Clastic Detritus

A relatively common sedimentary structure found in sandstone turbidite beds are dish structures. Take a look at the photo below and you’ll see why they are called that … they are concave-up, slightly darker laminations anywhere from a centimeter to a few 10s of cm across. Sometimes they are less arcuate (platters) and sometimes the concavity is more pronounced (cups).

How do these form? If you want to learn about what these structures are, how they form, and what they can tell us about the sedimentation mechanics go check out Hindered Settling’s fantastic post on this subject. I can’t say it any better than that post, so go there. Plus, he has some beautiful photos of more exquisite examples. Check it out.

As a side note, this paper from 1972 notes the first time dish structures were recognized and reported in the literature:

Dish structure was first described by Wentworth (1967) from the Gualala Formation of northern California…

As it turns out, the photograph above is from the Gualala Formation! Maybe Wentworth went to this exact spot … that would be cool.

Happy Friday!

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Eric from The Dynamic Earth had a post earlier this week about animals encountered while in the field. That reminded me about some photographs of Andean condors from my work in Patagonia that I’ve been meaning to post.

The Andean condor is huge … wingspan can reach 3 m (10 ft). On average, this is longer than its North American cousin, the California condor (although they are longer from beak to tail).

Because Andean condors like to take advantage of rising warm air to soar, you typically find them along mountainsides and big vertical cliff faces. As it turns out, this is also where you can find geologists … that’s where the rocks are.

The photo below zooms in on one while it is flying … although it is difficult to get a sense of scale of the bird against the blue sky.

The next photo below was taken a little bit later that same day. Condors are scavengers, part of the vulture family, and often circle around you (presumably waiting for something bad to happen to you). Once one or two start circling, many more show up. It may be a bit tough to see in this photo, but there are about 10 or so in this shot. We ended up counting 25 at one point!

Even though they are scavengers, they will sometimes fly only a few meters above your head checking you out … it’s a bit disconcerting when your precariously perched on a cliff trying to collect some data.

Finally, the photo below is from earlier this year and is the best close-up I have of a Andean condor not flying. Note the nice white ‘collar’, very preppy … also, note the Cretaceous conglomerate on which it is perched.

Happy Friday!

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See all Friday Field Fotos here.

See all posts tagged with ‘wildlife’ here.

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Man, this week has been crazy … just one of those weeks, I guess … I haven’t had a chance to sit down and catch up on any of the happenins in the geoblogosphere. I apologize to anyone I’ve left hangin’ in any discussions going on.

Instead of links and useful information (that stuff takes more time than you think!) … for this week’s Friday Field Foto, I’m simply going to post a photo and a bonus video of some antidunes. Everybody loves antidunes! My post about the submarine cyclic steps briefly discussed supercritical flow, which is what produces antidunes. One of the best places to find antidunes is where a stream comes out at a sandy beach. In the photo below, the current is going from left to right. In supercritical flow, the bedforms are “in phase” with the fluid wave and migrate upstream (from right to left).

The photo is okay … but the video is key (even if the resolution of the video function from my digital camera is craptastic). Watch how the standing waves grow and migrate (slowly) upstream (away from the camera) and then break. If the video isn’t embedded properly, go here.

Happy Friday!

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I did my master’s research on the Permian Brushy Canyon Formation in the Delaware and Guadalupe mountains of west Texas (I’ve shown some photos here, here, and here). The outcrop belt is adjacent to the Salt Basin Graben, which is the easternmost Basin and Range basin. In the photo below, the cactus-covered cliffs in the foreground are the Brushy Canyon outcrops and the mountains way in the distance on the skyline are other Permian rocks on the other side of the Salt Basin Graben.

Every once in a while, the storms that came through this area in the summer would create these ridiculously vigorous dust storms that would move up the axis of the valley. The photograph above is about an hour before the photo below (from almost the same spot). Note that light brownish ‘cloud’ very low in the sky. About 10 minutes after this photograph, this thing hit us in the hills like a freight train. We were taking cover in any little overhangs we could find. Good times.

Happy Friday!

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I was recently going through all my field photographs from Patagonia while putting together some material for a guidebook and found a nice example of an armored mudball.

Back in February I posted a geopuzzle Friday Field Foto showing a situation where just the armor was left, but the mud had eroded out.

Today’s example is a more straightforward example. This is within a mixed mud-silt-sand matrix-supported conglomerate in the Cretaceous of southern Chile. This is within a complex of clastic injectites, which you can read about here.

Happy Friday!

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See all Friday Field Fotos here

See all posts tagged with Patagonia here

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I’m headed out for a short trip out of town, so this is really quick.

If you ever find yourself within a day’s travel of Guadalupe Mountains Nat’l Park in west Texas and southeastern New Mexico (which you should), make sure you take a day to do the Permian Reef Trail in McKittrick Canyon. Not only is it beautiful, but the trail is marked with signs explaining the geology of the area.

