7 May 2018
Yesterday I finished a new GIGAmacro image, a view of a chunk of Tonoloway Formation showing what I thought were exquisite graded beds in these Silurian shallow-water carbonates. Closer examination revealed several other primary sedimentary structures with geopetal value: desiccation cracks (“mud cracks”) in cross-section, with their V’s opening upward, and a small instance of scouring and filling, where the pre-existing mud deposit was eroded by energetic sand transport, and sand filled the resulting hollow. Three for the price of one, in terms of determining younging direction. There were also some ambiguous mud cracks, and at least one example where the V pointed the wrong way. But the preponderance of the evidence was pretty clear to me, and I thought it was a good teaching example. I deliberately imaged it with bedding “vertical” in the frame of the image. Here it is:
I posted a link to the GIGAmacro on Twitter and asked:
Students: Which way is stratigraphic “up?” There are three primary sedimentary structures here to clue you in to younging direction: https://t.co/iyVo31fHB6
— Callan Bentley (@callanbentley) May 6, 2018
As you can see, about a third of respondents chose “left” and two thirds picked “right.” In this case, right is correct. (“It’s right!”) But since 1 out of every 3 respondents picked incorrectly, it appears there is a lesson to be taught here. To make the case for why stratigraphic “up” is to the right, consider this annotated copy of the image:
Click on the different annotations and see if you agree. I’ve marked the “right is up” ones with yellow circles, the ambiguous ones in pink, and the “right is wrong” one in gray.
Thanks for playing! More of this sort of thing in the future, utilizing the capabilities of the powerful GIGAmacro imagery viewing platform.