Tuesday, June 22, 2010

Bed material lithology puzzle.


View Big Creek, Current River in a larger map
I've spent many years looking at bed material size in the Missouri/Arkansas Ozarks, but know  little about how that relates to particle lithology, and haven't seen anything in the literature.

Last week I looked at this site on Big Creek north of Eminence, Missouri.

Surficial rock in the watershed is primarily carbonates and sandstones. 

Drainage area at the site is about 18 square miles.  Channel slope is 0.003, or 17 feet per mile.  There is a narrow but well developed floodplain.  As with most Ozark watersheds, there's a history of extreme disturbance on either side of 1900: clearcutting followed by overgrazing, free range hogs; riparian forest clearing and overgrazing in the last 75 years.

Most of the bed materials come from the Gasconade dolomite and Roubideax Formations.

These questions could have huge importance for studying bedload transport rates in the Ozarks with the aim of understanding the impacts of instream gravel mining.  And I've seen no analysis of them in the literature.

Click on the photos for larger versions.

Here's what I see:

Though bedrock is dominantly carbonate rock, I see large particles only in cases of extreme disturbance (e.g., urban areas).  Sometimes I see well rounded carbondates, but they're never very large ( bigger than fine gravel).  Lithology is dominated by chert, which is well rounded only when the particles are small, less than about 80mm.

When there are sandstones in the watershed, particles are usually well rounded, though sometimes brick-shaped with well rounded corners.

At this site, D50 is about 50mm.

Some hypotheses (sedimentologists, maybe this is obvious for you, help a brother out):

1.  Though these streams have suffered serious instability over the last 100 years, and bedload transport is clearly very high (this site was seriously disturbed by a county road crew, and rearranged itself over just a couple of floods), most particles above, say, 30mm, don't move downstream that much.  They are pounded and ground down by smaller stuff.  I picture a series of washing machines; the particles might move a bit during big floods, but most of their active weathering might take place in a small reach of stream as they are abraded by smaller particles.

2.  Of course there's storage; the particles might lay dormant for centuries in the floodplain.  Maybe this is why so few/small carbonate clasts?  They're eaten up by chemical weathering.  This could take place even in the streambed.  This is another dating method, though, right?

3.  Density plays a role, perhaps; maybe sandstones are less dense than cherts, and thus move more, and are thus more likely to take on well rounded, oval shapes?

4.  Sandstones are more mechanically homogenous and thus round evenly, while cherts are chemical/physically diverse (within a clast) and thus round unevenly.  Often you see holes in these particles, and color-wise they are very diverse, even within a particle.

What do you think?

1 comment:

Lost Geologist said...

The particle size of the carbonates may depend heavily on their cementation and microfacies. Lime-Mudstones that are not well cemented could rapidly disintegrate into small pieces.