These daily podcasts build upon previous episodes, so the best way to work through them is by starting with the oldest, January 1. But you don't have to do that.
Most episodes are two to 10 minutes long. It's December, so we're moving through the Cenozoic Era.

Saturday, December 20, 2014

December 20. Messinian salinity crisis & Ogallala aquifer

We’re getting really pretty close to the present, geologically speaking, and most of the rest of the episodes will focus on the most recent 10 million years of earth history. If our calendar was at a proper scale, these last 10 million years would have to be squeezed into the last 18 hours of December 31. We’ll spread it out some. 

Today I have two different water-related topics to discuss, both from near the end of the Miocene epoch, about 5 or 6 million years ago. The Messinian age is the last subdivision of the Miocene Epoch, from 7.2 to 5.3 million years ago. Near the end of this time, the Mediterranean Sea dried up. 

Mediterranean Sea bathymetry map from NASA (public domain) annotated by Gibson. Heavy black lines outline deepest parts which might have remained lakes or brackish areas when the rest of the Mediterranean was dry land.
It was actually multiple events in which the sea evaporated, leaving at most some large lakes in the deepest portions of the ocean basin. It was mostly in the latest part of the Messinian age, about 5.9 to 5.3 million years ago. The Strait of Gibraltar, the Mediterranean’s only connection to the Atlantic, closed as a result of ongoing tectonic activity there, essentially part of the Alpine Orogeny that reflects the complex interactions between North Africa and the complex bits of southern Europe.

With the Mediterranean cut off from oceanic circulation, evaporation could have become significant. The climate generally was not especially hot – in fact the planet was approaching the glacial period. But it’s been suggested that the evaporation took the lead because precipitation decreased, mostly due to orbital cycles that affected rainfall. The argument is a little bit circular, as it calls on reduced solar energy as a cause of lower evaporation in the Atlantic, and therefore less rainfall in Europe, and therefore more evaporation in the Mediterranean. There is no clear consensus on the role of climate changes in this event.

But it definitely did happen. Vast deposits of salt and gypsum dating to the Messinian age – named for Messina, Sicily, where such deposits are found – show it clearly. Drilling in the modern sea has also revealed evaporites in the deep-sea sediments as well. In places, the salts are interbedded with typical marine sediments including foraminiferal oozes, indicating that there were repeated times of drying and return of oceanic conditions.

A completely different line of evidence for the Mediterranean being dry at this time is submarine canyons, which must have been cut by rivers flowing across dry land, but which are now far below sea level. At the end of the Messinian, the end of the Miocene Epoch, the Mediterranean basin filled for the last time. The dam at Gibraltar was breached. There is debate as to whether the refilling was a gentle process or catastrophic – it might have taken place over many hundreds or thousands of years, although the image of a gigantic waterfall half a mile high with the water of 1,000 Amazon Rivers gets your attention. The only evidence regarding the nature of the refilling is in structures cut into the sediments around Gibraltar, beneath younger rocks – and the information there has been interpreted both ways.

But fast or slow, the Mediterranean did refill.

Saturated thickness of Ogallala aquifer, by Kbh3rd,
used under Creative Commons license
The second water topic from 6 million years ago that I have for today is the largest ground-water aquifer in the United States. The Ogallala Aquifer underlies much of the high plains, from Wyoming and South Dakota to West Texas and New Mexico. It contains about 30% of the groundwater in the U.S., and supplies water for irrigation, industrial uses, and drinking water.

Deposition of the rocks of the Ogallala Formation, which contain the aquifer, began in very late Miocene time, about 6 million years ago, and continued through the Pliocene, until about 2 million years ago when gentle uplift changed the setting from more depositional to more erosional again. The sediments that became the Ogallala were largely the sediments that were being stripped off the Rocky Mountains to the west, in the exhumation phase of their development, which we talked about December 14.

The entire package of Ogallala Formation rocks is up to 1,500 feet or so in thickness, and the aquifer itself, in nice porous sands, has a saturated thickness as much as 1,000 feet in places, but it’s mostly 50 to 400 feet of saturation. Most of the Ogallala formation was deposited by rivers, similar to the rivers of the High Plains today, but some parts of it are wind-blown silts and sands as well.

I was involved in a study of part of the Ogallala in the Texas Panhandle about 20 years ago, for an environmental project. The unit is really highly varied, and contains things like impermeable clay beds that can produce little perched aquifers, like bowls of water sitting above the main unconfined aquifer further down.

