Sunday, November 11, 2007

Neal Immega - What's this rock doing with red and green spots?

This is a fine grained quartz sandstone from a shallow creek that flowed during Permian times at what is now the Craddock Ranch. Notice that the rock is made up of layers that dip to the right. These are called forsets and are the leading edge of a ripple in the sand at the bottom of the creek. Moving water transports sand in the shape of a ripple. If you look closely, you will see that there are several of these angled layers and each layer truncates the previous one because the moving ripples tend to erode some or all of the previous one.

The red and green color comes from iron in different oxidization states. If you could examine the grains with an electron microscope, you would see that the quartz is coated with a layer of iron-rich clay. I think that the original color of the rock was red like all the other sediments in these redbeds. Rivers also transport organic material, like plant debris, along with the sand. Bacterial decomposition of the organic materials will also reduce the iron, changing the color of the iron from red to green. The green spots come from decomposition of isolated bits of organic material.

5 comments:

Samira said...

Awesome find! Can't wait to see it prepped out...tell Bob to WEAR THE SHIRT! I want to see it on the web...
Samira

Anonymous said...

Is it red and green below the surface or does it just get that way when it weathers?

Houston Museum of Natural Science said...

It is red and green below the surface, throughout the rock.

The green represents the presence of organic materials in the Permian dirt - like plant roots - that prevented the soil from oxidizing (and turning red) in that area, as the roots themselves did not contain iron.

As they dig down from the surface, the green will occassionally fall out of the soil amid the red dirt, indicating the presence of ancient organic material throughout the geologic layers.

kathavens said...

That's a cool rock Neal, where did you find it?

Zackery said...

Thanks for the post
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