The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages (link in index at right), and a few new episodes were posted from 2015-18. Beginning in May 2019, I'm adding short entries to the blog (not as podcast episodes, at least not for now, sorry!) mostly taken from the Facebook Page posts. Thanks for your interest!

Monday, June 30, 2014

June 30. The end of the Mississippian

As we discussed at the start of the Mississippian, it’s only in the United States that Mississippian and Pennsylvanian are treated as full periods of geologic time. In the rest of the world, the two together are called the Carboniferous.  

In the U.S. the distinction is made as much as anything because the two periods contain distinct kinds of rocks. The Mississippian is dominated by the shallow-water limestones we heard so much about this month, while the Pennsylvanian contains the coal that gives the Carboniferous its name. It’s true that the early Carboniferous in much of the rest of the world contains less-abundant coal than the later part of the period, the part that correlates with the Pennsylvanian in the United States, but it’s still treated as a single period worldwide.

I think the different terminology has to do with the history of usage as much as anything; in 1906, geologist T.C. Chamberlain pointed out that the rocks of Mississippian and Pennsylvanian age are not only significantly different in rock type but that there is a break – an unconformity – between them, so the two were recognized as periods in the U.S. That break more or less represents the onset of the Appalachian and Ouachita Orogenies which began to create significant uplifts in both the east and south, and to some extent the west of the North American Continent. The Variscan Orogeny was doing the same thing in southern and central Europe, but nonetheless, European geologists never embraced the idea of breaking the Carboniferous into two distinct periods. So, officially in terms of worldwide use, the Mississippian and Pennsylvanian are the equivalents of the Early Carboniferous and Late Carboniferous. Or you can call them sub-periods, or if you are in the United States, call them periods of geologic time. It really doesn’t matter as long as we can communicate!

So tomorrow we’ll enter the Pennsylvanian.

* * *

Today, June 30, in 1908, an object hit the earth near the Stony Tunguska River in Siberia. Something like 2000 square kilometers – 800 square miles – of forest were flattened. It could hardly have been in a more remote area, and consequently there were no known human fatalities. The explosion was almost certainly the result of the breakup in the atmosphere of a comet fragment or asteroid or meteor, although there have been any number of speculations as to the origin, ranging from black holes to alien space ships. The air burst probably occurred 5 to 10 kilometers above the surface, and the object was probably between 60 and 200 meters across, or about 600 feet at most. But it still represents the largest impact event on Earth in recorded history.

—Richard I. Gibson

Sunday, June 29, 2014

June 29. Mississippian fish


Rhizodonts were fish that grew to as much as 7 meters long, around 22 feet, the largest freshwater fish known. They got their start during the Devonian, but really proliferated during the Mississippian, only to go extinct by the end of the Pennsylvanian. 

The rhizodonts were a fish group whose limbs – fins – showed close affinities to the tetrapods, the primitive amphibians that were invading the land during the Mississippian. The fins, especially the front fins, were strong, with strong supporting tissues, and the rays of the fins were very much like fingers.

They had highly maneuverable jaws, and they were the greatest predators in the Mississippian lakes and rivers where they could have attacked things like lungfish and early amphibians. Some species had large tusks, and the name of the group, rhizodont, means “root-tooth” because their long teeth, some as long as 15 centimeters, or 6 inches, had distinctive root systems, another trait that connects them to the tetrapods and later animals.

Study of rhizodonts is an active area of paleontology today because of their relatively close relationship to the tetrapods, the ancestors of amphibians, reptiles, birds, and mammals. The fossil record of tetrapods, which began during the Devonian, still contains many gaps even millions of years later in the Mississippian.
—Richard I. Gibson

Image from Palaeos.

Reference: Gaining Ground: The Origin and Evolution of Tetrapods, by Jennifer A. Clack, Indiana University Press, 2012, p. 75-78.

Saturday, June 28, 2014

June 28. Corals

Lithostrotion basaltiforme
The extinction at the end of the Devonian decimated reef-building organisms such as corals. Extinction events typically see a rebound of life following the relatively brief period of extinction, and during the Mississippian corals did begin to make a comeback. We talked about the relatively small carbonate build-ups called Waulsortian Mounds on June 11. Those piles of mud may have been stabilized by the skeletons of animals such as crinoids and corals, but they were nothing like the huge edifices constructed by animals during the
Devonian, and the even larger ones to come in the Permian and even today.  

Corals were still abundant in those warm Mississippian seas, and both colonial and solitary types were common. Tabulate corals were colonial, tall columns growing together with horizontal segments, tabulae, providing the platform on which the individual zooids lived. One species, named Lithostrotion basaltiforme, has a remarkably similar appearance to the polygonal columns of basalt that form as a result of cooling – though the corals are much smaller.

There are biologic features in the Mississippian that can truly be called reefs, although purists might call them bioherms, meaning a relatively tall structure that might be a pile of material or a construction like a true reef, or a combination of both. The words “reef” and “bioherm” are sometimes used almost interchangeably, but to me a reef means a relatively large structure that is mostly composed of the actual skeletons of the animals that lived there, rather than a pile of debris. I’m not a specialist in this area, so take that as just my opinion about the terminology.
—Richard I. Gibson

Drawing from an old text (public domain)

Friday, June 27, 2014

June 27. Gastropods

Bellerophon gibsoni, Mississippian gastropod
We haven’t said much about gastropods, snails, in our journey. They have a long history, extending back at least to the Cambrian, and while they have a wide variety of shell forms, by Mississippian time many of the shapes that survive to the present day had been established.  

As with other marine invertebrates, gastropods thrived in the warm, shallow Mississippian seas. Some are known that were three inches or more across, relatively large for Mississippian gastropods – but today’s conchs are also gastropods and some of their shells grow to a foot long or more.

Gastropods are a class of the phylum Mollusca, which includes cephalopods – octopuses and squids – and bivalves including clams and scallops. Gastropods comprise about 80% of all mollusk species, and there are more than 60,000 known species today. More than 15,000 species are extinct and are known only from the fossil record.

During the Mississippian most or all gastropods were aquatic. Some of the earliest land-dwelling snails are known from the coal measures of Europe, the late Carboniferous or Pennsylvanian Period, which we will get to next month. Even then, land snails were rare for many more millions of years.
—Richard I. Gibson

Drawing of Mississippian gastropod from an old textbook (public domain)

Thursday, June 26, 2014

June 26. Leadville

Ore zones in black
Lead was discovered at Leadville, Colorado, in 1874, and has continued sporadically over time more or less to the present. The host rock for the ore is the Mississippian Leadville Limestone, which is one of the formations equivalent to the Madison Limestone that covered so much of North America during early to middle Mississippian time.  

As usual, the prospectors in the Leadville area were seeking gold, and the exploitation of lead was a later development.

The ore is related to igneous intrusions that forced their way into the limestone, forming sills, which are igneous bodies that are parallel, or concordant, with bedding planes in sedimentary rocks. The molten material came in along the weak zones that bedding planes created. The lead was most likely deposited by hot waters associated with the magmas. The water dissolved parts of the limestone, and the ore minerals, such as galena, lead sulfide, were deposited in the fissures and cavities that had been dissolved. The magmas are of Tertiary age, so the ores were deposited around 300 million years after the limestone formed during the Mississippian.

