Monday, June 6, 2011

Megabreccia III, the Continuing Saga

You'll have to see the second post in this mini-series on the Titus Canyon megabreccia to get a sense of scale (or the first, or see the next to last photo of this post). Very roughly, the horizontal fracture or shadow above the wash (most of the center of the photo, a quarter of the way up) is 3 to 4 feet high (that's at about the same level as the top of the lower right rectangle). First we'll look at the area shown by the large rectangle in the upper left, then we'll look at the area shown approximately by the smaller rectangle in the upper left, an area which goes farther left than the rectangle indicates. After that, we'll zoom to the lower left rectangle, then to the lower right.

Location Info: Titus Canyon general location; Titus Canyon narrows; megabreccia location. The megabreccia is in the first major bend of the narrows (first if traveling from the east), the first of two bends that define the southernmost latitude that Titus Canyon reaches. Also see Google Maps.
This is the one area where I could see the somewhat diffuse contact of the breccia with the carbonate wall rock. Contacts elsewhere were obscurred or I missed them (very possible — there is a lot to see!). I have no idea whether the contact is always gradational like this, or whether it is in some places sharp or some places irregular. Fracturing and incipient brecciation — sometimes in the form of crackle breccia, sometimes in the form of calcite veins or small masses — extends upward into the wall rock about 4 or 6 feet past the contact. The calcite veins might tell quite a story about any stresses affecting the rock during the fracturing and brecciation episode. (I'm taking the view that a tectonic origin is likely for this breccia, although one non-geological website was postulating a cave collapse origin.)
Moving into the breccia from the gradational contact (the smaller of the upper left rectangles), we see that breccia fragments decrease in size toward the more solid rock in the upper right, and it looks like it would be fairly easy to put the fractured rock back together in the zone right near that boundary.

Speaking of putting the pieces back together, I'm not sure of the exact definition of jigsaw breccia or jigsaw-fit breccia: some pictures I've seen seem to indicate a breccia that has gone just a wee bit beyond a crackle breccia in that the fragments are mostly matrix supported but the pieces would still be easy to fit back together; other pictures or descriptions seem to indicate *any* angular, matrix-supported breccia. Because of the confusion (which may entirely be mine), I'm not using jigsaw breccia as a descriptive term here (or jigsaw-fit or mosaic), although I suspect this breccia qualifies as a jigsaw breccia, at least in places.
Now to the lower left rectangle. The large white calcite mass in the left center is shaped roughly like a large breccia fragment and appears to show some sub-horizontal grooves. I think this calcite mass is a thin matrix zone that once cemented a fragment, now eroded, to the rest of the breccia mass. The upper edge of the large calcite mass bends outward, towards us, as if it were about to wrap around a large rock fragment; the lower contact with dark rock is quite sharp, possibly the edge of the now eroded fragment. The grooves may be a tectonic feature or may be something else, for example, grooving related to large boulders going downstream during huge flash floods. I favor the tectonic idea, but haven't examined the area enough to back up this hypothesis.

A second calcite mass to the right of the first one can be seen to be wrapping around the dark fragment below it — the large one a third of the way up the photo, a little right of center, which is casting an angular shadow downward — and you can see that the calcite seam looks relatively thin, although an unknown percentage of it has been eroded during formation of the Titus Canyon narrows.
Now on to the rectangle in the lower right. This part of the breccia is composed of subangular to rounded fragments of the white calcite that forms the matrix of the bulk of the breccia. Note that the white to cream-colored calcite is fractured and cut by hematitic veinlets; also note that many of these fractures formed prior to calcite brecciation but that some didn't. These calcite fragments are floating in a dark mass that is composed, at least in part, of sub-rounded to rounded fragments of carbonate wall rock and other carbonate material, including white unfractured calcite fragments, and possibly including carbonate rock flour. This is a dark breccia matrix for the brecciated calcite that is usually the light-colored matrix of the darker breccia fragments.

So, we've seen that the megabreccia of Titus Canyon is a multi-stage breccia. The most often reported causes of brecciation (tectonic of unknown or vague origin combined with phreatic or hydrothermal action and related brecciation) don't surprise me in the least after looking carefully at these many types of textures. Breccias can fool a person, however, and I haven't seen any detailed reports about the breccia or maps of the area showing the breccia. I spent a very small portion of a day snapping photos; would love to spend more time examining the breccia, even mapping it, though much of the terrain is precipitous.
If you want to see this breccia, besides driving one way into Titus Canyon on a long, narrow, one-way (open only to east-to-west traffic), sometimes rocky dirt road that may require 4WD or a high-clearance vehicle (I'd want both to dodge the rocks sticking out of the road uphill from Leadfield), you can drive to the Titus Canyon alluvial fan from the Death Valley side and walk in from there. It looks like it's about 1.5 to 2 miles in, and probably a magnificent hike. (Google Maps Location and additional photos).

A Few Links:
Introduction to Faults, p. 54 - the Titus Canyon megabreccia is shown as an example of a cataclastic rock, from the ES406 Structural Geology Lab of Dr. Stephen Taylor at WOU. (That is to say, the breccia is a product of brittle tectonic deformation.)

More photos of Titus Canyon and the breccia.

Virtual field trip guide to the geology of Death Valley - including the Titus Canyon megabreccia - Lots of information about Titus Canyon and Death Valley.

Titus Canyon Mini-Series:
Carnival of the Arid #4 is Up! - and Titus Canyon
Things You Find in the Field: Leadfield
Some Thoughts on Weirdness, and A Picture (or Two) (or Three)
Megabreccia II: More Photos
Megabreccia III, the Continuing Saga (this post)


David said...

You do not need 4WD or high clearance to make the Titus Canyon Drive, though high clearance makes it a lot easier. If you have good rough road driving skills, you will do fine. It is such a spectacular drive that you should not miss it even though your vehicle is not ideal. If you are too nervous to try, you can still easily reach the narrows and these limestone formations from the parking area as Titus Canyon Road emerges from the canyon onto the fan, and is 2-way the last two miles down to Death Valley Road. The upper portions of the canyon are different but equally spectacular.

Death Valley Natl. Park is an amazing destination. I first visited it and drove Titus Canyon 40 years ago, and have been back 20+ times (and down Titus Canyon 10 times or so - have walked in from the west when road was closed due to wash outs). There are many other great back roads in the now expanded park, as well as many great hikes to see amazing geology.

Silver Fox said...

We considered taking the Prius, but decided the truck wouldn't rattle to pieces quite so much, and if it did, it is, after all, only a truck. So that's what we took. So far, I've been down the road at least three times, always in a truck. Would like to hike up from the west side some time.