Question for you Geologist types.

[Ted]

On our way down the coast we stayed at South Beach State Park just south of Newport, Oregon. While walking out on the beach, I picked up the two clam fossils (?) pictured below. I say this with a question mark because my idea of a fossil is more like a split piece of rock with the imprint of a clam shell in the rock. The shell would long since have disappeared as it takes hundreds of thousands (?) of years to form.

But these are more like the clam shell filled with mud and then the mud hardened to rock over time. But it seems to me that in the time it took for the mud to become rock, the shells would have turned to dust. So how are these formed? Are they really fossils or are they formed by some other geological process I am unaware of?

BTW, they were quite literally all over the beach. I tried researching them on the net and found a blog where a guy filled his day pack with them to give to his students.

 

[MarkBC]

Ted -
I don't know the answer, but I, too, am interested in the answer.
I've seen similar stuff embedded in bluffs along the Oregon coast, seen as I walk along the base of bluffs along the beach...and I had similar questions as you do: Are these really fossils or just well-preserved shells-in-rock?

I've also seen similar things embedded in the banks of washes/dry-creeks cut in the playa of the east arm of the Black Rock desert. In that case the shells are embedded in rock-hard mud...not quite rock, but it seems very old. Wondering if those shells are from the relatively-near past -- late Pleistocene of a couple tens-of-thousands years ago? So...maybe not fossils yet, and yet not creatures that live there anymore.

I think a paleontologist as well as a geologist would be handy.

 

[Ted]

Mark, that is actually another thing that confuses me about where these were. I would expect them below a bluff. But there was no bluff here. In fact, the whole shoreline here was a couple of hundred feet further out than the historical shoreline because of the Newport jetty. So no bluff and the beach is less than 100 years old. I'm guessing they must be washing in from offshore.

 

[Gormley Green]

The larger shell looks like a cockle shell, and the smaller a clam. There are many different methods of fossilization. Usually in older rocks you'll find shells become replaced with other minerals like calcite. It's called "replacement".

In younger rocks, like what you have, you can often find shells perfectly preserved made up of their original material. They can also be preserved by a process called permineralization. Basically it preserves the original shell by filling in the small pore spaces with mineral material.

The sedimentary rock you are looking at in your photo definitely doesn't have to be hundreds of thousands of years old. In the right environment, rocks like that can form literally in years to tens of years. So if the cockle was deposited in thick, fine sand to clay, with low oxygen, it could be perfectly preserved, as is, for a long time. To shift to another kind of fossilization would take a change in environment (i.e. oxygen level, pressure, temp., etc.).

Hope that helps.

 

[Gormley Green]

I'm guessing they must be washing in from offshore.


Very likely, and in the process eroding some of the surface material which has been protecting the shell. Hence the exposure.

 

[GBW]

Aye, There are a ton of questions here...

Ted - you're thinking of a cast which is an imprint of sorts. The casts or molds can be filled in with any material afterwards, taking the exact shape of the original bone/shell/tooth/whatever.

Currently, the ocean is at its highest point in the last ~30,000 years but there have been tons of fluctuations over that time. The ~25,000 y.a. the ice sheets covering most of Canada and northern states dropped the sea level over 100'. As the water level drops, it erodes and moves sediments and material, exposing older surfaces. As sea level rises, it can uncover some of these older surfaces which were exposed. As a general rule though, rising sea level does not carry material upwards. There are exceptions but lets go with the general idea. It is good at exposing previous levels though. Argh, this is hard to write without scribbling over a napkin with a pint nearby...

Here's an idea how they came to be.

-Clam dies in amongst his other clam friends.
-covered with more sediment
-glaciation occurs, drops sea level
-melting, sea level rises and now exposing surfaces from when the clam originally died. Likely sand and silt covered the bed which is removed by the rising sea but leaving the heavier clam shells behind. This does not necessarily need to be a cliff of sorts, just a flatter beach surface of great lateral extent. We are at about present time.

Back to the fossils. I don't believe they've been fossilized. I think they are still in their original state. Gormley Green is right, you replace the original shell material (CaCO3) with Calcite but can also be Pyrite, Siderite, and silica (quartz). This usually takes longer than the 10k's time frame to occur.

Looking at the 'mud' in the clam shell, I'm inclined to believe its not mud. I'm not sure there would have been enough time and pressure to create a mudstone and yet still relatively preserve the original shell material. I'm thinking it is some sort of concretion inside the shell. Its not a concretion but similar, I'm drawing a blank at the moment. The shell was likely filled with a silt at time of death and over time, the silts became joined through mineralization from mineral laiden (possible silica) waters flowing through the ground. You don't need the large amounts of pressure, temperature and time for this to occur.

Question - is the 'mud' in the clam shell hard? Does it scratch easily?

Clear as mud?

I'm going to draw a picture on my lunch break now.

 

[GBW]

 

[SunMan]

Awesome explanation GBW, even a numbskull like myself can grasp that. Thanks. Awesome fossils Ted.

 

[Foy]

View attachment 13432


Holy Smokes, GBW, what kinda ultra liberal geology school did you go to? At Appalachian State, we NEVER talked about "redistribution". Dontcha know that's a Commie plot?

