Thursday, May 1, 2008

Death Valley Alluvial Fans!

Well, after witnessing my computer engage in Windows Vista-related computer seppuku, I am finally getting back to the ol’ blogging. And there is no better way to celebrate a return to the internets than through alluvial fan pictures.

During our Death Valley trip, we were lucky enough to see lots and lots of examples of alluvial fans, both stratigraphic (which I posted about earlier) as well as geomorphic. Today, I’ll share some pictures of some modern fans.

Alluvial fans, as you may or may not know, are a controversial subject. In the bad old days, researchers had a tendency to synonymize alluvial fans with braided rivers, effectively assuming that fans were extensions of the fluvial depositional model. Once the continuum of deposition had been set between Rivers and Fans, workers had to apply some intellectual contortionism in order to explain the disparate morphologies seen today. This trickled down into stratigraphic work, where humid-fan and dry-fan models were promulgated, and often used to justify incredibly complex paleoclimatic reconstructions based on this fan-moisture dichotomy.

A paper published in JSR, Blair and McPherson (1994) stands out as the counterpoint to this view, and points to several lines of evidence that distinctly define alluvial fans as discrete depositional environments that display marked hydrodynamic and stratigraphic differences from fluvial systems. These characteristics of alluvial fans include such things as fan slope (which is on average ~1.5°-6°, as opposed to the maximum of 0.4° or so for fluvial deposits), association with tectonically active range fronts, and the dominance of debris-flow and sheetflood depositional processes on fans.

Anyway, the two camps still rip each other’s throats out on occasion, which always makes for a fun time at conferences and in the Discussion-Reply literature. But enough of that! Let’s look at some pictures!

The picture below was taken from up top of Mosaic Canyon, looking down towards the parking lot at the trail head. You can see an older fan surface, abandoned and subsequently dissected by short channel networks. I think this picture displays nicely one of the problems in sedimentary geology, namely relating geomorphology to stratigraphy; If you were just walking along, trying to figure these things fan-shaped things out, you might be tempted to equate the surface process you are currently observing (the channelized system) to the formation of the fan itself. In reality, however, the geomorphic system observed has nothing to do with the deposition underneath, and represent the superposition of an erosive system over older deposits.

This next picture shows some active, coalescing fans; just goes to show you how much sediment is getting dumped out of these systems!

The third picture is pretty slick, if I say so myself. It shows an uplifted fan surface that has been subsequently trenched, which is acting to focus the incised channel a younger alluvial fan that is prograding out into the valley floor. The juxtaposition of active and inactive fans is a common feature, both in space and in time, which can make the stratigraphy of these things sort of complicated. This also might be a good example of the formation of growth strata, with different generations of fan deposits showing different dips as movement along the bounding fault readjusts the system, resulting in a “fan” of fan-strata that records this tectonic activity.

The fourth and final picture in this series is one of my favorite, though I can’t take credit for the photography skills on display. This picture below is a cropped picture taken by a fellow field tripper wielding a bajillion-dollar camera (which is why the shot looks so nice). This is an active (and probably fairly “recent”) debris flow that has slipped down this fan, depositing a remarkably coarse apron of sediment. And do you notice the band of REALLY coarse material in the center of the flow? Note that is has a fair number of big clasts, sort of floating there at the top of the flow. I’d interpret that to be the viscous plug of a flow, sitting right there, WAITING to be photographed. How cool is that!?!


BrianR said...

I also like the Blair & McPherson paper ... as you point out, it looks at alluvial fans from a more objective observational and process-based point of view.

Great photos, especially the last one. Wouldn't it be great to sit and watch (from a safe vantage) one of these events come down the canyon and then out onto the fan. Someone should set up a "fan cam" ... we might have to wait a decade (or two, or three...) to capture a big event, but it would be cool.

Silver Fox said...

Glad to see all these photos of fans - especially from Death Valley. "Fan cam" - a good idea!

Anonymous said...

Thanks for the great info, but do you have some actual names of these fans?