27 January 2013

Benchmarking Time: Mount Terrill, Fish Lake Plateau, Utah

Posted by Jessica Ball

The next benchmark in my collection is another from Fish Lake in Utah. This is one of my favorite places to do field work, despite the fact that quite a bit of it is vertical and I was cursed with a malfunctioning set of knees. Occasionally I do make it to the top of things, and as we know, geologists like to put benchmarks in high places. Mount Terrill, on the northern part of the Fish Lake Plateau, is one of them. It’s an interesting mountain – long and lean instead of round and bulky – and it’s one of the best places on the Plateau to get a look at the Osiris Tuff, which is the volcanic unit I studied as an undergrad.

Another missing elevation - this is a bit of an epidemic for Fish Lake! The summit of Mount Terrill is at 11,547 feet (about 3520 meters).

Mount Terrill viewed from the southeast.

Clarence Edward Dutton, a great 19th century geologist and explorer who I’ve written about before, does a superb job of describing Mount Terrill in his Report on the Geology of the High Plateaus of Utah (1880) - far better than I could do, so I’ll let his words fill you in on this bit of the plateau:

Upon the eastern side of Summit Valley rise two conspicuous masses, which present to the eye nothing suggestive of a plateau. The northern one is Mount Terrill, the southern is Mount Marvine, both being in the prolongation of the same axis. Although in external form they are great mountain piles, their origin is due to circumdenudation, just as a great butte owes its individuality to the removal of the strata around it. They consist of lavas, resting upon Lower Tertiary calcareous beds, and both the lavas and the sediments are nearly horizontal so far as stratification is concerned; but the lavas were obviously outpoured over a much eroded surface, with hills and valleys of some magnitude. The volcanic sheets may have been continuous with those of Fish Lake Plateau, since they have the same lithological characters as the more striking trachytic members, but are less numerous and of less thickness in the aggregate.

The "lavas" of Mount Terrill, which are actually a series of highly-welded ash flow tuff layers (or ignimbrite sheets, depending on your preferred terminology). The Osiris Tuff (a trachyte) makes up most of the upper part of the edifice, and is underlain in places by the Lake Creek trachyte. Photo courtesy of Chuck Bailey (W&M).

Looking south along the ridge of Mount Terrill at the "lavas".

Mount Terrill is a long narrow ridge, consisting of trachytic lavas, resting upon calcareous beds of Lower Eocene age. The trachytes are rather thin, their aggregate thickness being from 250 to 450 feet only. The varieties are very similart to those of Fish Lake Plateau. The extreme summit is a remnant of a light-gray clinkstone (not phonolite, but a sanidin-trachyte), which weathers into slabs about 3 inches thick by horizontal planes of cleavage and by vertical joints. Underneath is a large mass of light-red argilloid trachyte and several bodies of light-gray trachyte, and one dark mass which man be an augitic variety. The sedimentary beds upon which they lie are not well exposed. The altitude of the ridge forming Mount Terrill declines towards the south until a lofty col or “saddle” is reached, which divides it from Mount Marvine.

"Clinkstone" plates created by weathering and erosion on the summit. About-to-get-rained-on-geologist for scale.

Sitting on a fault on the northern end of Terrill, with more platy weathering morphology evident. I'm measuring the orientation of pumice fiamme in the Osiris trachyte (and sheltering a bit from the drizzle). Photo courtesy of Chuck Bailey (W&M).

What always impresses me is that Dutton, who did his exploring and writing in the 1870s without the aid of analytical equipment, nailed the composition of the rocks on Terrill. He did have a little trouble with emplacement processes, but that’s totally understandable, given that volcanology was in its infancy and most people didn’t know that explosive eruptions created rocks like this. Also, distinguishing a highly-welded ignimbrite from a lava flow is darned difficult unless you start counting up broken phenocrysts in thin section, which no one is going to do in the field, obviously. He was also a bit confused as to where the “lavas” were coming from, assuming that (because lava flows usually don’t travel great distances from their sources) the vent must be somewhere on the Fish Lake Plateau. We now know that the Fish Lake volcanic rocks probably originated from one of the calderas in the Marysvale Volcanic Field to the west (the Osiris can be traced to the Monroe Peak Caldera and is considered a regional marker layer).

Further Reading:

Dutton, C.E. (1880) Report on the Geology of the High Plateaus of Utah. U.S. Geographical and Geological Survey of the Rocky Mountain Region

Bailey, C.M., Marchetti, D.M., and Harris, M.S. (2007) Geology and Landscape History of the Fish Lake Plateau. Field Guide, 2007 Rocky Mountain GSA Section Meeting (Utah Geological Association Publication 35)