Counter point bars in fluvial AND deepwater channels?
The Geology on the Wing Wednesday post about counter point bars on the Brazos river has provoked a few questions, so I thought it would be a good idea to expand on it a little bit.
Meandering(or sinuous) channels migrate, and can do so one of at least two ways. The first is by translation, or “sweep” of the meander bend downstream/downslope. The second is by expansion or “swing” of the bend in a lateral direction, usually transverse to slope. These two methods are shown very simply below (the numbers are relative time steps).
Of course, natural systems are much more complex than this and show very complex and interesting patterns (just go check out you local river on google earth…). Generally, point bars result from ‘swing ‘, that is, erosion of the outer bank, which leads to deposition the inner bank. This produces the nice ‘scroll bars’ seen in so many rivers (including the Brazos) that represent the succeeding stages of lateral migration, also known as accretion. However, if there is more sweep than swing, then the channel can migrate downstream of its inner bend, leaving an empty space on the concave part of the inner bank. Picking the spot where a point bar ends and the counter point bar begins can sometimes be quite subjective, but here is a good image where I think you can see the difference.
Here is an annotated version of the photo I put up in the ‘Geology on the Wing Wednesday’ post, showing the point and counter point bars. The wet part of the river is about 20 m wide.
A great diagram that explain point and counter point bars is below, taken from a Sedimentology paper by Derald Smith and coauthors (pdf of that paper):
Lastly, there is a lot of discussion in the community about migration in deepwater (turbidite) channels. Before the advent of 3D seismic, scientists were skeptical that deepwater channels migrate like fluvial channels. However, recent investigations in many areas (Bengal, Indus, Nigeria, Gulf of Mexico, etc.) show that indeed sinuous channels are quite common and bear amazing morphological similarities to fluvial channels. Below is a nice image from Virtual Seismic Atlas of a slope channel system. The map is a time structure map that is draped with amplitude (i.e., the acoustic response of lithology to seismic waves). See if you think it looks like a fluvial channel to you, and whether or not there are counter point bars…