December 17. The Basin and Range Province

Today we’re going to a geological province that is almost unique in its nature – at least in terms of its size. The Basin and Range in Nevada and Utah is a region of broken, extended crust nearly 450 miles wide and even longer in a north-south direction. Areas of basin and range topography extend north into Oregon and Idaho, into southeastern California, southern Arizona and New Mexico, and cover a broad swath or northern Mexico. 

Shaded relief map (NPS)

The name basin and range is pretty descriptive. There are alternating narrow, high mountain uplifts separated from each other by long narrow valleys, or basins. In the core area of the Basin and Range Province, east-central Nevada and west-central Utah, there are dozens of mountain ranges and intervening valleys – 50, 60 or more. The topographic map of the region led one early geologist, Clarence Dutton, to compare the basin and range to an "army of caterpillars marching toward Mexico" – and that’s really not a bad way of thinking of it.  

The alternating uplifts and basins, technically called horsts and grabens, are the result of extension of the earth’s crust over this wide area. Take something brittle – continental crust – and pull it from the two sides, and it will break. The breaks are mostly steep normal faults – sometimes more than one – that separate the basins from the ranges. As with any mountain uplift, as soon as there is a variation in mountain relief, erosion starts, and the eroded material was shed into the adjacent basins. In some places, there is more than 10,000 feet of sediment filling the basins, all eroded from the adjacent mountains, which may stand 6,000 feet or more above the valleys. I’ve actually done quite a lot of work on this region because my specialty, gravity and magnetic data, is useful in figuring things out here. The sediments in the valleys are typically much less dense than the rocks in the ranges, so that density contrast is easy to see in gravity data – the denser stuff has a stronger gravitational pull than the less dense stuff.


This extension started in the Early Cenozoic or maybe even in very late Cretaceous time. It’s not as if these breaks all just happened suddenly – faulting, while it may generate catastrophic earthquakes, typically only offsets rocks by a few centimeters at a time – or a few meters in really huge quakes. That motion over millions of years can add up to a lot. The early phases of extension in Nevada produced low areas along low hills – nothing like today’s ranges. But the beginning basins were low enough for sediment and even lakes to form. For sure by Eocene time there were at least a few lakes in the region. It’s the Oligocene when the action starts to pick up, with ranges and basins starting to have higher relief, and more movement on faults. There was enough breaking to allow for some pretty vast volcanic activity as well – much of the region today is covered by sheets of volcanic ash falls and ash flows. Most of the volcanism is older than the most recent phase of mountain uplift, because the volcanics are cut by the faults that form the boundaries between basins and ranges, but there has been some volcanic activity in Nevada as recently as the past 5 million years or so.

OK, so stretching broke the crust into these long, narrow basins and ranges. What caused the stretching? This is a really big question, and we really don’t have a definitive answer. As with many complex processes, it’s likely to be a combination of diverse origins. One thought has been that the continent-scale uplift of the Rocky Mountains, centered to the east of the Basin and Range, was enough for gravity to drag the western slope of the mountains down to the west, like a gargantuan landslide, and the crust broke as it slid. But the details of the faults show that many of them are not simple straight line breaks dipping steeply into the earth. They are like that near the surface, but then they often curve at depth, merging into a possible deep, flat zone called a detachment. This is a hypothetical surface that would be a boundary above which the blocks – the basins and ranges – would tilt and slide into their present-day geometry. There’s quite a lot of support for some variation on this theme.

But still, what’s the ultimate driving force? That gravitational sliding idea doesn’t really work because the scales involved are too small. Is there something else that could drive uplift, and therefore the extension?  At about the time the basin and range faulting got going, the North American continent was overriding the oceanic spreading center in the Pacific Ocean, the rest of which is the East Pacific Rise mid-ocean ridge today. The spreading center itself subducted, and the whole tectonic framework changed. The San Andreas fault formed in California as a result – and it formed in Oligocene and Miocene time, about the same time as the Basin and Range started to form. Conceptually, it’s kind of easy to visualize that spreading center down there underneath the continental crust, subducting, but still with the pulling apart happening. Those forces might have translated up into the overlying crust, breaking it. You can think of it as an incipient continental rift, like the East Africa Rift system – but then we have to explain why the breaking is so widely distributed. You can maybe do that by saying the subducting East Pacific Rise has different properties than a normal rift, because it’s subducting, and maybe the nature of the crust in Nevada and Utah was such that it broke the way it did. The continental crust there is a lot thinner than normal continental crust, and heat flow is quite a bit higher than normal, but we’re getting into the realm of speculation now.

