the deposition of a distinctive sequence of rocks. There were two separate periods of marine inundation during which thick, uniform charity limestone sequences were deposited. These are the resistant cliff-forming units in the middle of the photo. During the times just prior to and following the marine invasions the region turned into a saline coastal plain known as a sabkah. Gypsum-rich silts were deposited during these times. These beds form the light-colored and recessive weathering slopes above and below each of the cliffs.

The red beds are mostly known as the Esplanade and Coconino Sandstones in both the Grand Canyon and Lake Mead areas. In the past they have also been simply called red beds or they were called Queantoweap and Hermit Formations. The cherty limestone and gypsum-rich beds are called the Kaibab and Toroweap Formations, both locally at Lake Mead and in the Grand Canyon. Over 670 meters (2,200 feet) of these beds accumulated in the region.

The Esplanade, Coconino, Toroweap, and Kaibab Formations are present at Frenchman Mountain, along the Northshore road between Callville Bay and Echo Bay, in the North Muddy Mountains, in many parts of the Virgin Mountains, in the Grand Wash area, and on top of the Shivwits Plateau at the rim of the western part of the Grand Canyon.

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