Roadside Geology

We begin our first field excursion to Death Valley.  Along the way, we pass by Nevada Test Site, which is reviewed using Google Earth (Maps).  Dr Hoaglund briefly describes the history of the nation's nuclear testing from 1952 to 1992, showing the testing locations that hosted 985 nuclear detonations.  The general public can take a bus tour of the Nevada Test Site led by guides authorized by the Department of Energy, tours arranged at the Atomic Testing Museum.

Devil's Hole is a groundwater discharge point in the Amargosa Valley Region.  The water supplies the Ash Meadows Natural Preservation Area comprised of wetlands and small lakes.  Devils Hole is a submerged cavern, and was the site of a cave diving tragedy in the late 1960's.

Dante's View is an overlook in the Funeral Mountains that gives you a perspective of the entire Death Valley.  It lies at an elevation above 5,000 feet, and sits directly above the Badwater Basin, which is 285 feet below sea level.  The relief, in other words the change in elevation, is thus over 5,000 feet, equivalent to the relief of the south rim of the Grand Canyon.

Zabriskie Point is an overlook within the Twenty Mule Team Canyon, with a history of borax mining and popular culture, including the movie Zabriskie Point featuring '60's music, and having served as a filming location for Star Wars.  The area is known for "badlands topography."  We climb into some of the arroyos (dry channels) and look at some bedding related geology in deposits of water reworked volcanic ash deposits.

The Badwater Basin is a point 285 feet below sea level, the 2nd lowest point on the land surface on Earth, second only to the Dead Sea in Israel.  Death Valley is an endorheic (endo = terminating; rheic = flow) basin where water flows into the basin and does not leave, instead it evaporates leaving many salt related minerals on the desert floor.

With a source of sand from alluvial fans in 3 different cardinal compass directions, the Mesquite Flat Sand Dunes have formed as Barchan dunes to impressive heights of over 150 feet high.  We hike the dunes IN FEBRUARY in 80 degree F (27 C) weather, at least 2 miles from the parking lot.  It is recommended that you do not attempt to hike the dunes in the summer when temperatures are above at least 110 F ( 43 C).

We leave Death Valley across the Panimint Range horst and into the Panimint Valley graben, then climb out of the west side of the valley to the Father Crowley lookout for a view of Rainbow Canyon.  The canyon exposes lavas and pyroclastic flows, sourced from volcanic pipes in the Darwin Hills to the southwest.

We cross one more little valley famous for a pop cultural photograph, then over the next little range into Owens Valley.  The valley hosts an incredible view of the eastern Sierra wall.  The valley was the location of California's largest earthquake, the Lone Pine quake in 1873, and more recently an earthquake occurred in Ridgecrest.  The valley also hosts the Coso Volcanic Field, a source of geothermal heat, all indications that the Walker Lane and the Basin and Range are still tectonically active.

We explore the boom and crash of the Rhyolite ghost town that rose from a gold strike and fell from a mine closure.

As we end our trip coming into the town of Beatty, we see Pahute Mesa in the background, the location of nuclear tests in excess of 1 Megaton.

This is the introduction to a little 9 hour road trip around Nevada Test Site / Area 51 and the Extraterrestrial Highway to Lunar Crater, a Pliocene / Pleistocene aged volcanic caldera, and along the way, discussing the structural geology and hydrogeology of the Pahranagat Wetlands area, Pleistocene pluvial lakes and wetlands, the Great Basin,  and controversies surrounding groundwater diversion from northern basins to southern Nevada.  First stop:  The Pahranagat Wetlands.

Likely fault scarps are seen in the photographs of the mountains around the wetlands.  They trend parallel to NE striking "tear faults" described in the area.  We review the geology from a geologic map and discuss both the compressional fold and thrust structures of the Sevier orogeny and the later Basin and Range normal faulting.  The structures have a role in localizing the wetlands by localizing sources of springs and damming the hydrogeology, forming both the groundwater and physiographic basin of the wetlands.

... continuing from the structural geology lecture, we look at the localization of springs upstream and along the Pahranagat wetlands, and we trace the intermittent Pahranagat flow to the Muddy River, then the Virgin River, and finally Lake Mead.

The Great Basin including major portions of Nevada and Utah is endorheic, meaning the basin is self contained such that surface water drainage and most groundwater does not reach the ocean.  Instead the flow ends up in "dry lake basins" and evaporates, leaving alkali salt deposits commonly referred to as "salt flats," or hypersaline lakes.  However, the groundwater in the Great Basin carbonate aquifer below the basins is generally "fresh" ( < 1000 mg/L TDS), though commonly compromised by water quality issues, including excess salinity (> 1000 mg/L TDS) and elevated levels ( > MCL) of naturally occurring inorganic contaminants, such as chloride, iron, manganese, and sulfate, as well as arsenic, antimony, thallium, and other heavy metals.  The water is likely fresh owing to recharge from the Pleistocene pluvial climate, including pluvial lakes.  The White River corridor provides a generally downhill course from northeastern Nevada groundwater basins.  A controversial groundwater diversion plan, including a $300 million pipeline, was proposed to move groundwater from these to urban demands in southern Nevada (Las Vegas).  After years of challenge, the plan was rejected in May of 2020, though the idea and its discussions have recently been renewed.

