Sunday, January 15, 2012

Sed Structure Sunday - Adhesion Ripples!

As previously mentioned, my visit to Sandy Hook in New Jersey was on a pretty damn windy day.  Big, 40 mph gusts were walloping the beach, coming in obliquely off the sea an onto the sandy foreshore.  These winds were mobilizing a LOT of material, saltating medium-grained sands along the damp shoreline, producing a perfect opportunity to catch some pretty neat eolian sedimentary structures.  Behold!  Adhesion Ripples (Camera case for scale)!

Adhesion ripples are formed when dry, wind blown sand gets glommed onto a wet, sticky surface; the fancy word for such glommification is "adhesion", et viola: Adhesion Ripples!  They differ from the traditional ripple cross-laminated sands in a variety of important ways, the most fundamental difference being that there isn't any evidence for the traditional grain-flow structure in the laminations, which would be expected if these structured formed as part of a migrating bedform.  More qualitatively, these structures just seem weird, with chunks and bits stuck onto a wavy surface in a way that just LOOKS different from the directional migration of sedimentary structures.

Kocurek and Fielder (1982) elucidated their genesis through a variety of flume/wind tunnel experiments, and also noted their occurrence in ancient eolian deposits.  In fact, the same quarries that produce the jellyfish impressions in the Cambrian of Wisconsin ALSO produce some pretty nice examples of adhesion ripples.  These are actually pretty handy structures, and have been used as evidence for a subaerial phase in some of these enigmatic mid-continent sandstones everyone seems to fond of.

Anyway, a little known but potentially quite helpful sedimentary structure to keep in mind when out looking at the rocks!

1 comment:

Unknown said...

Worked six summers as a lifeguard at Sandy Hook.

Tomorrow I will bring wounded veterans down the Colorado River through Cataract Canyon where the Cutler Formation contains such features.