EXERCISE ID: INTERPRETING THE GEOLOGIC HISTORY OF THE GRAND CANYON The Grand Can
ID: 288181 • Letter: E
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EXERCISE ID: INTERPRETING THE GEOLOGIC HISTORY OF THE GRAND CANYON The Grand Canyon in northern Arizona is one of the most spectacular examples of stream erosion in the United tates. The walls of the canyon are nearly 100 miles long and 1 mile deep. These excellent exposures of Paleozoic strata in the canyon make this one of the best areas in the world for illustrating geologic history (see Figs. 27 and 28). Answer the following questions by studying the geologic cross section of the Grand Canyon (Fig. 28). a. What is the name of the oldest rock unit in the cross section?Explanation / Answer
a) The oldest formation in the cross section is Vishnu Formation.
b) The Zoroaster granite is older than the Grand Canyon Supergroup.
c) There exists three types of unconformity in the cross section:
1. non-conformity : Between Vishnu formation and Grand canyon supergroup
2. Angular unconformity: Between Grand canyon Supergroup and Tapeats.
3. Disconformity: Between Muay and Temple Butte, Redwall and Supai, Tapeat and Bright Angel, Kaibas and Cedar Mountain Group
The ordovician , silurian age group of rocks is missing from the sequence . Hence an approximate 66 Ma years is missing from the sequence
d) The youngesst pre-cambrian formation is Tapeats.
e) Formations of the Palaeozoic era are : Bright angel, Temple Butte, muay, Redwall, Supai, Hermit, Coconino, Toroweap, kaibas
f) The youngest formation in the cross section is cedar mountain group.
g) There is no lateral continuity of the cedar mountain group formation on the lright side of the valley. This indicates that the erosion started before the deposition of the cedar mountain group. Hence we can infer that erosion processes began just after the Kaibas formation was deposited. i.e after permian,
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