So in the previous two installments, we reviewed ideas for how the High Plains got so high and some of the observations out there that bear on this question. Beyond satisfying some curiosity, what does this do for earth science? Why pay money to do this?
Let’s consider three outcomes: that the High Plains gained their elevation by the end of the Laramide orogeny (say, 40 Ma), that they gained their elevation after the deposition of the Ogallala Group (say about 5 Ma), and that they were high, went down, and rose again. Read More…
So earlier we saw that there are a number of different ideas for how the High Plains got high, but what matters are the observations. The oldest of these–the classical reason for saying the Rockies are young–is that the Miocene Ogallala Group/Formation has been deeply incised and removed entirely from large areas like the Denver basin. The classical interpretation of a switch from deposition to erosion is uplift and tilting, but another possibility is that the changes in climate going into the Pleistocene changed the ability of rivers to incise. This has opened the door to multiple lines of evidence. Below is a stab at trying to get a handle on some of this literature (happy to hear what GG missed in the comments). [Note: this is subject to updating]. Read More…
No, not high in that sense…high like “Mile High City”. This still is a problem GG is interested in and so for grins let’s quickly review the main ideas GG has seen with their pros and cons. The candidates are thickening the crust mechanically or by piling on sediment, thinning the mantle lithosphere, dynamic topography, hydrating the mantle or the crust, depleting the lithosphere, and emplacing depleted lithosphere. Whew! GG’s hot takes on these below the fold… Read More…
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….
GG has been piddling along though the Sierra (ostensibly to give a campfire talk in Mineral King) and in doing so stared a bit longer at a recent paper on the age of a pediment in the Sierran foothills by Sousa et al. in Geosphere in 2017. In a way this is a callback to concepts from far back in the geologic literature, namely the significance of an “Eocene erosion surface.”
Here, to be brief, low-temperature thermochronology from a low-elevation pediment in the western foothills of the Sierra yields very old ages–in fact, overlapping with the emplacement of plutons in the Sierran crest [this was not a unique observation; Cecil et al., 2006, had a pretty old point in their collection]. Sousa and coauthors model these data and get a cooling to surface conditions by about 40 Ma. Because these pediments abut noticeable topography, this means there was at least that much local relief in the ancient Sierra. While the pediments had been noticed by others, many suspected a far more recent age.
In some ways, this is old news. The Eocene sediments in the northern Sierra have long made clear the presence of significant local relief, and many workers had inferred that such relief was probably higher in the southern Sierra (e.g., Wakabayashi and Sawyer, 2001). But the southern Sierra lacked the Eocene sediments necessary to know what the Eocene landscape might have looked like, so this paper opens up a new window for us.
Where does this lead us? Kind of down a rabbit hole only to come up with no strong and useful statement–though perhaps future work could nail things down. This is more a personal attempt to try and grasp what is going on, so profound errors might exist and insights are few. So, proceed at your own risk….
It seems a bit odd, but yesterday had, on average, the coldest high temperature here in Boulder of any day of the year. Coming all of 11 days after the winter solstice, this seemed rather quick to GG. After all, shouldn’t there be more thermal inertia in the system? This got GG to wondering about these things, which led to an inability to locate this information trivially. So a few quick numbers lifted from Intellicast’s archive, which is clearly very smoothed…(except for Boulder, which is from NOAA’s ESRL page–Denver is from Intellicast for comparison)
|Place||Date Lowest High||Date Highest High|
|Boulder, CO (40N)||1 January (41)||17 July* (87)|
|Denver, CO (39.7N)||5 January (46)||21 July (89)|
|New York City (40.7N)||19 January (36)||24 July (83)|
|St. Louis, MO (38.6N)||12 January (37)||22 July (90)|
|Los Angeles (34N)||7 January (68)||8 August (85)|
|San Francisco (37.8N)||2 January (57)||28 September (72)|
|Phoenix, AZ (33.5N)||29 December (66)||12 July (107)|
(*-but several almost as hot days are later in the month)
There is in fact quite a range. Phoenix wins as the place which comes closest to echoing sunlight, telling us that part of the equation is humidity. Boulder and Denver are a close second, which isn’t too surprising given that the altitude limits thermal blankets and the absolute humidity is pretty low. But some of the rest are a bit surprising…
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…