#mudcracks

Mudcracks.

Surise over the California Desert

shainblumphotography

Mudcracks

Fossil mudcracks

These rocks come from the Belt Supergroup in Montana, a sequence deposited in the late Precambrian when a large sedimentary basin opened.

Probably a large majority of our readers have seen mudcracks when they’re fresh. Mudcracks form when fine-grained sediment undergoes cycles of wetting and drying. Fine-grained clays will expand and swell when they get wet, locking the water into their chemical structure. The wet clay will form layers that are damp but sticky; wet clay will stick to almost anything that touches it, especially your boots.

When the water dries up, those clays contract. Solids don’t contract easily; they have a habit of cracking and breaking. Drying out a layer of mud leads to the formation of mudcracks. If those cracks are later buried by more sediment, they can be preserved in the geologic record, preserving a record of the environment hundreds of millions of years ago.

This rock formed in an area that was undergoing cycles of wetting and drying. There weren’t strong currents that would bring coarse-grained sediment into the area, just piles of mud that went from wet to dry.

The cracks penetrate 2 different colors here. That hints to me that the color is not related to any original sedimentary bedding, but instead probably relates to processes that took place after the sediments were deposited. If there was a change in the abundance of oxygen across this unit that could cause a red color in a portion of a unit that ends at a line like we see here.

-JBB

Image credit: https://www.flickr.com/photos/29750062@N06/3150979627/