Wednesday, March 8, 2006

Paleovalleys and Ignimbrites

English is an Indo-European language, as are two "dead" languages, Latin, and the still older Sanskrit. From the Sanskrit "Agni," (god of the hearth, and mediator between gods and men) we move to the Latin "ignis," fire, from which we get our English "ignite," and also that vastly rarer word, "ignimbrite," meaning "fire rock" or welded tuff; and "tuff" (not tufa) is volcanic ash.

Rhyolitic volcanoes tend to have explosive eruptions in which a "glowing avalanche" of incandescently hot lava particles will spread out, perhaps flowing down a valley, and just so soon as that glowing avalanche stops, it freezes into solid rock. It is a cataclysmic event and will kill everything in its path.

There are basaltic tuffs, andesitic tuffs, and rhyolitic tuffs. In this part of the Sierra we have an abundance of rhyolite tuffs. Some are welded tuffs, some were air-borne ashfalls which did not weld, and were either immediately or subsequently remobilized by water; for a long time we have called all these rhyolite tuffs, the Valley Springs Formation, yet they are clearly composed of several to many different tuffs, spanning millions of years of time, beginning, say, in the Oligocene of ~30 m.y.b.p. (million years before the present) to the Miocene of ~20 m.y.b.p.

Hence we ought to split the Valley Springs into smaller, individual formations.

Strangely, although so close to major population centers and hence, to universities with geology departments, the Valley Springs Formation has not been exhaustively studied. Only now are some of its secrets being unlocked; similarly, only now is the much more recent (<3 m.y.b.p.) glacial era coming into focus; and there is so much remaining to discover here in the Sierra, on so many fronts.

It is a good time, a great time, to be a geologist in the Sierra Nevada. When one visits the web pages of these various professors and graduate students, and, sometimes, undergraduates, one sees they must live Indiana-Jones-like lives of high adventure, in lands of danger and mystery; for, hark, there they go, to the wilds of the Tien Shan mountains in northern Tibet, and they study some violent fault zone of critical import, in forming a model of Himalayan geology.

And then, in the summer, say, they come back to the Sierra and examine ignimbrites, in a certain paleovalley which seems to have run from near today's Honey Lake in the north, down past Soda Springs at I-80, and then crossing the upper North Fork to French Meadows reservoir on the Middle North American; thence joining the main Tertiary South Yuba near Michigan Bluff, and on past Yankee Jims and Iowa Hill to Gold Run and points north.

Since it is a paleovalley, predating our modern canyons, its originally continuous course has been broken not only by erosion (i.e., cut by our modern canyons), but by block-faulting: this paleovalley crosses the main Sierra Nevada fault, at a very shallow angle, just north of Castle Peak. To the west of this fault is the upthrown block: the Sierra. To the east is the downthrown block: Lake Tahoe, the Martis Valley, Highway 89 north of Truckee.

The main Sierra Nevada fault is really a stepped series of roughly parallel faults, all along the east side of the Sierra Crest, from south of Mount Whitney, north to about Yuba Pass. A little farther north, and we leave the Sierra for the southernmost of the Cascades volcanoes, Mt. Lassen. The thickness and extent of the "young volcanics" (of which these rhyolite tuffs form a part) is much greater here, in the northernmost part of the Sierra, and the volcanics often obscure bedrock and faults alike.

Back to the paleovalley. Around 1900, the USGS's Waldemar Lindgren mapped it as originating near Castle Peak; the idea that it may originate near Honey Lake (at Diamond Peak) results from field work by accomplished geologists and petrologists including student Dylan Rood, professor Cathy Busby of UC Santa Barbara, and David Wagner, a State geologist.

I should say that the idea of our Sierran paleovalleys having their headwaters east of the Sierra crest, in modern Nevada, is not new at all, but has yet to be widely demonstrated. The ~16 m.y.b.p. Lovejoy Basalt of the Northern Sierra, also presents the case of a paleovalley extending east into Nevada.

But in this particular "Diamond Peak" paleovalley, a little west of Honey Lake, they find five distinct ignimbrites, composed of nine cooling units. They correlate units 1, 4, and 7 of the Soda Springs paleovalley, with units 3, 4, and 5 of the Diamond Peak paleovalley.

Around ten years ago, I accompanied paleontologist Howard Schorn and geologist Dave Lawler to a site in the Soda Springs paleovalley, where we recovered some finely-detailed plant fossils from the river gravels beneath the ignimbrites. But the stratigraphic relations were not exposed there, in fact, we were finding the fossils in an anonymous creeklet threading through glacial till.

These ignimbrites are broadly similar to the famous Bishop Tuff of the Owens Valley, 35 cubic miles of welded rhyolite ash which erupted in an instant from the Long Valley caldera a mere 750,000 years ago; so our local tuffs are much older.

Rood et. al. remark that the lower three ignimbrites were confined to the paleovalley, while the fourth overflowed here and there, and the fifth and uppermost ignimbrite, the youngest, spread more widely still, scarcely at all confined by the paleovalley.

The sources of these tuff-beds seem all to be to the east, in Nevada, some rhyolitic calderas having been found in the central part of the state, of the same ages. At that time the Sierra had not been uplifted, and the source region of the calderas was higher in elevation than here, so the glowing-hot fiery avalanches of rhyolite ash traveled long distances, down the paleovalleys.

One of these five ignimbrites of the nine discrete cooling units is David S. Harwood's "pink welded tuff," dated to 22 m.y.b.p., as discussed in his USGS geological map of the Duncan Peak and Cisco Grove quadrangles. Generally speaking, rhyolite tuffs are light in color, white, cream, buff, tan, grey. This "pink welded tuff" is well-exposed in the vicinity of Palisade Lake, just west of Soda Springs. The westward continuation of Pahatsi Road, in the Serene Lakes subdivision, to Cascade Lakes, crosses the glaciated surface of this ignimbrite for about a half-mile, atop the South Yuba-North Fork American divide. Huge granite erratics are scattered everywhere, and one can drive right by thinking that the flat glaciated "bedrock" is granite, too; but it is a welded tuff, and it is Superjacent Series all the way, not Subjacent Series, like our granites.

At Palisade Lake this ignimbrite is seen, in cross-section, to have a massive vertical columnar structure; hence, I think, the name of the lake and of Palisade Creek, a south-flowing tributary of the North Fork. Which of Rood et. al.'s nine cooling units Harwood's "pink welded tuff" is, I cannot say.

These ignimbrites, these welded tuffs, are commonest near the Sierra crest, and are not found much if at all as far west as Dutch Flat. Here, the equivalent tuff-beds are sometimes well-consolidated and seem rather welded, but more typically, they are clearly water-deposited, and have sometimes weathered into a grey clay. The Dutch Flat store is built from the possibly-welded rhyolite tuff of this area. No, here in the lower elevations, so far away from the source calderas, our tuffs tend to be weak, so weak as to almost never be exposed at the surface, as outcrops; near the head of the Green Valley Trail is one small exposure, and there are others around Lake Alta, and west, at Chalk Bluffs.

Well, at any rate, I am delighted that new work is being done on our "young volcanics." This same team, Rood et. al., look to be involved in studying the next-younger members of the Superjacent Series, the andesitic lahars and all their variants, down in the Carson Pass area. With modern radiometric dating tools, we should gain a much more sophisticated portrait of our late-Tertiary volcanism.

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