There might also be consequences of a change in the angle of subduction that could have affected things here. You recall that back in the Cretaceous we called on a change in subduction angle to perhaps explain the breaking of the continental crust well into the continent, in the Laramide Orogeny. Maybe something similar happened here, even though the breaks in Nevada and Utah are mostly – but not entirely – within the Paleozoic and Mesozoic sedimentary cover.

Another idea is that as the San Andreas fault developed, it put a new kind of stress on this part of western North America. Instead of the fairly straightforward collision subduction produced, now we had a strong shear stress, essentially wrenching the continent so that dozens of breaks formed. Imagine a pile of wet napkins – that’s the sedimentary cover in Nevada and Utah. Put your right hand on the right side of the pile – that’s the strong, stable core of North America. Put your left hand on the left side, and push your left hand away from you, simulating the movement of the San Andreas Fault. All the country between your hands will wrinkle – and if you could do this with something brittle, it would break in many places. That’s the concept of this regional shear pattern generating the basin and range.

So obviously it’s complicated and there is no strong consensus as to how the Basin and Range formed. It is even more complicated by things that were going on as the Miocene phase of basin-range faulting got going, about 17 million years ago. Things like the opening of the Rio Grande Rift, in New Mexico, the eruption of vast flows of basalt in the Columbia River country of Washington and Oregon, and the first interactions between North America and the Yellowstone Hot Spot. We’ll tackle some of those things later this month.

* * *

Two geological birthdays today. Richard Alexander Fullerton Penrose Jr. was born December 17, 1863, in Philadelphia. R.A.F. Penrose studied the mining district at Cripple Creek, Colorado, for the U.S. Geological Survey, and invested in mining ventures that made him wealthy. He endowed the Geological Society of America with a gift of almost $4 million at his death in 1931 – a bequest that to this day funds significant grant programs for the Geological Society of America. The Penrose Medal, the highest award given by the GSA, is named for him. 

Nelson Horatio Darton was born December 17, 1865, in New York City. His long career with the USGS was quite varied, including important works on the hydrogeology of the Great Plains and Black Hills, the geology of the Big Horn Mountains, and paleontology studies, resulting in more than 200 publications. He received the GSA’s Penrose Medal in 1940.

—Richard I. Gibson

LINKS
Basin and Range (Idaho State U.)
Basin & Range aquifers
Basin & Range (USGS) 
Basin & Range (NPS)

September 15. Sonoma Orogeny

While Pangaea was more or less intact during the Triassic, together with the initial rifting we discussed a few days ago, there were places where Pangaea was still growing. Western North America was one such place.

Several long linear island arcs and zones of oceanic crust, and the sediment associated with them, was amalgamated to what is now northwestern Nevada and adjacent areas over millions of years, culminating in the Sonoma Orogeny in early Triassic time. The mountain-building event gets its name from the Sonoma Mountains in that part of Nevada.  

The Sonoma Orogeny was the second round of accretion, or adding of tectonic terranes, in this part of North America. We talked about the Antler Orogeny, in central Nevada, back in May, during the Devonian.

I visualize this something like a modern island arc, say, Japan, colliding with the nearby Asian continent. Slices of oceanic crust, piles of sediment, volcanic piles, and perhaps small bits of continents all became welded to the older North American continent. There is some controversy as to exactly when all this happened, and it did happen over millions of years. Some lines of evidence favor earlier or later culmination, but I believe the consensus is that it was complete by Early Triassic time, around 230 million years ago. It’s complicated by overprints of later tectonic activity as well as intrusive igneous and extrusive volcanic rocks, and a 2008 paper by Keith Ketner with the USGS even suggests that there wasn’t really a specific Sonoma Orogeny at all. The geology out there is complex enough that questions like this can be raised legitimately even now.

Sonomia Terrane in green (including parts covered by Cenozoic). Strontium 706 line in red.

The set of terranes in northwestern Nevada are usually combined together and called the Sonomia Terrane. The eastern edge of the Sonomia Terrane coincides fairly well with a geochemical boundary called the strontium 706 line. This value, 0.706, is the ratio between two isotopes of the element strontium, and it forms the western boundary of igneous rocks of continental origin, the main mass of North America to the east of that line. West of the Sonomia Terrane, the strontium isotope ratio reaches 0.704, indicating sources of magma in oceanic crust. Thus the Sonomia Terrane lies between the strontium 706 and 704 lines, a position that would be predicted for intermediate crust such as an island arc. All of this supports the idea that the rocks of the Sonomia Terrane came in from a considerable distance to be accreted, or attached, to the North American continent.

—Richard I. Gibson

Links:
Island arcs and oceanic crust  USGS Bull. 1857-B


Ketner 2008 paper

Map from Ron Blakey 

May 22. Grant Canyon Oil Field

For many years in the 1980s, the most prolifically producing oil wells in the onshore 48 states were in Nevada. Nevada? Yep – not the first place you think of for oil, but there’s oil there, in some pretty unusual traps.

