This is a Hypothesis to explain the presence of a large amount of oyster shells and other seashells in the Sediments 3 meters down in the Dee River, Mt Morgan.
AI grok says, These look like fossilised black-lipped pearl oysters (Pinctada sp.), common in Queensland's Pleistocene coastal deposits, thriving during higher sea levels ~10,000-120,000 years ago.
Grok AI says, These look like fossil oysters, likely Gryphaea sp. (devil's toenails), with characteristic curved, ridged shells and zigzag margins. Found in Jurassic rocks (~170 mya), when shallow seas covered parts of Queensland—uplift later raised the area to 1500ft. The "crinoid marking" may be a stem impression or encrusting bryozoan.
Claude AI says,Based on the visible features in your images (texture, color, shape, and growth lines), the specimens appear to be fossilized or subfossil oyster shells, likely from a Crassostrea species (such as Crassostrea virginica or a related type).
Here’s a breakdown of what I’m seeing:
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Surface texture and layering: The rough, wavy ridges and laminated structure are characteristic of oyster shells. The alternating purplish-gray and white coloration is also typical, especially in shells that have undergone partial mineral replacement or mild fossilization.
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Crinoidal impressions: The small circular or radiating marks you mentioned could indeed be from encrusting marine life—barnacles, crinoids, or tube worms—that settled on the oyster while it was alive or after death.
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Condition: The fact that they were found 3 m deep but don’t look heavily mineralized suggests they are relatively young subfossils rather than ancient fossils—perhaps from the Holocene epoch (less than 10,000 years old), depending on the sediment context.
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Shape variety: The twisting and irregular growth forms are normal for oysters that grew on uneven substrates or crowded beds.
If you found them in a river (the “Dee River”), they may represent ancient estuarine or marine deposits that became buried and later exposed by erosion or excavation.,
The Mount Morgan area—though best known for its famous gold and copper mine—also contains marine and estuarine sedimentary layers that date back to times when sea levels were higher. Around 125,000 years ago (Last Interglacial), sea levels were significantly higher, and marine deposits extended further inland along river valleys, including the Dee River system. Your shells may come from one of these ancient coastal or tidal deposits that were later buried and preserved.
🧬 Preservation and Appearance
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The purple-gray banding and chalky white outer layers suggest mild mineralization—probably calcite replacement of the original aragonite.
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The crinoidal or encrusting marks are consistent with marine organisms (barnacles, serpulid worms, bryozoans, or even small crinoid fragments) that lived on the shell surface.
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Since the shells still retain some of their original color and structure, they’re not ancient fossils (not millions of years old), but Late Pleistocene to Holocene (up to perhaps 100,000 years old).
There is a vast difference in classification. I agree with the Grok interpretation of the species. My research places them as Gryphaea in the Eocene, a leftover species from the Jurassic.
The look of the items probably doesn't have much to do with the age, as I have found out with subfossils millions of years of age from Port Alma peat bogs. These shells have had a sandy burial that has been cool and dry.
They have been dry all the time even though under a sometimes flowing river. These clay hearthclast items dug up in a small excavation, when dropped in water, dissolve.
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