Tilting Caves

GG was in Sequoia National Park recently and was reminded of an unorthodox paper from 2004 by Darryl Granger and Greg Stock. In some ways, it builds on an old and storied foundation.

One of the early tasks for G.K. Gilbert in working for the new U.S. Geological Survey was to understand the recent geology near Salt Lake City, specifically the history of the ancient lake that filled most of western Utah: Lake Bonneville.  To fully unravel its story, Gilbert had to measure the elevation of the shorelines of the lake–and reliably identify them.  Today you might pull out a GPS unit and use differential GPS or maybe work from a high-resolution LIDAR image.  Gilbert had none of that; what he had was a detailed survey along the Transcontinental Railroad and barometers (properly surveying in the shorelines was impractical). Trying to calibrate his techniques took time and effort (the existing application of barometers being too imprecise for Gilbert’s needs). In doing all this, Gilbert assumed that the lake shores were originally level.  When he completed his map he found that the lake shores were no longer level: they bowed up in the center of the ancient lake some 168 feet higher than at the edges.  From this he concluded, in essence, that there had been isostatic rebound: that a more fluid layer deeper in the earth had responded to the removal of the weight of the lake by flowing in under the area where the weight had been removed, pushing the land above up. The tilt of a paleohorizontal had revealed an unexpected deformation. This work forms an important part of Monograph #1 of the USGS.

Now geologists are taught Steno’s Laws, and one of them is that sedimentary rocks are horizontal when deposited. This is an approximation: alluvial fans can on occasion preserve primary dips up to the angle of repose of the material, something over 20 degrees. So over a small area you cannot really assume a bed was perfectly flat. Determining tilt for relatively small deformations requires an exceptional geologic tilt meter. And here is where we encounter the work of Granger and Stock. they recognized that pools in caves are rimmed by mineral deposits termed shelfstone, and that these rims are level with the top of the water owing to the way they are built. These deposits should be flat within a millimeter.  The two authors measured such deposits in three western Sierra caves, one being Crystal Cave (but, to GG’s dismay, not one of the deposits on the usual tour).

Exterior of Crystal Cave

Light-colored body is the marble body containing Crystal Cave in Sequoia National Park.

What they found was curious: two of the deposits were tilted–not a lot, but about 0.06 degrees (they could measure tilts down to about 0.006 degrees).  A third, from a more southerly cave, was untilted.  What was intriguing was that the two tilted deposits were younger than the untitled deposit: this seemed to require a cyclic process.  They proposed that this was caused by depression of the Sierra by the weight of ice during glacial times and showed that if the Sierran crust was sufficiently weak (only having an elastic plate thickness of about 5 km), their measurements were consistent with such an explanation. There is some surprise, though, if the elastic plate were really that thin (values in the Basin and Range to the east are higher over similar time periods); this suggests there could be something else going on.

Of course, the difference in location of the older cave from the 2 younger ones could suggest something different, but at present we have no indication of any tectonic structures that might be responsible for such a difference.

Recently Manny Gabet has pointed out another aspect of these measurements: the absence of tilt of the older deposit would suggest that there is no modern tectonic tilting of this part of the Sierra Nevada. Because we might expect tilting to be down to the west, very similar to the down to the southwest tilt produced by glacial unloading, it is unlikely that a tectonic tilt and a glacial tilt canceled each other out. Gabet states that “there is little geomorphic evidence, therefore, for active tectonic uplift in the southern half of the range.” Is this correct?

As far as it goes, it is reasonable, but the assertion assumes that tilt equals uplift.  While this has been the basis of many arguments in the northern Sierra, it is not nearly so airtight in the south where these deposits were measured. The range in the south is far closer to a horst than a tilted fault block: the Great Western Divide is nearly the equal of the Sierran crest to the east, as had been pointed out, for instance, by Francois Matthes in his posthumously published notes on glaciation in Sequoia. Furthermore, active faulting appears to be a much more prevalent aspect of the southern Sierra than the northern: Jason Saleeby and colleagues have highlighted and documented recent slip both on the west flank of the Sierra and along the Kern Canyon Fault within the range.  They have gone so far as to suggest that the Western Divide is the rapidly rising part of the Sierra and there are suggestions that more faults might exist on the west side than have been recognized to date.  If this is borne out, the long-term tectonic significance of the untitled cave deposit will be very unclear.

In any event, the study of Granger and Stock is one of the cleverest to emerge in an area where it is going to take cleverness to get hard data on what is going on in the Sierra.  It would be interesting to see if a similar story emerged from the caves farther north or if a longer tilt history might be present in these caves. If these features are sensitive enough to measure the tiny tilts from loading of the Sierra by glaciers, they are powerful constraints on our understanding of the deformation of the Sierra Nevada.

Tags: , , ,

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: