Thursday, September 3, 2015

195 Million Year Unconformity and an Archeological Site

I've been meandering around Petrified Forest National Park as Lisa has a stint as the Artist in Residence (artist-residency-at-petrified-forest) at the park. It has been an enormous learning experience figuring out Chinle Formation members and beds. I am very much out of my geologic expertise. A very different landscape.  
One exercise I have been doing is walking contacts simply because one can and one can learn a lot about a specific bed or unit by walking the contact. A 195 million year unconformity is located in the park where Bidahochi Formation or even younger sediments are unconformably over the underlying Chinle Formation. I took a walk along this contact to get a sense of the younger lake beds.
Possible Miocene-Pliocene lake sediments overlying Chinle Formation 

The much younger lake sediments are not readily discernible from the underlying pedogenic clay rich silts of the Chinle Formation. I did find what I believe to be little clumps of diatomaceous material within the reddish brown lake sediments.

Diatomaceous? material in lake silts 
During my walk along the contact I also spotted a spectacular bit of petrified wood.
Colorful piece of petrified wood

Using drift of sediment eroded out of a formation is also a way to trace outcrops when actual exposures are missing. The only problem with this bit of colorful petrified wood is that it is completely out of place. The petrified wood is supposed to be well below.

Anthropogenic processes need to be remembered. This bit of petrified wood represents an archeological site and shortly after spotting it I noted a few more and then noted that some of the pieces were marked with flagging. This site turns out to be a recently identified archeological site of unknown age. This landscaped has been occupied for a very long time.

Based on the petrified wood fragments I observed, the people that left these colorful shards had gathered colorful petrified wood from the valley below and then worked the material while camped or living on the old lake sediments which also happen to have good views across the plains in three directions.

A fun accidental observation that had already been discovered. It may be a while before much is known about this site. Over 2,000 sites have been identified in the park and much is still to be learned.

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