Fold Friday: Soft-Sediment Deformation

Folds and interesting structures within sediments prior to cementation into sedimentary rock formations.

Welcome to Let’s Talk Outcrop, your newsletter all about Earth Science. I send weekly Tuesday and Friday emails explaining cutting-edge research, interesting topics in Earth Science, or examining fundamental Earth processes.

This series, Fold Friday, is exclusively for paid subscribers and offers an in-depth summary of some amazing folded formations I have found.

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Many of the previous folds we’ve explored in this series relate to a specific folded structure and investigated the historic geology, deposition, and tectonics related to the formation of that amazing resultant fold.

This week, we focus on a specific type of fold, nonunique to a specific region. 

The structures shown above are in a category of folds within soft sediments, forming a classification of folded structures, called “soft-sediment deformation”.

Specifically, soft-sediment deformation refers to alterations of physical structures of geologic materials (sediments) that are still unconsolidated, or pre-lithification (still not cemented into solid geologic units).

Soft-sediment deformation structures are classified into subgroups based on the deformational processes forming them:

1. Endogenic processes, forming endoturbations: Structures forming from interactions with living organisms.

2. Gravitational processes, forming graviturbations: Deformational structures driven by gravitational forces (e.g., a solid load compressing and “squishing” sediments around it).

3. Exogenic processes, forming exoturbations: Structures forming from the movement of Earth’s exterior (e.g., tectonic forces, wind erosion).

An example of graviturbations is shown below. Originally, the now-folded sediments, still before solidification, were compressed by an overbearing load. The weight of the load and force of gravity pushed the semi-solid sedimentary rock out of the way and formed a depression. The increased load from extra sediments piled in continued deforming the bottom layers, eventually forming a fault within the lower layers to the bottom left of the dark-colored load (drip-like feature).

Credit: Figure from Loon (2009).