Oil was first discovered in Railroad Valley, in desolate central Nevada, back in 1954. It was kind of a fluke – the seismic data they had were pretty poor, and they drilled one thing but found another. The oil reservoir at Eagle Springs Field is mostly fractured volcanic rocks called welded tuffs – essentially, the result of hot ash erupted from a volcano perhaps 10 million years ago – just yesterday, geologically speaking, during the Cenozoic era. The ash fell and landed while still pretty hot, hot enough to weld itself together into a hard, almost glassy rock. Such rock is pretty easy to fracture naturally, and the fractures trap the oil.

Oil Fields of Railroad Valley (data from Nevada BuMines;
interpretation by Gibson)

The oil comes from a rich source, organic-rich black shale in the Mississippian-age Chainman Shale which is buried beep beneath the basins of Nevada. Some of the Chainman has as much as 8% total organic carbon in it, and if you recall some of our previous episodes on oil source rocks, you know that’s fantastic. Even 1% or 2% total organic carbon can make an excellent source rock.

OK, so Mississippian source rocks and Cenozoic volcanics as reservoirs. Aren’t we in the Devonian this month? Yes. Hang on, we’ll get there.

Fast forward to 1976. Another oil field was discovered in Railroad Valley. Trap Spring Field was also in fractured volcanic rocks, but it was across the valley from Eagle Springs. Eagle Springs was a small but steady producer, with today something like 5 million barrels total produced in 60 years. Trap Spring was better, and it has yielded around 15 million barrels in less than 40 years. For perspective, the United States today consumes close to 20 million barrels of oil every day.

The discovery of Trap Spring stimulated a renewed interest in Nevada. At the time, even major oil companies, like Gulf Oil where I worked, were interested. My first work on trying to understand the geology and to use geophysical data to predict where analogs to the existing production might be found began in 1978. And my most recent work on Nevada was this year.

In 1983 another oil field was discovered, in another corner of Railroad Valley. This one was entirely different from the others in terms of the reservoir. Instead of fractured volcanics, the reservoir was extremely porous dolomite – Devonian dolomite, buried within the Cenozoic sands and gravels that fill the basins of Nevada. Nevada’s basins and ranges are formed by long normal faults – the kind formed by pulling apart, extension of the earth. Think of the basins as the parts that dropped down, and the mountain ranges as the high-standing parts that were left back, that did not subside. As the faulting continues, and one side goes down and the other side goes up, relatively, you get these alternating high ranges and low basins. And of course you get erosion of the mountains, dumping sediment into the adjacent basins. Some of the basins in Nevada have more than 10,000 feet of sediment that was eroded off the mountain ranges, and most of that has happened in the past 10 to 15 million years. All of the known oil in Nevada is trapped in various kinds of rock that’s been dumped into the basins.

So back to the new oil field discovered in 1983, named Grant Canyon. If all the oil is in the Cenozoic fill in the basin, how can I say it’s in a Devonian dolomite?  Think of a fairly rapidly downdropping basin. Fairly rapidly means just a few million years. That can make a pretty steep scarp, the face of the mountain range. Steep scarps lend themselves to massive landslides on occasion – and that appears to be what happened here. A huge slice of the mountain range – composed of those Devonian dolomites and other rocks – slumped off the mountain and into the basin, maybe 6 or 8 million years ago. And then it was buried by more and more sediment coming off the mountain front, until that huge landslide was buried under around 3500 to 5500 feet of later sediment. You can think of it as a landslide, as I described it above, but it’s probably a little more accurate to think of it as another fault that dropped part of the mountain front down into the basin. Either a large landslide or a small fault block. The entire area of material is less than a square mile.

What’s the big deal? Well, in those highly porous Devonian dolomites, oil migrating up from the Chainman shale accumulated. Most of the time you should think of oil in rocks as simply filling the tiny pore spaces between grains of rock, but in this case it’s actually fair to visualize a real pool of liquid oil down there. Some of the porosity in these rocks is called cavernous porosity – essentially, little caves eroded out of the carbonate. With a really good seal, an impermeable layer of rock sitting above it, the Devonian dolomite became a small, but excellent oil reservoir.

How excellent? For about 9 years, from 1983 through 1992, the two wells in Grant Canyon Field yielded close to 6,000 barrels per day – the most of any wells in the onshore 48 states. I’ve said it before, but as a reminder and for perspective, the average US oil well produces 10 barrels per day. 6000 is Saudi Arabian levels. The total volume was nothing like a Saudi Arabian field, but Grant Canyon and the associated Bacon Flat Field produced about 25 million barrels over about 30 years.