Zinc and silver are often associated with lead deposits, and the total cumulative production at Leadville, Colorado, includes more than 240 million ounces of silver, making Leadville more famous for silver than for lead. The district also produced more than 700 million tons of zinc, and almost a billion tons of lead.

The Leadville Limestone in Utah was eroded and dissolved by a period of karst formation, and the resulting porosity makes the Leadville a target for oil and gas exploration in Utah.
—Richard I. Gibson

Leadville history 

Cross section from Argall, in Ries (1925). Public domain.

Wednesday, June 25, 2014

June 25. Glaciers in Australia and South America

After the end of the glacial period at about the end of Devonian time, most of the Mississippian was ice-free around the world until near the end of the period. There is evidence for episodic glaciation in Australia and South America at this time.  

It’s possible that the Mississippian glaciers were predecessors to the well-defined Permian glaciation, although that ice age was still at least 25 million years in the future as the Mississippian ended. The onset of glacial episodes would have contributed to sea-level changes that in turn would be contributing factors in the increase in clastic sediments in Late Mississippian time. The shallow seas so common earlier in the period would have become reduced in size as sea levels fell.

One possible contributing factor in the beginning of the glacial period is the tectonic closing of the Tethys Sea and the ocean between Gondwana and North America. Disrupting what was a major equatorial ocean circulation pattern could have impacted global distribution of heat in the oceans and therefore in the atmosphere, and could have helped continental ice sheets to form and grow.
—Richard I. Gibson

Technical paper

Tuesday, June 24, 2014

June 24. Cave passages

We talked about Mammoth Cave the other day, but here’s a little more about its development. Cave passages tend to be linear, in part because moving water preferentially dissolves limestones along lines of weakness, such as linear joints or fractures and along bedding planes.  

Sometimes in places where joints and fractures are regularly spaced and relatively rectilinear, forming rectangles in map view, a cave system might approximate that pattern too. But water isn’t usually strictly limited to the joints, and collapsing caves can create new patterns as well. The map above of Mammoth Cave was compiled in 1897. There’s a certain degree of regularity, and several directions such as northeast-southwest that seem to predominate, but there’s actually quite a wide variety in the development of the cave passages dissolved in Mississippian limestones. Regular patterns do happen, but natural systems tend to be complex enough that regularity is usually the exception rather than the rule.

* * *

Today’s birthday is David Dale Owen, born June 24, 1807, near New Lanark, Scotland. He was the son of Robert Owen, a reformer who established a social experiment at New Harmony, Indiana, in 1825. It was an attempt to establish a utopian society. It was an economic failure but New Harmony became a cultural and educational center, noted for scientific research. David Dale became a prominent geologist, doing some of the first work in the Midwestern United States. He was the State Geologist of Indiana, Kentucky, and Arkansas at various times between 1837 and his death in 1860.
—Richard I. Gibson

Monday, June 23, 2014

June 23. Appalachian in Iowa

The effects of continent-continent collisions can be felt far into the continents that are involved. Today’s collision between India and Eurasia, which has been going on for 30 or 40 million years, has helped to produce extension 2500 miles, 3700 kilometers away from the collision, at Lake Baikal in Siberia. 

The Appalachian-Ouachita Orogeny had impacts as far away as Iowa quite early in the collision. Early Mississippian marine carbonate rocks were tilted and folded and uplifted and eroded before Pennsylvanian coal-bearing strata were deposited. There’s a pretty good unconformity between those sets of rocks.
—Richard I. Gibson

Reference: Coal Deposits of Iowa, by C.R. Keyes, Iowa Geological Survey, 1894.

Sunday, June 22, 2014

June 22. Late Mississippian Lands and Seas

By late Mississippian time, around 325 million years ago, the extensive warm shallow seas that covered much of North America were becoming restricted. The sedimentary rocks were less often the limestones we heard so much about for most of this month, and more the shales we discussed on June 17. More clastic sediment reflects the onset of uplift of lands around the marine areas.

In North America, in both the east and south-central part of the continent, collision with the northwestern margin of Gondwana was beginning. In the east, this was largely along the same zone where previous collisions, from the Taconic Orogeny at the end of the Ordovician to the Caledonian and Acadian Orogenies in the Silurian and Devonian had produced episodic ongoing uplifts. This is the beginning of the Appalachian or Alleghenian Orogeny, the main continent-continent collision between North America and Gondwana.

Another aspect of that collision was beginning in what is now Oklahoma, Texas, and Arkansas, where it is called the Ouachita Orogeny. The uplifts there led to the deposition of the Barnett Shale in the Fort Worth Basin, which today is a significant source of natural gas production.

The Tethys Ocean between Eurasia and Gondwana was also closing, and aspects of that collision during the latter part of the early Carboniferous, the equivalent of the Mississippian Period, are the Variscan Orogeny in Europe. This was definitely not a simple collision, as there were many isolated microcontinental pieces, oceanic crust, and island arcs involved, so parts of the Tethys remained open while other parts were closing. Parts of the Tethys Ocean still exist today, beneath the Black Sea, the southern Caspian Sea, and in the deeper basins of the Mediterranean.

While the timing of all these events varies by a few tens of millions of years, I think it’s safe to say that the collision between Laurasia – North America plus Eurasia – and Gondwana was definitely underway by late Mississippian time. We’ll hear more about it over the next month or so.

—Richard I. Gibson

Reference: AAPG Memoir 43, Evolution of the Arctic-North Atlantic and the Western Tethys, by Peter A. Ziegler (1988).

Saturday, June 21, 2014

June 21. Mississippian amphibian tracks

The Mauch Chunk formation in Pennsylvania, Maryland, and West Virginia contains relatively coarse clastic sedimentary rocks – clastic means broken, and the broken pieces were sand grains, silt, and pebbles that became sandstone, siltstone, and conglomerate. The rocks are mostly reddish, indicating oxidized iron and indicating an environment that was alternately under water and exposed to air. The most likely depositional setting was a delta or the low shore of a shallow sea, or possibly a swamp that was periodically inundated and then drained. Some of the rocks are green, indicating reduced iron, which forms in the absence of oxygen in a setting like a swamp.  

The beds are late Mississippian, about 320 to 340 million years old, but the youngest layers may be from the early Pennsylvanian. The coarser sediments also tend to be from the upper part of the formation and may reflect the onset of the Alleghenian or Appalachian Orogeny, the “big crunch” of mountain building that represents the collision of Gondwana with eastern North America. That collision was probably underway by about 325 million years ago, near the end of the Mississippian.

from Bronson, 1910
One of the coolest things about the Mauch Chunk is that it contains indications of land-walking animals, very likely amphibians. Footprints of tetrapods, four-footed animals, have been found impressed in sandstone with ripple marks. The ripple marks mean the sediment had been under water recently, but the well preserved footprints were made in the sand when it was exposed to air – they would not have been so well defined underwater.

