Thursday, November 20, 2008

Breccia - A Popular Rock

Well, at least it's popular with me, and has been mentioned recently by a couple other geobloggers. As Andrew describes, breccias can form in many different ways - and he shows a picture of a breccia in an unknown rock (see comments) formed by unknown causes. Callan Bentley shows a breccia in limestone (his second photo), and he wonders if it might be a collapse breccia.

The breccias I photograph are often associated with faults, like this one, but it often is hard to tell what caused a breccia to form while out in the field. The breccias shown below were all found as loose rocks on a steep slope, a slope covered with lots and lots of breccia, probably not all formed the same way.
This breccia, which is the brownish portion to the left in the above rock, is a heterolithic breccia (more than one type of rock fragment) composed of white to gray, matrix-supported (the fragments don't touch each other), subangular to angular fragments in a brown siliceous matrix. The brown coloring is from iron oxides. The white to light brown portion of the rock on the right is dolomite, and it may be a large fragment in the breccia - but that is impossible to tell from this small rock - one would have to see the outcrop. I would suspect that this is either a fault breccia or a hydrothermal breccia, but would really want to see the outcrop before saying for sure.
The breccia above is monolithic breccia (one type of rock fragment) composed of mostly matrix-supported, subrounded to (maybe) subangular fragments of light gray dolomite in what appears to be a matrix of ground-up dolomite, or rock flour, suggesting that this is a fault breccia.
The above gray breccia is heterolithic, composed of mostly subrounded fragments of white to light gray quartzite and dark gray to black dolomite and limestone. I'm not sure if the funny-looking, almost white (cream-colored) fragment in the upper right of the rock is actually part of the breccia, or was something plastered on loosely. I didn't have all my geology tools with me (like my pocket knife). The gray breccia matrix is composed of smaller breccia fragments, angular to subrounded, in the gray carbonate matrix (limesone or dolomite). It's unclear to me what kind of breccia this is. It is strongly fractured by closely spaced fracture sets that are almost turning the rock breccia into a type of breccia sometimes called a crackle or crackled breccia. These close-spaced fracture sets can be seen better in the next photo of the same rock, below:

Here, above, is another breccia, with a field boot on top of it! This rock is almost not a breccia, being mostly a fractured rock to a crackle breccia. The fragments are sometimes not separated from the adjoining rock by more than a fracture, and the fragments haven't rotated or moved much from their original position, except, perhaps, in the lower right part of the rock. The dark gray to black fragments are clast-supported (touch each other), are subangular to subrounded, and are composed of dolomite or limestone.

In the lower left of the above photo, the black, fractured to brecciated rock is resting on a rock slab or outcrop of gray breccia. (Trust me, breccia is everywhere on this particular slope!)
The above rock, and the next, speak a little to the question of what were the rock holes seen in a previous post of mine. The light brown veinlets of silica and iron oxide stand out with respect to the somewhat weathered out black marble and marble fragments. These irregularly shaped to rounded-looking fragments may or may not have been rotated as in most breccias.
The above breccia is composed of subangular to rounded gray marble fragments, partly weathered out from the brown, iron-oxide-rich silica matrix and silica veinlets. At first, I thought the light brownish gray fragments, which are particularly shiny in the sun, were composed of pyrite or marcasite, but no, they are marble. There are a few square holes completely weathered out in the silica matrix where cubic pyrite crystals used to live. This breccia could be a tectonic or hydrothermal breccia, or some combination thereof - but, again, an outcrop rather than a loose rock on a scrabbly slope would be more helpful in determining the origin of the breccia, and so would a little microscope work.

More on fault breccias here - although the link goes only to an abstract, it does have some breccia definitions.


Unknown said...

I'm drafting a map where one of our geos put 'crackled' as part of a description for one of her units. Cool!

Silver Fox said...

I told you it was popular!

GeoBeed said...

I have the coolest breccia. A brecciated shale, well-rounded cobble from Titaus Canyon in Death Valley. It's my favorite rock in my collection.

Silver Fox said...

Probably lots of cool breccias in Death Valley! And Titus Canyon is a great place.