May 31. End Devonian extinctions

We’ve talked this month about the abundance of life in the Devonian – the spreading of plants across the land, and even the first forests. Both diversity and abundance resulted in oil and natural gas source systems that add up to one of the largest in all of geologic history.  It all came to a rather screeching halt with two major extinction events toward the end of the Devonian. 

The first occurred before the end of the period, at the boundary between the last two subdivisions of Devonian time, the Frasnian and Fammenian stages, at about 374 million years ago. A second event took place at the end of the Devonian, about 359 million years ago.

Together these two events wiped out something like 75% or more of all species – some estimates say as much as 87%. Depending on how you look at it – diversity, abundance, marine, non-marine, the end Devonian extinctions might be second to the end Permian extinction, but in any case, the end of the Devonian is one of the “big five” mass extinctions in earth history.

It’s quite clear that the extinctions were associated closely with anoxic periods in the world’s oceans – times when the oxygen content of the water decreased drastically. That gives us a starting point for trying to determine the causes.

One possibility is climate change. You may recall that western Gondwana was situated over the south pole during much of the Devonian, but there’s not much evidence for glaciation until late in the period. A Late Devonian glacial epoch would have decreased sea level, and as we’ve heard in previous extinctions, that reduces shallow water niches for life as well as cooling the average water temperature. Life that lives in warm, shallow, well-oxygenated water suffers.

Another possibility is that all the life on land – plants including trees, for the most part – would have resulted in a dramatic increase in the volume of nutrients washing from the land into rivers and ultimately into the sea. Enough nutrients could have led to immense algal blooms that could have produced eutrophication – stagnation – in shallow or restricted seas.

Plants on land would have accelerated chemical weathering of rocks as they form soil. By burying organic matter, it would have been a form of carbon sequestration, and reducing carbon dioxide in the atmosphere would have reduced the greenhouse effect and resulted in cooling. That in turn might be tied to the glaciation, or the glaciation might have begun for some other reason such as changes in the earth’s orbit, but it might have been accelerated by the removal of CO2 from the atmosphere by land plants.

There’s some speculation that an impact or impacts from an asteroid or other bolide might have triggered, or at least contributed to, the climatic effects that resulted in extinctions, but even though there are a couple known impacts at about the right time, they really don’t seem to have been big enough to have had the dramatic effects that are observed in the fossil record. Nor would meteoric impacts have necessarily produced the global oceanic anoxia that we know was present and the timing isn’t really right, either.

The anoxic conditions contributed to the organic-rich black shales we’ve talked about this month, so to an extent, the extinctions that led to trapping of organic matter in the rocks helped generate some important oil source rocks.

All the phyla that we have today survived the extinctions in the Late Devonian, but several sub-groups did not. The arthrodires, the huge predatory fish, went extinct, and so did the primitive armored fish, placoderms and ostracoderms. Corals suffered badly and while most groups survived, abundance decreased so that reef-building pretty much ceased and there were no substantial reef ecosystems for the next 100 million years. 

The late Devonian extinctions seem to have clearly come in at least two distinct pulses, each one lasting a million years or so, but as more information has become available, it is also looking like the “events” might have been the culminations of a longer period of crisis for life that spanned as much as 15 million years. To my mind, it’s much easier to explain things like this when they are gradual, since changes like the oceans becoming anoxic and the atmosphere losing carbon dioxide would be expected to take time. With some exceptions, of course. Catastrophes CAN happen. But it seems to me that the end Devonian extinctions probably result from changes that took a lot more time than something like a cosmic impact, and that they came about from multiple, interconnected causes.

* * *

On this day, May 31, 1970, an earthquake in Peru killed more than 67,000 people, perhaps as many as 100,000. It was a subsea quake just off the northern coast of Peru, but there was no tsunami. The death toll was mostly the result of a landslide and avalanche in the mountains that took ice, rock, water, and debris 11 miles down a river valley to bury several towns beneath as much as 80 million cubic meters of material. The quake was related to the subduction of the Nazca Plate – part of the oceanic crust beneath the Pacific Ocean – and the South American continental plate. It was part of the process that continues to build the Andes Mountains. 

—Richard I. Gibson

Link:
British Natural History Museum

May 30. Novaculite

Do you remember conodonts, the tiny tooth-like fossils that are often the only remnants of an eel-like animal? We first talked about conodonts in March, during the Ordovician, but they were abundant in Devonian time as well. Like ammonites, conodonts are so specific in nature that they serve as excellent index fossils, and because they are tiny, often no more than a millimeter long, they can be identified from cuttings in oil and gas well drilling. They’re important to the science called biostratigraphy, which helps oil explorationists know exactly where they are as the well drills down.  

We’ve also talked about chert, really fine-grained silica, and how it can preserve even microscopic fossils. Combine chert with conodonts and you’ve got something to hang your hat on, in terms of detailed stratigraphy.

Caballos novaculite ridges (USGS photo).

There are several layers of mostly chert in the United States, including the Arkansas Novaculite and the Caballos Novaculite. Novaculite is the rock name given to a special kind of chert that is hard, tough, and dense. Its broken edges can be sharp, and the name comes from Latin meaning “razor stone.” Native Americans valued novaculite as a resource for making projectile points. Chert is definitely a sedimentary rock, but most geologists would consider novaculite to be a very low-grade metamorphic rock, where heat and pressure have tightened the crystalline structure of the silica even more than in typical chert.

Novaculite such as that from the Devonian of Arkansas has been used for whetstones and abrasives. In West Texas, the Caballos Novaculite serves as a good reservoir for oil and natural gas where it is fractured in the subsurface. These novaculite beds are generally a lot thicker than the chert beds and nodules we talked about earlier this month. Those discontinuous layers might be a few inches thick, typically, while the Arkansas and Caballos Novaculite can be as much as 60 feet of almost nothing but silica. One possible origin for the novaculites is thick accumulations of the shells of diatoms – planktonic or floating algae whose cell walls are made of silica. Even though they are microscopic, these algae in their billions could create quite a layer of silica on the sea floor as they died over many tens and hundreds of thousands of years. Radiolarians, animals with silica shells, also likely contributed to the silica accumulations that became chert and novaculite.

—Richard I. Gibson

USGS Photo from U.S. Geological Survey Professional Paper 187.

May 29. Ammonites

Today is my 150th daily podcast. I very much appreciate your continued interest. As I said back on episode 100, I’ll be trying my darndest to keep the daily postings going, but just be aware that the summer is my busy season doing history tours and such. Please accept my apologies in advance if I miss a day or so here and there. And also, let me encourage you to post questions on the Question of the Week page on the blog, and also if you have suggestions for ways I can improve the program, feel free to post a review on iTunes or send me an email at rigibson@earthlink.net.

Goniatite

I think we’ve only talked about cephalopods once in this exploration of the history of the earth. Cephalopods are mollusks, a diverse group that includes clams and snails as well as cephalopods. In contrast to clams and snails, the cephalopods have an internal shell or none at all. They also have an array of arms or tentacles extending from their heads. Cephalopods today include octopuses, squid, and cuttlefish, and the chambered nautilus, a spiral-shelled marine invertebrate.