Given its importance to the region, it’s no surprise that a lot of attention is being paid to the drawdown caused by intense use of the groundwater in the Ogallala. In places, the drawdown is more than 300 feet – definitely a problem, because recharge to the aquifer, replenishing the water that is removed, comes almost entirely from precipitation, and this region is pretty much arid in terms of climate.
—Richard I. Gibson

Image sources:
Saturated thickness of Ogallala aquifer, by Kbh3rd, used under Creative Commons license.

Mediterranean Sea bathymetry map from NASA (public domain) annotated by Gibson.

Friday, December 19, 2014

December 19. A Hotspot Breaks Out

Columbia River Basalts (yellow) - see below for source.
During the Miocene epoch of the Cenozoic, about 16 or 17 million years ago, the Pacific Northwest of the United States was a busy place. 

A hotspot, a relatively small location where heat is focused upward from deep in the earth’s mantle, either reached shallow depths, or North America in its movement westward encountered one. At about what is now the common corner of Oregon, Idaho, and Nevada, the hotspot’s heat brought out lava – lots and lots of lava. The Columbia River Flood Basalts are comparable to those in Siberia, and the Deccan in India, and the Parana Basalts of South America. Over about two or three million years, 17 to 14 million years ago, something like 40,000 cubic miles of basalt was erupted, mostly in what is now Washington and Oregon. There are at least 300 individual flows stacked upon each other. In area and volume, the Columbia River basalts are tiny compared to the Siberian flows, and about one-third the size of the Deccan, but still pretty large, and they are among the youngest of these flood basalts. 

It looks like there was a north-northwest trending zone of weakness that extended away from the center of the hotspot – or maybe the hotspot was asymmetrical, or bigger than usual – so that the cracks through which the lavas came were focused to the northwest in Oregon and Washington. Another big crack extended to the south-southeast, through northern Nevada, producing the Northern Nevada Rift, a narrow zone of igneous rocks of Miocene age. Flood basalts didn’t flow there, though, perhaps because the region was a little stronger, a little more a part of the North American craton than the country in Washington and Oregon.

Hotspot origin of various features (see below for source)
At the site of the hotspot itself, that corner where Oregon, Idaho, and Nevada come together, huge explosive volcanism took place. While the flood basalts came out relatively quiescently, like the flows in Hawaii today, the center was a scene of violent activity. A caldera developed. This is a huge collapse feature that forms when a magma chamber erupts much of its lava, leaving a void behind. That empty space may collapse, with the surface rocks falling down into the old magma chamber. The first caldera related to this hotspot, near that corner of Oregon, Idaho, and Nevada, is about 35 miles across. As North America continued to move southwestward, the position of the hotspot was progressively further and further to the northeast. Today, it is under Yellowstone National Park – the Yellowstone Hotspot. 

The trace of North America’s movement over the hotspot is clearly defined by a series of calderas that get younger and younger as you go from the southwest corner of Idaho to Yellowstone. They are in the Snake River Plain of southern Idaho, which is covered by basaltic and other volcanics associated with the various calderas.

Ages of Yellowstone Hotspot Calderas (illustration by Kelvin Case at English Wikipedia, used under Creative Commons license)

Our other discussions of extensive volcanic events have often found some correlation between the volcanism and extinctions. Was there one with this one? About 14.5 million years ago, about 2 million years after the flood basalts started and while they were still in progress, there was a marked global cooling event that coincided with a major growth spurt in the Antarctic Ice Sheet. And it does correlate with an increase in extinction rates, though I don’t think we’d call it any kind of mass extinction like the great ones in earth’s history. This may have been mostly a result of the change from what’s called the Miocene climatic optimum, a warm period 17 to 15 million years ago, and part of a more general change to cooler conditions that eventually led to the ice ages. It’s not obvious that the Columbia River Basalts played a major role in this minor extinction event at 14 million years ago.