We take the Extraterrestrial Highway to our next geology stop, but in this video only get as far Rachel before we're abducted by The Little A'le'Inn Cafe.  Students will learn how not to run out of gas, how not to get lost, and how not to get shot by Area 51 armed guards.

The rainy pluvial climate during the Pleistocene filled many of these desert basins with pluvial lakes.  The lakes today are dry, leaving alkali salt deposits or hypersaline lakes, but the groundwater below is commonly fresh.  We have geochemical evidence that at least some of the water in the pluvial lakes recharged the groundwater below, which implies much of the groundwater is Pleistocene "fossil water."  This has enormous implications for the development of groundwater supplies in Nevada.

In the "Resources" section of this "Lesson" there is

1) a link to my YouTube video presentation of my National Groundwater Association conference abstract on the scientific evidence for the different origins of groundwater flowpaths flowing under Pahute Mesa of the Nevada Test Site.   The groundwater flowpath sourced from the Gold Flat paleo pluvial lake (or wetland) is lighter and more saline than the groundwater sourced from Pahute Mesa itself.  Both flowpaths continue under Pahute Mesa and down Thirsty Canyon towards Beatty.  Though the talk is more technical than intended for this class, you may find it interesting. 

The YouTube description is as follows:

The following video is a narration of a poster presentation delivered to the National Ground Water Association (NGWA) Groundwater Summit in December of 2022. The video highlights a technique to quantify carbonate disequilibrium in groundwater, and use the measure as a groundwater provenance (source) indicator. The measure is shown to be consistent with other provenance indicators such as the d13C stable isotopes in the dissolved bicarbonate (HCO3). The presence of zeolites and volcanic glass in the felsic rocks removes divalent cation and acid from solution, forming a Na-K-HCO3 water and raising pH to abnormally high levels. The acid neutralization without involving carbonate components (HCO3, CO3, CO2 degassing, etc.) leads to the disequilibrium in the carbonate system. The water is readily re-equilibrated when the water encounters sources of divalent cation, carbonate, and CO2. This occurs when groundwater encounters carbonate rock at depth.

and

2) a link to the conference abstract itself,

titled:

Carbonate disequilibrium and isotopic groundwater provenance in faulted felsic volcanic terrain underlain by carbonate rock

and reproduced here:

Over 70 years of groundwater study at NNSS=NTS provided detailed study of Great Basin geology and aqueous geochemistry, where primarily felsic pyroclastic volcanic rocks overlie the GB-carbonate aquifer. Faulting subdivides and largely controls the distribution of both aquifer hydraulic properties and hydrogeochemical facies. The combination of felsic rock composition and the presence of zeolites have largely starved the water of divalent cations, with zeolites exchanging univalent cations into solution, and along with devitrification of volcanic glass, also consuming acid (Hoover, 1968), without exchanging dissolved CO2 as usual with carbonate or a gas phase. The result is high ANC water, both high pH and supersaturated wrt bicarbonate, but undersaturated wrt DIC, specifically the CO2 component.

Groundwater with ANC in excess of its equilibrium with DIC for a given pH (Deffeyes, 1965) occurs on the high volcanic mesas. When the groundwater moves into lower canyons it encounters carbonate rock and other sources of divalent cation, causing precipitation of carbonate, with the consummate lowering of pH to equilibrium values. The results are predictable changes in the levels and associations of pH, ANC, DIC, the ratio of univalent to divalent cations, and bicarbonate in groundwater, and in relative levels of disequilibrium in the groundwater carbonate system. Together these parameters can be diagnostic of the evolution of groundwater in both open systems, in the presence of atmospheric CO2, and closed systems, and in the presence of felsic rocks, volcanic glass, zeolites, and carbonate rock. Combined with carbon isotopic data (d13C) indicating the source of carbon, these parameters make effective groundwater provenance indicators.

Paleo-recharge was identified, sourced from a playa behind Pahute Mesa. Identifying paleo-waters, especially those recharged from Pleistocene paleo pluvial playas and lakes must be considered in estimating the true recharge rates of SW aquifers and availability of groundwater supplies in the desert Southwest.

"In the field," cool nerd in obligatory sunglasses and Led Zeppelin T, Dr Hoaglund, walks a paleo shoreline of Pleistocene Pluvial Lake Reveille. "Back in the office," we discuss Glacial Lake Lahontan and the broader network of pluvial lakes extending to Glacial Lake Bonneville in Utah. The presence of deep Pleistocene lakes as the most likely source of groundwater in the Great Basin has extreme implications for managing any sustainable groundwater development. Over extraction will amount to "groundwater mining" of limited duration.

Both Tulle Springs in Las Vegas and the La Brea Tar Pits in Los Angeles were pluvial wetlands during the Pleistocene, and preserved a diverse Pleistocene wetland fauna, including mastodons.