Mudstone with folded layers, Tucson Mountains, Arizona

The folding is marked by changes in thickness of individual layers. This material must have been soft and non-lithified when the folding took place. This type of deformation is known as soft-sediment deformation. It is the lack of regularity of thickness that is the give-away. The mud probably was very shallowly buried but then disturbed. The cause of the disturbance and slumping could have been earthquake activity, or sediment loading atop this particular layer.

I collected this rock in the early 1970’s, a time during which I was running a lot of geology field trips and mapping projects in Arrastre Wash, Ripple Wash, and Cabin Wash, while teaching field-based structural geology.  I like the way the folded layers are outlined by pink mudstone.

Age & Formation
Lower Cretaceous Amole Arkose Formation, ~120 Ma.