Spiral cephalopods became abundant during mid- to late Devonian time, around 400 million years ago. They included the first ammonites, coiled animals that lived in chambers within a spiral shell. Each chamber was separated from the next by a wall, called a septum, that had amazingly complex convolutions. Some of the traces of septa on fossil shells’ surfaces, called suture patterns, form an intricate fractal design and can be used to identify various species. This helps make ammonites excellent index fossils, fossils that are characteristic of a specific and typically short interval of geologic time.

Ammonites get their name from the Egyptian god Amun, who was often portrayed with tightly coiled ram’s horns, which resemble ammonites.

Early ammonites such as those from the Devonian often have simpler suture patterns than later species. The commonest general type from the Paleozoic is called goniatitic, for the genus Goniatites. Most Devonian goniatites have a gentle waving or zigzag suture pattern rather than the incredibly complex patterns that came later.

Ammonites survived until the extinction at the end of the Cretaceous, a run of about 330 million years. We’ll talk about them several more times in the course of our journey through the history of the earth.

* * *

Today’s birthday is Hollis Hedberg, born May 29, 1903, in Falun, Kansas. Hedberg worked in the petroleum exploration business and he made major contributions to understanding sedimentation and stratigraphy. He was employed mostly by Gulf Oil Company, and worked extensively in Venezuela. Gulf Oil named one of its marine seismic exploration vessels for him.

—Richard I. Gibson

Goniatite photo by Rama under terms of the CeCILL license.  

May 28. Antler Orogeny

Antler Orogeny

Toward the end of the Devonian Period and continuing into the Mississippian that followed, parts of western North America collided with something, perhaps a volcanic island arc like the modern West Indies.

It does not appear to have been the intense kind of mountain-building event that results from continents colliding. There isn’t much evidence of extensive metamorphism and igneous intrusion. The best evidence is found in the state of Nevada, where coarse conglomerates indicate that there was an uplift in what is now central to east-central Nevada. Something was nearby, and eroding to make those thick, coarse conglomerate layers.

To the east, the Devonian rocks are the typical Paleozoic carbonate shelf deposits, the same kinds of limestones we’ve heard about for much of the Ordovician, Silurian, and Devonian across the middle of North America. But to the west the rocks are more siliceous, those coarse conglomerates, and there are some interbedded volcanic rocks. The western rocks are thrust eastward on top of the eastern rocks in the Roberts Mountains Thrust Fault, a major feature which is additional evidence for the tectonic activity we call the Antler Orogeny.

Could an impact event have triggered some of this tectonic activity? About 367 million years ago, Late Devonian time and about the time the Antler Orogeny was getting underway, something did crash into the waters that were forming the Devonian Guilmette formation, near the town of Alamo, Nevada. There is no crater for this impact, but the rocks record the results of the impact. Smashed and deformed rocks, huge broken blocks forming a rock called breccia, shocked quartz grains, and high iridium levels. There are even deposits that can reasonably be interpreted as tsunami deposits. This is some relatively recent work, mostly in the 1980s and 1990s. John Warme was one of the first geologists to recognize the nature of these rocks and to interpret them as a bolide impact.

But one impact, even one that made impact breccias and the rest, really could not have made a mountain range. We really have to seek tectonic causes for something that’s hundreds of miles long.

I said earlier that it might have been a volcanic island arc that was colliding. But that’s definitely not certain. There are various theories to put the Antler Orogeny into a plate tectonic context, but there are problems with all of them. It’s just not clear what was west of what is now central Nevada back in the Devonian. One reason for that is that much later, a lot of stuff was added to western North America – including most of California. And that process obliterated some of the rocks and structures that were there previously.

As always, research continues.

* * *

Glacial Lake Agassiz

Today’s birthday is Jean Louis Rodolphe Agassiz, born May 28, 1807, in Motier, Switzerland. Louis Agassiz worked extensively on fish, both modern and fossil, and his reports published in the 1830s and 1840s established him as a renowned scientist. But he is probably best known as the first to propose scientifically that the earth had had an ice age. He worked extensively on the glacial deposits of both Europe and the United States. Glacial Lake Agassiz, of which Lake Winnipeg in Manitoba is a remnant, is named for him.

—Richard I. Gibson

Glacial Lake Agassiz map from Upham, Warren, "The Glacial Lake Agassiz". Plate III. Monographs of the en:United States Geological Survey: Volume XXV, 1895. 

Antler orogeny map drawn by Richard Gibson.

May 27. Hunsrück slate

I think it’s time to visit another lagerstatten – one of those amazing collections of fossils that are remarkable in their preservation and completeness. This time, it’s the Hunsrück slate, in southwestern Germany near the border with France and Luxembourg.

Brittle star from Hunsruck slate. Photo by James St. John (creative commons license)

Slate is a hard, dense rock with fine partings that make it come apart easily into flat slabs. Huge flat slabs of black slate used to make chalkboards in schools, and smaller pieces have been used for centuries to make roofing shingles. Slate is a metamorphic rock, formed by subjecting fine-grained shale to heat and pressure. The main difference between shale and slate is often just how indurated it is, how hard and tightly bound together the tiny grains are. The heating and pressure might drive off any water in the rock, and might mobilize the chemicals just enough to make the rock hard, rather than crumbly.

So how do you get fossils in slate, a metamorphic rock? Well, there are all levels of metamorphism, and even the process of going from loose sediment to solid rock might be called really low-grade metamorphism, but we usually reserve the word to mean a later cooking or pressurizing that changes the minerals at least a little. But not necessarily a lot.

The Hunsrück slate was laid down as mud and clay that became shale in a sea along the southern margin of the Early Devonian continent of Europe. As we’ve learned, fine-grained sediments are great for preserving details in fossils, and dark shales often indicate anoxic conditions, where even fragile organic structures might not oxidize and decay.

The Hunsrück fossils include more than 260 animal species, and even soft parts are preserved. Most of the kinds of marine life of the Devonian are present, including spectacularly preserved crinoids and brittle stars with long delicate arms intact, as well as fish, trilobites, and sea cucumbers. The strange and extinct carpoids, a kind of echinoderm, are also found with outstanding preservation.

The process of turning fossil-rich shale into slate must have been quite gentle, as metamorphism goes. The shale dates to the Emsian Stage of the Lower Devonian, about 400 million years ago. Sometime during the Carboniferous Period, perhaps 80 to 100 million years later, the rocks were metamorphosed to slate without significant destruction of the fossils.

—Richard I. Gibson

Photo by James St. John under Creative Commons license.  

Carpoids

May 26. Fish scales

In a group as diverse as fishes, it’s no surprise that they developed differing kinds of scales. You recall the placoderms and ostracoderms, which had bony plates covering parts of their bodies. That model was unsuccessful – but flexible bodies, covered in scales, not only let fish survive for 400 million years, but might have led to the development of a distinct neck – an important anatomical feature for living on land.

