Friday Field Foto #37: Ancient submarine channel

Wow … I haven’t posted a Friday Field Foto since November! Sorry about that … December was kinda busy.

This week’s photo is from my PhD field area in southern Chile. I’m currently working on revisions to a paper that was submitted last summer and is also a major chapter in my soon-to-be-officially-completed dissertation. So, these rocks are back in the forefront of my mind.

The title of this post is “ancient submarine channel” … to be more specific (and anal-retentive), what you are looking at is what I am interpreting as the deposits that filled channelform accommodation. That is, it is very difficult to point to a preserved sedimentary body like this and simply label it a “channel” because the deposits that fill a channel are sometimes quite different from the processes that created the channel.

But, I digress, I don’t want to split hairs too much here…that’s not the point, I can do that in papers or at conferences.

What I do want to do is point out the noticeable features of these rocks. First, notice the sandstone-rich body in the center of the photo. The thickness of the sand on the cliff face in the foreground is about 20 meters. Second, notice how this sand body thickens away from you in this view. The sand body on the cliff face in the middle ground is about 30 m thick. It may be difficult to see on this photo, but note how the beds in the lower part of the middle-ground cliff face pinch out against the base of the body. Here’s a zoomed-in version.

So, picture a “scoop”-shaped depression on the sea floor that is subsequently filled in by turbidity current deposits that lap onto the side of depression. This is what we mean by “channelform” … it has the shape of a channel, but in many cases we can’t determine from a limited outcrop if the feature is indeed an ancient channel fill. In this case, there are numerous other smaller-scale features that lead us to interpret the composite body as a channel fill. We climbed all over these cliff faces, measured sections, measured paleocurrents, and so on.

What’s even more telling is context. The characteristics of similar-looking sand bodies stratigraphically below and above this one were analyzed similarly. You’re gonna have to wait until the paper is in press (hopefully in another couple months) to see all the details.

Happy Friday!

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