Thursday, April 19, 2012

SW to SWxS of Center: West Northumberland Canyon

In our backwards trek away from The Geographic Center of Nevada, we've reached one of the last stretches of dirt road prior to the start of our now long ago January day in the center and south-center of the Northumberland caldera. And what stretch of road would that be? That would be the Northumberland Mine Road, the section running from the mouth of West Northumberland Canyon on the west side of the Toquima Range, east and up West Northumberland Canyon past the springs, the old barite mine, and the Northumberland gold mine, to Northumberland Pass. Beyond the pass, the road continues eastward, down into East Northumberland Canyon, past another old barite mine (the larger of the two, both of which operated in the 1970s and 1980s), finally ending at the Monitor Valley Road about 7 miles southwest of our last stop.

Compass Card, on Wikimedia Commons

The mouth of West Northumberland Canyon, up on the west pediment of the Toquima Range, is about 27 miles southwest of center; the springs are about 26 miles southwest by south (SWxS or SWbS) of center; Northumberland Pass is about 27 miles south-southwest of center (see Boxing the Compass).

I had planned to post this section of our trip — that is, the entire Northumberland Canyon section, in one post — but due to limited editing & writing time (I did my initial editing of this on a funky phone while riding one of the better (read: smoother) buses back from a day's work at the mine) — I'm now posting just the first part: West Northumberland Canyon from it's western mouth to just below the springs. The second part will post in a few days.

After returning from our 5 mile excursion up some canyon near Mt. Ziggurat, which we'll hopefully see later, we hightailed it north on the range-front road, turned right on the Northumberland Mine Road, and entered West Northumberland Canyon. (MSRMaps location of the area.)

Northumberland Canyon cuts southeast through the northern part of the Northumberland caldera, a large caldera about 21 miles in diameter that erupted a moderately large volume (15 cubic miles) of rhyolitic quartz-sanidine ash-flow tuff about 32.3 million years ago (McKee, 1974a). The canyon wall is cut into typical exposures of the typically goethitic golden brown, strongly welded Northumberland Tuff, which usually shows intense vertical fracturing and imperfect columnar jointing.

Driving up the canyon, one can see a good deal of geology related to the formation of the caldera, including large, often irregular, and at least sometimes internally brecciated landslide blocks of often dark gray to black Ordovician Vinini Formation (Ov).

A couple of the smaller slide blocks or masses are shown in the photo above; these particular masses are apparently at least partly contained within welded tuff.

The reddish hill in the distance to the north is capped by the tuff of Hoodoo Canyon, K-Ar dated at 31.4 Ma (McKee, 1974b). The reddish portion of the hill is underlain by moderately to strongly welded tuff; a black vitrophyre can usually be found near the base of the upper reddish zone, above a white, relatively thin, poorly welded zone. The tuff of Hoodoo Canyon (Hoodoo Canyon is the canyon north of West Northumberland Canyon), overlaps and rests on intracaldera sediments (what we called Ts2 through Ts4; what was sometimes called Ts5 turned out to be the poorly welded portion of the tuff of Hoodoo Canyon). These intracaldera sedimentary units were deposited on the Northumberland Tuff and on the Ov landslide blocks in an irregular intracaldera moat lake that may have been deeper on the north side of the caldera, resulting in a sedimentary package that was thicker to the north (it was thinner to the south, or maybe partially to entirely eroded in places).

The fractured dark brown to black mass capping this hill of golden brown Northumberland Tuff on the north canyon wall, is one of the larger Ov landslide masses or blocks.

Several of the largest of the landslide blocks and masses occur close to the mouth of the canyon. At least one near the canyon mouth can be seen from the road, others can be reached by hiking north of the road. Several, if not all, of the dark patches and outcrops in this MSRMaps airphoto are landslide blocks or masses composed of black chert of the Ordovician Vinini Formation (Ov). I keep saying landslide "masses" and not just landslide "blocks" because we found from mapping and drilling that many or most of the masses join in what is probably a basal layer to the intracaldera sedimentary section, a unit we called Ts1. A couple of these larger masses can be seen in the first photo here and the second photo here.

Here's a somewhat closer view of the irregular contact between the same Ov landslide mass sitting on top of the Northumberland Tuff. The internally brecciated fragments of the black chert in Ts1 are often glued or welded together, presumably from the heat of the tuff beneath it. Where drilled at depth, the angular to subangular breccia fragments are sometimes rimmed or cemented by pyrite. In places, the brecciated Ov landslide masses or layers look more like cemented talus deposits than landslide blocks.

In the central part of the caldera, these intracaldera landslide or talus deposits are thinner and are overlain by a later pulse (or more than one pulse) of Northumberland Tuff, which in the central part of the caldera show strong welding and compaction foliation like the tuff in West Northumberland Canyon. This upper part of the Northumberland Tuff correlates with what we called Ts3 in the northern intracaldera moat sediments: an ash flow or series of ash flows that flowed into the intracaldera lake, retaining some semblance of ash-flow tuff in places, and looking more like disrupted ash-flow tuff or even waterlain air-fall tuff in other places.

And now we've arrived at the springs in West Northumberland Canyon.

References:
McKee, E.H., 1974a, Northumberland caldera and Northumberland Tuff, in Guidebook to the geology of four Tertiary volcanic ceters in central Nevada: NBMG Report 19, p. 35-41.

McKee, E.H., 1974b, Road log to Austin-Northumberland caldera-Carver Station, in Guidebook to the geology of four Tertiary volcanic ceters in central Nevada: NBMG Report 19, p. 3-5.

Related Posts (in order of posting):
The Geographic Center
South of Center: Potts Ranch Hot Springs
South by West of Center: Monitor Valley Salt Flat or Dry Lake
Backroads: Too Cold to Change a Flat, and other Considerations
SW to SWxS of Center: West Northumberland Canyon - this post

SWxS to SSW of Center: the West NU Canyon Springs and Northumberland Pass - a coming post

2 comments:

Hollis said...

I want to go there! but then I want to go everywhere I read about, especially with such great photos. Winter light is so wonderful.

Silver Fox said...

I guess you'll have to make a (long) list, and organize it by state or highway or region, or something! :)