Help. Disc fossil or sedimentary deposit?

[Desertdaze]

Hi,

I'm seeking info. I found this about 15 kilometers inland from the Sea of Cortez and 15 kilometers south of Los Barriles, Baja Sur, Mexico. It was on a steep hillside above a small sandy wash or arroyo which will later flow into another and then the Sea, maybe 75 meters elevation. The person who took me to this gully said he has found Megalodon teeth here. 

It literally looks like a one kilo stack of tortillas, same size, dimension and layering. It's about 20 centimeters in diameter and 5 centimeters tall. I have found whale discs before but only on fresh skeletons and this does look similar but perhaps it's just a sedimentary deposit. I also found what appeared to be a rib about one meter in length but broken into about 8 pieces along with a few other pieces that didn't look like they belonged. I'm not sure about the geology. Appeared be banded horizontal layers with a surprising amount of some type of crystal that looked like selenite or calcite. 

[ynot]

It is either an odd geologic structure or an algae.

My money would be on odd structure.

[Rockwood]

2 hours ago, Desertdaze said:

sandy wash or arroyo which will later flow into another and then the Sea, maybe 75 meters elevation.

Sounds like a good spot for a mineral rich spring fed by sporadic rains.  

[Plantguy]

Very neat find, especially the round shape/layering. Looks sedimentary/geologic as Tony said--an example of differential weathering..harder layers being more resistant/softer less cemented layers eroding faster..Whether something organic also helped out in their formation I dont know.  

 

I also cant decide from the photos if the layers are clastic (of different grain sizes) or are they alterating limestone/dolomite layers? looks kind of grainey....Have you looked at this up close with a magnifying glass? Does it fizz with vinegar? Could the crystals you are seeing be individual sand grains in sandstone/siltstone? that make up the individual layers and not calcite? 

 

Neat specimen...reminds me of some dolomites from Death Valley, CA a bit further north and inland. Thanks for showing us. Good luck with the fossil hunting in the area!

 

Regards, Chris 

[Desertdaze]

Thanks for all your help. I probably shouldn't have mentioned the crystals. They were just in the area and seemed odd. There are none visible in the tortillas. They were scattered about on the hillside, sheeted, banded or small 1 centimeter crystals in pieces up to 10 centimeters. Softer than my fingernail.

Under magnifying glass I would say I didn't see any discernible crystal grains, only 4 different shades of the colors you see. All the layers are the same and 1/4 of the stack separated when I was handling it. Some dust came out. I could take another photo if interested. The left edge in the side view photo above, looks like erosion intruded there. The other side is flatter and more like a tortilla stack.

I googled hundreds of images of vertebrae and discs and the dot in the center and concentric ring seem consistent with the ridges and grooves on a variety of species, in some cases. I also could not find an image on google that looked similar. It is so round. The only other thing I have seen in my life that is that round geologically would be a pothole that is swirled out of bedrock by flowing water and gravel. I googled concretions, dolomite formations, circular rock formations and  a bunch of other stuff. I'd really like to see a photo of something similar if anyone comes across one.

I went back to the same area today, though, and found what I was looking for, my first megalodon tooth, or well, shark tooth! It's broken and only about 5 centimeters long but it's priceless to me. It took about 20 days of looking over the last 5 or so years. Thanks again folks.

 

Edited March 18, 2019 by Desertdaze
I made a mistake.

[Plantguy]

Glad you were able to find a meg tooth! Sounds like you might be on to something. Continued success there! 

 

Here's a document that has some good stuff about the geology in that part of the peninsula. Hoping you can use it to figure out what formation your in and perhaps narrow what rock types originate there and maybe narrow down what your unknown is...It has a geologic map in Figure 2

and shows some stratigraphic sections of the formations in there along with the various rock types making them up.  