There have been several other important oil discoveries in Railroad Valley and some in Pine Valley, further north. The last large discovery came in 1986.

In 2009 and 2010 I did some work for an Irish oil company in Hot Creek Valley, across one mountain range to the west of Railroad Valley. I used a predictive model based on analysis of gravity, magnetic, and geologic data to point to possible analogs to the existing production in Railroad Valley. The company used my recommendations to do a lot of additional work, including geochemical surveys and other approaches, and in 2012 they drilled the second exploratory well ever located in Hot Creek Valley. The 400 barrels per day that they tested was deemed non-commercial, but I can tell you that as far as I am concerned, I felt like I had found oil. It was for me a proof of the concept used to identify analogs to existing production, and I was really happy!  The last I’ve heard, the company is using the information it gained in the first well to plan a second well. Stay tuned.

—Richard I. Gibson

Link:
My Nevada oil exploration page

April 29. Carlin gold

The Roberts Mountains Formation in Eureka County, Nevada, is mostly carbonate, limestone and dolomite that was laid down in a shallow Silurian sea. Interesting rocks, certainly, but geologists and prospectors alike walked over those rocks for decades without realizing it was the host to one of the largest gold deposits on earth. 

They missed the gold because it’s in the rock as tiny tiny grains – often smaller than a micron. A micron is one-one thousandth of a millimeter. Pretty small. Gold was discovered in 1961, and the first mine began production in 1965, near Carlin, Nevada, which gives its name to these “Carlin-type” gold deposits. 

Goldstrike Mine, Carlin Trend, Nevada

What happened is that the limestones and dolomites, which are soluble in even slightly acidic water, were pretty much turned into a really finely porous sponge – many tiny holes were dissolved in the rock. And the water that dissolved the holes – or maybe later water – was mineral rich, and carried gold in solution that precipitated into those little holes. That happened long after the Silurian rocks were laid down around 417 million years ago. The gold mineralization of these rocks probably happened more like 40 million years ago, when Nevada was beginning to be pulled apart and big normal faults were starting to form. Those faults probably served as conduits for the hot, acidic, mineral-rich waters to percolate through the Silurian strata.

Exactly where the mineral-rich waters came from is controversial – did they pick up gold as they passed through older rocks, leaching out the gold and then redepositing it here? Or did the water come out of magma, molten rock, deeper in the earth? There’s geochemical and geophysical evidence to support the magmatic idea, that the stuff came from molten rock deep down in the earth’s crust, but I don’t think this question is fully resolved.

The gold ore at Carlin is typically only 1 to 10 grams of gold per ton of rock. A gram is about the mass of a paper clip, so you can see how the gold must have been really thinly scattered through the tons of rock – but there was a lot of it. The Carlin Trend in Nevada has produced way more gold than the Mother Lode in California, and today The Silver State – Nevada – produces about 80% of all the gold mined in the United States, and more than 10% of world gold production. The Carlin mines passed the 50-million-ounce mark in 2002 and 70 million ounces by 2008, and they’re still going strong. New mines continue to be opened along the 5-by-40-mile zone, whose production is more than $85 billion at 2010 gold prices.

Much of the gold at Carlin is produced from open-pit mines. It’s the second-largest gold district in the world, second to the Witwatersrand in South Africa, where the gold comes from underground mines almost 2½ miles deep, the deepest on earth. With Carlin approaching 100 million ounces of gold, it’s a distant second to Witwatersrand, which has produced about 1.5 billion ounces of gold since it was discovered in 1886. That’s about half the gold ever mined on the planet. The origin of the gold there is about as different as possible from Carlin. The South African gold is related to a huge meteorite impact more than two billion years ago, back in the Precambrian.

The main use – 38% – of gold in the United States is in electronic components, because gold is an excellent non-reactive conductor. Things like computers are among the main consumers. Jewelry takes another third, coins amount to about 19% of consumption, and all the gold in all the new gold crowns and other dental uses adds up to about 5% of US gold each year.

Thanks to Carlin, Nevada, the US is a net exporter of gold, but China produces twice as much. Australia is #2 in world gold production, and the US and Russia are usually about tied for third place. 

—Richard I. Gibson 

Photo of Goldstrike Mine, Carlin Trend, from USGS 

Links:

http://en.wikipedia.org/wiki/Carlin%E2%80%93type_gold_deposit
http://www.geotimes.org/apr06/feature_GoldOrigins.html
http://minerals.usgs.gov/west/projects/nngd.htm
http://goldinvestingnews.com/6070/the-geology-of-the-carlin-trend.html