Seriously, nice work on the diagrams. I was always a metallic minerals and energy minerals exploration guy--the bugs never really melted my butter.

Foy

 

[GBW]

Holy Smokes, GBW, what kinda ultra liberal geology school did you go to? At Appalachian State, we NEVER talked about "redistribution". Dontcha know that's a Commie plot?

Seriously, nice work on the diagrams. I was always a metallic minerals and energy minerals exploration guy--the bugs never really melted my butter.

Foy


haha, liberal's & commie's? Have you heard of this right-winged province! Don't get me started on how ignorantly right-winged this place is... Anyways, being in Alberta, there is quite the oil and gas influence here. Searching for O&G makes me study a lot about sediment transport and sea level fluctuations, sediment traps etc. All that fun stuff.

I did do a little of the mineral exploration gig but the sites are so remote and removed that I was spending an entire 4 month summer away. I liked the hard rock work better than my office job but the office work allows me escape on a more regular basis. I've tossed out the idea to my wife about going back to work those schedules and she says I better pack my camper because I won't be coming home to anything. She'd rather have me work at the local hardware store than have me gone 4 months at a time.

 

[Casa Escarlata Robles Too]

Thanks GBW. We get a lot of that type of "fossil"here in the Monterey Bay area. Lots of shells in a "mud" type stone. The beaches around Santa Cruz are full of them. I have mentioned it in other threads,there is a provincial park in the Smithers area of B.C. that is so cool that you can keep the fossils you find,up to a certain size.We stopped there in 1990 and the kids found some nice ferns and fish. I think you can still collect and keep them.

Thanks for the info.

Frank

 

[Casa Escarlata Robles Too]

Thanks GBW. We get a lot of that type of "fossil"here in the Monterey Bay area. Lots of shells in a "mud" type stone. The beaches around Santa Cruz are full of them. I have mentioned it in other threads,there is a provincial park in the Smithers area of B.C. that is so cool that you can keep the fossils you find,up to a certain size.We stopped there in 1990 and the kids found some nice ferns and fish. I think you can still collect and keep them.

Thanks for the info.

Frank


Not to steal this thread,but here is a rock we find around our beaches.
Frank

 

[Lighthawk]

Clear as mud?



Yup. I'm impressed. Now can you explain all the crazy stuff we see in DV next?

Honestly, we need a geologist to understand more than half of what we see in our Nevada / Eastern Sierra travels. Heck, right here in the Gold Country of No Cal, we've got accreted island chains with serpentine crashing into granite plutons covered with a good dose of volcanics. At least that's what Assembling California by John McPhee tells us.

 

[Ted]

Wow, you guys are great! Fantastic info with drawings to boot. GBW, I laughed at the comment of needing a pad to draw on and a pint. I need to draw out all my ideas when explaining them to people. Thanks for replies. Another friend supplied a link to an Oregon State sheet on fossils you can find on Oregon Beaches that has some info and photos. You can see it here . Makes me want to go back and spend more time looking. Thanks again for all the help.

Ted

 

[GBW]

Ted-

I swear, more geological papers have been written and proposed on the back of pub napkins than paper themselves. When we were going through school, there were always a good percentage of geo's present at the pub at any one time. Because we were a smaller class, we knew everyone from about 2nd year to masters students and onto the Prof's. We'd all meet up, and after a few pints, the academic arguments would start to fly, napkins were always in short supply. They were scribbled on, discussed, ripped, re-drawn. I recall an office of a masters student, he had about 15 napkins pinned up on a corkboard with all the drawings and notes ordered in some sort of fashion.


Lighthawk-

California is a mess... geology wise. I haven't studied much of the west coast US geology but its a little similar to southern British Columbia. Once upon a time, things were simple, the some accretion, a few volcanoes, couple granite plutons, little heat, pressure and time, and throw in a dash of faults and its all gone to hell. Its interesting stuff to say the least.

My wife is starting to hide rocks I collect though... She always asks why I need to pick up a unique rock from whatever location we go to on holidays. She just doesn't understand.

 

[MarkBC]

I love learning about this stuff!

I was introduced to the wonders of southeast Oregon (that is, the non-biological wonders) -- and by extension, the Great Basin Desert in general -- and by extension, basin and range terrain in general, by a class I took at the local community college about 25 years ago: "Geology of Southeast Oregon". (I think I've posted this somewhere else here...). We didn't learn much about mineralogy - other than basic rock types found in SE OR -- basalt, rhyolite, granite, etc. -- but we learned about the big story that made the land that we saw there, mainly volcanism and faulting and the resultant up/down movement, and also some about erosive processes.

I buy layperson (mostly) books about the places I go -- "Geology of the Great Basin", "Geology of Oregon", "Geology of Utah"...this last one (which was more like a textbook than layperson) I bought in the Capitol Reef NP Visitor Center on my last-May trip to southern Utah, and I read a lot of it while camped there, at night and on the one rainy day I had.
And yes, the John McPhee books are great reading, too -- I think I've read most of them.

I love understanding what I'm looking at...so I can incorporate it into the model of the universe that I'm building in my head.