Devonian ganoid fish (Osteolepis)

The ganoid fishes where bony fishes abundant during the Devonian, and they have many modern descendents including sturgeons, paddlefish, and gars. If you’ve ever gone fishing in the waters of Arkansas and caught a gar, you know how bony they are. One of my few clear memories from the time I was around 7 years old is fishing with my grandfather. I caught a gar that seemed to be as long as the boat – in reality it was probably 3 feet long – with a sharp nose, silvery scales, and lots of teeth. My grandfather fought with it for what seemed like an hour – probably 2 minutes – until he broke it in half. Fishermen in the backwaters of Arkansas in the 1950s didn’t like gars.

Scales are similar to human hair and fingernails. They’re composed of collagen, the structural protein that makes up lots of connective tissues in animals, combined with calcium phosphate, the mineral apatite, which is found in bones and teeth. Some scales also include calcium carbonate. In ganoid fishes, much of the mineral matter in the scales is ganoine, glassy, rod-like calcium phosphate. That’s what makes gars so shiny and silvery.

Other types of fish scales are somewhat more bony in the case of primitive fish like coelacanths. Most common modern fish have the overlapping flexible scales that you’re probably familiar with.

—Richard I. Gibson

Ganoid fish drawing from an old textbook (public domain)

May 25. Devonian glass sands

Shales like the Marcellus Shale are usually deposited out in quiet, relatively deep water. Closer to shorelines, and on shorelines, you get coarser sediments including sand. Sands on beaches tend to get washed and winnowed by wave action, so unless there is some source for other materials than the quartz grains in the sand, you can end up with a rock, sandstone, that’s really pretty pure quartz, silicon dioxide.

Photo by Kevinaj, public domain via Wikipedia.
Caudy’s Castle (Oriskany Sandstone), West Virginia

In what is now Pennsylvania, Maryland, West Virginia, and Virginia, during the Devonian, such a sand was laid down early in the period, around 400 million years ago. The clean quartz sand forms a layer that is 100 feet or more thick, spread over a large area. The purest portions were mined historically for making glass, which requires quartz of high purity. Even a trace of iron, less than a fraction of a percent, will color glass.

The historical name for these glass sands was Oriskany, but that’s really a large group of related, similar, but not necessarily exactly equivalent sandstones. The glass sand miners didn’t care of course – they just went after the sand that gave them the best glass. These sandstones in the subsurface can also serve as excellent reservoirs for natural gas.

—Richard I. Gibson

Photo by Kevinaj, public domain via Wikipedia.  Caudy’s Castle (Oriskany Sandstone), West Virginia. 

May 24. The Adirondacks

Today is a break from the Devonian, and a thanks to listener Thatcher Hogan for suggesting the Adirondacks as a topic.

I mentioned the Adirondacks back in the Precambrian, because the rocks are part of the Grenville Terrane which collided with the Superior Craton, the ancient core of North America, about 1.1 billion years ago. But the modern Adirondacks are much younger than that old mountain-building event. The rocks were buried deeply – perhaps as much as 15 miles below the surface – during the Grenville Orogeny. They were also intruded by various magmas. The result of the high pressures and temperatures was a wide variety of both igneous and metamorphic rocks.

The Adirondacks of northern New York are a strange little range, almost circular in shape. It’s really a large dome, a circular geological uplift. The oldest rocks, uplifted the most, are in the center, with younger rocks draping the flanks of the dome. And this uplift is really quite young, beginning around 5 or 10 million years ago – just yesterday, geologically speaking – and continuing to the present. So the present mountains have nothing to do with the ancient Grenville mountains, and also nothing to do with the Appalachians – which today are relatively low, eroded hills, a remnant of the mountain building events of the Ordovician, Silurian, Devonian, and Carboniferous. So why are the Adirondacks there?

The circular dome shape suggests some kind of force pushing up from depth, and you get domes where things like magmatic intrusions, cylinders of molten rock, like the neck of a volcano but down within the earth, rise. They push the rocks above them up like your fist pushing up in the middle of a blanket. Salt, which is not usually molten in the earth, can flow plastically under pressure, and salt domes can do the same thing to the rocks they rise up against and through.

The Precambrian core of the Adirondacks is exposed because those old rocks were uplifted, along with a thick pile of younger sedimentary rocks. It’s probable that sedimentary rocks, including the Cambrian Potsdam sandstone (February 19) and strata from the Ordovician, Silurian, and Devonian were laid down over the region where the Adirondacks now stand. But those rocks were eroded off the rising Adirondacks, mostly in the past 5 or 10 million years or so.

It would have to be something big to rise from great depth to produce the huge dome at the Adirondacks. Not a salt dome, and not the small uplift around a rising magmatic intrusion. Those kinds of things make domes that are maybe one to 5 miles across, maybe 10 miles at most. The Adirondack Dome is 160 miles in diameter.

To be honest, we really don’t know why the Adirondack Dome began to rise, and why it continues to rise – by some estimates, one of the fastest-rising mountain ranges on earth, perhaps as fast as 1 or 2 millimeters a year, which is actually incredibly fast. There is controversy over uplift rate estimates, so stay tuned for more research on that.

The best guess – and it really is a guess – is that there was a hotspot beneath the Adirondacks. Hotspots are regions of relatively low-density mantle, many tens of miles within the earth, that tend to rise buoyantly through denser parts of the mantle. Such a blob, pushing up, could make the broad dome that we see in the Adirondacks. Hotspots are well known, especially those that get shallow enough that reduced pressure allows the hot rocks to melt. Then you can get volcanoes. There is a hotspot beneath Hawaii, one under Iceland, and one under Yellowstone. There are a few dozen around the world.

Out in the Atlantic Ocean there is evidence for a hotspot that the Atlantic Oceanic Plate has been moving over for some time. The track is represented by seamounts, essentially flat-topped eroded volcanoes that might have once been something like today’s Hawaiian chain. The hotspot that made those volcanic seamounts is called the New England Hotspot or Great Meteor Hotspot. Don’t let that name throw you – it has nothing to do with a meteor impact. The seamount that gives its name to the hotspot, Great Meteor Seamount, was named for the German research vessel Meteor, whose scientists discovered the seamount in the 1920s.  It’s possible that the Adirondacks represent an uplift above that hotspot when it was beneath the continent. There are problems with that idea – like why is the only domal uplift at the Adirondacks? Maybe the hotspot hung out there longer. Or maybe the crust was a bit weaker there. There are problems with the timing, too – the hotspot would have been under the Adirondacks well before the modern uplift. So maybe the hotspot weakened the crust, so something else made it rise in the past 5 million years or so. Or maybe something else.

This is an interesting and enigmatic question, and the bottom line is we really don’t know for sure why the Adirondacks are there.

