Tempe Butte Stop 6 - Superposition

Look at the bottom of the image. You can see some the dipped Tempe Beds sticking out from a covering of colluivum. You have already learned that the bottom of the Tempe Beds are older than the layers at the top. You have also learned that the tilting of the beds came after they were deposited in a flay-lying position.

Superposition of sediment, however, has not stopped just because Tempe Butte was titled. Angular pieces of colluvium are superimposed over the titled Tempe Beds. The andesite caprock on top of Tempe Butte weathers into angular pieces of colluvium, and the colluvium moves down the slope (mass wasting) and covers the Tempe Beds in places. This process of colluvium covering the bedrock is shown below. The "yellow" andesite in the diagram is shown superimposed over the rocks underneath.

This cover of andesite colluvium does not occur in all places on the north side. In fact, the andesite colluvium makes a complete covering in only a few places (you can see it best on the northwest side of Sun Devil Stadium).

If you visit this place in person, you may also be able to see old sandy flood deposits of the Salt River that mix with the Andesite colluvium. If you find those sandy flood deposits, you will see that the flooding took place during the same time framework of the andesite mass wasting.

It is actually quite common to have very long time periods separating geological deposits. At Tempe Buttes, the andesite, sedimentary (sandstone, shale), and rhyolite rocks were deposited about 25-15 million years ago. In contrast, the colluvium and Salt River flood materials were deposited hundreds to thousands of years ago.