New Red Sandstone

NEW RED SANDSTONE The Triassic witnessed the retreat of ocean waters completely from land as continents continued to rise. Abundant terrestrial redbeds and thick layers of gypsum and salt are deposited in abandoned basins. Also, the amount of land covered with deserts was much greater in the Triassic than it is today. This is indicated by the predominance of red rocks consisting of terrestrial sandstones and shales exposed in the mountains and canyons of the western United States. Terrestrial redbeds cover an area from Nova Scotia to South Carolina and the Colorado Plateau (Fig. 142). Redbeds are also common in Europe, where in the northwest of England they are very well developed. In northern and western Europe, the terrestrial red layer is characterized by an almost fossil-free sequence called the New Red Sandstone, named for a sedimentary formation in Scotland famous for its dinosaur footprints. This unit shows a continuous gradation from the Permian to the Triassic in the region, with no clear demarcation between the two periods. The widespread occurrence of red sediments probably resulted from a large accumulation of iron supplied by one of the most intense intervals of frozen activity the world has ever known. Air trapped in ancient tree sap shows a greater abundance of atmospheric oxygen, which oxidizes iron to form the mineral hematite, so named because of its blood-red color. The Cordilleran mountain belt in North America, the Andes in South America, and the Tethyan in Africa-Eurasia contain thick marine deposits from the Triassic epoch. The Cordilleran and Andean belts were created by the collision of the east-Pacific plate with continental margins, the American plate was formed when Pangaea broke apart. The Tethyan belt was formed when Africa collided with Eurasia, lifting the Alps, which contained parts of the Triassic that contained abundant fossils. The Tethys Sea, located in the tropics during the Triassic, contains widespread coral reefs that were uplifted to form the Dolomites and Alps during the collision of Africa and Eurasia in the Cenozoic. During the Triassic, when North America was still part of Pangea and the ocean began in present-day Nevada, an ancient large river system called the Chinle flowed through the western United States. The source of Chinle is the Texas plateau, known as the Amarillo-Wichita Uplift. From there it flows west across northern New Mexico and southern Utah. It empties into the ocean in central Nevada. Sedimentary rocks from the Chinle Formation produced important uranium ore deposits responsible for the uranium boom in the American West that began in the 1950s. Eventually, the Texas plateau eroded onto the plains, and the Pangea moved away from the equator to drier middle latitudes. By the time North America separated from other continents about 200 million years ago, the great Chinle River was drying up. Late in the Triassic, the inland sea began flowing into the westcentral part of North America.Accumulated marine sediments eroded from the Cordilleran plateau to the west, often referred to as the ancestral Rockies.They deposited into the terrestrial redbeds of the Colorado Plateau. Important reserves of phosphate used for fertilizer were deposited in the late Permian and early Triassic in Idaho and adjacent states. Large deposits of iron sediments are also laid. Ore-bearing rocks from the Clinton Iron Formation, a major iron producer in the Appalachian region from Alabama to New York, were deposited during this time. The accumulation of evaporite peaked during the Triassic, when the supercontinent Pangea was just beginning to crack. Evaporite deposits are formed when shallow saltwater pools recharged by seawater evaporate. However, some evaporite deposits date back 800 million years, possibly because most of the salt was buried or recycled into the ocean. Ancient evaporite deposits exist as far north as the Arctic region. This suggests that these areas used to be closer to the equator or the global climate was much warmer in the past.