In 1908 E.B. Branson and his colleagues from Oberlin College found such prints in late Mississippian rocks in Virginia. The animal’s feet were about 6 centimeters long, about 2½ inches, and they had five toes, at least some of which were webbed. The animal was probably lizard-like, although in the early discoveries they found no drag marks from tails.

In addition to footprints, impressions of nearly complete animals have been found. There’s a link on the blog to a 2007 report on those body impressions. The report draws possible conclusions about mating behavior based on the nature of those impressions.

The name Mauch Chunk is from Native American Lenape words meaning “bear place”. Today the community that once had that name is called Jim Thorpe, Pennsylvania, for the Olympic athlete.  

* * *

On June 21, 1887, Norman Levi Bowen was born in Kingston, Ontario. N.L. Bowen was an experimental petrologist who studied the ways minerals crystallize from molten magma. Much of his work was at the Geophysical Laboratory at the Carnegie Institution in Washington, D.C. He established Bowen’s Reaction Series, an explanation of how various minerals form based on the temperature of a cooling magma. The reaction series helps explain why some minerals are commonly associated with each other while others, such as quartz and olivine, are rarely if ever found together. It also has implications for the stability of rocks and how they are likely to erode and alter under weathering conditions on the earth’s surface or during later metamorphic events.

Today is also the anniversary of a devastating earthquake in 1990, in northwestern Iran. The death toll was 40,000 to 50,000, and several large cities and many villages were almost completely destroyed. The quake was centered in the western Alborz Mountains along the Caspian Sea, where a relatively small continental block is trapped between the colliding Arabian Plate to the south and the Eurasian Plate to the north. The southern Caspian Sea is a small bit of dense oceanic crust, a remnant of the Tethys Ocean that lay between Gondwana and Eurasia. The microcontinent that forms today’s Iran is being pushed up and over that little oceanic crustal fragment. This has uplifted the Alborz Mountains – and puts volcanoes in them – while the Caspian Sea and most of its margins are below sea level. The high point in these mountains, Mount Damavand, is about 5600 meters or 18,000 feet above sea level, and it rises to that height within about 80 kilometers or 50 miles from the below-sea level coast, making this one of the steepest mountain fronts in the world.
—Richard I. Gibson

Amphibian fossil body impressions (2007 report)

Photo taken from Journal of Geology, v. 18, #4, p. 357, 1910, report by Branson (volume in author’s collection).

Friday, June 20, 2014

June 20. Mammoth Cave

Photo by Navin75, via Wikipedia, under CC-by-SA license
Mammoth Cave, Kentucky, with more than 400 miles of mapped passages and connected to the Flint Ridge cave system, is the longest known cave in the world. Dissolution of the St. Louis and St. Genevieve Limestones of Mississippian age, about 325 to 350 million years old, produced the cave. The process may have begun as much as 30 million years ago, but most of the cave’s development is less than one million years old. Mammoth Cave is a “living cave” where dissolution and precipitation of cave features by running water are still continuing today.  

The cave includes an underground river and large rooms dissolved from the limestone, but which are often partly filled by redeposited calcium carbonate, calcite, in the myriad of forms you know about in caves – stalagmites, stalactites, flowstones, and much more that give the cave its beauty.

In 1972, members of the Cave Research Foundation exploring the Flint Ridge cave system discovered the connection between those caves and Mammoth Cave, making it by far the longest cave in the world. Steve Wells, a fellow geology student with me at Indiana University, was part of the team that pushed the connection. Steve and I were teaching assistants at the Indiana University Geologic Field Station, and he went on to become the President of the Desert Research Institute in Nevada.

Mammoth Cave was a commercial resource in the early 19th century, and during the War of 1812 it was sold several times as a source of nitrates to make gunpowder. Slaves mined the ore, but when the price of saltpeter, calcium nitrate, fell after the war, it had a long history as a private tourist attraction. It didn’t become a national park until 1941.

* * *

Francis Pettijohn was born June 20, 1904, at Waterford, Wisconsin. His work focused on sedimentology, the processes that result in sedimentary rocks.

—Richard I. Gibson

Reference: Geology of Mammoth Cave National Park by Ann Livesay, 1962 

Photo by Navin75, via Wikipedia, under CC-by-SA license

Thursday, June 19, 2014

June 19. Crinoids

Actinocrinites gibsoni, Montgomery Co., Indiana
Crinoids really thrived in the warm shallow Mississippian carbonate seas of central North America. At Crawfordsville, Indiana, there’s a lagerstatten – an assemblage of fossils that is remarkable in the completeness and preservation of the fossils. It has lots of crinoids, with the fine details of their arms and other structures remarkably well preserved. They lived about 340 million years ago, a time called the “Age of Crinoids,” at least in Indiana and much of the rest of the region covered by those seas. Crawfordsville crinoids are found in museums of paleontology around the world.  

A picture is worth a thousand words - check the links below. 

—Richard I. Gibson

Photo galleries:
Virtual Fossil Museum 

Indiana Fossils website 

Wednesday, June 18, 2014

June 18. Karst topography

Karst topography develops in areas underlain by soluble carbonate rocks like limestone. In some areas, the rock is mostly dissolved in the subsurface, forming caves and sinkholes, common in Indiana and Kentucky and Florida and many other places around the world. 

Karst topography in China
In parts of Puerto Rico, extremely high rainfall, more than 65 inches a year, helped dissolve most of the limestone leaving behind the parts that were slightly more resistant. The residual hills are called haystacks or pepino hills and sometimes they can reach mountainous proportions, hundreds of feet high and often with nearly vertical flanks. China also has some amazing karst mountains.

The name karst comes from the Karst Plateau in the Dinaric Alps of Slovenia, whose name comes from a German word of probable Slavic origin meaning “a bleak, waterless place.” That’s something of an ironic name since it was an abundance of water that caused the topography. But in many karst areas, much of the water flows underground because of the subterranean caves and even rivers. Lost River, in southern Indiana, is lost because it flows on the surface for a distance, but then plunges underground for several miles. Because of the subterranean drainage, the surface may have few or no lakes as well, so karst plains can indeed seem waterless. 

—Richard I. Gibson

Photo of karst topography in China by chensiyuan via Wikipedia, under Creative Commons license.

Tuesday, June 17, 2014

June 17. Shales

You’ll probably be happy to have a break from all the Mississippian limestones. Today’s topic is shales. Mississippian shales in the United States have become pretty important as oil and natural gas source rocks and as tight reservoirs in various places.  

The Heath Formation in central and eastern Montana is a world-class source rock, containing as much as 26% total organic carbon. You may recall me raving about the Chainman Shale in Nevada, which has as much as 8% total organic carbon. The Heath formation has some interbedded carbonate rocks, similar to the interbedding of shale and dolomite in the Bakken Formation, and using horizontal drilling techniques, some wells in the Heath produce close to 500 barrels of oil per day. But it’s not nearly as uniform and prolific as the Bakken in North Dakota, leading to a 2013 headline in the Billings Gazette about the Heath, indicating that the “Shale formation in Montana frustrates oil drillers.” In mid-2013 there were only a dozen producing wells, and the most recent discoveries only yielded around 20 barrels a day. It’s not worth spending $5 million on a well for that kind of return. Explorers haven’t given up, but for now the Heath is on the back burner.

