Sunday, February 11, 2018

Mystery sand spheres

A fellow geologist recently sent me the following pictures and information about "sand spheres" he found in the general area of the shoreline region of the late Pleistocene Lake Manix, in the Mojave Desert, between Barstow and Baker, southern California. Lake Manix formed by overflow of the Mojave River between 500,000 and 25,000 years ago.

The spheres are all of small size and range from 0.51 to 1.5 cm in diameter (e.g., they are about the size of a U.S. dime). They consist of friable (= fragments come loose from the spheres when rubbed) and angular, coarse-grained material called grus, which results from the granular disintegration (weathering) of granite in an arid climate. The material making up the spheres is slightly cemented by calcium carbonate. 

The above picture is a cross-section of a sliced sphere. Compared to armored mud balls, the "sand spheres" do not have a mud core and are too uniformly of small size. Armored mud balls form when a clump of gooey mud begins to roll around under a flow of water and fragments of rock adhere to the mud surface.

The "sand spheres" shown above, in the background, might have formed in place. The "loose" ones in the foreground are derived from this more concentrated mass of them.

If you "lean in" on this far-away shot, you can make out how numerous the small spheres are (thousands and thousands) among the much larger fragments of angular rock. The "sand spheres" litter the ground sort of like rabbit or deer droppings. 

The "sand spheres" are at an elevation of 563 m, which is slightly above the Pleistocene shoreline, thus they were most likely not formed by a shoreline process. There has not been any lake water at this elevation in 18,000 years. The "sand spheres" look fairly fresh. They are on the surface and although they are not very delicate, thus it is doubtful that they have been around since the Pleistocene. My colleague believes that they might be a product of a local (recent?) downpour. 

Although the nonmarine "sand spheres" of Lake Manix resemble "sand balls" created by small "bubbler" crabs on modern beaches in the tropical ocean waters of the Indo-Pacific, they cannot share the same origin. By the way, if you have the interest you will be amazed and amused by a BBC Blue Planet video (online) that shows how these crabs form the "sand balls."

In closing, we do not know the origin of these Lake Manix spheres and could not find anything in the literature about them. Determining how they formed would be a worthy project. 

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