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Geopuzzle: What’s the scale? (ANSWER)

March 19, 2008

ChrisR was the closest, and Ed wasn’t too far off either … the unknown scale bar from the photo in this post is about 36 cm, or two field notebooks long.

The reason I picked this photo is because these structures look very much like climbing ripples, except that they are climbing dunes. The lighter material is siltstone to very fine-grained sandstone and the brownish stuff is fine to lower coarse-grained sandstone. Note the consistent direction of climb (from right to left), especially in the interval just above the field notebook.

whatsthescale_1b.jpg

To get climbing cross-stratification you need a combination of migration and aggradation. In other words, these structures form where sediment is being transported in suspension and by bedload such that some grains fall out of suspension (aggrading the bed) while others are transported as bedload (migrating the bed).

Climbing ripples are everywhere in turbidite sequences, but climbing dunes are not nearly as common. It takes some fairly sustained sediment gravity flows to produce such structures.

Thanks for playing!

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