* * *

Harry Hess was born May 24, 1906, in New York City. He spent most of his geological career as a member of the faculty at Princeton University, and he is considered to be one of the founding fathers of plate tectonics theory. He came up with the concept of sea-floor spreading, based on the discovery of mid-oceanic ridges and rift valleys. Even though it was not completely understood as a mechanism, it was the lynchpin in the idea of continental drift, which had been largely ridiculed, at least in the United States, because there was not only no mechanism to explain it, there was no evidence that even started to explain it. Mid-ocean ridges and sea-floor spreading provided that important basic starting point as a hypothesis, and work addressing that idea uncovered the evidence for the mechanism – convection currents of heat within the earth’s mantle that drive the motion of plates in the crust.

—Richard I. Gibson

Good overview of Adirondack geology 

Map from USGS

May 23. Acadian orogeny

The long-lasting complex collisions that created the Appalachian Mountains and their equivalents in Greenland, Britain, and Scandinavia continued into the Devonian. You may recall that we called that the Caledonian Orogeny, or mountain-building episode near the end of the Silurian last month, when Greenland and Scandinavia collided, as well as the bits of North America and Europe that would become most of the British Isles.

The long, narrow microcontinent called Avalonia, for rocks in the Avalon Peninsula of Newfoundland, did not encounter North America at the same time in all places. South of Maritime Canada, a lot of the accretion of Avalonia to North America didn’t happen until the Middle Devonian and later. It’s usually called the Acadian Orogeny, for Acadia, the French name of Nova Scotia where some of the colliding happened. It was likely underway in the northern terranes by late Silurian, but it culminated in a major crunch in the Devonian.

Bits of Avalonia or related terranes were accreted to North America as far south as Georgia and Alabama, and some of those rocks are in the subsurface today, while some are exposed in the Appalachian Mountains. This collision was another one that wasn’t really head-on, but more oblique in nature, involving some strike-slip faulting like that along the Pacific Coast of Canada today. But there was also enough subduction to generate magmas that are scattered through the Acadian mountain belt.

Besides the well-known orogeny in eastern North America, there was some ongoing tectonic activity in parts of Europe at about the same time. In Europe, it’s called the Variscan or Hercynian Orogeny, just to confuse matters, and there, it continued well into the Carboniferous Period that followed the Devonian. This was the result of Africa, or really Gondwana, or even more accurately, some pieces of Gondwana moving to collide with Europe.

The diagram shows the 200-million-year sequence of events that add up to the Appalachian Mountains, spread over at least four distinct tectonic collisions spanning parts of 5 periods of geologic time. It’s not over yet.

—Richard I. Gibson

Diagram from USGS 

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

May 21. Vertebrates come ashore

Yesterday we talked about insects and the fact that they are not at all common in Devonian rocks, although they were there, on land. The bigger deal from the point of view of us vertebrates is the emergence on land of walking, air-breathing vertebrate animals. That also happened in the Devonian.


To understand this evolution, the best thing you can do is read Neal Shubin’s book Your Inner Fish. I’ve recommended it before, but it’s very pertinent in this connection. Shubin and his team found an animal fossil called Tiktaalik, in 2004 in Arctic Canada. It represents an evolutionary transition between the lobe-finned fishes, the sarcoptyrigians that we’ve discussed previously, and the true tetrapods, four-limbed animals that walked on land. Tiktaalik has essentially, wrist bones and fins with bones getting much closer to fingers than fins. There are also structures that suggest it had lungs and skeletal features that made Shubin and others pretty certain that it must have spent time on land. It was the earliest known animal with an amphibian lifestyle. 

Amphibians – frogs, toads, salamanders, and some other groups, live in both worlds, the land and sea. The name itself, amphibian, means “both kinds of life.” While some amphibians are largely aquatic, others are almost entirely terrestrial, living on land. But all amphibians return to water to lay their eggs, and some of their embryonic development, such as tadpoles in the case of frogs and toads, is also in water, even if the adults spend virtually their entire lives on land.

Tiktaalik lived about 383 million years ago, the Late Devonian, 15 million years before the Devonian ended. Its well-known successor, Ichthyostega, was also Devonian, but from later in the period, probably about 365 million years ago or so. Neither Tiktallik nor Ichthyostega can be classified with certainty as an amphibian. The limb-like fins were probably still fins. But they very likely did spend some time on land, even if it was very near the water, perhaps in ways similar to mud-skippers today, which use their strong fins to scuttle across open land between puddles of water.

Ichthyostega was pretty big – more than a meter long, and Tiktaalik wasn’t much smaller. These were not tiny critters.

* * *

Mary Anning was born May 21, 1799. In the early 19th century, women were pretty much precluded from the realm of scientific study, but Mary Anning became a paleontologist and expert on the Jurassic fossils around her home at Lyme Regis, in Dorset, England. She discovered the first ichthyosaur – the marine reptile – to be correctly identified. Even though as a woman she was excluded from membership in the Geological Society of London or other scientific organizations, a few years ago the British Royal Society named her one of the 10 British women who have most influenced the history of science.

—Richard I. Gibson

Link:
Nice discussion of vertebrates coming onto land


Graphic by Dave Souza under GNU Free Documentation License 

May 20. Wingless insects

Modern springtail

Given that we had forests developing by late Devonian time, you might suspect a proliferation of insects as well. You’re probably right. But insects, fragile as they are, are notoriously difficult to preserve, so the record is fairly sparse during most of the Devonian, especially the early and middle epochs of the period. The bugs in the Rhynie Chert, which we talked about on May 8, do include some primitive insects related to modern springtails. Springtails are hexapods, like insects, but technically they fall into a different group of arthropods. The first complete Devonian insect was not described until 2013 – this link is to the paper in Nature about it. That discovery helps fill in a 45-million-year gap in the fossil record of insects.  

All the few insects known from the Devonian are probably wingless – wings didn’t develop, so far as we know, until the Carboniferous, which we’ll cover in June and July. In terms of the rock record, we’ll have to wait until next month before we can really start to talk about insects. Conditions during the Devonian, including a warm climate and all those proliferating plants on land, would lead you to expect a lot of bugs, too, but the fossil evidence is poor. Either there is a preservation problem – there certainly IS a preservation problem, but we don’t know how big a problem it is – or insects were taking longer to evolve and expand into terrestrial ecologic niches than plants did. Or both.

* * *

Sometime about May 20, in the year 526 a.d., an earthquake hit Antioch, in northwestern Syria. The estimated death toll was 250,000 – maybe as many as 300,000, making it among the deadliest earthquakes in recorded history. The location is near the intersection of the tectonic boundaries between the African and Arabian Plates and the smaller Anatolian Plate that occupies most of modern Turkey. Most of the damage was actually caused by fires that followed the quake and burned for many days. The Great Church of Constantine was a victim of those fires.

—Richard I. Gibson

Modern springtail, similar to wingless insects of the Devonian. Photo by Sarefo under GNU free documentation license.

