Author Question: What sequence of events would have to occur in order to produce the unconformity shown here? What ... (Read 97 times)

james0929 · **on:** Jul 14, 2018

What sequence of events would have to occur in order to produce the unconformity shown here?

What will be an ideal response?

Question 2

Examine the geologic time scale shown here. Which is the longest-lasting period of geologic time? Which is the shortest period? Why should these periods differ so much in duration?

What will be an ideal response?

makaylafy · **Reply #1 on:** Jul 14, 2018

Answer to Question 1

Answer: First the lower units (purple/white) would have had to be deposited as horizontal layers. Then they got folded. The top got eroded off by some amount. A layer of pebbles was deposited atop this eroded surface (making it an angular unconformity), and then some shale or other fine-grained sediment was deposited on top of that. Finally, I suppose, something had to erode or cut into all three layers and the unconformity to expose the cross-sectional view we see here.

Answer to Question 2

Answer: The answer should not confuse eons or eras with periods. The period with the longest duration was the Cretaceous at 80 million years. The Silurian was the shortest period of the Phanerozoic, at 28 million years. (These numbers come from subtracting the start and end dates for each period from the geologic time scale image. The length of the period is based on changes in the fossil record, so the Cretaceous represents an extended time of relative stability in Earth's ecosystems and therefore its fossil record, while the Silurian was relatively a flash in the pan, a biosphere that was stable for only a (relatively) short time.