Saturday, February 6, 2010

Bedforms and Cross-Bedding

It has been several years since I have had sed/strat, and when I chanced across this webpage I was struck with the thought that it would have been a nice (scratch that, AWESOME) resource to have had during that course. It is via the USGS - not sure how I could have missed it, as it was an active site at the time of my course. I think perhaps the title was such that I mentally discounted it as useful because it says 'Western Coastal and Marine Geology'. However, this animation set they have is applicable to all coastal environments/paleocurrents. It (the site) is truly a masterpiece of work, and a lot of time and effort went into its development.

As those of us who have gone through a sed/strat class, it can (at times) be daunting to learn the associations between bedforms, cross-bedding, and environments. It isn't so much the absorption of knowledge that hangs a lot of us up, but rather looking at an outcrop or rock sample and associating the type of cross-bedding it exhibits. Even when I had that part pretty well mastered (of which at this present time I have only vague memories, hence my personal reason for appreciating this website) it was still difficult to visualize the environment in action. This is why these animations are so fantastic to look through!

To compliment the animations, there are detailed explanations on how to classify bedforms; transverse, oblique, or longitudinal, via various mathematical computations. Considering the aspect of unknown variables, implementing the wrong formula can produce less than desirable results. This paper explains certain pitfalls, and how they can be precluded in order for all variables to be considered.

Conceptually,the approach is to determine the unique transport vector that simultaneously would cause the observed migration of two sets of bedforms. Algebraically, this is accomplished by solving equation (2) simultaneously for the transport represented by two sets of bedforms. The solution is given by

Equation (3) can also be applied to a single set of bedforms, if they are three-dimensional. In such a situation, β is equal to 90°, V2 is the along-crest migration speed of the plan-form sinuosities, and H2 is the mean height of the bedforms meassured along profiles parallel to the generalized trend of the bedforms. In the computer-generated depositional situations, H2 was measured from contour maps of the bedform topogrraphy. Although equation (3) cannot be used with perfectly two-dimensional computer-generated bedforms, most real bedforms, including many that would be considered two-dimensional, are probably three-dimensional enough to use this approach.

 Relations between cross-bedding, bedforms, and flow as well as  two dimensional bedforms/cross-bedding are also touched upon via pdf files. The real gem of the website was refreshing my memory on the different bedding types and how they develop. I had intended upon adding more to this post, but I lost myself in browsing the site as I was composing this blog and now am too short on time. I mainly just wanted to share resource I had found, if you had not already discovered it yourself. (I may be behind the powercurve in regards to that ;0)




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