Sunday, January 31, 2010

Mudball clasts

Over the holiday break, I had the chance to visit Providence Canyon, GA, sometimes called the "Grand Canyon of Georgia", since it's the biggest hole (other than Zell Miller) in all of the state. Anyway, the canyon is a big ol' erosional feature caused by extremely poor land management in the 1800's; basically, someone irrigation ditches got a "little out of hand", and resulted in a 200 foot canyon that exposes Cretaceous shoreface sandstones. The picture below is a view out into the canyon from the rim.

Anyway, while tromping around in the canyons proper, we came across some features that made me think of my Mud as Sand post from last year. In that post, I discussed some work by Wright and Marriott, a Sedimentary Geology article from 2007 entitled "The dangers of taking mud for granted: lessons from lower Old Red Sandstone dryland river systems of South Wales." In this paper, Wright and Marriott (2007) point out that mud, when glommed together into larger mudballs, can behave hydrodynamically as sand grains. Thus, some of the mud that we so glibly ID as "fine-grained suspension fallout" is actually the result of traction current sedimentation in turbulent flows, and probably has some sort of bed structure, cross-stratification, etc. in it.

In addition to all the sand in Providence Canyon, there is a lot of muddy sediment exposed as well. We had been hiking around after a series of pretty big rainstorms, and I was busy sticking my nose into every rill, cut, and stream channel I came across. Mostly, they conformed to expectations, like the nice-little bit of braid-plain self-similarity evidenced in this erosional feature below:

I also came across some rather larger clasts, hanging out as lags in the bases of the little creeks that drain the main arms of the canyon. Like so:

HOWEVER, upon closer inspection, I found that these clasts were, in fact, entirely made of clay aggregates! In the picture below, you'll appreciate how sticky they are:

So just think about that next time you come across a muddy lens in some channelform sandstone body, eh? Mayhaps it's not finer grains settling out during waining flow, but something even cooler: mud aggregates behaving as sand!


Tammy said...

I was wonder if I could use your story in an art project I am doing on Providence Canyon.

Eric said...

Tammy; feel free!