Wednesday, March 10, 2010

Virtual Field Trip - Inverted Stream Channels

So I just went down to Capitol Reef NP for my Depositional Environments class and we stopped along the way to view some inverted stream channels and I figured I would try my hand at a virtual field trip guide. (Click on all images for larger versions).

Over view of the site above. Just off I-70, exit 149. The lat-long of the pin above are:
38deg 55.545' N
110deg 22.711' W

Above are highlighted 3 of the known inverted stream channels. The long one to the lower left I did not visit but we were able to see from our vantage point at A. A was the one we spent the most time at. It is a thin (~2 m thick) body of sand on top of a thick shale deposit. B we spent the shortest amount of time at and the actual areal extent is not very easy to discern on the aerial image so I just highlighted the approximate location.

An inverted channel is where a stream deposits sand along the base of the channel and mud along the floodplain. Several million years pass, this all gets buried and lithified (made into rock) at a later date. Then sometime in the future this is re-exposed and starts to weather. Since the mudstone of the floodplain erodes a lot easier than the sandstone of the stream channel the streams begin to become elevated in comparison to the surrounding terrain, producing what we see here.
Stop A: Overview of the inverted channel standing on top of it looking south towards I-70. The upper sandstone unit is likely part of the Salt Wash Member (or Brushy Basin Member) of the Morrison Formation.

Looking at the side of one of the larger sandstone blocks you can see characteristic trough cross-bedding indicative of fluvial systems. The current in this picture is to the left which is south (picture taken facing west).
This photo taken about 10 feet north of the previous photo, still facing west, you can see a draped structure (just left of the hammer) where the cross-bedding went over a harder rock, then during compaction was deformed around the rock. Currently the rock has been lost due to weather but there is still a hole indicating where it was.

In the above photo you can see what are called deformation bands, where during compaction of the sediment the layers were deformed where softer rock underlies well cemented rock causing little tiny "faults" to be produced.
Stop B:
Stop B is located further to the north (~300 ft-ish) from Stop A where the outcrop is not very visible from either the air or the ground. It contains much courser material (more gravel and course sand than A) and is not as well cemented (more friable). It is also located down section of A, so it might be in a different member of the Morrison as well. In the above and below pictures you can see nice examples of more trough cross-bedding with some linguloidal beds. Rock hammer in both for scale.

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