What geological event could have created this environment ?

[ScottInTexas]

There is a stretch of super rocky embankment on our local Army Corps of Engineers built lake that has thousands of common marine fossils and slip rock fossils where these bolder configurations lay. 

 

My question is - A few of the huge boulders are blackened as if they had been exposed to fires while some that are planted nearby are white as can be so I am curious as to what could have caused this type of situation. Seems to me that a huge forest fire would have scorched the stones right next to the darker ones but it's not like that at all. 

 

 The bank is littered with these huge boulders and are mixed charred blackened and snow white stone as noted in the attached pictures. I added a picture of the bank looking toward the water for further reference. The bank stretches just over a mile. 

 

 

I made the uneducated assumption that it was somehow related to the earthquake that occurred at the spot  evidenced by the slip rock fossils that are littered all over the place that I am showing with the photo set but otherwise I have absolutely no clue. 

 

 

[Ludwigia]

Just thinking: If these are all slip rocks from an earthquake or landslide, then the dark ones could have originated from another spot than the white ones. The dark ones appear to have more clay content and the white ones seem more like pure limestone. Are we in the cretaceous here?

[Fossildude19]

What are "slip rock" fossils? 

[JohnJ]

These appear to me to be weathered differently due to prior lake levels.  Rocks usually in the water will not be exposed to the same elements.

[Ludwigia]

Another remote possibility is that the dark stones are more in the shade and prone to retaining moisture which would favor the development of lichens, while the white ones are directly exposed to the sun which bleaches them out with time.

[ScottInTexas]

4 hours ago, Fossildude19 said:

What are "slip rock" fossils? 

That would be the terminology of someone like myself who has absolutely no idea what to call rocks that emerged from a prehistoric earthquake lol.

Definitively, at least in my mind the effect on stones when they slip against each other during the quake....if that makes any sense at all. 

[ScottInTexas]

7 hours ago, Ludwigia said:

Just thinking: If these are all slip rocks from an earthquake or landslide, then the dark ones could have originated from another spot than the white ones. The dark ones appear to have more clay content and the white ones seem more like pure limestone. Are we in the cretaceous here?

Yes assumptively cretaceous I think late IDK.

 

[ScottInTexas]

4 hours ago, Ludwigia said:

Another remote possibility is that the dark stones are more in the shade and prone to retaining moisture which would favor the development of lichens, while the white ones are directly exposed to the sun which bleaches them out with time.

That could be true and as far as I know could be totally right however there are even larger boulders around there that have no shade at all. These stones are much more black than the pictures show at least others around the place are. 

[Fossildude19]

1 hour ago, ScottInTexas said:

That would be the terminology of someone like myself who has absolutely no idea what to call rocks that emerged from a prehistoric earthquake lol.

Definitively, at least in my mind the effect on stones when they slip against each other during the quake....if that makes any sense at all. 

 

They wouldn't be called fossils, ... as fossils are defined as : "the remains or impression of a prehistoric organism preserved in petrified form or as a mold or cast in rock".

 

 

 

You would be looking for a geologic term. Striations, ... scratches from glaciers or tectonic movement, ...  or slickensides.

 

That said, the items you show in your last image look similar to Rudist fragments, to me.  Or some other sort of bivalve.

[ynot]

There are two types of rock there. One is a light colored limestone and that is overlaid by a darker stone that (probably) has a high iron content that has formed a 'desert varnish" on the stone. erosion has allowed some mixing of the layers (evident in the first 2 pictures).

The marks left by rock movements during earthquakes are called "slickensides" as Tim said above.

Your pieces are not examples of slickensides..

Edited September 28, 2023 by ynot

[SPrice]

Here's a PDF  regarding Texas geologic timetable with a map image excerpt showing Austin, Texas area to be upper and lower Cretaceous layers.  Maybe reading the file will be enlightening.  I'm kinda in with the "ynot" reply.  Limestone... some lighter/drier, some darker w/different mineral content.  Look at the lake shore pic. The wet rocks at the waterline are not longer white but medium gray. Hope this helps.

 

http://ccgcd.org/Reports/NPSOTpaper-Bill Ward.pdf

 

 

[ScottInTexas]

On 9/27/2023 at 6:22 AM, Ludwigia said:

Another remote possibility is that the dark stones are more in the shade and prone to retaining moisture which would favor the development of lichens, while the white ones are directly exposed to the sun which bleaches them out with time.

Took a hike down a trail a couple miles from the spot I am curious about and now I think you might be right about it because I noticed the same darkening of stones that were deeper in the forest than those along the trail. Anywhere there was shade the stones were like that thank you for your answer. 

[ScottInTexas]

13 hours ago, ynot said:

There are two types of rock there. One is a light colored limestone and that is overlaid by a darker stone that (probably) has a high iron content that has formed a 'desert varnish" on the stone. erosion has allowed some mixing of the layers (evident in the first 2 pictures).

The marks left by rock movements during earthquakes are called "slickensides" as Tim said above.

Your pieces are not examples of slickensides..

Thank you for your reply, any idea what these stones might be then? There are hundreds of them where I got these and some are a couple feet in diameter.  When I posted them after collecting another member stated they were quake related and called them slickensides which is why I made the assumption. 

[ynot]

8 hours ago, ScottInTexas said:

any idea what these stones might be then? 

No, but they have inconsistent markings for slickensides.

[erose]

Certain limestones in the Texas Cretaceous weather almost black.