May 19. Devonian plants: the first forests

The Devonian saw the most fundamental change in the appearance of the land in several hundred million years. Plants, which had gotten started on land maybe by very late Ordovician time and for sure by the Silurian, began to spread across the landscape. For the first time, soils with organic matter began to form in abundance. You can visualize the development of soil as a sort of symbiotic relationship with plants – chemicals from plants, plus the mechanical action of their root systems, broke up rocks and changed them to the stuff we’d call soil. Soil in turn served as a reservoir of nutrients for future plants, as well as a substrate that was softer than hard rock, a place for plants to grow. The cycle of plant growth, death, and soils had begun.

Devonian forest

Early Devonian plants were still pretty primitive, but by the end of the period many diverse plants with true leaves and root systems were covering large areas. Many were relatives of modern ferns and horsetail rushes, but early varieties of other plants, such as pro-gymnosperms, spore-bearing plants that eventually gave rise to conifers, were also around. And the first true seed-bearing plants had evolved by the end of the Devonian.

You remember Cooksonia, from April 25, back in the Silurian? In contrast to those spindly stalks a couple inches tall, the Devonian saw the development of the first woody plants – trees – and the first real forests. The oldest known tree is called Wattieza, a fern-like tree from New York dating to about 385 million years ago, the Middle to Late Devonian. Some of these early trees were more than 30 feet tall. The oldest known forest, at Gilboa, New York, has upright stumps, roots and trunks that are interpreted as part of an extensive ecosystem that can only reasonably be called a forest. The stumps and trunks had been known since 1870, but it wasn’t until 2007 that the crowns of the trees were found and connected to show the geometry of the entire tree.

Apart from bragging rights for being the oldest forest, this discovery has huge consequences for the history of the earth. If large plants were widespread on the earth’s surface, it would have had a significant impact on the atmosphere – carbon dioxide in, oxygen out, and dying plants would be returning their elements – largely carbon, into that new product on the land, soil. All of this is part of the carbon cycle, the shifting of carbon around in the atmosphere, hydrosphere, and soils and rocks of the solid earth. That, in turn, has a great impact on the nature of climate and the kinds of life that can inhabit various ecological zones.

* * *

May 19, 1871, was the birthdate of Reginald Aldworth Daly, at Napanee, Ontario. Daly worked as a geologist surveying the Canada-U.S. boundary for many years, leading to a massive report entitled North America Cordillera: Forty-Ninth Parallel. His work also resulted in a definitive book called Igneous Rocks and their Origins. Daly served as the head of the geology department at Harvard for 30 years. He worked on impact theory, and there are craters on the moon and Mars named for him.

Carl Beck, my graduate school major professor of mineralogy, was also born on this day in 1916. He put me on the convoluted path that took me from kidney stone mineralogy to geophysics in oil exploration.

—Richard I. Gibson

Devonian landscape painted by Eduard Riou, 1872 (public domain)

Link: Gilboa fossil forest

May 18. Devonian trilobites

Trilobites – again? Well, trilobites are cool and each period seems to have some that are pretty distinctive. For the Devonian, at least in the United States, I’d say it’s the genus Phacops. There are at least 26 species of Phacops, and like all trilobites, they’re all extinct. They had large heads and bulbous glabellas – that’s the nose-like section in the middle of the head. And they had really large compound eyes. 

Phacops

The eyes of some Phacops species are sort of like frogs’ eyes, and one common species, Phacops rana, takes its name from a large group of frogs. Rana means frog in Latin. The eyes were mounted on little turrets and stood above the basic level of the head, so Phacops probably had pretty close to a complete 360-degree range of vision. They likely lived in muddy sea floors, so it’s possible that their eyes evolved to serve in a sometimes murky environment.

Phacops rana grew to as much as 6 inches long, but a lot of specimens are rolled up like pill bugs. Enrolling was probably a defense mechanism. Phacops rana is pretty much Middle Devonian in age, dating to around 385 to 400 million years ago.

Fossils of Phacops rana are found in Devonian rocks of the US Midwest, in the northeastern states, and adjacent parts of Canada. It’s the state fossil of Pennsylvania.  If you recall that northwestern Africa was getting closer and closer to the northeastern United States during the Devonian, it may come as no surprise that Phacops rana is also abundant in Morocco. A word of warning however – if you are interested in collecting trilobite fossils, be aware that there are a lot of very well-done fakes coming out of Morocco. Not just Phacops, but lots of different trilobites including delicate spiny trilobites, mostly from the Devonian. They make casts in resin of one original and attach it to natural rock. If you don’t really care if it’s real, and want something cool and decorative, $10 or $20 is a reasonable price to pay. Just be wary before you lay out $200 or $500 or more for something that has been mass produced – and not by nature 400 million years ago, but by humans within the past decade or so. If you are planning to invest in collectable trilobites, be sure to check around for information about how to identify fakes. Some of them are really well done.

* * *

Today, May 18, is the anniversary of the eruption of Mt. St. Helens in Washington State in 1980. It was the first eruption of a volcano in the 48 United States since Lassen Peak in 1915 to 1917. 57 people were killed in Mt. St. Helens’ eruption, the deadliest volcanic eruption in U.S. history. Mt. St. Helens and the Cascade Range are part of the volcanic arc related to the subduction of part of the Pacific Oceanic Plate – the part called the Farallon Plate. It’s still heading down under North America, and the small remnants of the Farallon Plate beneath the ocean west of northern California, Oregon, and Washington, are called the Juan de Fuca and Gorda Plates.

—Richard I. Gibson

Photo by Didier Descouens under Creative Commons Attribution License.

May 17. Bakken formation

Today’s topic, the Bakken formation of North Dakota and Montana, is probably familiar to most listeners, and there’s a vast amount of easy-to-find information available, so this will just be a summary. I actually started the Wikipedia page for the Bakken, back in 2007 when it was just taking off – if you go into the history of the article, and go back to the start, you’ll see it was started by Geologyguy – that’s me. The announcement a couple weeks ago that the Bakken had produced its one billionth barrel of oil also indicated that most of that production had come since 2008.

First, let’s talk a little about terminology. The Bakken is not an oil shale. I talked about oil shale in Estonia on March 27. Oil shale is a solid rock, no liquid. It contains a lot of organic material in it, which can be cooked to covert the organic stuff to liquid oil. The confusing, but somewhat better term for the Bakken is shale oil – liquid oil that is tightly trapped within very fine grained shale. The Bakken is the oil version of the Marcellus Shale that we talked about a few days ago – the Marcellus has natural gas, and the Bakken has oil. In both cases, the hydrocarbons are trapped in tiny, tiny pore spaces that are poorly interconnected, if at all. Consequently, techniques like horizontal drilling and hydraulic fracturing, which I described for the Marcellus on May 11, are used to extract the oil or gas.

In the Bakken formation, the oil bearing horizons are about 9,000 feet down, a little short of two miles. So the well is drilled that far, pretty much straight down, then the drill bit is turned to nearly horizontal and navigated through the oil-rich part of the Bakken, only about 140 feet thick. The horizontal portion of a typical Bakken well may extend for two or three miles, 10,000 to 15,000 feet. The kinds of production that such wells yield range from around 100 barrels a day to more than 1000 barrels a day – obviously 1000 is better than 100, but 100 is pretty good considering the average production of all oil wells in the United States is around 10 barrels per day per well. These wells in the Bakken are also a lot more expensive than conventional oil wells, costing $2 million to $5 million or more, when a standard conventional well might cost $1 million or even less.

