concretions - ironstone concretions from the Lindavista Fm - an earth analog of the Mars "blueberries" in a Pyrex tube

Ironstone concretions, an analog to the Martian "blueberries"

Lindavista Formation,  Late Pleistocene,  San Diego County, California

These are found in the iron-rich red sandstone at the top of the Lindavista Formation that caps the roughly 400 ft elevation mesas near San Diego. About a million years ago, this was submerged as part of the continental shelf during high stands of the sea, an area which was later uncovered by dropping sea level and a rising San Diego coastline and then dissected into mesas.

During the Ice Age, San Diego's climate was wetter and cooler. Acidic groundwater picked up iron and manganese which was then precipitated in a subsurface horizon as limonite around tiny nuclei. Microbial and bacterial activity played a role in concentrating the iron into spherical shells. Analysis by SEM on thin sections of these ironstone concretions reveals the presence of colonial coccoidal microfossils. The concretions are enriched in several elements, particularly in biologically mobile iron and manganese. The enrichment may be due to concentration or secretion by microbial life.

These concretions serve as geological analogs to the Martian "blueberries" discovered by the Spirit and Opportunity rovers, since they are chemically and structurally almost identical.  These are chemical concretions cemented with iron oxide, formed after the surrounding sediments were already deposited. As mineral-rich groundwater percolated through the porous sandstone, chemical fluctuations and biological activity caused iron oxide to precipitate and bind the sand grains together in these self-organizing spherical shapes.

Shipping: By Priority Mail or USPS Ground Advantage, whichever is cheaper. Click > here < for shipping rates. See comment at Note! Use back button to return to this page.Click > here < for shipping rates. See comment at Note! Use back button to return to this page.

Making multiple purchases? Click on the "combine shipping" button in the shopping cart. We'll send an invoice with combined shipping. A link in that invoice will bring you back to checkout, no hassle.