Understanding the Formations of Charleston and Dorchester Counties, SC

[cthamon 2]

Okay, I posted this yesterday and I’m not sure if it was that it was too long winded, in the wrong spot, or both. So, I will attempt to boil it down.

 

There was a post on this topic in 2011 but I feel like there’s certainly more knowledge on this now.

 

1. What formations are megalodon teeth coming from? The plausible ones are the Parachucla (22ma), Marks Head (18ma), and Goose Creek Lime (3.5ma), all within the umbrella Hawthorn formation. The CofC Museum lists almost every specimen as coming from the Goose Creek Lime, yet the hottest spots at best have the Raysor formation(2.5ma) exposed.

2. Are said spots only good underwater where the river has cut through to the former three?

 

3. Is material between the Marks Head and Goose Creek era extant in any areas? People have suggested that the size of some teeth would place them in the middle of these two time periods, unless there’s reason to believe they’re reworked.

 

4. Wanting to see pictures of the formations mentioned (excluding Marks Head which is only subsurface), in addition the Wando and Chandler Bridge formations if anyone has pictures lying around.

[Boesse 1,754]

Howdy! So I probably wrote whatever you saw at the Mace Brown MNH. Here's some points to make:

 

1) Carcharocles megalodon is middle Miocene to early Pliocene in age (~15-3.5 Ma). We don't really have any marine units in the Charleston area that are middle or upper Miocene. The only rock units therefore producing unreworked C. megalodon teeth are the Raysor and Goose Creek Limestone; the two units are only barely differentiated and there has historically been some confusion about mapping these. Some of the only 'good' Raysor exposures with marine vertebrates in the entire area were in places like Giant Quarry and Cross Quarry (both now closed to collecting). The Raysor doesn't really have many exposures in the Summerville area.

 

2) Older units will produce older species of Carcharocles. Rare Carcharocles chubutensis are likely derived from the Marks Head Formation, and the Oligocene Ashley and Chandler Bridge formations produce Carcharocles angustidens teeth.

 

3) Most localities where C. megalodon teeth are being collected in good numbers are actually Pleistocene lag deposits that overly the Pliocene or Oligocene 'bedrock'. In the vicinity of Summerville and Ladson it is typically the base of the Ten Mile Hill beds (middle Pleistocene); closer to the air base or in West Ashley, it's the base of the Wando Formation (late Pleistocene). One locality I collected at quite a bit this summer I found numerous meg teeth in the basal bonebed of the "Ten Mile Hill beds" directly overlying the Chandler Bridge Formation - which does not produce C. megalodon teeth - indicating that either A) the original Miocene or Pliocene unit had been completely eroded away and the teeth left behind, and mixed with older (Oligocene) fossils and younger (Pleistocene) fossils; B ) the meg teeth were transported horizontally for some distance from a different outcrop (now also eroded away); or C) the teeth were never buried and they were shed during a period of nondeposition, sitting on the seafloor for several million years (unlikely, owing to the lack of a phosphatic hardground).

 

4) I've never seen an exposure of the Parachucla or Edisto Fm and am skeptical about many fossils being derived from these units.

 

5) Lastly, I've attached a photo of the "best" exposure of the Oligocene units in Charleston I've ever seen (vicinity of Ladson SC). It's a construction site I prospected extensively at in the summer of 2018, now completely filled with water and built upon. The Runnymede Marl member of the Ashley (A(r)), the Givhan's Ferry Member of the Ashley (A(gf)), the typical Chandler Bridge (ChB), an unusually clean sand facies of the Chandler Bridge I've termed the "upper unit" (ChB(U)), and the base of the Ten Mile Hill beds (TM). As you can see, the units are quite difficult to tell apart from a distance, as they're all 'gooshy' and the contacts between them are only obvious if you scrape through them with a shovel (and even then, some contacts are not erosional so even that doesn't work). It took me repeated visits to this locality to figure out the stratigraphy - and I am a trained geologist/paleontologist familiar with regional stratigraphy!

 

For more reading on where your fossils are coming from, take a look at this post I wrote a while back: http://coastalpaleo.blogspot.com/2018/05/the-ashley-phosphate-beds.html

 

 

[cthamon 2]

Boesse, thank you so much! You told me everything I was curious about and more. I got worried no one would respond.

Any time and/or day people can drop by the museum and chat with you? I’d be interested in picking your brain at some point.

[Boesse 1,754]

Absolutely! I have an open door policy - though do note that my daily schedule is quite hectic and unpredictable, so don't be disappointed if you show up and I say "give me about 15 minutes to wrap this up" (and no, half the time I do not know what I'm going to be doing 24 hours in advance, so texting me ahead of time is generally not fruitful). I am generally available from 2-4 on Mondays, Tuesdays 12-5:30, Wednesdays 11-5:30, Thursdays 12-4, and Fridays 11-4.