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.

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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)

December 16. East African Rift

We’ve been talking about the dismemberment of Pangaea and its biggest piece, Gondwana, for months now. The process is still going on, and the newest break within the old Gondwana continent is in its largest surviving portion, Africa. 

Map from Digital Tectonic Activity Map of the Earth (NASA)
with annotations by Gibson.

The East African Rift System is a present-day break that extends from the Dead Sea in Israel and Jordan, south through the Red Sea, separating Arabia from Africa, and into the African continent through Ethiopia, Kenya and Uganda, eastern Congo and Zambia, and into Mozambique and on offshore. All told, the system is more than 5,000 miles, 8,000 kilometers, long. It’s a big, complex break in the continental crust. 

As much as we have a pretty good handle on how rifting proceeds, our understanding of how and why such rifts begin is still pretty poor. There are multiple ideas for how the East African Rift started, ranging from some deep-seated mantle plumes, whose upwelling heat broke the crust apart, to crustal thickness variations that allowed magma to flow upward in some locations preferentially to others, initiating the rift process. Differences in crustal density might have the same effect as thickness variations. Whatever started the rift, it has since followed a pretty standard and expectable development.

Early in the process, around 30 million years ago, early Oligocene time, the upwelling magma breached the surface and flowed as extensive flood basalts in what are now Ethiopia, Somalia, Yemen, and adjacent areas. This point is called the Afar Triple Junction, because it is the focus for three branching rifts. To the northwest it’s the Red Sea, a young ocean basin where sea-floor spreading has just barely begun, to the northeast is the Gulf of Aden, true oceanic crust, and the mid-ocean ridge there continues into the Indian Ocean as the Carlsberg Ridge, the divide between the Indian tectonic plate and the African Plate. The third branch of the system extends from the Afar Triple Junction into the African Continent. 

Where the rift is in continental crust, in East Africa, the result is long narrow down-dropped troughs, called grabens. They are bounded by normal faults that have large offsets, many thousands of feet in some cases. The situation is very much like eastern North America must have been back in the Triassic as the Atlantic Ocean began to open. In Africa, it’s not one simple linear zone, but it curves and branches into two major segments on either side of Lake Victoria.

The fault-bounded troughs, the grabens, are obviously lower that the uplifted flanks, which tend to be mountainous, and the grabens or basins accumulate thick piles of sediment eroded off the mountains. In East Africa, the long, narrow lakes, such as Abaya in Ethiopia, Turkana in Kenya, Lakes Albert, Edward, and Kivu along the eastern border of Congo, Lake Tanganyika between Congo and Tanzania, Lake Rukwa, and Lake Malawi all lie in the down-faulted basins of the East African Rift. Lake Victoria, the second largest freshwater lake in the world, after Lake Superior, isn’t in a narrow fault basin, but it is related to the tectonic activity. It lies between the two big branches of the rift system, and formed when the uplifts to east or west dammed rivers flowing into the central basin. Victoria is a young lake, only about half a million years old or less, and it has dried up completely at least three times in its history, a reflection of changing climate conditions during the recent ice ages. Victoria is a shallow lake, less than 300 feet deep. In contrast, the deep troughs of the rift system hold some of the deepest lakes in the world. Lake Tanganyika, for example, reaches a maximum depth of more than 4,800 feet, and holds about 18% of all the fresh water on earth.

Volcanic activity continues in the region related to the rift process, including Mt. Kenya and Mt. Kilimanjaro, and the active volcanoes of Ethiopia and the Mountains of the Moon in Congo. Over time, the rifting has been sporadic. After the initial pulse of rifting and flood basalt eruption in the Oligocene, the faulting and real rift formation began in early Miocene time, around 22 million years ago or so. There was a period of several million years without too much going on, and the volcanic activity and earthquakes in the region today began in earnest about 5 million years ago or thereabouts.

The present rate of extension, about 6 millimeters per year, is slow, compared to typical oceanic crustal spreading rates, which are more like 20 millimeters a year, two centimeters. The slow rate is probably at least partially related to the strength and thickness of continental crust – it is more brittle, and harder to move than oceanic crust. But, at this rate, we should have a narrow ocean similar to the Red Sea separating the two parts of Africa by about 10 million years from now. The dismemberment of Gondwana continues, at least on this side. But to the north, Gondwana – India, Arabia, and North Africa, are more or less in a state of collision with Eurasia. All these things go on simultaneously.

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Today we have the anniversaries of two significant earthquakes. On December 16, 1920, a quake hit Haiyuan County in Gansu Province, central China. The death toll estimate, 200,000, has been increased by modern estimates to almost 275,000, making it one of the most deadly earthquakes in human history. Its magnitude has been given variously from 7.8 to 8.5, but whatever it was, it shook the earth enough so that seiches – basically, a sloshing of the water in a relatively enclosed body, such as a lake – were recorded in the fjords of Norway. The location was along the Gansu thrust fault, a major fault where one of the continental blocks of North China is being pushed over the rocks to the south. Or maybe it’s better to think of it as the rocks to the south being pushed under the rocks of North China, because this quake is a result of the ongoing collision between India and Eurasia.

