NSS 75th Anniversary Convention

Geology Field Trip: The Great Basin

Sunday, July 17, 2016
Trip coordinator: John H. Peck

Cavers - This trip is now Sold Out!
Incoming registrations will be placed on a wait list. There are still seats left for the geology steam train trips on Tuesday evening.

Take a bus trip through the geology of the classic Basin and Range of Nevada. This geological excursion will be a one day trip starting and ending in Ely, Nevada. From Precambrian metasedimentary rocks to the only glacier still present in Nevada, participants will get some hands on geology as well as panoramic views of spectacular geomorphology.

The geologic history of the Ely region spans more than 600 million years. Quartzose sands and gravels deposited on a marine shelf of Laurentia, a continent extending across much of the present northern part of North America, record the earliest sedimentary rocks of the region. These rocks have been metamorphosed by the pressure exerted by overlying Paleozoic and Mesozoic sedimentary rocks during their long history before being exhumed by the uplift and block faulting of the Cenozoic tectonic episodes forming the Basin and Range structures. These tectonic features continue to be active today along range-bounding faults and smaller, but related, Quaternary faults in the alluvium-filled basins.

Stops along the field trip route will examine the Precambrian rocks, the sedimentary marine rocks of the Paleozoic era, and igneous rocks of the Mesozoic and Cenozoic eras. Tens of thousands of feet of marine sediments were deposited on relatively shallow marine shelves during the Paleozoic era. They are thicker, but laterally equivalent to the rocks exposed in the Grand Canyon further south in Arizona. Some of these marine rocks are recognized as persistent key beds in the section. The Pioche Shale of Lower Cambrian age is correlative with parts of the Bright Angel Shale of the Grand Canyon and yields trilobite fossils. The Dunderberg Shale and its equivalent in the Snake Range is another key bed of Upper Cambrian age that serves to break up the rather monotonous limestone sections above and below.

Great Basin Geology

Video compliments of Great Basin National Park

The Pole Canyon Limestone is the host rock of the famous Lehman Cave in Great Basin National Park. Another field trip will visit the Lehman Cave. The Pole Canyon overlies the Pioche Shale.

Another key bed in the Paleozoic section is the Eureka Quartzite. The Eureka is a prominent cliff forming sedimentary quartzite of Ordovician age. In the Egan and Schell Creek Ranges it forms cliffs 300 to 500 feet thick of white to light gray very resistant fine to medium grained, sometimes cross-bedded, sandstone cemented by quartz.

Silurian and Devonian limestones and dolomites make up a thick section of rocks above the Eureka. These are overlain by a thick, but non-resistant shale of Mississippian age called the Chainman Shale. The Chainman is dark gray to black carbonaceous shale with little limestone or siltstone as interbeds. The Pennsylvanian and Permian rocks of the region consist of carbonates, sandstones and gypsum bearing evaporite deposits. Most of these rocks are correlative with the Park City Group of Utah.

West of Ely, Mesozoic rocks consist of igneous intrusive bodies of porphyritic quartz monzonite of Jurassic and Cretaceous age. The rocks were crystallized below the surface and have been exhumed since their emplacement. They host ore deposits that have yielded huge amounts of copper and also gold, silver and platinum precious metals. The Robinson Mining District near Ruth was once the most prolific mining district in Nevada.

The history of the rocks of the Robinson District and their ore deposits is very complex and will be discussed on the trip as we overlook the open pit mines. Mineralization of the igneous rocks and their surrounding intruded Paleozoic sedimentary rocks, occurred in Cenozoic time during the eruption and cooling of volcanic tuffs and flows. These volcanic eruptions and associated hydrothermal soaking of the preexisting rocks yielded the porphyry copper deposits that made Ely the center of a huge mining and smelting complex in the twentieth century. Copper mining continues today in the Robinson District.

Cenozoic volcanic rocks from separate caldera complexes overly the older Paleozoic marine strata in many of the mountain ranges surrounding Ely. These caldera complexes are typical of the Basin and Range extensional regime as are the fault bounded mountain ranges.

Intermontane valleys are filled by alluvial, colluvial and sometimes large landslide deposits that have been shed from the rising mountain chains in late Cenozoic time. These valleys contain very important aquifers that hold a significant groundwater resource. The mountain ranges receive substantial amounts of snow in the winter and provide the source of the groundwater beneath the valleys.

Lexington Arch, Great Basin National Park

The 4wd road required to reach Lexington Arch precludes it from being being available for the geology field trip. You may, however, get directions to this interesting feature from our trips team at the convention. Video compliments of Great Basin National Park

Mountain glaciers were present in the higher ranges during the Pleistocene and the valleys were wetter than they are now. Marshes and shallow lakes supported a relatively lush savannah that saw mammoths, mastodons, camels, horses, and other animals that are now extinct. Glacial moraines are present in the Snake Range and the only remaining glacier in Nevada is present on the east side of Wheeler Peak in Great Basin National Park.

Geologic structures seen on the field trip are principally the mountain ranges bounded by steep faults. Low angle thrust faults in the Paleozoic sedimentary section are older and formed in reaction to the collision of the North American plate with the Pacific plate near the end of the Mesozoic era. Subduction of the Pacific plate beneath the North American continent caused thrusting within the Paleozoic section and subsequently, the pulling apart of the upper crust of the Basin and Range province resulting in block faulting and volcanic eruption.

All in all, participants on this field trip will get a journey through an area of beautiful country, complex geologic history, mining geology, ranch country with precious water resources, and the grandeur of Nevada's Great Basin National Park.