Today’s photo is a view from near the top of ridge looking out to the northeast. Essentially, the relief shown there is a product of going from reef crest to adjacent basin. In other words, the modern-day topography reflects the paleotopography of a reef rimming an ocean basin during the Permian!

Happy Friday!

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see all my posts tagged with west Texas here

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Today’s Friday Field Foto is from the Cretaceous of Utah and is a nice example of a type of cross stratification called swaley cross stratification. The key to recognizing this type of sedimentary structure is that the angle of the laminae decrease upwards. In the photo below, you’ll notice the very obvious scour surface cutting down through roughly horizontal laminae. The fill of that depression has sandy laminae that start off steep and then systematically decrease in steepness upwards. I will admit that this criterion is not always foolproof … you can sometimes get tricked by the cut of the outcrop and such. In this case, you’ll have to take my word for it as there were plenty of examples in this stratigraphic section.

This sedimentary structure is thought to be a product of a combination of both bedload sediment transport and sediment falling out of suspension. Reworking and resuspension of sand as a result of vigorous storm waves are a common interpretation of swaley cross-stratified sandstone. Another structure that is, in a way, the inverse of this - a mounded structure, called hummocky cross-stratification - can be quite common in close stratigraphic association (to see a nice example of hummocky bedding, check out front page of The Dynamic Earth blog).

Happy Friday!

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Inspired by Dynamic Earth’s latest posts on modern/Recent sedimentation in Death Valley (see here and here), I decided to post one of my own photos from Death Valley for this week’s Friday Field Foto.

I forgot exactly where this location is (I’ll have to look at my notes), but it’s representative of the kinds of modern deposits you’ll find in the fans in the Death Valley/Owens Valley region.

Similar to the last photo in this post, deposits from the most recent events stand out nicely as a function of color (the light gray). What I like about this particular deposit is that you can pick out the levees in the upper right part of the photo. In these types of flows, the chunky stuff (i.e., coarser-grained material) get pushed to the front and the sides of the flow. As the flow moves down slope, levees of coarser material develop. Next time you are in Death Valley, go walk around on almost any alluvial fan and you should find this (some better developed than others).

Now move down slope from the leveed area … note how the distribution of the lighter material is starting to spread out a bit. Finally, at the terminus of the deposit, you’ll see some very nice debris lobes. One of the coolest aspects of visiting these features on field trips is that you can see the products of a single event (or series of multiple events closely related) in a relatively small area.

Happy Friday!

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We went for a really nice hike this weekend to the southern part of Point Reyes, a bit north of San Francisco. The destination for the hike was a place called Alamere Falls. The captions for the following photos are below the image.

A dense grove of Eucalyptus trees.

Looking south towards the very southern end of the Point Reyes area.

Point Reyes proper off in the distance. It was an absolutely gorgeous day … perfect weather for a hike.

Another view looking north. The trail came in and out of the woods giving us these spectacular views.

The wildflowers are still good this time of year, although not as numerous and dense as they were a few weeks ago. Does anyone know what kind of flower this is?

Yet another view northwest with Point Reyes in the distance.

The poppy, the California state flower.

The end of our 3.5 mile hike was this waterfall coming out at the beach, called Alamere Falls.

No hike is complete without a little sedimentary geology … this is the outwash of the stream and waterfall at the beach. My next post will show some movies from that stream.

Here’s a view from the beach looking back up at Alamere Falls. That cliff is about 20 meters tall, give or take. It was a great hike.

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If you ever find yourself around Guadalupe Mtns National Park in west Texas and southeastern New Mexico, take a day to do the hike up Shumard Canyon on the Western Escarpment. It’s a full day and parts of it quite strenuous, but worth it. To do it, you need to get a key from the park ranger for the Williams Ranch trail. The trailhead is at the end of 4-wheel drive road that can get washed out sometimes … takes about a half-hour to drive it.

When you get up into Shumard Canyon you’ll be within an ancient submarine canyon succession (the siliciclastic and turbiditic Brushy Canyon Formation overlying the carbonate Cutoff and Victorio Peak formations). One particularly spectacular exposure of the Brushy Canyon is a vertical cliff of nearly 100% plane- and ripple-laminated fine sandstone.

The photograph below is a vertical mosaic taking advantage of the vertical layout of a blog post. You’ll have to scroll down to see a person for scale. In addition to the within-bed sedimentary structures, there are a few intervals of inclined stratification that one might interpret as a large bedforms or barforms. For a slightly bigger version, click on photo.

For you turbidite geeks, one thing to consider … if you didn’t have the context of this area being within a submarine canyon-fill, do you think you would interpret it that way?

Happy Friday!

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check out more photos from a trip to this region last September

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