The second big earthquake on this day, December 16, was in 1811, at New Madrid, Missouri. It was the first of three quakes over a two-month period there that had magnitudes of about 7.5. They are among the largest historic earthquakes ever in North America. There were few deaths because the region was so sparsely populated, but the sequence of three quakes resulted in the formation of Reelfoot Lake in Tennessee, and the Mississippi River temporarily flowed backwards as a result of the forces. The December quake was strong enough to awaken people in New York City and to damage buildings in Cincinnati, Ohio. There’s a nice book chronicling these quakes, titled When The Mississippi Ran Backwards, by Jay Feldman (2012, Free Press Publishing).

—Richard I. Gibson

East African Rift 

Map from Digital Tectonic Activity Map of the Earth (NASA) with annotations by Gibson.

December 15. The Badlands of South Dakota

First today, I think I have been remiss in not pointing you to a great paleontology podcast, Palaeocast. All things fossils. The most recent episode is about ceratopsians, the group of dinosaurs that included Triceratops. Check it out. And speaking of ceratopsians, the oldest one known was just announced last week. It was a raven-sized little thing that lived in Montana about 105 million years ago. LINK • Link2

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Yesterday I promised that today we’d talk about some of the Oligocene debris that helped bury the Rocky Mountains. We’ll go to South Dakota to do that. 

You can find the sediments that were shed off the mountains up in the mountains, but because of the Miocene and later erosion, a lot of that stuff is gone, eroded and washed onto the plains or down the Missouri River. In South Dakota’s Badlands, a nice pile of the Oligocene sediment has been preserved.

It’s eroded, to be sure, into fanciful angular shapes – that’s why the place is called Badlands, the word for a region of exposed rock, often fairly soft rock, that has eroded into steep slopes separated by an intricate meshwork of canyons and ravines. They tend to exist in arid country today, where erosion rates are relatively low and the erosion that does happen is usually catastrophic, as in flash floods. The rocks eroded tend to be relatively young, because younger rocks are often more poorly consolidated than older rocks, but there’s nothing sacred about that. 

Exposure of Oligocene Brule Formation in South Dakota Badlands (National Park Service photo)

The White River Badlands of South Dakota preserve a thick wedge of the Oligocene sediments that eroded off the nearby Black Hills, a Laramide uplift. The sediment was carried to the east by rivers of various sizes, some that flowed strongly enough to carry gravels, and some that were parts of shallow braided streams carrying sand and silt. Flood plains saw extensive mud deposits. Over time, from the late Eocene into the early Oligocene, the deposits built up a pile that totaled more than 100 meters thick – 300 feet – in the Badlands National Park area. Across the eastern Rockies region, equivalents of these Oligocene deposits reach thicknesses of 700 feet or more.

The first package of rocks that makes up the South Dakota Badlands is called the Chadron Formation, gravels, sands, silts, muds. As the sediment was being laid down, this area was certainly not badland, but harbored abundant life. The Chadron is famous for its titanotheres or brontotheres. The nature of the cement and porosity in the Chadron formation is such that the rocks weather into smooth, rounded hillocks.

River and flood deposition wasn’t continuous over the 11 million years of the Oligocene, and in the Badlands of South Dakota, a lot of the reddish layers interbedded with the white rocks are paleosols – ancient soil horizons, red because iron was concentrated in them. There were also occasional lakes in which limestones formed, and one of them marks the break between the Chadron Formation and the overlying, younger Brule Formation. It’s really quite similar to the Chadron, but the cement in it includes considerable volcanic ash and calcite, so it holds together well and tends to form steep slopes and knife-edge ridges. The Brule is famous for turtle fossils and oreodonts. Oreodonts had teeth that were suited for eating oreo cookies – well, no, not really – oreodonts were fox-sized mammals that are not very closely related to any modern animals, but camels and pigs are not too far from oreodonts on the evolutionary tree. Technically they are called artiodactyls, even-toed herbivores. Lots of their fossils have been found in the Badlands rocks.

There’s a distinctive volcanic ash bed at the top of the Brule Formation, erupted about 30 million years ago, and it was followed by more deposition in the mid- to late Oligocene of the Arikaree formation, generally the highest unit in the Badlands National Park area.

Try to envision a vast sheet of these Oligocene strata laid out across the intermountain basins of the Rockies as well as much of the western Great Plains. There were rivers, certainly, and both erosion and deposition were both still going on, but at a rate that was nothing like it had been in Eocene and Oliogene time. The region was relatively level for millions of years – close to 25 million years. The modern badlands topography is a very recent development, within the past million years and more like just 500,000 years ago. Significant rainfall and snowmelt associated with the glacial period provided the water that cut aggressively into the Oligocene sediments, creating the modern badlands. They continue to erode and develop today.

—Richard I. Gibson

Links & References
Chadron, Brule, Arikaree
Arikaree 
Reference: Geology of National Parks, by Ann Harris and Esther Tuttle, Kendall/Hunt Publishing.