During much of the early Paleozoic Era, the Williston Basin in western North Dakota and eastern Montana was a deep depression, much like the Michigan Basin that we’ve talked about several times. The water was deeper, and because it was kind of like a deep bowl in the sea floor, the water was also restricted in terms of circulation, so it became anoxic, at least at times. That happened on multiple occasions during the Devonian.

The Bakken lies above, and is therefore younger than the Devonian Jefferson Formation that we talked about the other day, and it’s also younger than another formation called the Three Forks. Both the Three Forks and the Bakken contain black shales, similar to those of the Marcellus, that accumulated in the deep, quiet, anoxic waters of the Williston Basin. But the Bakken is multiple layers, including typically a lower black shale, a middle dolomite, and an upper black shale. The dolomite, calcium magnesium carbonate, probably represents a change to a shallow, more well-oxygenated environment.


A few minutes ago I talked about the Bakken shale being tight – low porosity and low permeability, or interconnectedness of the pores, and that’s true, but it’s not 100% tight. Some of the oil has been squeezed from the two black shales into the dolomite, and the dolomite is in fact the main oil reservoir for the Bakken, so even calling it shale oil is misleading. The shale has some oil in it, and it most definitely served as the source for the oil that’s in the dolomite. Even in the dolomite, the porosity is quite low – maybe 5% of the rock – and the permeability is also very low. And it’s not all uniform – there are definitely “sweet spots,” places where there’s significantly more oil in the reservoir than elsewhere. It’s not a case of drill anywhere.

How much oil is there in the Bakken? Lots of guesses, and some of them are actually intelligent guesses. But you should ignore the hype that says there’s a trillion barrels of oil there. Or, more accurately, you should dig a little deeper. There might be a trillion barrels of oil in place – but by no means can all that oil be produced, and certainly all of it cannot be produced economically. The U.S. Geological Survey and the North Dakota Department of Mineral Resources give pretty reasonable estimates of 150 to 400 billion barrels of oil in place.

How much is producible? That’s the more useful question, and the answer is a moving target based on changes in technology and increasing understanding of both the volumes present, how they are distributed geologically, and how the production declines over time. Again, take various estimates with a few grains of salt. If a company producing oil estimates that there might be 20 billion barrels to produce, maybe that’s correct, but remember that it would certainly be to the company’s advantage to have such a high value for its reserves.

The U.S. Geological Survey, a reliable if somewhat conservative organization, estimated about 3.6 billion barrels producible in 2008, and they’ve more than doubled that estimate now, to 7.5 billion. Proved reserves – that’s a more reliable number, based on actual drilling rather than projections – amount to about two billion barrels or so.

All this is very good. The Bakken has propelled North Dakota from about the #10 state in terms of oil production to #2, after Texas. And its production continues to grow. In November 2013, according to the journal World Oil, North Dakota was producing an average of 972,000 barrels per day. That’s out of about 8 million barrels a day for the entire US, so North Dakota is producing more than 12% of all U.S. oil. North Dakota, combined with significant production increases in the Gulf of Mexico, has dramatically increased U.S. total oil production, from less than 5 million barrels a day 5 or 6 years ago to 8 million today. That’s still well short of the U.S. peak of oil production, at more than 10 million barrels a day back in 1970. Will the surge continue, and bring the U.S. to a new peak? Time will tell. You can find many headlines that say U.S. oil production will exceed that of Saudi Arabia within a year, or a few years. That’s not impossible, of course – Saudi Arabia produces about 9½ million barrels per day, only 1½ million more than the U.S. today. Personally, though, I wouldn’t bet the farm on it. Your mileage may vary, of course.

 * * *

On May 17, 1776, Amos Eaton was born at Chatham, New York. He was a geologist and botanist, and he significantly influenced education in the United States through a philosophy of applying science to daily life. In 1824 he co-founded the school that became the Rensselaer Polytechnic Institute in Troy, New York. Among his students was Mary Mason Lyon, founder of Mount Holyoke College, and James Hall, first state geologist of New York.

—Richard I. Gibson

Cross section from USGS

Map from Energy Information Administration

May 16. Devonian sharks

Just a quick one today, to mention sharks, which were very much coming into their own during the Devonian.

Devonian fish. Shark at top.

Sharks are remarkably successful fish. Although there have been some changes, the basic plan of sharks has survived, apparently quite well, for more than 400 million years. Sharks are one of the major subdivisions of fish – the ones that have cartilaginous skeletons rather than bony skeletons. Cartilage is a lot more challenging to preserve in rocks than bones, so a lot of sharks are best known from fossils of their teeth. But there are still some body fossils that give us a glimpse into a strange and diverse group.

Stethacanthus, from the late Devonian, had a weird projection on its back – its dorsal fin – that looked like an ironing board or anvil. When you think about the modern hammerhead shark, maybe such an evolutionary adaptation isn’t so weird, but we don’t really know what it was for. Speculation focuses on mating displays.  

—Richard I. Gibson

Links:
Sharks of the Devonian 

Devonian shark teeth

Drawing by Joseph Smit, 1905, public domain

May 15. Devonian Gondwana

I gave a general description of the distribution of lands and seas during the Devonian back on May 7. There’s a map on the blog for that episode that probably is an easier way to grasp that than from my words. But I want to say a little more about Gondwana, the supercontinent. 

Laurentia or North America, and Baltica (Europe) and Siberia were all getting pretty close to becoming a second supercontinent, but Gondwana had been pretty much one big continent for tens of millions of years. But it wasn’t just sitting there.

The heart of Gondwana was today’s Africa and South America, which is called West Gondwana, plus Arabia, India, Antarctica, and Australia making up East Gondwana. Australia projected away from the continent as a wide peninsula. At the start of the Devonian, the south pole was somewhere in southern Brazil or perhaps in adjacent Namibia, but there doesn’t seem to have been much if any glaciation there, at least not until near the end of the Devonian. One problem is that continental environments, including glacial areas, are typically less well preserved in the rock record than marine environments, simply because the terrestrial environments usually cover smaller areas and receive fewer sediments. There are exceptions, of course, and both the Old Red Sandstone and the Catskill Delta are examples of terrestrial environments that are indeed well preserved in the record.

Gondwana was evidently pretty unstable, at least along its northern margin. That’s where the long microcontinent called Avalonia, including parts of what are now Britain and Newfoundland and Nova Scotia, rifted away. Other relatively small continental fragments appear to have been rifting away from what is now North Africa, and they continue to do so. They’ve become much of southern Europe, including Iberia and Italy.

