Cenozoic Marine Snail

[KevinG]

Here is a snail which is about 3/4" long by 3/8" wide, it is along with some clams also. I was wondering about the snail, the clam is just for reference.

Thank you.

[Auspex]

Attractive fossils; they look pretty fragile...

[Shamalama]

Welcome to the Forum! Look like some interesting fossils, about what age are the sediments? Are they all molds and casts or do you find shell material too?

Dave

[Auspex]

...about what age are the sediments? Are they all molds and casts or do you find shell material too?

I'm guessing Late Miocene or Pliocene, and I'd probably keep my eyes peeled for shark's teeth too

[KevinG]

All those pictures are of the original items as is on the rock face. There is no clam shell material left at all, just the impressions. I did not touch the snail but that sure looks like shell to me. As far as shark teeth are concerned I bet there would be something (hope). It looks like it was a clam bed because some chunks I do have look like a clam .

Shamalama, the sediments are older than me, which is all I know. They are very soft too.

Thank you for the hint Auspex I will check into that theory.

For anybody interested in the lat/long it is posted in the Hello/introduction forum.

Edit: According to the USGS maps it looks like I am sitting on either a Pleistocene or late Miocene deposit.

Edit: There is also some darker grey rock around which is considerably harder. Is this worth looking at or is it just rock, the clams are definitely in sedimentary rock.

[Boesse]

Hi there Kevin, there are a lot of Miocene formations in the east bay, and that snail looks like Nassarius, which I believe is suggestive of Late Miocene.

There are many different Miocene units in the east bay, and I can't remember off the top of my head which ones.

Bobby

[KevinG]

Excellent Bobby, off the top of your head do you think there would be anything else of interest in that formation or will it just be clams and snails? I am also wondering if it could be fresh water clams and snails.

Thanks for the hint.

[Boesse]

Well, those look like marine snails/clams. That whole area in the oakland hills has historically yielded some important terrestrial mammal localities (Mio-Pliocene), and in the 1930's, there was a Miocene mastodon trackway exposed on a hillside somewhere within 1/4 mile within St. Mary's college.

Bobby

[Auspex]

In another thread, the cliff face was said to have it's bedding plane tilted at 45 degrees; that makes me think that it is sediment that was bulldozed off the ocean floor by the N. Am. plate as the Pacific plate was subducted, and is thus both marine and older than the Pleistocene.

[KevinG]

in this picture the central of the three sections is where I have looked. There is nothing to be seen on the right hand section and I have never looked at the left hand section. The right hand section has no riprap, the middle section has a fine (1/2") erosion base with the 45 degree slant (looks like cracks) and the left hand appears to fall off in large chunks.

This is also within 1/4 to 1/2 mile from St. Marys college.

[Boesse]

Auspex,

Most of the Cenozoic sediments in CA were deposited on continental crust. Most of the accretionary material is Mesozoic (if I remember correctly); also, the Miocene witnessed the change from subduction to strike-slip faulting, and as this rock unit is Late Miocene (due to Nassarius and my prior knowledge of East Bay stratigraphy), this was deposited after strike-slip faulting had been initiated.

If this were an accretionary deposit, it would be waaay more screwed up than just dipping at 45 degrees; it would have contorted bedding (if any bedding remained), and would look more similar to a melange.

There are a series of different tectonic blocks in the east bay bounded by strands of the San Andreas fault that probably allowed for small microbasins to form by downdropping of the various blocks.

Bobby

[KevinG]

~20 years ago in Geology I would have understood more of that...

[tracer]

Auspex,

Most of the Cenozoic sediments in CA were deposited on continental crust. Most of the accretionary material is Mesozoic (if I remember correctly); also, the Miocene witnessed the change from subduction to strike-slip faulting, and as this rock unit is Late Miocene (due to Nassarius and my prior knowledge of East Bay stratigraphy), this was deposited after strike-slip faulting had been initiated.

If this were an accretionary deposit, it would be waaay more screwed up than just dipping at 45 degrees; it would have contorted bedding (if any bedding remained), and would look more similar to a melange.

There are a series of different tectonic blocks in the east bay bounded by strands of the San Andreas fault that probably allowed for small microbasins to form by downdropping of the various blocks.

Bobby

i keep trying to figure out a way to invest in your brain. i mean, ninety percent of the time i have no idea what you're saying, but you really sound freaky smart. and i mean that with all accretions.