East Muav Lime Tapeats Vishnu Schist and associated rocks Cross-section of Cambr

Question

East Muav Lime Tapeats Vishnu Schist and associated rocks Cross-section of Cambrian sedimentary rocks exposed in the Grand Canyon. Red line 0 marks the top of the Low Cambrian Olenellus trilobite zone, and red line G marks the Middle Cambrian Glossopleura trilobite zone. (From Levin, H, 2013, The Earth Through Time (10 edition), figure 10-12, p. 285. This material is reproduced with permission of John Wiley & Sons, Inc.). Adapted from Mckee, E. D.,1945, Cambrian Stratigraphy of the Grand Canyon Region, Washington, Carnegie Institute, Publication 563. 1. Label the position of the unconformity 2. What type of unconformity is this? 3. What does the vertical sequence of rock types indicate about sea level changes? Is this a transgressive sequence or a regressive sequence? 4. What is the most likely depositional environment of the Tapeats Sandstone in this 5. What is the most likely depositional environment of the Bright Angel Shale in this trilobite species (Olenellus and Glossopleura) sequence? sequence? Notice the two red lines, one labeled O and the other labeled G. These lines indi- cate the times at which two different lived, along with their geographic distribution. These trilobites are index fossils or indicators of very specific intervals of geologic time. Olenellus lived during the Early Cambrian and Glossopleura lived during the Middle Cambrian. These red lines can be considered time lines. 6. What is the age of the Bright Angel Shale in the western part of this diagram? 7. What is the age of the Bright Angel Shale in the eastern part of this diagram? 8. By examining these trilobite distributions, we can see that the Bright Angel Shale is diachronous (it is older in some areas and younger in others). Is the Bri Angel Shale older in the east or older in the west? ght

Explanation / Answer

1. The unconformity is marked by a surface of erosion or with a plane of time gap. Therefore it can be marked in between the 2 red line G and O depicting the time gap between these formations.

2.This type of an unconfirmity is said to be angular unconfirmity.

3. This layers indicate a fining upward sequence, hence it is a trangressive event.

4. Since the trilobite fossils are dominated in the upper regions of the sandstone, therefore it might be formed in a marine environment because the trilobites are exclusively marine fossils.

5. The Shale in the figure is also formed a marine environment but the difference from that of the sandstone facies that the time gap between the early cambrian and late cambrian generally specifies the difference in their overall temperature of deposition. Since the early cambrian was comapratively colder due to the ocean currents in the Gonadwanaland and the late cambrian was a but warmer due to the retreat of the ice due to the increasing temperature. Therefore it can be said that the shale deposited in this areas are formed in warmer marine environments.

6. The relative age of the western oart of the bright angel shale can be said of lower cambrian because the western part of the shale doesn't intersect the line of the mid cambrian.

7. The eastern part of the bright angel shale may be of late cambrian because the facies has passed out the line depicting the mid cambrian zone.

8. The diachronous Bright Angel Shale is older in the western part.

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