Tuesday, August 11, 2015

"Cantaloupe" concretions

A few years ago, while doing field work just west of Simi Valley, Ventura County, southern California, I collected some spherical concretions that are quite interesting. They are mostly five to six inches in diameter and have a distinctive pitted appearance that strongly resembles cantaloupes, thus I have dubbed them the name “cantaloupe concretions.” They are fun to collect, and other members of my field party fell victim of trying to find perfect ones (like the one shown below).  

A spherical concretion (approximately 11 cm = 4.5 inches) that looks like a cantaloupe.
 The black-and-white rectangles of the scale are in centimeters.

Some of the concretions are elliptical shape, as shown below.

An elliptical concretion (approximately 14.5 cm = 6 inches).
The concretions occur in sandstones in the nonmarine Sespe Formation and are approximately Oligocene (30 million years) in age. These concretions were first noted in 1924 by W. S. W. Kew, who did some of the earliest geologic work in the area. He also reported that they have a pitted appearance.

Concretions form around some nucleus (an impurity, possibly a shell or plant fragment) when groundwater flows through sands.  The sand subsequently becomes strongly cemented. Differences in the strength of cementation of the sand grains in these Sespe Formation concretions were made apparent by weathering of the surface, thereby producing the pitted appearance. Some of the concretions were broken open, but there was nothing organic nor unusual in the central (core) area.

In one of my earlier posts dealing with "Pseudofossils," in July, 2014, I briefly discussed what concretions are and illustrated a common shape.

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