Derek Field (M.Sc) is a geologist from Calgary, Canada. After getting his B.Sc in Geology from California Lutheran University and working as a volcanology intern for the University of Colima, Mexico, he migrated to Flagstaff, Arizona, to pursue Sierra Nevada research under Nancy Riggs at Northern Arizona University. Derek specializes in doing fieldwork in remote, rugged areas of the US, Mexico, and South America, focusing on continental volcanic arcs and large silicic systems. He recently earned his Masters in Geology degree from Northern Arizona University.

My eyes crack open to the eager rays of backcountry dawn. I find myself nestled cozily in sleeping bag on bare metavolcanic slab, a solid day’s walk into the enchanting John Muir Wilderness of the High Sierra. With a deep inhalation of frosty mountain air, I sit up, emerge from my down cocoon, and stroll about the campsite attending to various chores. Eventually I go bumbling down towards the glassy, smooth lake to fetch water for coffee. Ray, my sprightly field assistant, is already sitting peacefully down by the shore with notebook in lap. After issuing me a warm greeting punctuated by a trademark one-liner, he kindly indulges me his thoughts. For the past half hour, he’s been ruminating on our working hypothesis that the tuff (solidified volcanic ash) of Skelton Lake, a voluminous rhyolite tuff dated at approximately 216 million years old, was originally erupted from a supervolcano similar to the nearby Long Valley caldera. Instead of an isolated volcanic center like Long Valley, our hypothetical caldera would have existed as part of a chain of volcanoes known as the Sierran arc, whose subsurface magma chambers are today exposed as granite peaks and domes of the Sierra Nevada Batholith. I seat myself beside him on the placid shore and we ruminate together.

The Sierra Nevada Batholith is one of the largest of its kind in North America, constituting a 65-km-wide band of pristine white granite stretching some 650 km from Tehachapi to Chester. Speckled throughout the vast ocean of granite are islands of metamorphosed volcanic and sedimentary rocks, called pendants. If the batholith represents the crystallized and cooled magmatic plumbing system (think magma chamber) of the Sierran volcanic arc during the Mesozoic era, then the pendants record the surface geology when the system was active, including the products of volcanic eruptions from those magma reservoirs. This allows scientists to interpret the geologic history of the Sierra Nevada during the Mesozoic era, and use those interpretations to make inferences about active continental volcanic arc systems around the world. The focus of my research is the Mount Morrison pendant, a 100 km² patch of volcanic rocks whose stratigraphy went largely neglected since the initial 1959 USGS survey.

As Ray and I sit by the lake, it’s now our second field season (2017) in the Mount Morrison pendant, and we’ve been exploring the finer details of the caldera hypothesis, among other areas of research involved in the greater project that is my Master’s thesis. How far away was this caldera, and how big was it? Can we look to other pendants to find clues as to the source of the tuff of Skelton Lake? At its most fundamental level, the idea of a caldera source for the tuff of Skelton Lake is supported by the overall regional extent of the unit: the tuff occupies the entire 10-km length of the pendant at a constant thickness of ~1.2 km. The monotonous nature of the tuff wasn’t telling us much, however. We’d been hoping to find some sort of variation, either laterally along strike or vertically through the stratigraphic column, that would indicate the style and/or direction of eruption. But the tuff of Skelton Lake proved to be rather boring, and presented itself in the same form everywhere. Despite making headway on other aspects of the overall project, the caldera story remained untold.

That is, until our final trip to the backcountry. We’d structured our fieldwork schedule into a series of successive 3- to 6-day trips, which proved to be a fruitful tactic for covering such a large field area (nearly 100 km³) in such rugged alpine terrain. Now that our final trip was upon us, it was time to investigate the last unmapped section of the field area: an isolated, 10-km² exposure of metavolcanic pendant rocks in the vicinity of the North Fork of Fish Creek headwaters. To add intrigue, this area was left untouched by the original (1959) USGS surveyors, and they could only speculate that the strata were related to those in the main body of the pendant. A later (1972) study by a pair of structural geologists (Benjamin Morgan and Douglas Rankin) identified the rocks as part of the Duck Lake sequence, part of the central Mount Morrison pendant. As such, I was excited to gain new insight into this fascinating assemblage by documenting it in this isolated corner of the pendant.

