13 November 2014

Gypsum casts? You be the judge — UPDATE: Syneresis cracks!

Posted by Callan Bentley

I’m curious to hear what you sedimentologically-inclined readers think of these features:

IMG_0156(with flash)


IMG_0158(without flash)

I collected this sample in May on Corridor H in West Virginia, at an outcrop of the Silurian-aged tidal flat carbonates of the Tonoloway Formation. It’s got a nice mud crack (dessication crack) triple-junction (yellow in annotation below). It has a calcite vein that cross-cuts the mud crack (pink). And it’s got lots of little lenses (in the geometric sense of the word) a few mm long, all over the “plates” of the mud cracks (blue). What are they? What do they tell us?


My colleague Joshua Villalobos (El Paso Community College) snagged one then, too. Josh imaged his the other day with a jerry-rigged “macro” GigaPan set up. Here’s the result: It’s not as high-resolution or focused as a MAGIC macro GigaPan, but it does impart an additional perspective on these structures.


I posted a link to Josh’s GigaPan on Twitter the other day, and asked what these things were. I got reactions that ranged from the ichnological to structural, but mainly clustered around the idea of gypsum casts.

Gypsum casts would be something that we would expect to find in tidal flat carbonates – an arid, dessicating environment would encourage the evaporation of seawater and the precipitation of evaporite minerals in the resulting brine. Halite casts are well known from this same outcrop of the Tonoloway.

The key question is then: Why are they all aligned? Any insights?


UPDATE: Matt Kuchta has provided a compelling explanation: Syneresis cracks – dewatering structures, essentially. Click the link to the Wikipedia article and compare. Looks like it to me!