Saturday, March 26, 2005

Sawtooth Ridge Basalt Dated!

Over the past few years I have occasionally gathered samples of basalt, from this area, and sent them away to Professor Brian Cousens, in Ottawa, Canada. Brian has been investigating the petrology of basalts and andesites north of Lake Tahoe, and south of Truckee. He has spent quite a lot of time in Squaw Valley, which had been a major eruptive center, in the closing phases of Tertiary volcanism.

My interest goes to geomorphology: how old is the North Fork canyon, say?

If we could answer that question, we would then guess the same age for all other major canyons in this part of the Sierra.

It is clear that all these canyons--all the forks of the Yuba and the American, in particular--are relatively young, and have been incised into a generalized volcanic mudflow plateau. The flat-topped ridges seen everywhere in middle elevations are relicts of this surface. And beneath these "young volcanics" are buried remnants of the "ancient bedrock" land surface.

In more detail, there was an "ancestral Sierra," in which a system of broad valleys had evolved over millions of years, a somewhat over-mature landscape in which streams aggraded (built up thicknesses of sediments in flood plains) rather than eroded.

We have a fine fossil flora from remnants of these old (55 m.y.) river channels, preserved on the divides between the modern canyons, as for instance at Gold Run.

The ancestral Sierran landscape was developed upon a complex of different bedrock types, from granites to serpentines to slates, with chert and limestone and more. A system of ridges ran north and south, along strike of the more resistant rocks, which were often metavolcanics called "greenstone."

About 30 m.y. ago, rhyolitic eruptions blanketed the region in volcanic ash, and some of the shallower of the existing valleys were filled outright. New patterns of drainage would then form. More volcanic ash would fall, filling these brand-new valleys.

Then, say, 15 m.y. ago, the eruptions became andesitic, and andesitic mudflows or lahars swept over the area, often filling the valleys, whereupon new drainage patterns would arise, but then new lahars would fill those new valleys.

Over millions of years, a generalized plateau of andesitic mudflow developed. Only a few parts of the Ancestral Sierra stood above the volcanic sea. Banner Mountain, beside Nevada City: and Dutch Flat's Lovers Leap, and it only by the skin of its teeth.

And then, finally, at long last, volcanic activity began to wind down. Eruptions of basalt sent streams of lava down the valleys draining high and mighty places like the Squaw Valley Eruptive Center.

Summarizing, the "standard sequence" of the Tertiary volcanics, from old to young, is

1. Valley Springs fm. (rhyolite ash, some welded tuffs).
2. Mehrten fm. (andesitic lahars etc.).
3. Basalt.

Now, most of these basalts have been eroded away entirely. In the high country they are fairly common, most notably of all at Devils Peak, made of columnar basalt.

The thing is, since they are youngest, and since basaltic lava is famously liquid and runny, these basalts ran down and partially filled valleys which almost must have preceded the incision of our modern canyons. I don't know if this is at all clear, but my thinking is, the modern canyons developed *after* the basalts. So if a patch of basalt on Sawtooth Ridge, for instance, is 5 million years old, then that too is the age of the adjacent North Fork canyon. And then we could say, "Broadly, the canyon is five million years old and two thousand five hundred feet deep, hence the average rate of incision is six inches per thousand years."

The nascent North Fork likely made quick work of the young volcanics and began incision into the ancient bedrock quite early on.

But how early? Oh, if only we could date the basalts!

And just where are these supposed basalts?

Well, it turns out that on a ca. 1900 geological map of this area, by Waldemar Lindgren of the USGS, quite a few small patches of "Pliocene basalt" are identified on ridgecrests between the modern canyons.

And among the most southerly of these little basalt patches, is one on Sawtooth Ridge, and another on Lowell Hill Ridge. I sent samples of these to Brian a couple years ago or so. He did a detailed chemcial/petrological analysis of the basalts, which varied significantly, one from the other; so it began to seem we had two very distinct and disparate lava flows on our hands, although topographically and stratigraphically, the two sites were equal.

Both sites are on ridges where no continuous volcanic plateau remnant remains, but smaller remnants persists as knolls, while the passes are commonly at the level of the ancient bedrock, on both ridges, the Shoo Fly Complex metasedimentary rocks.

Both sites show the basalt in direct contact with andesitic mudflow, but both flows sit directly on Shoo Fly basement.

It should be noted that the topgraphy has completely reversed: both flows were in the bottoms of valleys, now, they are on the crests of ridges.

OK. To make a long story short, dates have finally come in on the two samples. They are indeed disparate, as the Lowell Hill basalt came in at 16.3 m.y., while the Sawtooth basalt came in at 3.82 m.y.

Hence *iff* the assumption is correct, that incision of the North Fork canyon began after the Sawtooth basalt, we have a nominally 2500-feet-deep canyon which is only 3.82 million years old, and now we have .654 feet/thousand years, or roundly, eight inches of incision per thousand years.

A few miles upstream, the North Fork canyon is over 3000 feet deep.

So what are we to make of the Lowell Hill basalt, at fully 16 million years? It is coeval with the andesitic mudflows, so far as I know, so it is not much of a stretch to imagine some basaltic eruptions from time to time. The age of 16.3 m.y. is indistinguishable from the age of the Lovejoy Basalt, so widely exposed farther north in the Sierra and nearby areas. It would really be a hoot if it turned out that the Lowell Hill flow is in fact the Lovejoy; but Brian seems to think that's impossible, for its chemical signature is much different than the Lovejoy's.

Clearly more samples should be gathered, from the Lowell Hill and Sawtooth sites, as well as from other potentially related flows, farther north, near Bowman Lake, etc. I sent Brian samples from Lyon Peak, Needle Peak, and another nearby flow, and also from Devils Peak, last summer. It will be exciting to hear what dates come in for these flows.

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