Spent many hours in November sitting in on sessions and perusing posters at the Geological Society of America annual meeting; one goal was to see what’s up with the evolution of elevation of the U.S. Cordillera.
First a quick recap. There are two camps, more or less, on each side of the Cordillera. The old mountains camp on both sides points mainly to oxygen and hydrogen isotope variations in proxies for precipitation. There are also attempts to retrodeform the lithosphere resulting in thick crust and high elevations. The dominant counterargument is that the paleometeorology used to interpret the isotopic values is flawed. On the young mountain side, classical geologic observations are invoked, including apparent tilting of river channels and the recent incision events in many places. The counterargument to this is that the appearance of a tilted channel may be biased by the depositional environment and that changes in climate can drive incision as easily as uplift. In between in some ways are geophysical observations of the lithosphere; recent changes in the lithosphere seem likely in much of the region, supporting younger mountains, but seem older east of the Southern Rockies.
Well, a meeting in Indianapolis isn’t one to bring out all the western geologists (next year’s meeting in Phoenix is a whole different matter), but a couple of things popped up. Did anything look to change the landscape, either by opening up new vistas or overturning old results? Not that GG discerned. Below are some notes probably only of interest to the most interested….
Well, time to catch up on the evolution of the Sierra Nevada. Although a large collection of paleoaltimetry papers has bolstered a case for the elevations in the Sierra having been created by the Eocene (most based on Rayleigh distillation of precipitation), a couple of other recent works, one geodetic and the other geomorphic, seem to indicate that Sierran topography has grown over the last few million years.
First up is an update on vertical GPS velocities in California and Nevada by Hammond et al. in the Journal of Geophysical Research. They find “…the Sierra Nevada is the most rapid and extensive uplift feature in the western United States, rising up to 2 mm/yr along most of the range….Uplift patterns are consistent with groundwater extraction and concomitant elastic bedrock uplift, plus slower background tectonic uplift.” This in some ways is trimming the sails a bit on the earlier Amos et al. paper in Nature; as we previously discussed this wasn’t entirely unexpected. Their money figure would be this:
The red blob in most of eastern California is the Sierra Nevada. For most of the range, the pink colors correspond to uplift rates of 0.5-1.0 mm/yr. The presence of the pink/red colors in the central to northern Sierra, where there are no blue colors to the west, would indicate uplift is not being caused by groundwater withdrawal to the west (which is the cause of most of the dark blue south of 38°N and was the focus of the Amos et al. paper). Given the these rates would produce the modern mean elevation of the Sierra in under 6 million years, this would seem to strongly support the young Sierran story and be broadly consistent with the geologic story of a young uplift caused by removal of a dense root.
But, hmm, let’s look more closely…
Recently, GG concurred in the observation that myths can persist in the scientific community and added his own story of the “ignorant sheepherder” comment supposedly directed by Whitney at Muir. Some readers might have said so what, these are innocent little pieces of color commentary independent of the march of science. So for those skeptics, a more significant example.
A lot of recent work has been done on the Auriferous Gravels. These papers pretty uniformly assign a middle-late Eocene age to these rocks. For instance, Cassel et al. (2009, Int. Geol Rev.) said “Middle – late Eocene flora from within the upper half of the sequence are the only dateable material in the prevolcanic gravel (MacGinitie 1941).” A later paper gets a bit more precise (Cassel and Graham, 2011, GSA Bull):
The “Chalk Bluffs flora,” from the auriferous gravels at You Bet Diggings (Fig. 1), has been used to estimate the depositional age. Originally described as Capay stage and interpreted as middle Eocene by MacGinitie (1941), the Chalk Bluffs flora is now considered to be early Eocene (48.6–55.8 Ma; Wing and Greenwood, 1993; Wolfe, 1994; Fricke and Wing, 2004), which is consistent with comparable floral assemblages in other recently dated sections (Meyer, 2003; Retallack et al., 2004; Prothero, 2008).
Hren et al. (2010, Geology) similarly date these rocks: “Plant fossils are classified as Chalk Bluffs Flora after their best-preserved occurrence, and are dated at 52–49 Ma by faunal and floral correlation (MacGinitie, 1941; Wing and Greenwood, 1993).” It would seem that these sediments are pretty firmly dated to 49-52 Ma.
Except that in fact there is no firm floral date for these rocks.