The Heath Formation is late Mississippian in age, about 320 to 340 million years old. It’s about the same age as the Barnett Shale in Texas, the Fayetteville Shale in Arkansas, the Chainman Shale in Nevada, and other shales. They all formed in basins along tectonically active margins of North America, although some were more active than others. The standard view of these organic-rich shales is that they formed in deep water, poorly oxygenated, and stagnant. Recent studies seem to suggest that it was more complicated than that, and that there might even have been some periods of wave erosion in the deposits at times, so the water must have been relatively shallow for that to happen.

In 2013, the Barnett Shale in the Fort Worth Basin of Central Texas produced about 4.5 billion cubic feet of natural gas per day, almost 7% of all the natural gas produced in the United States. The Marcellus Shale, which we talked about last month, was the nation’s leader in natural gas production.
—Richard I. Gibson

Multiscale Erosion Surfaces of the Organic-Rich Barnett Shale, Fort Worth Basin, USA
Mohamed O. Abouelresh, 2013, Journal of Geological Research

Map from USGS

Monday, June 16, 2014

June 16. Indiana Limestone

Much of the Mississippian limestone in southern Indiana is uniform in its color and texture, properties that make it an excellent building stone. It’s used in many monument facings too. The Empire State Building, National Cathedral, Chicago Public Library, Metropolitan Museum of Art, the new Yankee Stadium, and the roof of the immigration building at Ellis Island are all Indiana Limestone. 35 of the 50 state capitol buildings feature Indiana limestone. 

Sanders Quarry in Salem Limestone
The rock is more technically known as the Salem Limestone, which formed in the warm, shallow Mississippian seas about 335 to 340 million years ago. The region was distant enough from land that very little detritus washed into the lime, so the resulting rock is more than 97% pure calcite, calcium carbonate. The rock is fossiliferous, but many of the fossils are of tiny, even microscopic animals called foraminifera. They are one-celled organisms that made calcareous shells. Most are no more than a millimeter across.

From the building stone point of view, irregularities like fossils aren’t as important as the fact that the Salem Limestone has few bedding planes to disrupt the rock. It’s massive, so blocks of the limestone can be cut out for building stone and for decorative facings. Stone used for these purposes is called dimension stone in the industry, to differentiate it from crushed stone used for things like aggregate in concrete. The rock has been quarried since 1827, and by the 1920s, something like 80% of the limestone quarried in the United States for dimension stone came from southern Indiana, mostly around the towns of Bloomington and Bedford. There are still 9 active quarries in the area.

* * *

Today’s birthday is George Gaylord Simpson, born June 16, 1902, in Chicago. He was a prominent and influential paleontologist who contributed greatly to evolutionary theory regarding the details of how evolution takes place. He spent most of his career at Columbia and Harvard and the University of Arizona.

—Richard I. Gibson

Reference: Indiana limestone 
Sanders Quarry photo by Sphinxcat via Wikipedia (public domain).

Sunday, June 15, 2014

June 15. Mississippian Plate Tectonics

I’ve said for months that Gondwana is coming toward southeastern North America. It’s still coming, but the remaining ocean between the two continents was really pretty narrow by middle Mississippian time.

The Supercontinent of Gondwana included South America, Africa, Arabia, India, Antarctica, and Australia, plus a few additional bits. One of the most interesting bits, from North America’s point of view, was a triangular zone between what is now west Africa and northern South America, along the Venezuelan coast. That triangular zone included what is now Florida and southern Georgia and parts of Alabama and the Bahamas. It was absolutely part of Gondwana at this time, but fear not, we’ll get it fairly soon.  

North America was tilted relative to its present geography, so that the Mississippian equator ran more or less through the middle of the continent, with what is now the east coast forming a southern coastal edge to the continent. The northeastern part of North America – the Maritime Provinces of Canada, Newfoundland, Labrador, and Greenland, were still pretty firmly attached to Europe by the remnants of the Caledonian Mountains, even though they were fairly old by Mississippian time.

The central part of Europe, France, central Germany, and points east, were assembling as a result of small continental blocks that had been shed off the northern flank of Gondwana colliding with the margins of Baltica. What’s now the Iberian Peninsula, Spain and Portugal, was involved in a complex mountain belt called the Variscan Orogeny, which won’t really culminate until the late Carboniferous and Permian Periods, 20 to 50 million years after the end of the Mississippian or early Carboniferous.

* * *

On June 15, 1991, Mt. Pinatubo on Luzon Island in the Philippines reached the climactic phase of a multi-day eruption. Pinatubo had been inactive in recent memory until the 1991 events, and the mountain was deeply eroded and supported extensive forests and human populations.

The eruption ejected huge quantities of ash, and the reported death toll of about 900 was largely the result of buildings collapsing under the weight of the ash.

Evacuations saved thousands, but the lives of pretty much everyone in the region of the eruption were severely disrupted. The volume of ejected material was the second greatest in the 20th century, estimated to be a bit less than the eruption at Katmai, Alaska, in 1912, which we discussed a week ago. The ash and especially the aerosols Pinatubo put into the atmosphere did have global climatic impacts, reducing the average global temperature by almost a degree Fahrenheit or a half degree Centigrade.

—Richard I. Gibson

Early Mississippian map

Reference: AAPG Memoir 43, Evolution of the Arctic-North Atlantic and the Western Tethys, by Peter A. Ziegler (1988).

U.S. Geological Survey Photograph of Pinatubo eruption in 1991 taken by Richard P. Hoblitt.

Saturday, June 14, 2014

June 14. Mississippian chert

First, my disclaimer about the time scale of this calendar of earth history. It’s not at a proper scale. If it were, we’d be in the Precambrian until mid-November. So I’ve arbitrarily assigned the Precambrian to January, the Cenozoic Era to December, and the months between are the periods of the Paleozoic and Mesozoic Eras. It’s June, and that means we’re in the Mississippian Period.

We’ve talked a bit about chert before, but today’s short episode is to say a little more about it.

Chert, fined-grained impure silica, SiO2, appears in sedimentary rocks in two ways. It can form when silica-rich waters percolate through the rocks after they have solidified, with the chert deposited in openings within the rock. And it can be an original part of the rock, deposited at the same time as the rest of the sediment.

Chert does appear in sedimentary rocks earlier in the Paleozoic era, but it’s really quite rare until the Mississippian Period. Tiny animals called radiolarians secrete siliceous shells. Radiolarians span the entire time from Cambrian to present, but they may have increased in abundance during the Mississippian so that their skeletal remains could have contributed to the increased abundance of chert in the Mississippian rock record, but I’m not sure if that is the explanation for Mississippian chert.

Radiolarian remains today on the deep ocean floor cover large areas with siliceous ooze, soft sediment made mostly of radiolarians and diatoms, algae that also make siliceous shells. Siliceous ooze on the ocean floor is probably chert in the making. It will take many tens of thousands of years, maybe even millions of years, for the ooze to be buried by additional sediment, for the water to be driven off, and for it to lithify into chert.