Because Ray had hastily fled the continent to spend a semester in France, I called in my fiancée Giselle Fernandez, whose enthusiasm and curiosity for the natural world compensate for her lack of specific geologic education, to take over the role of field assistant. We hiked more than 15 km over snowy passes, across swollen rivers, and through thick mosquitoed groves to reach our promised land of untouched, essentially virgin geology.

Immediately upon arrival, it became quite obvious to me that this was not the Duck Lake sequence. In fact, the Duck Lake sequence was completely absent. Here, with initial confusion that gradually morphed into sheer excitement, I confidently identified our beloved tuff of Skelton Lake. At first glance, I was puzzled as to why its thickness here in this corner of the pendant is a mere fraction of the thickness elsewhere in the pendant. My initial thought was that we had finally encountered distal ash deposits, and that a tapering of unit thickness may represent the furthest reaches of the ash flow. Closer inspection would be necessary to confirm or deny this.

The next morning, we set about mapping the basin and promptly discovered that the top of the tuff of Skelton Lake is cut off by a fault with Jurassic-aged tuffs, hence explaining the decreased thickness here in this part of the pendant. One part of the mystery: solved! Weeks of careful field work had trained my eye for these features, which tend to confound scientific observations when left undetected. Even the original surveyors failed to recognize major faults within the Mount Morrison pendant due to their subtlety. It was only after receiving the first batch of uranium-lead zircon age data (collected during the initial 2016 field season) that I noticed rocks were “out of order” at a large scale in the Mount Morrison pendant, and that the only reasonable explanation was faulting. Armed with the newfound knowledge that faults simply had to be present in certain spots based on the high-precision age data I collected, I began the second field season with a hunt for the faults themselves. Features that previously seemed inconsequential (namely, zones of highly deformed material between different rocks) now served as bright red flags to identify faults – both large- and small-scale – within the pendant.

Now, in the North Fork of Fish Creek area, I recognized this particular fault as the same one cutting through the middle of the pendant. Standing on glorious ridgetop with wind flowing though my shoulder-length hair, I channelled my inner Swiss geologist by tracing the fault line on my map exactly how I saw it on the glacially-polished basin below me. With glee, I noticed that it aligns perfectly with the fault I had traced in the main part of the pendant, given a slight offset that I attribute to the intrusion of magma bodies during the Late Cretaceous period. Rest assured that the following days were spent ground-checking this fault in addition to taking loads of measurements and samples on the tuff of Skelton Lake.

Compiling a detailed geologic record of the North Fork of Fish Creek area led to the discovery we had been waiting for: that the tuff of Skelton Lake does indeed display variation that indicates proximity to a volcanic source. Here in this basin, the typical monotonous tuff incorporates large (up to two meters in diameter), angular fragments of darker-colored volcanic rock. Chaotic distribution of these fragments indicate that they did not travel far before being entrained within the ash flow. Several months later, after compiling all relevant data and consulting with geologists from many different backgrounds, I would come to the interpretation that the breccia bodies represent fragments of the caldera walls and/or roof that collapsed into the vent during eruption of the tuff of Skelton Lake. The implication here is that the North Fork of Fish Creek area contains vent material, and what could feasibly be part of the vent itself.

Caldera vents are commonly located adjacent to, but still inside, the margin of the structure. Therefore, the presence of vent material indicates that we had been working inside the caldera, and not outside it as we had previously assumed. The fact that the tuff of Skelton Lake was so thick and boring could now be explained on the basis of it being an intra-caldera tuff. Sure enough, characteristics of the tuff of Skelton Lake correspond well with younger intra-caldera tuffs in Nevada and Colorado. I calculated the total estimated volume of Skelton Lake tuff at ~200 km³, which is within the accepted range for the volume of an intra-caldera tuff.

Finding such critical and exciting geologic information is a due reward for our efforts accessing such remote, rugged countryside. Sometimes it works out like this, and other times you go through all the trouble only to get nothing in return. But weeks of scientific nothings can be erased by finding the special something that you needed all along. Or better yet, you find that the nothings were somethings after all.

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