What is the approximate age of the Pierre Shale and how do geologists know? You

Question

What is the approximate age of the Pierre Shale and how do geologists know? You may find the links in the Overview most useful for info on this rock formation. Select one: a. 35 million years ago based on sediments deposited by rivers and streams b. 68 million years old, based on iridium in the soil c. 80 million years, based on the marine fossils found in the grey cliffs d. 35 million years old, based on volcanic ash layers deposited during eruptions in the ancestral Rockies The White Cliffs consist of Navajo sandstone preserved in Zion and numerous other AWESOME national parks in Utah. Yep, it's worth a visit. Which best describes the evidence/setting (or depositional environment) in which this sand accumulated? You may find the links in the Overview most useful for info on this rock formation. Select one: a. Sweeping crossbeds indicate the sand accumulated by the air movement across dunes. b. This clean sand accumulated on the abyssal floor of a broad ocean c. Planar (flat) thin layers of sand show this was once a spot where rivers flowed d. Planar (flat) thin layers of sand show this was once a spot where waves crashed on a beach This question is review of the geologic principles that you learned earlier in the course! You may look them up again in your textbook. Now, use your critical scientist thinking caps...) The rocks at Bryce Canyon (the Claron Formation) lie at the very top of the Grand Staircase. They are thus at the highest elevation, above all the Not yet answered Points out of other sedimentary layers. The stratigraphic position (place within the entire stack) of these rocks indicates which of the following, when compared to the other layers lower down in the stack? Select one: a. The sediments deposited in the Claron are the youngest in the sequence b. The sediments deposited in the Claron are the oldest in the sequence c. The sediments deposited in the Claron are the youngest preserved on the Grand Staircase (i.e. younger ones may have been washed away) d. The sediments record the disappearance of the Interior Seaway

Explanation / Answer

Question 8

Pierre Shale are a rock unit division, located in the United States. This division rocl belongs to the Upper Cretaceous Age. Remember that Cretaceous period lasted from about +/- 146 million to 65 millions of years. The occurence of this rock unit is located in South Dakota, Montana, Colorado, minnesota, New Mexico, Wyoming and Nebraska.

This rock unite is compoused by 600 m (about 2,000 ft) of dark gray shale, some sandstone interlayers, and many layers of bentonite (altered volcanic-ash falls that look and feel much like soapy clays).

Geologist have found in the Pierre Shale, South Dakota location Archelon , a cretaceous marine turtle fossil and largest known turtle species. This fossil and the rock composition show off that is of a marine origin and it was deposited in this paleo-environment, so the option A into the question is not possible because rivers and streamns are of a continental origin. According to these fossil rests, Pierre Shale has been dated to 80.5 million of years.

Remember that the relevance of the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally. Thus, any deformations of strata must have occurred after the rock was deposited, and the fossils organisms were also deposited in the same period of time. Fossils are the clear evidence of paloe-environments then Pierre Shale is a rock fossilliferous unit, where have been found rests of marine vertebrates as the turtles mainly and invertebrates organisms as ammonites. The bentonite layers are related as a consequence of tectonic processes into the foreland basin durind or subsequent to the deposit rock.

To summarize, the correct answer is C. 80 million of years, based on the marine fossils found in the grey cliffs.

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