Silicified Microbreccia
Marietta-Tryon Graben
North & South Carolina
By Mike Streeter
February 2011

According to Furman University geologists William A. Ranson and John M. Garihan and others, tabular zones of silicified microbreccia are present in the crystalline rocks of the Piedmont of North and South Carolina. Ranson and Garihan have documented an east-northeast-trending set of brittle faults, some as much as 70 km long, that defines a complex graben-within-a-graben structure named the Marietta-Tryon graben. The faults extend from just northeast of Tryon, North Carolina to just southwest of Marietta, South Carolina. This graben apparently formed during early Mesozoic rifting. Movement along the series of faults and associated fractures caused pre-existing country rock to be crushed and shattered into breccia. Repeated phases of fault activation caused the breccia to be re-brecciated forming what may be referred to as microbreccia. Sometime during and probably after the faulting took place, quartz-rich groundwater solutions infiltrated the highly fractured rocks as evidenced by banded chalcedony fragments and quartz crystallization. The resultant rock, as it is found today, has been referred to by Ranson and Garihan as silicified quartz-feldspar microbreccia. The microbreccia differs in composition and appearance across the region, mostly due to differing compositions of the pre-brecciated rock, groundwater solutions and degrees of brecciation and silicification.

For over six years, I have been using geologic maps and other resources to track down the locations of the aforementioned silicified microbreccia throughout the Marietta-Tryon graben. My original intent was to find mineral specimens, but since I started cabbing a couple years ago, my interest has widened to include hunting down lapidary materials.

As you can see by the above cabochons I made prior to my latest silicified microbreccia hunt, I have enjoyed some success tracking down some nice lapidary material. But, several weeks ago, while tracing on foot yet another Marietta-Tryon Graben fault, I found a bunch more excellent rocks that make beautiful and unique cabochons.

Since faults can cause lines of weakness on the Earth's surface, creeks will often follow their traces. Such was the case in my latest hunt.



Any keepable rocks found along such a beautiful stretch of creek on a sunny day could only be a bonus. And, and as it would turn out, there was a BIG bonus as I found a bunch of silicified microbreccia with some small banded chalcedony clasts widely scattered in the mix. The following three pictures demonstrate some of what I was looking for and found.

Click on each rock picture above to enlarge

The following pictures show some of the many cabochons I've made from my latest batch of rocks (cab pictures do not enlarge).

While I was hacking up the rocks, one slab screamed out at me to be made into a pretty framed picture - so I did what I could to accommodate! And, another slab with quartz druse on one side also looked like a beautiful scene that needed to be saved, so I cut it down just enough to be able to polish it on my 8" flat lap.

You can bet I'll continue to hike area streams and beat the bushes for more silicified microbreccia and especially banded chalcedony - I guess you could say this is a rock type with a lot of faults, but I like it!