Chert comes in a wide variety of colors. It’s often black, but it can be brown, yellow, reddish, and even white. The colors reflect impurities incorporated into the silica, including organic matter and iron.
—Richard I. Gibson

Image from Kunstformen der Natur (1904), by Ernst Haekel, via Wikipedia

Friday, June 13, 2014

June 13. The Redwall Limestone

In the Grand Canyon area, the rocks equivalent to the Madison Limestone that we talked about yesterday are called the Redwall Limestone. It forms one of the highest near-vertical cliffs in the canyon, as much as 500 to 800 feet high. The Redwall is a typical gray to tan limestone – so why is it called Redwall? It is stained red by erosion of the overlying rocks, especially the Supai group and Hermit Shale, which is red because of oxidized iron in the shale. The shale was deposited in a low mud flat that was periodically exposed to air and at other times underwater, which allowed the iron in the rock to oxidize. Shale erodes quite easily so it washed down the cliff faces and stained the underlying Redwall. 

Redwall Cavern, Grand Canyon
Fairly soon after the deposition of the Redwall – and this applies to the Madison limestone in many places, too – gentle uplift brought the limestones above sea level. Then the action of moving water, percolating through the rock, did what it does to limestone – it dissolved out caves. In a good number of places, caves collapsed, and sinkholes and other features typical of karst topography developed. This was a pretty widespread development, because we can see the evidence of the collapses in the Black Hills, in Montana, and in the top of the Redwall limestone too. This was not remotely related to the cave formation that made the modern Lewis and Clark Caverns we talked about yesterday, which are at most a couple million years old, but was something that happened probably quite soon after these Mississippian limestones were lithified. How do we know that? Because the collapse structures don’t include much of the overlying rock layers. The collapsing had to take place before those layers were laid down.

I can assure you that the Redwall does make really steep cliffs. Probably the most difficult hike of my life was on a perfectly smooth, level trail about three or four feet wide along the Redwall’s face. The problem was there was a drop-off on the left that went straight down about 400 feet, and the wall on my right went straight up another 400 feet. If you’re interested in the travelogue of that 1987 backpack trip, here’s a link to my report on it.
—Richard I. Gibson

Photo in Redwall Cavern, Grand Canyon, by Richard I. Gibson

Thursday, June 12, 2014

June 12. The Madison Limestone

The Madison Limestone of the Rocky Mountains and Great Plains is formally a geologic group that includes several formations, including the Lodgepole Formation and Mission Canyon Formation.

It was formed in those warm, shallow Mississippian seas that covered a lot of the interior of North America about 330 to 340 million years ago. The calcium carbonate deposited in those seas accumulated to as much as 2,000 feet thick in parts of Montana, where the Madison forms prominent gray cliffs. The Gates of the Mountains, named by Lewis and Clark for dramatic cliffs along the Missouri River east of Helena, Montana, are made of these Mississippian limestones.  

Madison Limestone at Gates of the Mountains
The Madison was named back in 1893 for outcrops along the Madison River near Three Forks, Montana, or perhaps for the nearby Madison Mountain Range, but it has equivalents with different names as far away as the Black Hills of South Dakota, central Colorado, and Arizona. It was an extensive shallow sea.

You probably won’t be surprised to hear that the warm shallow water supported lots of life. The Lodgepole Formation especially has loads of fossils, ranging from crinoid stem columnals to brachiopods and small horn corals and much more. The Lodgepole Formation has nice bedding sometimes distinguished by a little silt or mud interbedded or mixed with the limestone. The beds are often two to four inches thick and sometimes show color differences that give the rock a distinct striped appearance – but don’t visualize bright colors, we’re talking about shades of tan and gray.

The Mission Canyon formation, younger than and on top of the Lodgepole Formation, is massive. That’s a technical term that means we seldom see the discrete beds like the ones in the Lodgepole, or for that matter in most layered sedimentary rocks. The Mission Canyon is just limestone. Lots of it. Microscopically, the Mission Canyon is called bioclastic – that means “life, broken” – and the rock is often made up mostly of tiny broken pieces of shells and crinoid stems and such, all cemented together by more calcite. It’s usually gray, and because it’s thick and doesn’t have planes of weakness, bedding planes, the Mission Canyon sometimes makes really prominent cliffs like those at the Gates of the Mountains.

Limestone is easily soluble in rainwater, which is normally slightly acidic because of the reaction between water and carbon dioxide in the atmosphere, which makes carbonic acid. Even in arid country like Montana, caves can develop. Lewis and Clark Caverns, a state park along the Jefferson River east of Whitehall, Montana, probably formed when Montana was a lot rainier than it is today, probably during the glacial periods of the past couple million years. Lewis and Clark were on the river just below the caverns, but they never saw them. The caves were discovered by non-Native Americans in 1882, and they were designated our second National Monument in 1908, but because the park service couldn’t manage it, the cave was transferred to the State of Montana in 1938 and became Montana’s first state park.

The porous, easily dissolved limestones of the Madison Group serve as important oil reservoirs in places like the Williston Basin of eastern Montana and western North Dakota, and elsewhere they are valuable groundwater aquifers.
—Richard I. Gibson

Photo of Madison Limestone at Gates of the Mountains by Richard I. Gibson.

Wednesday, June 11, 2014

June 11. Waulsortian Mounds

During the Mississippian, a special kind of carbonate build up evolved, called Waulsortian Mounds. They were not reefs – they did not contain the kinds of organisms like corals that could build large complex structures. You may recall that the end-Devonian extinction decimated the reef-building organisms, and we don’t find true reefs in the geologic record for a hundred million years after the Devonian extinction. These mounds are the only constructions known during the interval until reef builders recovered. They are not simple piles of sediment – they can reach 200 meters in height with very steep flanks. They seem to really be constructions of some kind.   

And they are found in a restricted geographic setting – the southern margin of the Laurussian Continent, formed by the collision of North America and Baltica, the core of Europe. Today, this zone is in present-day Belgium – the community of Waulsort, Belgium, gives its name to these mounds – southern England, Wales, and Ireland, Illinois, Missouri, and Kansas, and north up to North Dakota and Alberta. During the Mississippian, that margin was very near the paleo-equator, so the mounds probably relate to processes and life in warm, tropical seas. The entire southern margin of Laurussia was probably a relatively narrow but continuous ocean, in part the Tethys Ocean between Gondwana and Eurasia and in part the ocean that remained between Gondwana and southeastern North America.

Waulsortian mounds are mostly lime mud, including some broken skeletal fragments of things like crinoids and bryozoans. It is not clear exactly why the mounds formed, but there may be a connection to the likely storm tracks of hurricanes at the time they formed in the Mississippian Period. Or more specifically, the lack of storms. The mounds seem to be in regions that would likely have been protected from the most common tracks of hurricanes, so that they would have had time to grow by whatever mechanism. David King, at Auburn University, shows a connection between the mounds and low wave energy, another way of suggesting that major storms were rare or absent. I have a link on the blog to King’s interesting report on Waulsortian mounds.

