Monday, February 6, 2012

ENE of Center: Breccia at Devils Gate

About 35 miles east-northeast of The Geographic Center of Nevada, both Highway 50 and Slough Creek go through a gap in the south end of Whistler Mountain, a gap or notch known as Devils Gate.

Note: Devils Gates are scattered all over Nevada, including a second Eureka County Devils Gate located north of Eureka, west of the paved road between Eureka and Carlin. Our Devils Gate, the one in this post, is located about ten miles east of Lone Mountain and seven miles northwest of Eureka (MSRMaps location).
Coming into Devils Gate from the west just before sundown.

A large breccia can be seen high on the carbonate cliffs north of Highway 50 when you approach the gate from the west. MOH and I first noticed this breccia several months back; we pulled over to check it out with binocs, then mosied on, filing the place away for a future hike. Finally, coming back from Serbian Christmas, the time was right.

We pulled off the highway onto a little dirt entryway that accesses the old highway south of the main road, parked the truck, and started walking. By the way, we've had the Prius on this old road, so presumably the pullout is suitable for most vehicles. After making it across the highway, we walked down a rabbit-brush infested two-track road, crossed Slough Creek, and then bypassed a fence by walking over to its abutment against the cliff face.
As we skirted around the base of the cliff toward our goal higher in the cliffs, we made a discovery: we found a lower breccia associated with small caves and solution cavities. In the photo above, the upper, larger breccia is in a smallish, bright white patch in the far, upper left part of the cliff; the lower, smaller breccia is clearly visible in the lower right of the photo, closer to Slough Creek.
This labeled Google Earth image shows the location of the upper and lower breccias and our convenient pullout. Re: pullouts, YMMV.
The lower breccia, shown above, is easier to reach than the one high on the cliff, and is recommended if you don't feel like scaling the hillside. The lower breccia contains some large, generally elongated and subrounded carbonate breccia fragments in white calcite cement of varying thickness. The thicker, upper part of the white calcite looks vein-like to almost bedded.
Our goal, the white patch with one conspicuous dark gray fragment, is now obvious in the center of the photo.
Side-hilling up this scrabbly, brushy, talus-covered slope was tricky in places, especially where talus formed only a thin veneer over the bedrock, and where limestone beds jutted out unexpectedly, sometimes forcing a downward retreat.
MOH, on a ledge to the left of the breccia, provides a bit of scale.

This is a large breccia body with large fragments, somewhat remindful of the Titus Canyon breccia in Death Valley, though not as complex. The large to giant fragments are angular to subrounded, and are set in a matrix of coarsely crystalline white calcite. The breccia body, as exposed, appears to be mostly in one or two layers or beds of the Devonian Devils Gate Limestone; its shape could be subspherical (possibly cavern-like) or cylindrical (pipe-like).
Nearly the entire cliff face in this alcove is composed of breccia. For scale, the dark olive green ephedra bush near a curved breccia fragment (right of center) is about 3 feet high. Also, see the next photo for a closeup of the curved fragment and the same bush.
The curved fragment behind the ephedra bush shows primary layering or bedding, as does a second curved fragment below it.

Vertical to overhung cliffs of Devils Gate Limestone are towering behind me in the next shot, and I'm having trouble finding solid footing in the talus-ridden alcove. Talus was almost slithering down the slope, about to cascade over cliffy beds of limestone to the tear-a-pants rocks below. No boot-skiing allowed here, unless you know how to ski jump.
The breccia is mostly exposed in this one wall. The alcove may have formed from erosion of breccia; maybe it formed from erosion of solution caverns or caves no longer apparent and partly associated with the breccia. Here, I'm just speculating, as I don't really know the genesis of the breccia or the alcove. The curved fragment from previous photos is now above the 3-foot high ephedra bush and to the right, below and to the right of the large central fragment. From this angle, the curved fragment looks oval. Numerous smaller, angular pieces of dark gray limestone float in white calcite below it.

The ephedra bush suggests an apparent height or thickness of 20+ feet or 6+ meters for our breccia, but viewing angles are deceptive in the alcove.
Fragments are clast supported to matrix supported. One example of matrix support can be seen in the upper part of the cliff exposure, where the mass of calcite appears to be greater than the mass of fragments. It's possible that solution caving caused collapse of pieces and large blocks of limestone, which then concentrated toward the bottom of the collapsed area, leaving the upper area partly open after collapse and before later calcite deposition or infill.

The white breccia matrix consists of large, intergrown calcite crystals. White calcite veinlets in some fragments appear to be mostly, but not entirely, older than than formation of the breccia, and therefore older than the white calcite infilling event. I noted only a few places where late calcite veining appeared to cut across both the breccia fragments and its coarse calcite cement.

The age of all this activity — breccia formation, fragment cementing, and early and late calcite veining — is unknown to me.
An unidentified plant grows on coarse-grained white calcite in a narrows below the alcove.
Update: Identified by Hollis in comments as Petrophytum caespitosum.
Here's some more coarse-grained white calcite with a nicely reflective calcite crystal above my boot.

NOTE: More pictures of the calcite matrix can be seen here, along with one more shot of the petrophytum.
The same unknown plant is growing on dark gray Devils Gate Limestone in the ledgy area outward from the alcove.
We take one last look at the breccia before starting downhill.
The view from the ledge, looking west: Highway 50 and the partly parallel old highway angle across in the upper left; Slough Creek, surrounded by lots of rabbit brush, meanders through the foreground; Lone Mountain stands alone in the distance.
After making our way downward on some decent, though imperfect, boot-skiing slopes, we looked back at the breccia and wondered if the way we came down would have been easier as a route up than the scrabbly sidehill across loose talus.


Hollis said...

The unidentified plant looks like Petrophytum caespitosum, in growth form and habitat. I'm assuming it's in winter garb -- brown and dry? The common name is mat rockspirea according to PLANTS, around here we just call it petrophytum (rock plant). I've always seen it on limestone.

Have a look at the P. c. PLANTS page:

especially the first photo:

Silver Fox said...

Thanks, Hollis! It does indeed look like a Petrophytum. Too bad I didn't get any real closeups. Interesting if it only grows on limetone.

Ann said...

Thanks for posting this. In 1977 I went to Ohio U field camp in the Eurkea area. We took an afternoon off to look at this Devils Gate formation. It totally fascinated me. I didn't have my camera with me at the time and always regretted it. I just remember it being an 'Ah-ha' moment seeing that brecciation. I had never seen anything like that before.
Please be careful, because when we were there we saw cougar tracks near the caves and then the animal itself. Needless to say the when we came across each other we both went in opposite directions. I was glad I was with a group of people and not there by myself.
I'm not positive about this but John Mahoney did his thesis @ Ohio U (1980) on some of the formations in the Eureka area. I remember us talking about Devils Gate and he may have studied it. Sadly to say its been a long time since I was there and don't remember much about it. He borrowed my field camp notebook, and when he returned it, I failed to put it with my other geology stuff to keep and no longer have it. So I can't help you with what I had learned.
I love reading these posts because it brings it all back to me and then some. Its nice to see that the geology is as complex as I remember it to be.

Tom Dill said...

Thanks for the great blog. I ran across this entry while searching for geology photos of Devils Gate. We mapped it for University of Nebraska field camp in 1978. I clearly remember the sills and numerous small faults that we mapped, but had forgotten the breccias until I saw your photos. Would be curious if you have been to the top of Anchor Peak, a small hill to the N of Devils Gate, and what you saw there.

Silver Fox said...

Tom, I haven't been up to Anchor Peak.

Did you by chance discuss the genesis of the breccias?