The significance of erosion
Still pondering some Sierran things that Gabet brought up, and one is the significance of the post 3-5 Ma erosion in the Sierra, and that is how significant 1000m of erosion through Mio-Pliocene volcanic rocks really is. Basically, Gabet takes John Wakabayashi to task for assuming that rivers were graded to the top of the volcanic pile on the canyon rim:
The assumption that volcanic rocks on canyon rims define the channel bed elevation at the time they were deposited is therefore falsified by the presence of volcanic rocks in the S. Fork of the American River canyon (p. 1245)
The main observation is of basalt flows found down to 5440′ in elevation, about 720′ above the modern South Fork of the American River just below. And the basal contact of these volcanics varies within a mile from that 5440′ up to nearly 7000′, revealing that the volcanics buried at least 1500′ of topography. Does this mean there was no uplift?
Now Gabet’s point is fair: just because you have volcanic rocks on the rim doesn’t tell you anything about the river’s position, unless of course you have river gravels present [in that same place-2/3/17]. (Wakabayashi makes no such claim). But there is a bit of apples and oranges comparison here: Wakabayashi was describing the total incision since the burial of the western slope of the Sierra under Neogene volcanics and volcaniclastics, while Gabet is pointing out that much of that subsequent exhumation simply removed that Neogene cover. His statement “… all the canyons were cut before the period of volcanism and then blanketed by volcanic rocks that, over time, were mostly eroded away” is one most Sierran geologists would accept with the provisio that “all the canyons” be modified somewhat. What is actually in dispute is the significance of that erosion.
Except that Gabet claims that the mapped basalt flows are “Pliocene-age pyroclastic debris”and prove that the canyon was more or less in place as it is today at 3 Ma [
this, interestingly, is not explicitly stated but clearly meant to be inferred]. [OK, this was really unclear and evidently unfair–see comments below. Gabet certainly said that this meant the canyon in this location dated back well into the Tertiary, and GG took the way this was written to imply that this meant that the canyon all the way downstream was also this old, which is Schaffer’s interpretation but perhaps not Gabet’s. It is that geographic inference that was not explicitly stated–well provided GG hasn’t skipped over it elsewhere–and which GG felt was intended to be inferred-2/3/17]. This would seem to imply that the canyons have actually been present throughout the Tertiary; Gabet offers a couple diagrams as to how that might be possible. Now, GG has an issue here: the geologic mapping available ( none is cited by Gabet [GG didn’t read the figure caption, where the preliminary version of the relevant map was cited; realizing this would have clarified things and revealed where the Pliocene pyroclastic statement came from]) shows the deposits at the coordinates given to be Miocene basalt flows, not Pliocene pyroclastic rocks, and there is no indication they have ever been dated. Unfortunately the Cousens et al. study a few years back didn’t extend quite this far south, but basalts as old as 16 Ma (and some older andesite) are found to the north a short distance. So unless some reference or date was left out of the Gabet manuscript, there isn’t enough evidence to support Gabet’s claim that
“…for Wakabayashi’s interpretation to be correct, the S. Fork of the American would have to have been incised before the andesitic eruptions, ~20 Ma, while its northern branch and the nearby Mokulumne River (30 km to the south of the S. Fork) would have been spared. Then, 3 Ma, the N. Fork of the American and the Mokulumne River would have carved canyons nearly a kilometer-deep but, this time, the S. Fork of the American would have been spared.” (p. 1245)
Truly, neither author has been totally forthright about this. Both have chosen locations toward the range’s crest (admittedly started by Wakabayashi), where pre-Miocene topography has been recognized for a long time (e.g., Lindgren’s report describes how the Pyramid Peak area near the spot Gabet highlights was clearly an ancestral piece of relief that was not buried). Both are not far from Miocene and younger volcanic centers that quite possibly have led to local topographic changes. Thus it is quite likely that the kilometer of erosion found along the Mokelumne highlighted by Wakabayashi might overestimate the total erosion since the highest volcanic was erupted or volcaniclastic deposited. Similarly, the presence of 1500′ of paleotopgraphy at the base of the volcanic pile on the South Fork of the American River is only news if, in fact, the material deposited is of the same age as the stuff at the top of the pile that Wakabayashi discusses.
Look, everybody agrees there was a lot of burial in the Miocene into the Pliocene and then a lot of erosion. What, if anything, does it mean? Does it mean the Sierra rose up late in the Pliocene? Or does it mean that a pile of goop was just dumped on an already high range and it is in the process of being removed?
Frankly, GG isn’t sure that these observations require uplift c. 3-5 Ma, though it is certainly possible. Deposition of a sheet of volcanic rock some hundreds of feet thick should have led to isostatic subsidence equal to a significant fraction of that thickness; combined with the beheading of the drainages as normal faults developed on the east side of the modern Sierra, it seems plausible that, absent any other changes, that little if any incision of the volcanic/volcaniclastic cover would occur. However, given climatic changes, increased incision even with reduced drainage sizes could well occur, and as incision would progress, some isostatic rebound would steepen river grades some, so recovering the pre-volcanic river grades seems plausible too. Incision of the Miocene-Pliocene pile is, in and of itself, not diagnostic of the creation of topography along the Sierra crest.
But the issue is that this is not what happened: the rivers have cut well below the pre-volcanic river grades which were themselves above the base of the Eocene Auriferous Gravels. What is more, the river grades through the Oligocene and into the Miocene were very stable, not only in the Sierra but to the east in the Basin and Range, where despite repeated deposition of significant ash flow tuffs, rivers only cut down to the pre-volcanic valley floor, not below. What is more, the Sierran rivers are not in valleys like the pre-volcanic ones; modern Sierran rivers are in the bottom of steep-sided, narrow, V-shaped canyons over most of the west flank of the range, while the pre-volcanic rivers were in broad valleys with strath terraces. When the modern rivers return to that geometry we will know just exactly how different the modern and pre-volcanic landscapes truly are.
Now along these lines, Gabet does point out that incision in the last few million years, as measured by Greg Stock and colleagues, has only amounted to a few hundred meters; there was, particularly in the southern Sierra, a lot more topography clearly in existence prior to 3 Ma in places like Kings Canyon and the Kern River canyon. There clearly was something older (and GG is guilty of overemphasizing the 3 Ma story in a paper some years ago)–but how old? 10 Ma? 50 Ma? We’ll have to save that for another day.