Miocene Pleistocene sediments within the San Jose Cabo basin, Baja California Sur, Mexico. GSA Special Paper 318, 1997. Martinez-Gutierrez and Sethi. 

http://activetectonics.asu.edu/BAJA/Martinez-Gutierrez_and Sethi_1997.pdf

 

I wish I still had some weathered layered dolomite and sandstone samples to show you from the desert from eons past geology trips but they are long gone somewhere!! Sorry you werent able to find any images online--there are some pretty wild things that mother nature does with sphere/round and flattened shapes.....from oolites, to concretions, to spheroidal weathering to name several. So, here's a photo of  some of the circular examples I do have....starting top left going right to left...a calcareous concretion, limestone matrix adhering to a fossil echinoid, and on the bottom row--a phosphate nodule and a worn fossil whale vertebra. 

The only really close example to yours with that fine layering I found on the web  (and wont share the url here...not sure what kind of site it is--looks a little iffy), I cant seem to find credit attached to give the photographer his/her due....It says sandstone concretion...probably buried in some pub/document somewhere. I'm not saying that's what you have but it does have some similarity but it seems finer grained. 

Rockwood's suggestion made above also seems plausible...

Might be interesting to know if yours has any calcareous component/cement. 

 

Regards, Chris 

 

[Mark Kmiecik]

It looks like it would fit almost perfectly in that blue bowl behind it.

[Mark Kmiecik]

On 3/20/2019 at 9:40 PM, Plantguy said:

There are structures similar to this in the Francis Creek Shale of Mazon Creek in Illinois. Some are light colored like this, while others are various darker shades of reddish brown tending towards the color of an iron siderite concretion, as if they were concretions in the making and for some reason the process had been interrupted.

[Plantguy]

3 hours ago, Mark Kmiecik said:

There are structures similar to this in the Francis Creek Shale of Mazon Creek in Illinois. Some are light colored like this, while others are various darker shades of reddish brown tending towards the color of an iron siderite concretion, as if they were concretions in the making and for some reason the process had been interrupted.

Interesting. I have a bunch of Mazon creek material but my experience is admittedly very limited and the examples have no grain to them at all...nothing with any silt/sand sized grains or larger. If you run across any pics I'd love to see. thanks. 

Regards, Chris 

[Mark Kmiecik]

Exactly like the photo in my post with the color as described in sizes ranging from 1" in diameter to 18" or more, flattened to the same degree as the one in the photo and ranging from nearly perfectly round to elongated oval, basically the same shape as any Mazon Creek concretion could take. Like the concretion in the photo they quite fissile and delicate in that you can crumble them by hand when a light color and become more coherent with added color.

[Plantguy]

On 4/9/2019 at 10:34 PM, Mark Kmiecik said:

Exactly like the photo in my post with the color as described in sizes ranging from 1" in diameter to 18" or more, flattened to the same degree as the one in the photo and ranging from nearly perfectly round to elongated oval, basically the same shape as any Mazon Creek concretion could take. Like the concretion in the photo they quite fissile and delicate in that you can crumble them by hand when a light color and become more coherent with added color.

Hey Mark, if you ever get the chance and you are out agaom I'd love to see those structures...snag a closeup and the outcrop. 

Regards, Chris 

[Mark Kmiecik]

2 minutes ago, Plantguy said:

Hey Mark, if you ever get the chance and you are out agaom I'd love to see those structures...snag a closeup and the outcrop. 

Regards, Chris 

I doubt that I'll ever get out there again because of restricted mobility and other health issues, but I'll scrounge around for photos of what I'm talking about. That one photo that I've included with my replies here is so near to spot-on I can almost swear I'm looking at a Mazon Creek specimen.

[Plantguy]

23 minutes ago, Mark Kmiecik said:

I doubt that I'll ever get out there again because of restricted mobility and other health issues, but I'll scrounge around for photos of what I'm talking about. That one photo that I've included with my replies here is so near to spot-on I can almost swear I'm looking at a Mazon Creek specimen.

Sorry to hear that. Yeah, I'm racking my brain and I seem to recall some in Northern California that I wish I had brought home or taken a photo of. What I would have given to have the convenience and features/functions of having a cell phone/camera 30 years ago...

Regards, Chris