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UCMP's 2017 Basin & Range field trip, page 2
Thursday, May 25
Our original plan had been to visit the bristlecone pines in Great Basin National Park and to camp in the Park, but Seth had heard a weather report that called for cold temperatures; fearing that we might be unprepared for cold weather weather (and that the trail would be blocked by snow), he decided to skip the Park and head straight to Utah. We stayed on U.S. Route 6/50 and made a stop at the Border Inn (at the Nevada-Utah border, of course) to fill our water containers. We then entered Utah and Mountain Time (losing an hour). About 34 miles into Utah, we turned north onto Tule Valley Road, an unmarked dirt track that runs roughly north-south along the west side of the House Range. We made one stop to take photos of the dramatic western face of the range, dominated by the cliffs of Notch Peak, before turning east on the Old Route 6 & 50 Road and entering Marjum Canyon.
We stopped near the west end of the canyon, with high cliffs rising on each side, and got out to look at the Cambrian rocks. Seth pointed out large red masses in the rocks on the north side of the canyon. The red masses are infilled caves within the Howell Limestone, recording karsting and a drop in sea level during the Cambrian. On the south side of the road, Seth showed us gray limestones that were micrite (essentially massive carbonate mud), devoid of layering because of heavy bioturbation (mixing of the sediment by animal activity). In some places, orange patterns in the rock provided evidence of infilled burrows, created by arthropods and various worm-like creatures. Chunks of rock that had fallen from higher up on the cliff exhibited a sharp contact between the carbonate mud and an oncolitic rock. Oncoids are layered, spherical structures that form by the growth of cyanobacteria around a nucleus such as a piece of shell.
Continuing up the road, we made a second stop to look at trilobites and other fossils, including algae, in the Wheeler Shale; it too is of Middle Cambrian age but stratigraphically above the Howell Limestone. But first: lunch. During our lunch break, Erik Sperling, an Assistant Professor of Geological Sciences at Stanford and Richard's advisor, drove up. He had flown into Salt Lake City a few hours earlier, rented a car, and somehow managed to find us.
Near the road, small agnostid trilobites, sponge spicules, and unidentifiable organic matter were found in the rocks of the Wheeler. These rocks were deposited in deep water where oxygen concentrations were variable. Higher up the hill the rocks were very laminated, indicating no bioturbation, a sign that oxygen levels were too low for most bottom-dwellers. However,the trilobite Elrathia kingi was quite plentiful. Apparently, it was well-adapted to the low-oxygen conditions. In fact, there are layers in the Wheeler Shales in which Elrathia occurs in dense accumulations, often 1,000 individuals per square meter.
When done with our trilobite hunt, we returned to the vehicles, left Marjum Canyon behind, and stayed on Death Canyon Road before turning southwest onto another dirt track, 3c Road, and took it back to 6/50. We drove west for a bit with the mostly dry Sevier Lake on our left before picking up Tule Valley Road again on the south side of the highway. We drove due south along a very dusty track, then turned west around the southern end of the Barn Hills. Seth steered us to a camping area at the base of Fossil Mountain in the Confusion Range. Only one couple with an RV and a little dog were camping there. The group fanned out to set up their tents while Peter and Emily set about making Peter's jambalaya recipe for dinner. While looking around the camping area, Seth noted that arrowheads have been found here; seconds later Mackenzie bent down and picked one up! The jambalaya took a while to make so we ended up eating in the dark. Seth, Ivo, Franzi and others went off to collect wood for a fire. We sat up and talked to around 11:00. When I went off to find my tent in the darkness, I followed Daniel since his tent was on the same line from the vehicles as mine. The stars this evening were amazing.