Were these mounds constructed by life processes such as sediment binding by bacteria, or by some inorganic process of selective cementation? As near as I can tell, the answer is not certain. It may have been a combination of processes, made possibly by the fortuitous absence of large storms.
—Richard I. Gibson

David T. King, Jr.’s report

Tuesday, June 10, 2014

June 10. Tri-State Lead-Zinc District

The Tri-State lead-zinc district lies around the common corner of Missouri, Kansas, and Oklahoma. Lead and zinc were produced from Mississippian rocks that historically were called the Boone Formation, but have been subdivided and given other names including Reeds Spring, Keokuk, and Warsaw formations.

The rocks that host the lead-zinc deposits are mostly limestones that were deposited in warm, shallow seas, the more or less standard for middle America during the Mississippian Period. It’s not 100% limestone – there’s a lot of chert, fine-grained silica, interbedded with it and as nodules in the limestone, as well as some shale in places. 

The mineralization appears to be related to faults and fractures in the limestone, complicated by dissolution and collapse of the limestone in some places. The fractures provided abundant pores and passages for mineral-rich waters to flow through, but the ultimate origin of those fluids is debated. The most common view is probably that the mineral-rich fluids rose from some deep, magmatic source until they were stopped by an impermeable layer and found the open fractures in which to crystallize the lead and zinc minerals.

Sphalerite photo by Rob Lavinsky,
The deep magma from which the hot mineral-rich water rose might have been related somehow to the flank of the Ozark Uplift, which got its start as long ago as the Ordovician, but there was some ongoing, relatively gentle tectonic activity over many millions of years, including during the Mississippian. As is often the case, it’s difficult to pin down the time the minerals came in with accuracy. It has to be younger than the rocks that contain the minerals, of course – they and their fractures had to be there first. And there’s good evidence that the deposition of minerals must have taken a long time, probably in many episodes, possibly spanning as much as many tens of millions of years.

The most common minerals are galena, lead sulfide, and sphalerite, zinc sulfide. The minerals were so important to the early history of this region there’s even a town in Kansas named Galena. Mining began about 1848 and continued until the Eagle-Pitcher Mine in Oklahoma shut down in 1967. Over the century from 1850 to 1950, the district produced about half the Zinc mined in the United States and 10% of the lead. One mine complex, the Pitcher Field in Oklahoma, was the most prolific producer, yielding about 60% of the total which amounts to more than 15 million tons of lead and zinc over the history of the mining district.
—Richard I. Gibson

Report from Oklahoma Historical Society 
Report from Kansas Geological Survey part 1; part 2

The Geology and Ore Deposits of the Tri-State District: D.C. Brockie et al., in Ore Deposits in the United States, John D. Ridge, ed., American Inst. Of Mining, Metallurgical and Petroleum Engineers, 1968, p. 400.

Sphalerite photo by Rob Lavinsky, – CC-BY-SA-3.0

Monday, June 9, 2014

June 9. Deltas of the Midwest

With all the talk about limestone, you might get the impression that all of America was a big beautiful tropical seaway with nice white carbonate sand everywhere. While the shallow seas were extensive, there were other kinds of sediments deposited as well. In what is now the eastern Great Lakes, Ohio, Kentucky and West Virginia, during early Mississippian time some huge river deltas formed, as large as the Mississippi or Nile Delta today.

from USGS Prof. Paper 259
Remember that the Great lakes weren’t there – it would be hundreds of millions of years before they would form. The complex upland in eastern North America, the result of collisions with Baltica and other pieces of Europe, led to a huge river system in what is now southern Ontario. The Ontario River flowed south, into the eastern edge of the sea that covered most of the United States west of the Appalachians.

The river carried a lot of sand, silt, and mud into the branch of the sea called the Ohio Bay, and built a long linear delta complex, a peninsula that extended all the way from present-day Lake Erie to the border between Kentucky and West Virginai – almost three hundred miles long and something like 75 miles wide. The sediments in the delta are known today as the Bedford Shale and Berea Sandstone.

There was another peninsula to the west, in west-central and southwestern Ohio, but this one wasn’t a delta. It was all that was left of the Cincinnati Arch – the broad upland that got its start 130 million years earlier, during the Ordovician. As the Mississippian Period progressed and sea levels became higher and higher, the peninsula sometimes called Cincinnatia was reduced to a string of islands and shoals and eventually was pretty much completely submerged. West of Cincinnatia, in present-day Indiana and Illinois and points west, most of the Mississippian rocks are the shallow-water limestones with abundant fossils that are so common from this time across much of North America.

The Berea Sandstone and Bedford Shale represent oscillations in sea level that produced variations in the sedimentation pattern over time at various places. Modern deltas do this too, changing the position of the main channel and smaller distributary streams sometimes on an annual basis. So you can get coarse river channel sands at one time, but muds from floods over the banks of the channels at other times in the same place, and many variations in between. Deltaic systems can make pretty good reservoirs for oil and natural gas when coarse, porous sandstones are encased within impermeable fine-grained shales. And in fact the Berea Sandstone was historically an important oil and gas producer in Ohio.
—Richard I. Gibson

Reference: Geology of the Bedford shale and Berea sandstone in the Appalachian basin, USGS Prof. Paper 259 (1954) Also source of map above.

Sunday, June 8, 2014

June 8. Brachiopods

Just a short one today, to mention the brachiopods again. 

Two of the major groups of brachiopods, the productids or spiny brachiopods, and the sprifers, with elongate, wing-like shells, continued to thrive during the Mississippian.

Productus longispinus (left); Spirifer glaber (right)

In 2008 a report by Chinese and Polish paleontologists described an unusual assemblage of silicified brachiopods, including 4 new species, from the Muhua area of southern China. LINK

* * *

Laki Eruption

Beginning on June 8, 1783, and continuing for eight months, a fissure – a crack in the earth – and 130 eruptive vents began to pour basaltic lava over southern Iceland at Lakagigar, often called Laki. By most estimates the volume of erupted material, more than 14 cubic kilometers, is the greatest on record during historic times. The lava was accompanied by poisonous fumes composed of hydrofluoric acid and sulfur dioxide which killed half of all the livestock in Iceland and devastated the countryside well beyond the lava flows themselves. The resulting famine killed an estimated 25% of Iceland’s human population. Worldwide, the ash and acid aerosols in the atmosphere caused dramatic climatic effects – crop failures in Europe, droughts in India, extremely low flows in the river Nile, and a North American winter cold enough for the Mississippi River to freeze over at New Orleans. The sulfurous fumes were strong enough in France to kill a few dozen people, and the U.S. ambassador to France, Benjamin Franklin, speculated on the connections between the poisonous fogs in Europe and the Icelandic eruption – correctly, as it turned out. The total death toll worldwide from this eruption may have been as high as 6 million when all the impacts of the famines are included.

