Saturday, April 21, 2012

Sandy Troughs!

All this nice weather is makin' me hanker some field work!  Instead, I wither away, chained to the ol' Word Processor, processin' words.  Oh well.

However, a few of us grad students were recently able to slip off for a little local rock hammering.  An old railroad cut here in town actually chops right through some pretty awesome Ordovician sandstones, actually providing some pretty okay exposure, for the midwest.  These sandstones are stupid-rich with well sorted, well rounded, upper-medium quartz grains; despite this, there's actually some pretty decent sed structures to enjoy, accentuated by weathering.

 The picture above shows some fairly nifty cross-stratification.  Don't be tricked by the apparent bi-directionality of the cross bedding, though! These are trough cross-stratified sets, formed by the migration of sinuous-crested 3-D dunes.  Paleoflow is actually broadly oblique into the picture, with troughs chopping into each other at slightly different angles. 

The picture below is a close up of the juxtaposition of two different sets of cross-strata.

If you're not careful, you might interpret these structures as herring-bone cross-stratification, which would imply a tidal sort of paleoenvironment dominated by the bidirectional ebb- and flood-tidal cycles.  However, as you can see in the picture above, and with some careful observations on the rocks, the apparently different directions are actually just obliquely cut troughs, with the internal cross-laminae conforming to the immediately local conditions formed by the variable geometries of individual dunes spilling into the erosional scoops forming on their lee-side.  Careful paleocurrent measurements from these rocks would actually show a fairly consistent and unidirectional suite of directions, with no evidence of tidal back-and-forth. 

To be honest, I've never actually seen ANY convincing "herring-bone" cross-stratified sandstones; EVERY example I've ever been shown in the field turned out to be, upon closer and more careful inspection, the result of obliquely flow-perpendicular exposures of troughs.  I'm not saying that herring-bone x-strata don't exist, but just that they're probably grossly over interpreted.  A little careful, nose-on-the-rocks geology will rapidly disabuse us of our misapprehensions.

So don't be tricked by crafty sed structures!  Do as the geologists in the picture below, and get on those rocks!      

1 comment:

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