It’s not at all clear what was happening with the margin of Gondwana that contained what are now Arabia, India, and Australia, but it seems that various blocks that are now in Central Asia, Tibet, and China, were probably breaking off of Gondwana in Devonian time and starting a relatively independent motion. A good modern analogy for this would be Madagascar today – it has rifted away from the east coast of Africa and is moving independently.

While all those pieces were rifting away from Gondwana on its northern and northeastern margin, the entire continent was rotating, pretty much in a clockwise direction. That meant that South America, at the other end of Gondwana from Australia, was moving to the north and northwest. Northern South America and the adjacent part of Africa – northwest Africa today, as well as what’s now Florida – were all approaching North America. I’ve said it before and I’ll say it again: Gondwana was coming. The ocean between North America and Gondwana was closing, and a big crunch was on its way.

—Richard I. Gibson

Reference:
Nice map of early Devonian Gondwana


Map above based on public domain map by Peter Bøckman

May 14. Chert

Earlier this month, we talked about the Rhynie Chert, in Scotland – a hot spring or geyser deposit that preserves a remarkable array of Devonian plants and animals. 

Chert, very fine grained silica, the same as the mineral quartz, is found in lots of places around the world. The Rhynie Chert is special because it was deposited by geysers. Chert forms in other ways, too. Often it’s found as thin discontinuous layers in sedimentary rocks, or as nodules that might range from a few millimeters to maybe as much as a meter, but most nodules are smaller.

Devonian chert (dark bands) from Pennsylvania (photo by Jstuby, public domain)

Since silica, in the form of quartz, is so common in the earth’s crust, it shouldn’t be too much of a surprise that silica is a common material that’s dissolved in water. When that water percolates through rocks or sediments, the silica can precipitate – sometimes into an open space, say one that’s been dissolved in limestone, or sometimes it might precipitate on something that serves as a nucleus, like a fossil. All of this can be part of the process called diagenesis, the change from loose sediment to solid rock, or it might happen later, with the chert precipitating in pore spaces in the solid rock.

Some layers of chert probably develop from concentrations of silica in the sedimentary environment. For example, sponge spicules, which we talked about back in February, are mostly silica. If you had a whole lot of sponges living in an area, when they die their siliceous spicules might accumulate enough to actually make a sediment that is mostly silica. That could lithify into a discontinuous bed of chert.

Likewise, microscopic animals called radiolarians make siliceous shells, and enough of them could also turn into a layer of silica-rich sediment that might become chert.

As with the Rhynie Chert, most chert is extremely fine grained, and often replaces things on a molecular scale. The rich Devonian fossil beds at the Falls of the Ohio, which we talked about on May 5, are mostly in limestones, but there are also interbedded chert layers that contain a wealth of microscopic plant and animal fossils.

There are also thick bedded cherts, especially in the Permian of the western United States. We’ll talk about them in August.

—Richard I. Gibson

Devonian chert (dark bands) from Pennsylvania, Public domain photo by Jstuby via Wikipedia. 

May 13. Jefferson formation

After hearing about all the organic material in the Marcellus Shale, you may start to think that there was a lot of life in the Devonian, dying in the oxygen poor water and falling apart and getting trapped in sediment to become oil and gas millions of years later. On the whole, you’d be right. The Devonian – mostly middle to late Devonian – is one of the six primary oil source systems on earth, and by far the largest one in the Paleozoic era. It depends on who’s making the estimate, but Devonian source rocks worldwide may contain as much as 60 billion barrels. For comparison, Cretaceous source rocks might hold as much as 500 billion barrels. 

When I called the depositional environment of the Marcellus Shale a stagnant sea, I more or less mean that literally. Organics in the rock generate hydrogen sulfide gas – the rotten egg smell associated with a lot of oil and gas.

Out here in Montana, you may recall that the Devonian rocks rest directly upon the Cambrian rocks, and all the Ordovician and Silurian rocks have been eroded away. If there were any early and middle Devonian rocks, they were eroded away too – the oldest Devonian, generally here in western Montana, is Late Devonian in age. The most prominent rock unit is called the Jefferson Formation, named in 1893 for exposures along the Jefferson River, near the three forks of the Missouri in southwestern Montana. It consists of massive dolomite – calcium magnesium carbonate, like limestone except for the magnesium in the mineral’s crystal structure. It’s anywhere from 200 to 800 feet thick, and there are some interesting things in it that make it a geology student trying to map the rocks happy to see it.

For one thing, there's a really prominent member at the top, called the Birdbear. It's usually pretty distinctive. Then there are conglomerates in places, rocks with really large grains, as much as several inches long. But these pebbles are made of the same kind of rock, dolomite, as the rest of the formation. And they are long, but flat – maybe a half-inch thick but 4 or 5 inches long. These things are called intraformational conglomerate, which means that the pebbles came out of the formation itself, rather than being washed in from some eroding land source. They’re also called flat-pebble conglomerates, for the shape of the pebbles. They probably originated when storms ripped up bits of the sea floor that had started to lithify from sediment into solid rock. The stuff the deep storm waves tore up was a lot more solid than loose sediment, but not as solid as hard rock. So the waves tore flat slabs of semi-solid carbonate off the sea floor, and when things calmed down, those chunks fell back into the sediment and were incorporated into it. It can make for a distinctive, and rather pretty rock. But there are intraformational conglomerates in lots of carbonates, so that alone doesn’t tell the geology student that he or she is in the Jefferson formation.

The Jefferson formation is often black. That may seem surprising for a limestone or dolomite, but it’s actually fairly common for the impurities in limestone to make it gray, and more and more impurities – especially if those impurities are organic matter – can indeed make it black.

In the deep Williston Basin of eastern Montana and western North Dakota, the Jefferson formation is an important reservoir for oil, and even here in western Montana if you whack a piece of the rock with a hammer, you often get a distinct rotten egg smell – hydrogen sulfide, the decay product of organic material. That’s a pretty good clue that you’ve found the Jefferson, but it doesn’t ALWAYS have that smell.

If the Jefferson started out as a limestone, and mineral rich waters percolated through and converted it to dolomite – and that’s a reasonable possible way for dolomite to form – you might expect it to have fossils. And it does. You can find little horn corals, and in some places there are these little white things that look a lot like short strands of spaghetti. Both the corals and the spaghetti-looking stuff are silicified – turned to quartz, which stands out in marked contrast to the black dolomite. The spaghetti stuff is probably the remnants of algal mats – the things that made stromatolites back in the Precambrian, some of the earliest large evidence for life on earth. Those mats might have gotten torn up in the same storms that made the flat-pebble conglomerates, and they probably became silicified because they preferentially were replaced by silica from the water, just as petrified wood becomes silicified but the surrounding rock does not, at least not necessarily.

One of my professors at Indiana University’s geologic field station here in Montana, Lee Suttner, called the “spaghetti rock” one of your few geologic “friends” – if you found it in a dolomite, you were definitely in the Jefferson. If the dolomite was black, and smelled like rotten eggs, and had flat-pebble conglomerate, and was between the Cambrian rocks and what you expected to be above it, well then, you really had it nailed. Put it on the map with great confidence.