There’s a new book out on the Laki eruption, just published in March 2014. Island on Fire is by Alexandra Witze, an award-winning science journalist. I haven’t finished it yet, what I have read is an excellent report on the eruption and its worldwide consequences.
—Richard I. Gibson

Brachiopod drawing from an old textbook (public domain)

Saturday, June 7, 2014

June 7. Lands and Seas

The Mississippian Period was much like the earlier part of the Paleozoic Era in North America – much of the continent was covered by warm shallow seas in which abundant life contributed to the thick sheets of limestone that were laid down. Eastern North America was still a highland, the result of the ongoing collisions with Europe and the microcontinents that were rifting away from Gondwana, and the ocean between North America and Gondwana was getting narrower and narrower.  

In western North America, the Antler uplift that we discussed at the end of May was still there, and it was shedding sediment into the western seaway. The combined weight of those sediments and the piles of thrusted rocks pushed over the western edge of North America by the Antler Orogeny pushed the earth’s crust downward, creating what’s called a foreland basin. The Antler foreland basin extended through what is now eastern Nevada, western Utah, and into parts of Idaho and California. It was a relatively deep trough, and the mud that found its way into it contained a lot of organic material, washing in from both the west and the east. The resulting rock is called the Chainman Shale – a rock we mentioned in the Devonian, in May, as the source rock for the oil fields of Nevada. The Chainman is an excellent source rock, as much as 8% total organic carbon in some places.

There was some tectonic activity in what is now Alaska’s North Slope and across the Arctic islands of northern Canada – it’s called the Innuitan or Ellsmerian Orogeny and it was mostly taking place toward the end of the Devonian and into the Mississippian Period.

In Europe, Gondwana was pretty much encroaching on the southern margin of Baltica, but it was a complex interaction with lots of smaller blocks colliding. The seaway between Gondwana and Europe, called the Tethys Sea, is actually still there. We call it the Mediterranean Sea today. 

—Richard I. Gibson

Links to Paleogeographic maps:
North America
Western North America


Innuitan Orogeny

Friday, June 6, 2014

June 6. Echinoids

Echinoids are another class of echinoderms. Echinoderms today are probably most familiar as starfish, but they also include sand dollars and sea urchins, and those groups are part of the class Echinoidea. Fossil echinoids are common in carbonate rocks from the Mississippian Period. The oldest known echinoids are from the late Ordovician. Most of the classes of echinoderms appear to have become established during the Ordovician, and many, including echinoids, have survived to the present.

photo by Debivort via Wikipedia, under GFDL
Modern sea urchins have a globular body with five-fold symmetry, typical of all echinoderms, and a forest of spines encrusting the body. Fossil echinoids usually show only the body, often with a distinct 5-point star design on top. Ancient echinoids probably had spines as well, but they are not usually preserved intact with the body. 

Echinoids began to decline during the Pennsylvanian or late Carboniferous period, which we’ll cover next month, and by the Permian extinction there were only six species that we know about, and only two survived into the Triassic. But that was enough to give rise to the modern varieties of echinoids, which total about 950 species today.

* * *

On June 6, 1912 and for several days thereafter, Mt. Katmai in the Alaska Peninsula erupted. The series of eruptions devastated the area and created the landscape known today as the Valley of 10,000 Smokes, for the many steaming and smoking fumaroles and vents that were formed, including one large volcano called Novarupta. The summit of Mt. Katmai collapsed to make a caldera more than two miles across. The eruption was related to the subduction of the oceanic Pacific Plate beneath Alaska, and it was probably the largest volume of material erupted in the 20th century, at about 11 to 13 cubic kilometers, or about 3 cubic miles of ash and lava. That volume is about 30 times the volume erupted by Mt. St. Helens in 1980. The only other 20th century eruption that comes close was that of Mt. Pinatubo in the Philippines in 1991, which is also estimated at about 11 cubic kilometers of ejected material. Today, the Valley of 10,000 Smokes is part of the Katmai National Park and Preserve.

—Richard I. Gibson

photo by Debivort via Wikipedia, under GFDL

Thursday, June 5, 2014

June 5. Blastoids

First, an update. Some new work seems to have pinned down the cause of one of the multiple mass extinctions during the middle and late Cambrian period. The extinction at 510 to 511 million years ago correlates well with voluminous volcanism in Australia. I’ve put a link on the blog episode for February 28 to an article about this.

Now, back to the Mississippian. It’s June 5, and today’s topic is blastoids. Blastoids are a class of echinoderms, quite similar to crinoids. Crinoids have decreased tremendously from their peak during the Paleozoic, but they’re still with us today. Blastoids however are extinct – they didn’t make it past the Great Dying at the end of the Permian Period. But since they began during the Ordovician, or possibly in the late Cambrian, blastoids had a run of more than 230 million years.

Photo by DanielCD via Wikimedia Commons under GFDL.
Like crinoids, blastoids were animals that had a plant-like stalk and a system of holdfasts, root-like structures that held them to the sea floor. Unlike crinoids, blastoids’ skeletons were held together by solid interlocking calcareous plates. In most crinoids, their bodies were held together by muscular tissues, so after death, crinoids tended to fall apart. Blastoids are often found intact. It also seems that blastoids and crinoids may have had different systems for moving water through their bodies to provide oxygen to the animal.

The pentagonal symmetry typical of echinoderms is usually well displayed by blastoids, and the body fossils often look like a flower bud or some kind of nut. There’s a wide range in size, of course, but some of the most common genus, Pentremites, are on the order of a half inch to an inch long.

Blastoids reached their peak of diversity and numbers during the Mississippian, and in some places, such as the fossil locality known as Pentremites Hollow, near Bloomington, Indiana, they are exceedingly abundant.

—Richard I. Gibson

Photo by DanielCD via Wikimedia Commons under GFDL.

Wednesday, June 4, 2014

June 4. Limestone

We’ve talked about limestone quite a few times, and we’ll talk about it a lot this month. I thought we should focus a bit on the rock itself. I imagine most people have a concept of limestone. It’s a rock, not a mineral. A mineral is a compound with a distinct chemical composition as well as a specific crystalline structure, and a rock is an aggregate of minerals.   

Sometimes, a rock might be just one mineral, and that’s often the case with limestone. It’s usually mostly the mineral calcite, CaCO3, calcium carbonate. Calcium, carbon, and oxygen are all pretty common in the earth’s crust, and carbon and oxygen are obviously also in the atmosphere. They all also get into the hydrosphere, the world’s oceans and other waters.

Limestone quarry in Italy
Photo by Michael J. Zirbes
via Wikimedia Commons
, under Creative Commons license.
As a sedimentary rock, limestone is often mostly grains of calcite that are cemented together – often by more calcite. The grains can come from several different sources – they might be broken pieces of older limestone, or they might be broken pieces of the calcareous shells produced by a great many organisms including clams, snails, crinoids, bryozoans, and many more. Obviously you could not get broken shells until shelly animals had evolved, and that didn’t really happen in large volumes until the Cambrian Explosion that we talked about in February. But there are Precambrian limestones too.

It’s also possible for calcium carbonate to precipitate directly from water that’s saturated with calcium that can react with carbon and oxygen – an inorganic process not related to life. That chemical process happens today, in places like caves. Stalagmites and stalactites are chemically precipitated calcium carbonate, usually resulting from pre-existing limestone being dissolved by water.

