Quaternary pseudo/fossil from LA area

[Scherzer]

The strange objects seen below were found at a depth of ~8 ft below ground surface in what is mapped as "Quaternary alluvium," adjacent to a channelized river or creek. At the same depth, we found a horizon of tiny terrestrial snail shells and a few ?oyster shells. All were found in unconsolidated clay/silt, or at a contact between unconsolidated clay/silt (below) and unconsolidated vfgr-fgr sand (above).

They are certainly not fossilized elements anymore - you can see in one fractured portion (small ovoid in top center in photo 2) that they are composed of silt or very fine-grained sand. But I'm wondering if they are secondary infilling of a void left by an element - mainly because at first glance, I thought they were weathered vertebrae. Do they look biologic to anyone else, or are they likely just random calcrete growth?

[DPS Ammonite]

Where did you find it? I am curious about the oyster shells. Do you think that they are modern or older?

I wonder if they might be a concretion formed around a root mass from an old soil horizon. Does the rock fizz in acid which would indicate a carbonate cement?

Edited April 28, 2016 by DPS Ammonite

[Scherzer]

Oh right, I forgot to mention - they did react strongly with acid, which is why I was leaning towards some type of calcrete growth.

As for the shells, I suspect they are no older than late Pleistocene - structurally, they're still pretty "soft." Everything was found in the Chino Hills area, in the foothills of the Santa Ana Mountains.

[tmaier]

They remind me of the oddly shaped rocks we call "live rock", found in lagoons and brackish estuaries. I've smashed a few open, and they are often a conglomerate of shells and other bits glued together with a weak limestone that I've always assumed was secretions of the organisms that live on the surface. Under the "rock" is usually a pistol shrimp, crab, wrasse or blenny, and on the upper surface of the "rock" are anemones, tube worms, sponges, and other benthic invertebrates. When these are found in a lagoon or estuary, it is best to leave them alone because they are an oasis for marine creatures in the barren mud flats. Collecting "live rock" is banned in most all states of USA, for that reason. They are beautiful when viewed underwater. You can use a glass bottomed bucket and a chair to watch the activity around it. If you pull one out of the water (don't!) and pop it into an aquarium, the "rock" will bloom before your eyes as the worms and anemone flower out of it, turning a dull grey rock into a beautiful bouquet. It's better to view it undisturbed in it's native habitat, though. Also, all the critters are difficult to feed, so they do better in the wild.

Anyway, that's what I suspect it to be.

http://www.google.com/search?q=live+rock&btnG=Search&hl=en&gbv=1&tbm=isch

Aquarist have been successfully breeding their own legal "live rocks" since most collecting in the wild has been banned.