Field of Science

The Rock Point Member of the Chinle Formation in the Chinle Valley

On of the great things about this weekend was seeing the entire Chinle Formation type section.  H. E. Gregory divided the Chinle into four divisions A, B, C, D; which since that time have been given formal names.  A = Rock Point Member, B = Owl Rock Member, C = Petrified Forest Member (which in our modern usage also includes the Blue Mesa and Sonsela Members), and D = the lower red member (or Bluewater Creek).  We were able to see all of these divisions as well as the underlying Shinarump Member (which Gregory did not include in hs Chinle). We spent 4th of July evening looking at the Rock Point Member:

The type section of the Rock Point Member is the slope forming base of Little Round Rock.

Rock Point Member on the side of Round Rock.

Close-up of Rock Point strata on Round Rock.

Jeff Martz examining the Rock Point Member at Round Rock.

Photo from a little more than half way up the Rock Point section on Round Rock.  Valley below is the Owl Rock Member.  Note our vehicle to the bottom left of the photo.  These exposures are really high up.

2 comments:

  1. Can you discuss a bit how you define the boundaries between the Petrified Forest/Owl Rock, and Owl Rock/Rock Point? Some might be obvious, but it would be cool to hear exactly what you look for. For example, is the Owl Rock simply diagnosed by the carbonate beds, or can it be distinguished using only the pastel colors?

    ReplyDelete
  2. The boundary between the Owl Rock and the Petrified Forest Members is a distinct color change from more brick reds to more pastels and oranges. In PEFO Russ Dubiel placed it at the base of a grey gastropod producing horizon. We concur with this.

    The boundary between the Owl Rock and the Rock Point is traditionally placed at the top of the highest limestone in the Owl Rock) shown in my post on that unit. However, at Round Rock the interval is mostly stripped off and/or covered but there is a small outcrop that has characteristics of both units, so the Owl Rock may extend slightly higher.

    ReplyDelete

Markup Key:
- <b>bold</b> = bold
- <i>italic</i> = italic
- <a href="http://www.fieldofscience.com/">FoS</a> = FoS