MUD RIPPLES FROM A PLAYA LAKE IN EASTERN CALIFORNIA

A previously unreported sedimentary structure has been observed on the dry bed of a small playa lake in California. Usually, dry playa lakes present a flat surface dotted with desiccation cracks but in this case, relatively big (dm- to m-scale) ripple-like structures (dunes?) were present.

During several visits to the site, mud cracks were repeatedly observed but never in an embricated ripple-like pattern. Upon subsequent visits the structures were not preserved, and their only record consists in a series of pictures taken upon the first visit.

Coso Playa is a small basin, connected through a narrow opening to a bigger lake to the east and limited by the volcanic cinder cone of Red Hill to the west. Strong winds often blow at this location, as evidenced by sand dunes, composed of volcanic materials from Red Hill, that surround the lake. Water occasionally fills the lake, coming from precipitation, but there is no evidence of drainage. When water motion ends, clay deposition occurs, then water evaporates and mud cracks form. There is no recent evidence of evaporites at this location.

Ripples are mostly known from silts and sands but ripple-like structures have also been signaled in mud, even if never from a desert playa environment. Since individual grains of clay move in suspension and not as bed load, they can not originate cross-bedding. Lab experiments showed that clay can often form flocs, or aggregates, that increase their size up to coarse silt or even fine sand. These flocs, while still retaining the cohesive properties typical of clay, would then behave like bigger size grain and be subject to traction.

A possible origin for the ripple-like structure at Coso Playa would then imply flocculation and a current strong enough to carry these flocs. Since there is no drainage at Coso Lake, the only other possible agent of transportation is wind. A strong wind blowing on a dry playa, or even a wet playa but with no water, would not be able to cause flocs traction. The most likely case would be one where strong winds blow over a playa covered by a relatively thin body of water, while clay deposition was not yet complete. The grains of clay still in suspension could be forming flocs, and these flocs settle to the bottom, where they would be subject to water traction. In this way, flocculated mud could actually originate cross-bedding as observed at Coso Playa.