Monday, February 18, 2008

Nonmarine Waltherian Successions?

The key to much of our understanding of stratigraphy relies heavily on Walther’s Law. The vertical juxtaposition of deposits that represent various depositional environments allows us to pick surfaces, correlate wide-spread geological processes, and determine time-rock and time-surface relationships; in other words, it’s kind of important to the whole sedimentology and stratigraphy thing.

While Walther’s Law works pretty well for shallow marine successions (for instance, the shoreface succession), we start to get into hazy territory in nonmarine environments (and deep marine, too). What is a Waltherian succession in an evaporative lake? How about in fluvial/alluvial successions? More to the point, when do we pick a surface that delineates a non-waltherian succession?

Once again, I present some pretty pictures gleaned from NASA World Wind! ENJOY!

This first picture is an overhead view of a lake basin north of the Ordos basin; a river, sourced from nearby mountains, feeds a large lacustrine delta, with distributary network, near a pretty good-sized dune field in the lower left. Alluvial fans make an apperance as well. If the lake levels drop, how will the migration of these disparate features be recorded, and how would we interpret them? Could get kinda complicated.

This second image is an oblique view (at 10x VE).

This last picture is a pretty nifty image of some Kelvin-Helmholtz instabilities developing on the edge of a hypopycnal plume.


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BrianR said...

I work on deep-marine stuff a lot and have had similar thoughts.

There are some cases in turbidite systems where different 'environments' occur in a semi-organized fashion. One of the best examples is the Amazon submarine fan - where you see a vertical succession related to the avulsion of the submarine channel and then subsequent construction of a new channel-levee. But, this seems to be an exception rather than a rule.

C W Magee said...

I have no idea who Walther is, but on the Sydney/Alice leg of my flight to work, we often go over a salt pan that has a clastic delta prograding out onto it. I guess that means either a heavier baseload or bigger flows, but I haven't thought heaps about it. Assuming I actually have a camera, should I try to snap a picture when the new season starts?

Eric said...

Chuck - nifty pictures of depositional environments are always appreciated!