82 Historical Geology Exercise 2-6 INTERPRETING THE GEOLOGIC HISTORY OF THE ORAN
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82 Historical Geology Exercise 2-6 INTERPRETING THE GEOLOGIC HISTORY OF THE ORAND CANYON erosion in the United States. The walls of the canyon are nearly 100 miles long a r examples of stream nd 1 mile deep. These excellent exposures of Paleozoic strata in the canyon make this is one of the t areas in the world for illustrating geologic history (see Figs. 2.27 and 2.28). swer the following questions by studying the geologic cross section of the Grand Canyon (Fig. 2.28) a. What is the name of the oldest rock unit in the cross section? b. Which is older, the Zoroaster granite or the Grand Canyon supergroup? c. For the rocks exposed in the Grand Canyon, number each of the unconformities pre- sent and label its type. For each of these unconformities, list any geologic systems that may be missing and make a calculated estimate as to how much geologic time (in years) is missing. (Use the Geologic Time Scale, inside back cover.) d. What is the name of the youngest Precambrian unit shown in the cross section? e. List the units that were deposited during the Paleozoic era. f. What is the youngest unit of rocks shown in the cross section? g. Based only on the evidence in the cross section, during what geologic period did the Colorado River begin to carve the Grand Canyon? h. In brief narative form, describe the geologic history of this region, beginning with the Precambrian and ending with the Cretaceous. Be as detailed as the evidence permits. (Use a sheet of notebook paper.)Explanation / Answer
(a) The oldest rocks in Grand Canyon cross section are "Vishnu Basement Rock"
The age is at about 2.5 and 1.8 billion years ago in Precambrian time, sand, mud, silt, and ash were laid down in a marine basin adjacent to an orogenic belt.
(b) The oldest rocks of the Grand Canyon are the Vishnu Schist. These metamorphic rocks have were intruded by the Zoroaster Granite perhaps 1.5 to 1.6 billion years ago. The granite is indicated by dotts at the bottom of the figure 2.28.
(d) Grand Canyon Super group is the youngest precambrian time from 1.2 billion and 740 million years.
(e)
(f) The youngest unit is Triassic period CEDAR Mountain group.
(g) Colorado River carved to begin in Grand Canyon from 5.5 million to 5 million years ago, headward erosion to the north and east consolidated the streams into one major river and associated tributary channels.
(h) The geology of the Grand Canyon area includes one of the most complete and studied sequences of rock on Earth. The nearly 40 major sedimentary rock layers exposed in the Grand Canyon area range in age from about 200 million to nearly 2 billion years old. Most were deposited in warm, shallow seas and near ancient, long-gone sea shores in western North America. Both marine and terrestrial sediments are represented, including fossilized sand dunes from an extinct desert. There are at least 14 known unconformities in the geologic record found in the Grand Canyon.Uplift of the region started about 75 million years ago during the Laramide orogeny; a mountain-building event that is largely responsible for creating the Rocky Mountains to the east. In total, the Colorado Plateau was uplifted an estimated 2 miles (3.2 km). The adjacent Basin and Range Province to the west started to form about 18 million years ago as the result of crustal stretching. A drainage system that flowed through what is today the eastern Grand Canyon emptied into the now lower Basin and Range province. Opening of the Gulf of California around 6 million years ago enabled a large river to cut its way northeast from the gulf. The new river captured the older drainage to form the ancestral Colorado River, which in turn started to form the Grand Canyon.
Wetter climates brought upon by ice ages starting 2 million years ago greatly increased excavation of the Grand Canyon, which was nearly as deep as it is now, 1.2 million years ago. Volcanic activity deposited lava over the area 1.8 million to 500,000 years ago. At least 13 lava dams blocked the Colorado River, forming lakes that were up to 2,000 feet (610 m) deep. The end of the last ice age and subsequent human activity has greatly reduced the ability of the Colorado River to excavate the canyon. Dams in particular have upset patterns of sediment transport and deposition. Controlled floods from Glen Canyon Dam upstream have been conducted to see if they have a restorative effect. Earthquakes and mass wasting erosive events still affect the region.
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