Friday, May 26
The hillside before us presented a partial cross-section of what was once a thick carbonate platform built up in shallow water on a passive continental margin (i.e., there was no plate boundary in the vicinity). Numerous invertebrate fossils were present in these rocks, all the way to the top of the ridge. More than 60 years ago, geologist Lehi Hintze, who studied these Lower Ordovician rocks extensively, first measured this section and painted his measurements directly on the rocks. The measurements would have been erased by the elements long ago, but geologists periodically repaint them. Seth led us up the slope, following Hintze's trail. We began by looking at the Wah Wah Limestone, a brownish-gray formation of mixed shales, micrites and grainstones. Again the students split into groups to describe two-meter sections. The Wah Wah was probably deposited at a depth similar to that of the House because of the presence of ripple marks, but there was a more diverse biota. In addition to silicified trilobites and echinoderms, there are also sponges with holdfasts, small sponge-microbial mounds, nautiloids, brachiopods, high-spired gastropods, and some graptolites in the shales.
Above the Wah Wah we moved into the Juab Limestone, generally similar to the Wah Wah, but a more medium to dark gray in color. It contained unsilicified trilobites, brachiopods, crinoid stems, and high-spired gastropods. Although the same major groups are present as in the Wah Wah, Seth pointed out a sharp shift in faunal dominance whereas the Wah Wah is numerically dominated by trilobites, the Juab is dominated by brachiopods. This ecological shift occurs gradually in many places during the Lower-Middle Ordovician, but in this area it is particularly striking and abrupt.
Overlying the Juab Limestone is the Kanosh Shale, which records a local increase in water depth. The Kanosh is composed of a variable mixture of shales and limestones, but we found the shales to be obscured by soil and rubble. The limestones are often very fossiliferous, and in the rock-strewn surface there were abundant brachiopods, gastropods, trilobites, ostracodes, nautiloids, echinoderms, and more.
Since it was getting close to 5:00, we climbed back down to the vehicles and returned to camp. It had become quite windy. Seth attempted to fix his bad tire with Fix-a-Flat but it had no visible effect. While he was out doing that, Eric, Richard, Emily and Franzi hiked over to the cliffs northwest of camp to have a look at the upper Kanosh Formation. Seth's spare also turned out to be low on air, so he would have to go into Ely tomorrow to get the tires serviced. Everyone but Seth, Emily, Franzi and Richard would be returning to the Bay Area the next day so Seth would caravan with us as far as Ely.
For dinner, Cindy and Ivo made a big pasta dish with a sauce containing eggplant and olives; I assisted by peeling some garlic. I went out to gather wood for a fire but didn't find much. I'd been told that Seth would want to keep the Expedition that I'd been driving so I removed all my stuff from the back and moved it to my tent. It remained very windy and became overcast we tried to position the vehicles so that they'd provide some shelter. We all headed to bed before 10:00. Shortly after getting to my tent, it began to drizzle and was still drizzling when I turned out my light.
Saturday, May 27
It wasn't until 6:30 that we finally got underway. Seth, with Richard, Franzi, and Emily, led the way to Ely in the SUV with the nearly-flat tire. It turned out that Seth would not be wanting my Expedition after all, so with Peter and Daniel as passengers, we made to leave camp when wouldn't you know our low-tire-pressure warning light came on again. We ignored it. Cindy, Ivo, Mackenzie and Jun were in another SUV. The fourth vehicle would remain at Fossil Mountain since Seth would be needing two vehicles to get his three passengers, all their gear, and all the food bins, coolers, etc. back to Berkeley. At the Border Inn, we found air, so both Seth and I filled our tires. We made it to Ely without incident, meeting up at the Shell station to top off our gas tanks. We said our goodbyes to Seth and the three students remaining with him they were staying a few days longer to do some field research in the area and began the long drive back to the Bay Area. We made a few stops for gas and food along the way; I was back at my house in Berkeley by 6:00 PM.
And thus ended the 2017 UCMP field trip! It was not so much about the origins of the Basin & Range itself, but about some of the individual rock formations and what their rocks and fossils tell us about what was going on in terms of changing sea levels, oxygen and carbonates in the oceans, and climate.
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