—Richard I. Gibson

Resources:
USGS – Jefferson formation
Flat-pebble conglomerate photo 

May 12. Catskill delta

Toward the end of the Ordovician, last March, we talked about the beginning of tectonic collisions in what is now eastern North America – the Taconic Orogeny. The rivers that eroded those mountains shed a vast pile of sediment into what is now Pennsylvania, West Virginia, and Ohio and beyond, creating the Queenston Delta. As collisions continued, mountainous terrains continued to be uplifted and erosion continued. In Middle to Late Devonian time, ongoing collision between parts of the Avalonian extension of Europe with northeastern North America created more uplifts. This is sometimes called the Acadian Orogeny, which we mentioned last month during the Silurian. It pretty much reached its peak during the Devonian. 

The upland was located in southeastern Pennsylvania and northern New Jersey and points further north and south. The sediment that was eroded off the mountains was dumped in especially thick piles in central Pennsylvania and southern New York. River floodplains and deltas lay along a shallow sea. The whole complex is called the Catskill Delta, because the sediments are now exposed as rocks in the present-day Catskill Mountains, as well as the Pocono Mountains of Pennsylvania.

This was a long-lived environment, spanning probably 20 million years about 380 to 360 million years ago. The rocks are similar to those of the Ordovician Queenston Delta – red beds indicating oxidation of iron, river sandstones, flood plain shales, and ephemeral lake deposits. There’s 3,000 feet and more of these rocks. They were deposited on top of the older Marcellus Shale that we talked about yesterday, so that means there was a pretty noteworthy change in the environment, from the oxygen-poor sea waters of the Marcellus to the above-sea-level terrestrial environment of the Catskill Delta.

Catskill delta rocks are to the east (right). Cross-section from USGS.

Because the sea level was fluctuating, there are also some layers of marine rocks interbedded with the deltaic sediments, just as you would find in the Mississippi Delta today.

The sandstones that formed from the distant limits of the Catskill Delta serve as oil reservoirs in western Pennsylvania – the first oil deposits that were exploited in the United States beginning in the 1850s but really taking off in the 1870s.

 Because they are Devonian in age, and many of the rocks are red with oxidized iron, American geologists in the 19th century initially thought the rocks of the Catskill Delta were part of the Old Red Sandstone of Europe. But they’re not, really. The Catskill sediments were eroded off a rising mountain uplift, and deposited in a coastal flood plain. The Old Red Sandstone was deposited within intermontane basins, pretty much entirely terrestrial in origin, except in a few places.

* * *

Maurice Ewing was born today, May 12, 1906, in Lockney, Texas. He was a pioneer in the field of oceanography, especially the use of seismic studies to understand the nature of the ocean basins. Much of his work was fundamental knowledge that led to the comprehensive theory of plate tectonics.

—Richard I. Gibson

References
Catskills Geology 
Catskills
Appalachian maps and cross sections
Cross-section from USGS

May 11. Marcellus Shale and natural gas

In the early part of the Middle Devonian, between about 385 and 390 million years ago, what’s now western New York and Pennsylvania, much of West Virginia and some of Ohio were under water. There was a relatively deep and relatively restricted sea out there, where generally only the finest grains of sediment could reach –mud and clay particles much smaller than a grain of sand, together with fragments of organic material that might get carried that far from the land that lay to the east.

Click to enlarge

Because the sea was constrained between rising land on the east and a low region above sea level, in the Cincinnati Arch area to the west, it was a relatively stagnant sea. It must have been something like the Black Sea today, though somewhat smaller. The fine mud and organic material accumulated to form a black shale called the Marcellus Shale, for its outcrops near the town of Marcellus, New York – but it and similar rocks extend over a region from New York into parts of Kentucky and Tennessee on the south and into Ontario on the north. The map shows the extent of the Marcellus Shale.  

It’s possible that the anoxia, the stagnant, low-oxygen sea, was related to a global event and not just to a restriction here in this part of the world. Evidence for a period of anoxia has been found at many places in Europe, as well as in Morocco, China, and Australia.

The organic material in the Marcellus Shale ranges from 1% to 11% of the rock. Anything more than 1 or 2% is an incredibly rich hydrocarbon source rock. Some of the best source rocks in the world are around 5% organic content. So the Marcellus should have generated lots of oil and gas that migrated into appropriate reservoirs somewhere to make great resources, right?

Not right. The problem is the fine-grained shale has only tiny pore spaces to hold oil or gas, and the spaces, such as they are, are very poorly interconnected – we say that the rock has very low permeability. But the oil or gas is still there, stuck in the shale.

If you drill a well vertically through a rock like this, the well bore only touches the rock where it goes through it, maybe a few hundred feet at best. That’s not much exposure to get at those tiny pores with oil or gas in them, and there’s no way to suck the oil or gas from further away, since there are no connections – no permeability.

Two relatively modern hydrocarbon exploration technologies have turned the Marcellus into a huge target for natural gas. First, there’s horizontal drilling. The drill goes straight down, maybe several thousand feet until it hits the target in the Marcellus, then the drill string is steered to turn it 90 degrees, to near horizontal. Then you can navigate through the formation, along its length, so to speak – and go through thousands of feet of the good stuff rather than just punching a hole through it. You’ve exposed the well to a lot more gas-bearing rock.

But you still can’t get at anything away from the hole. Enter hydraulic fracturing. If there are no connections to suck the gas through, make some by breaking the rock. There are variations, but basically it’s a system of forcing water containing chemicals and sand into the rock, creating fractures and propping them open so the natural gas can flow into the well.

This is, as you probably know, a very big deal. The need for natural gas in the populous states of the northeastern United States is great. But there are environmental concerns associated with hydraulic fracturing – fracking in the vernacular usage. There may be introduction of unwanted chemicals, and the potential does exist for polluting drinking water aquifers, although most of the fracking is thousands of feet below any such aquifers. Fracking has been used for more than 60 years with minimal impact. I’m definitely not saying it’s 100% safe – nothing is, and there is evidence that some problems can be related to it. Even some possible generation of earthquakes. As with all resource extraction, these things have to be evaluated carefully – is the need and desire for cheap natural gas worth whatever the trade-off may be, in terms of the environment? I’m sure many residents of Pennsylvania might say no, it’s not.

This is an ongoing controversy, and the concerns are not limited to environmental issues. It’s also not at all clear how sustainable these wells may be. Calculating the impact, and the trade-off, depends on a certain life time for natural gas production – but if it’s much less than projected, the decision might be different. There are many things in play, and if you expect me to come down solidly for or against fracking, sorry, I’m not going to do that. There are good arguments on both sides, and it’s not a simple choice. The Marcellus Shale is today the largest aggregate producer of natural gas in the U.S. and still growing. Almost all this production has been established since 2008, and given our incredibly consumptive lifestyles, that’s not something to discard easily.

—Richard I. Gibson

Resources:
U.S. Geological Survey Open-File Report 2005-1268
Fracking debate