Page 253 - Laboratory Manual in Physical Geology

ACTIVIT Y 8.6 CSI (Canyon Scene Investigation) Arizona

Name: ______________________________________ Course/Section: ______________________ Date: ___________

A. This is a photograph of part of the bottom of the Grand Canyon, which runs east to west across northern Arizona. You are standing
west of Grand Canyon Villiage, on the south rim of the Canyon, and looking at the north side of the bottom of the canyon.

Carefully analyze the photograph for rock layering. The very bottom rock layers in the foreground are folded Precambrian
metamorphic rock called the “Vishnu Schist,” which contains narrow bodies of granite (colored white). The Vishnu Schist is
overlain here by relatively horizontal layers of sedimentary rock.

1. Using a pen, draw a line exactly along the contact (boundary) between the Vishnu Schist and the relatively horizontal
sedimentary rocks above it.

2. Based on FIGURE 8.1 , what specific kind of unconformity did you trace above?

3. REFLECT & DISCUSS The Vishnu Schist has an absolute age of about 1700 million years. The Lower Cambrian

Tapeats Sandstone sits on top of the unconformity that you drew in Part A. If you assume that the Tapeats Sandstone
includes strata (sedimentary layers) that were deposited at the very start of the Cambrian Period, then how much of a gap
in time exists at the unconformity (where you traced it with a pen)?

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