We’ll talk a bit more about Yellowstone in a few days when we talk about supervolcanoes. There is of course a vast amount of information about the Yellowstone Hotspot and the Columbia River Basalts. One of the best resources in my opinion is a book by Robert Smith and Lee Siegel, titled Windows into the Earth – the geologic story of Yellowstone and Grand Teton National Parks (Oxford University Press, 2000).
—Richard I. Gibson

Links and image sources:
Miocene climate

Hotspot breakout model and Columbia River basalt map both from Camp, V.E. and Ross, M.E., 2004, Mantle dynamics and genesis of mafic magmatism in the intermontane Pacific Northwest: Journal of Geophysical Research, v. 109.  doi:10.1029/2003JB002838, used under Creative Commons license 

Hotspot track illustration by Kelvin Case at English Wikipedia, used under Creative Commons license  

Thursday, December 18, 2014

December 18. Oil at Baku

The Caucasus Mountains, between the Black and Caspian Seas, hold one of the most important and early-produced oil provinces in the world. This area is part of the Alpine-Himalaya collision between pieces of Gondwana and the southern margin of Eurasia. Specifically, it’s the northern prong of Arabia that’s squeezing a small bit of continent, more or less part of the main Iran block, which itself was part of the Cimmeride continent, all that is being pushed into the south side of Eurasia.

Geographically, the Caucasus is taken as the boundary between Europe and Asia, and it contains some high mountain peaks, including Mt. Elbrus, a dormant volcano that reaches more than 5,600 meters above sea level, more than 18,500 feet. It last erupted about 2,000 years ago, showing that this area is still tectonically active.

Photo: Baku oil wells, Asbrink Collection.
One of the effects of the ongoing Alpine-Himalayan collisions was the development of fold belts along and within the Caucasus Mountains complex. Rocks of Miocene age were pushed into large asymmetrical folds, anticlines and synclines with strata arched upward and downward, respectively. This shows certainly that the tectonic action was going on after the Miocene rocks were laid down, since they are involved in the folding. This isn’t a surprise, since we know the collision is still going on today. The early Miocene rocks were probably folded in Miocene time, 5 to 20 million years ago, and in the Pliocene, 2 to 5 million years ago.

These anticlines trap lots and lots of oil. Oil was known in the area around Baku from the time of Marco Polo, and was supposedly used by locals for lubricants and fuel in the time of Alexander the Great. Baku oil was produced in quantity from hand-dug wells in the 1830s, and the world’s first paraffin factory began there in 1823. The first mechanically-drilled well in the world was drilled at Baku in 1846, 13 years before America’s first oil well in Pennsylvania, in 1859. By the 1870s, oil demand was surging worldwide, and outside investors came in to develop the oil fields around Baku. Two of the many fortunes that came from Baku oil were those of the Nobels, of Nobel Prize fame, and the Rothschilds. In 1900, half the world’s oil was coming from Baku, much of it from rocks of Miocene and Pliocene age.

Further west along the northern front of the Caucasus Range, additional fields were discovered. Grozny, in Chechnya, became Russia’s #2 source of oil until after the Revolution in 1917, and the Grozny area still produced about 7% of the Soviet Union’s oil as late as 1971. The Grozny field is in an anticline in Miocene rocks, with multiple sandstone reservoirs with impermeable shale seals. The Caucasus oil was a major target of Hitler’s forces in World War II, and it still plays a significant role in the geopolitics of the region.

Pliocene deltas (that form oil reservoirs)
coming into the South Caspian Basin.
From USGS Bulletin 2201-I
It’s no surprise that this oil was found so early, because it is practically at the surface in many cases, or just a few feet beneath the surface in the relatively young Miocene and Pliocene rocks. Marco Polo reportedly saw a natural gusher of oil. The organic rich source rocks are largely of Miocene age, called the Maykop Suite. There was a restricted seaway extending through this region, on the north side of the approaching continental blocks before they collided to raise up the Caucasus, and the marine carbonates of the Maykop Suite were deposited there. By Pliocene time, just four or five million years ago, the region became isolated from the sea, and rivers brought sandy sediment into the basin. Some of the most productive reservoirs around Baku are from Pliocene rocks deposited in deltas around the margins of the South Caspian Basin, which is an entrapped bit of old Tethys Ocean floor. The ongoing tectonic activity has created plenty of traps for the oil. 

Azerbaijan, where Baku is located, still produces about 900,000 barrels of oil per day, about 10% of what the US produces. But it’s only about the size of the state of Maine.

—Richard I. Gibson

Cenozoic oil – Azerbaijan 
Photo: Asbrink Collection.

Pliocene deltas (that form oil reservoirs) coming into the South Caspian Basin. From USGS Bulletin 2201-I, by Linda Smith-Rouch, 2006.