Just what, if any, significance is there to the paleochannels from the Eocene on the west side of the Sierra Nevada? These have been held up as demonstrations of post-Eocene uplift of the range and demoted to insignificant artifacts of a landscape developed on metamorphic rock. Consider these conflicting statements from the abstracts of two recent papers:
Eocene paleochannels show lowest gradients parallel to the range axis, steepest ones perpendicular, and reaches with significant “uphill” gradients that rise in the paleo-downstream direction. Modern Sierran rivers lack this relationship. The azimuth-gradient relationships of paleochannels, especially the uphill gradients, require late Cenozoic tilting and uplift.-Wakabayashi, Geosphere, 2013
and the counterpoint:
The studies supporting recent tilting in the northern Sierra Nevada are inconclusive and rely on observations not unique to tectonic forcing. Indeed, much of the evidence based on the paleogradients of the Tertiary channels is consistent with an early trellis drainage network formed across alternating bands of resistant and weak lithologies. –Gabet, Am. J. Sci., 2014
Now to be transparent, GG has published the view that the drainages do support post-Eocene uplift, but that was then and this is now; given the work done in the past decade, reexamining this is worth some effort. (Hopefully sometime we’ll take a long look at the Gabet paper, which is a more comprehensive attempt to consider the surface geology of the Sierra).
As a reminder, arguably one of the most promising means of estimating paleoelevation (and one being used a lot) is to measure the isotopic composition of rainfall that has been stored in the rock record (in altered volcanic glasses, in bones and teeth, in clays, in soils, etc.). The idea is that as rain-bearing clouds rise up terrain, they rain on that terrain and the farther up they go, the more rain they have lost. Since heavy isotopes tend to rain out first, the ratio of heavy to light isotopes in water decreases. And if you look at river water or rainfall today, you broadly find that the most depleted water is falling in the highest areas–but it is a noisy record and so even if you were handed a modern water sample you might be hard-pressed to determine its elevation.
For the moment, let’s assume that the measurements of isotopic composition of paleo-rainwater is robust. Can we just use some regression of depletion vs. elevation to get paleoelevation? There are several who have argued no; at its heart, the basic problem is that it is not elevation that you are measuring but the amount of water that has been wrung out of the clouds. What else might control rainfall? These two articles point out two elements that are very challenging in the paleo-realm: air trajectories and rain nucleation. (We’ll leave out a lot of other issues for today).
Back when GG was young and scrambling up and down mountains willy-nilly, one of the gadgets he craved was an altimeter. No more trying to triangulate on peaks to figure out how close to the top of switchbacks. Or trying to locate yourself on a nondescript mountainside when in need of putting a pencil line on a map to show where a contact was.
Funny thing was, once you had one of these joys in your pocket (and GG tried both analog and digital versions), you discovered that there is a lot more that affects barometric pressure than just altitude. Sure there are weather systems, but it turns out that small pressure changes associated with winds or thunderstorms or such not would limit the accuracy. When GPS came in, the elevation errors were comparable to barometers but the horizontal errors were small enough that you could get your elevation from the position on a decent topographic map (e.g., the TopoMaps app on an iPhone works really well in this regard).
So GG was a bit puzzled to see the rumors that Apple is putting a barometer into iPhone 6s (and apparently there are Android phones with such a sensor). Really? The most likely suggestion from the comment stream is that this would be used to determine how high you were within a building, where GPS doesn’t work (other suggestions, such as group-sourced barometric pressure maps and elevation information for hiking, seem better done in many other ways). It will be interesting to see how well that pans out or if Apple has some other clever trick for such a sensor.
This all reminds GG of the special place barometers held in geology for a very long time. The initial surveys of much of the west relied on barometers to get elevations; in general, you had one barometer somewhere fixed and another on the traverse to control for weather. But when you really needed accurate elevations, barometers had limits that were hard to get past. A very hefty part of the second annual report of the U.S. Geological Survey is dedicated to the lengthy set of experiments run with barometers by G. K. Gilbert, presumably in large part to improve his measurements of the elevation of the terraces left by Lake Bonneville.
And, of course, there is the ongoing search for a successful paleobarometer. We would love to know how high some places were at different times in the past, but the only true paleobarometer proposed uses the variation in size of bubbles in basalt flows; the technique has some issues and conflicts with some other more indirect measurements.