Sunday, August 21, 2011

Sole-Saving Sed Structure Sunday!

Brian shared an image of some completely rad flute marks on Friday, which has inspired me to dedicate this Sunday Sed Structure to saving some SOLES!  Sole marks, that is!  As Brian pointed out, sole marks are a whole suite of erosional features generally preserved on the lower boundaries of beds.  Sole mark taxonomy can get complicated, because the features are caused by a range of subtle processes that grade into each other; however, you can safely subdivide sole marks into those produced by fluid scour versus those produced by tools or objects interacting with the substrate.  Below is a quicky flowchart I made for a sed class I TA'd in the past, describing some of the salient features of the different "kinds" of sole marks.

Sole marks are pretty cool; they're some fairly solid paleocurrent indicators, even when they only give you a trend of motion without a sense of direction.  More important than that, however, is the fact that, at least for the scours like flutes marks and gutter casts, these sole marks actually represent pretty dramatically high energy conditions, much higher than for any measly ol' cross-stratification your going to find.  That ability to start subdividing energy states in a stratigraphic section is key, and sole marks are incredibly useful when interrogating strata at fine scales of resolution.

Anyway, the picture below is from some Wilkins Peak Ss (Eocene, Green River Formation), showing a sole mark on the base of a channelform.  This is an example of a groove cast, meaning that a big ol' chunk of something (probably some wood) scrapped along the base of this channel.  At some point later, river sediments filled in the erosion gouge, casting the feature we see below.  Importantly, these features that record erosion, exposure, and high energy are often found in association with things like lag deposits.

 This picture below shows some smaller groove casts in a parallel grouping, from the same succession of rocks.  

Show some love for some erosional features, and they'll love you back, I promise.

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