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Impact Breccia?

Posted by Jerry Montgomery  
Jerry Montgomery December 03, 2011 05:19PM
I found this rock in a field in Jasper, GA. No river in site and no smelting facilities. Shows signs of shock. Just wanting opinions on whether or not this could be impact breccia. The center appears to be a tightly packed , very hard sandstone.
Thanks for any input.
open | download - DSC_8974.JPG (866.3 KB)
open | download - Side 1.jpg (977 KB)
open | download - Closeup of edge.jpg (817.4 KB)
Don Saathoff December 03, 2011 05:57PM
The clasts are too rounded to be a breccia....looks like a conglomerate to me. Thw clasts in a breccia tend to be much more angular with sharper edges.
Jerry Montgomery December 03, 2011 08:05PM
I found a picture on this website

As an amateur, I didn't read it well and assumed the one photo was Breccia. This rock actually looks like the photos of the oncolitic limestone. I attached a picture of the clast with more detail (8993). They are of the same material that the entire rock is made of. It doesn't appear to be a conglomerate. i also attached a photo with lighting from the side to show the details in the side opposite of the clasts (8992).

When I revisited the web page for the odessa impact, I realized that I have many rocks on my property that look like the breccia that they show. I have attached one labeled 8999.
open | download - DSC_8993.JPG (964 KB)
open | download - DSC_8992.JPG (909.1 KB)
open | download - DSC_8999.JPG (771.5 KB)
Reiner Mielke December 03, 2011 09:17PM
Look likes sandstone in which concretions have run together.
Rick Dalrymple December 03, 2011 09:59PM
Oncolites are fossilized algal balls and have nothing to do with impactites or breccia or conglomerate. The link to Ohio state shows oncolites not impactite, breccia, or conglomerate.

I know I am in my own little world, but everyone knows me here.
Nathalie Brandes December 03, 2011 10:57PM
I don't think the rock is conglomerate and it does not look like any algal oncolites I have ever seen. This rock actually looks like an arkosic sandstone. There appear to be lots of small, shiny flecks of mica in the rock, which is often the case with arkose. The rounded shapes that look like clasts could very well be concretions. Concretions are formed when some parts of the rock are cemented more than others. In the photo numbered 8993, some areas appear to be preferentially cemented with haematite, which could be causing concretions. The photo numbered 8992 shows an interesting sedimentary structure known as ripple marks. As these appear to be asymmetric ripple marks, it is likely this rock was formed in an ancient river system.

Nathalie Brandes
Professor of Geoscience
Milton Dye December 03, 2011 11:02PM
If the rock you show is from Jasper GA it is metamorphic .It appears at a first and distant glance to be a metaconglomerate or a metasandstone.Do some web searching on the geology of the Jasper GA area; there is a lot of very good information out there!The Jasper area has some very interesting geology!
Norman King December 03, 2011 11:07PM
Rick is correct. The rock illustrated on the Ohio State website shows the rock that was disrupted by an impact, not an impact breccia.

Your samples do not show any features associated with impacts. Some of your samples are rippled sediments (8992) with dessication (drying) cracks (8974;8992). Others show intraformational conglomerate (Side 1; 8993). Those are features that go together (rippled sediment, dessication cracks, intraformational conglomerate), suggesting deposition, erosion, and redeposition on a tidal flat. Sample 8999 shows metamorphosed clayey/silty sediment that might be a side view of any of the others, especially as they come from near the Appalachian Mountains with its history of metamophism. I have seen exactly that assemblage of lithologies at many sites throughout the Appalachians.

To claim evidence for an impact, your samples should have impact written all over them. No equivocation--all of us raving over them as impact artifacts rather than atributing them to normal sedimentary/metamorphic processes.
Norman King December 04, 2011 02:27AM
In looking at those specimens again, I think what I called dessication cracks in 8974 and 8992 are actually trace fossils--marks made by some critter crawling on the sediment. Sample 8993 also shows some trace fossils, but of less certain geometry, both to the upper left and upper right in the photo among what Nathalie said might be concretions. I will still place my bet on intraclasts, however. These samples may be from the Chilhowee Group, the basal Cambrian transgressive interval in the southern Appalachians. The sandstones are arkosic and micaceous, and often display signs of low intensity metamorphism in this area.
Jerry Montgomery December 05, 2011 01:52PM
Thanks for all of the input! It gives me a lot to go on as an amateur. I do live in an amazing place for a rock hound. The diversity is incredible. My wife and I have an open door if anyone ever wants to visit and dig around.
Olivier L. October 22, 2013 10:37PM
Looks to me like a worn out piece of man made garbage.
Show me a thin section

Edited 1 time(s). Last edit at 10/22/2013 10:37PM by Olivier L..
Andy Lawton October 23, 2013 12:27AM
I'm just off to bed, but to my tired eyes the second & third pictures look like load casts to me.
Thaddeus Gutierrez October 23, 2013 01:21AM
Interestingly, the feldspathic autobrecciated clast (DSC_8999.JPG (771.5 KB)) seems to show that it fractured in situ, but not necessarily suddenly, during the initiation and maintenance of ion-rich hydric alteration of its cleavage structure, differentially conducting as a dielectric under the influence of a magnetic or electrical field, which is ubiquitous and intrinsic to groundwater flow. Also, don't underestimate the power of electrical discharges propagating into the water table - the very essence of the neutralization of geologic-atmospheric electrical disequilibrium . Many relict Pleistocene landforms, such as playa bluffs, positive features in badlands, and spring-bearing perched alluvial aquifers (as found on fluvioglacial moraines) only survive due to their ability to concentrate resistant precipitates as they conduct underground discharges, creating greater negative charge during frechet distribution-constrained discharge spikes ahead of powerful thunderstorms, as well as more directly, due to surface/sub-surface zone vitrification effects. What aspect is your property? what kinds of storms do you get? Can you describe the geology in your specific area (age, composition, degree of weathering)?
Doug Daniels October 23, 2013 02:30AM
uh...say what????
Olivier L. October 23, 2013 03:00AM
Doug Daniels Wrote:
> uh...say what????

I think he's saying that this is melt produced by a lightning discharge
but I really doubt it.

It really looks like old industrial or construction garbage that got dumped
there... I dug a hole in my backyard to plant a tree and I stumbled upon
a concrete mass, leftovers from houses built way before my area was

Edited 1 time(s). Last edit at 10/23/2013 03:03AM by Olivier L..
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