Thursday, March 5, 2015

What a Geolgist Sees - Part 2




This scanned-slide was taken during my UTEP Geology Summer Field Camp field trip 30+ years ago. I don't remember exactly where it was, but it is probably in the vicinity of Canyonlands National Park, near Moab, UT.

This is to further illustrate the definition of an "Angular Unconformity" and while we are "at it", the concept of  "Superposition", and "Slope-forming" vs. "Cliff-forming" sedimentary rock units.

To address "Superposition" first, that is the concept that when observing a sequence of layered rocks (usually sedimentary), if the sequence has not be severly deformed, then the oldest layers are at the bottom and the youngest are at the top. Perfectly logical when you think about it, but it was a big deal when conceived by Nicolas Steno in the 1600s.

Another of Steno's principles was "Original Horizontality", i.e., when originally deposited virtually all sedimentary rocks are horizontal.  So when we look at the above photo, we can see that the reddish-colored layers are slightly inclined to the right and are overlain by the horizontal grayish-colored layers. [It may be a little more complex than this, but as I don't remember the exact location, I will keep it simple.] 

In reconstructing past events, the reddish-colored shales (which form slopes in dry climates) were deposited, then the area underwent some minor geologic deformation, which resulted in the gently tilting shown here. Then after some weathering and erosion of the reddish layers, the gray layers were deposited above the eroded surface (the Angular Unconformity).

Above the gray shales are the cliff-forming layers, probably siltstones and sandstones in this case. In dry climates, limestones can form cliffs, too, but these are likely siltstones and sandstones. The differences in colors relate to the amounts of iron in the sediments and the oxygen conditions in the environment at the time of deposition, as related to atmospheric exposure in a river delta or floodplain setting. Generally, the more oxygen there is, the redder the sediments are. [Again, I am leaving out some details for the sake of brevity.]  So using these concepts, without knowing exactly which rock units (and ages) are present here, at least we can define some of the events that led to the particular landform that we see in this photo.

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