Calcite is easily soluble in acids, and in fact geologists use the fizzy reaction between calcite and weak hydrochloric acid to test a rock or mineral for the presence of calcite. But there are acids in nature as well, including the acids produced by chemical weathering of rocks and by the reaction between rain and the carbon dioxide in the atmosphere. The latter is called carbonic acid, and it’s really a very weak acid, but a weak acid is enough to dissolve limestone when you’re talking about millions of years. The earth has had acid rain – slightly acid rain – pretty much forever. Modern acid rain that comes from human pollutants can be much more significant over shorter periods.

So we can get limestone, calcite, deposited in vast layers through both chemical precipitation and as a result of the activity of marine organisms that secrete calcium carbonate to make their shells, which can become the main part of some limestones. Usually the rock is a combination of both factors.

Something like 10% of all sedimentary rocks are limestones, but in some places it can seem that they are a lot more than that. In part that’s because in arid country, such as western North America, limestone does not dissolve as much as in areas where it rains a lot. It’s just that carbonic acid reaction again – lots of rain, more weak acid, more dissolution of limestone. In arid country, limestones often form prominent ridges and cliffs.

Limestones can be pretty complex rocks, including grains that are really broken shells, as well as little grains that are chemically precipitated calcite forming tiny round balls maybe a half-millimeter across. Those things are called oolites – from the Greek word for “egg” because they are round or oval – and they often show concentric layers of calcite deposited on some nucleus such as a sand grain. As they get swirled by waves, they roll around but grow as thin layer after thin layer of calcite is deposited.

While I said earlier that limestone is usually mostly calcite, that’s by no means the only thing that you can get in limestones. Sometimes you get calcium carbonate with a different crystal form – aragonite is the same chemical composition as calcite, CaCO3, but is has an orthorhombic molecular crystal structure in contrast to the hexagonal arrangement of calcite. And you can definitely get the whole spectrum of impurities in the sediment that becomes limestone. Quartz sand grains, or any other kind of grains, can be washed in, and you can get traces of iron that may color the rock.

Sometimes even more than chemical variations, the texture of the rock can show wild diversity. Texture includes things like the size and shape of grains, nature of pore space, what kind of cement is present, and structures in the rock. That diversity can tell us a lot about the depositional environment, the setting in which the sediment was laid down. That’s one of the main goals of looking at rocks – figuring out the nature of the world that produced those rocks.

We’ll be visiting several specific limestone formations during this month.

* * *

Beno Gutenberg was born June 4, 1889, in Darmstadt, Germany. He was one of the most prominent seismologists of the 20th century, and he founded the seismological laboratory at CalTech in 1930. In 1935, together with his colleague Charles Richter, he developed the magnitude scale for evaluating earthquakes that was used until other methods were established in the 1970s.
—Richard I. Gibson

Photo by Michael J. Zirbes via Wikimedia Commons, under Creative Commons license.

Tuesday, June 3, 2014

June 3. Archimedes

Today I want to talk about a bryozoan, a colonial animal somewhat like corals but probably more closely related to brachiopods. You may recall that they were the only phylum that, so far as we know, was NOT established before or during the Cambrian explosion but developed later, during the Ordovician. And they do survive to this day.  

During the Mississippian, the warm shallow seas of North America harbored plenty of life, including diverse bryozoans. But the screwiest of all is a group named Archimedes.  

Archimedes was the Greek philosopher who invented the screw as a way to lift water.

Archimedes wortheni
Fossils of the bryozoan genus Archimedes look just like screws. In life, the individual zooids that comprised the colony had lattice-works extending away from the long, spiraling skeletal substrate of the colony, but the fossils usually preserve only that skeletal backbone, and it looks like a screw. The name was first applied by the American geologist David Dale Owen in 1828.

They’re really pretty common, and you find them typically something like one to four inches long, or shorter broken fragments. Screws can be left-handed or right-handed, and so are Archimedes bryozoans. They began and were abundant during the Mississippian and survived about 108 million years, until the mass extinction at the end of the Permian Period.

Archimedes bryozoans also look an awful lot like fusilli pasta, and when you get a lot of geology students together in a place like geology field camp and serve it to them, you can bet that they’ll start calling it Archimedes noodles. I’ve had it dozens of times.

* * *

Two geological birthdays today. James Hutton,, who formulated the theory of uniformitarianism and many of the concepts that laid the groundwork for the modern science of geology, was born June 3, 1726, at Edinburgh, Scotland. Check the posts for March 7 and May 4 for more about him. Lee Suttner, one of my professors of geology at Indiana University, was born June 3, 1939, in Wisconsin. If I’m a decent teacher of geology, I owe it to Lee for sharing his style of asking leading questions, rather than telling. It’s a style I admire and try to emulate when I can. Happy birthday, all.

—Richard I. Gibson

David Dale Owen and the naming of Archimedes

Drawing from an old textbook (public domain)

Monday, June 2, 2014

June 2. Mississippian time

The time span of the Mississippian is about 36 million years. It began with the end of the Devonian about 359 million years ago and it ended about 323 million years ago. As with all the subdivisions of geologic time, there are error bars on these dates, in this case about a half million years, plus or minus, on both ends.  

Mississippian time (from Wikipedia)
In Europe, the Early Carboniferous, the equivalent in time to the Mississippian, is divided into three ages which correspond to three stages in the rock record. The Tournaisian and Viséan are named for rocks in Belgium and the Serpukhovian is from outcrops in Russia. They span from 7 to 17 million years each.

In North America, there are four subdivisions of Mississippian time. From oldest to youngest they are Kinderhookian, Osagean, Meramecian, and Chesterian. These names are used a lot in the literature on Mississippian rocks, and I may use them too – but I will try to avoid too much jargon and I’ll try to always refer any names like that to the part of the period we’re in, and about how many million years ago it was.

As I’ve said before, international agreements are often needed to assign specifics to the breaks in geologic time. Since rocks in one part of the world don’t necessarily record the same events as other parts – or the same event may span some time and may occur at one time here, and another there – because of that, it’s not surprising that we really cannot make an exact, world-wide time scale that applies everywhere. But things do usually work out so that they’re pretty close.
—Richard I. Gibson

Sunday, June 1, 2014

June 1. The Carboniferous begins

…or is it the Mississippian?

The next period of the Paleozoic Era is called the Carboniferous, which means carbon-bearing in reference to the coal beds in the upper part of the system in many parts of the world. The name was invented by British geologists William Conybeare and William Phillips in 1822, making it the first to be established of the names we use today for the geologic periods.  

Now we have a nomenclature problem to deal with. In the book I put together in 1994, the month of June corresponds with the Mississippian Period. In the United States, the Carboniferous of Europe is divided into two distinct time spans, the Mississippian and the Pennsylvanian. Technically, in terms of international geologic names, the Mississippian and Pennsylvanian are sub-periods of the Carboniferous, but in part because of long-standing usage, in the United States the two are treated as full-fledged periods of geologic time.

In the U.S., the period takes its names from rocks of this age exposed along the Mississippi River, especially in western Illinois where these rocks are hundreds of feet thick.

For purposes of the original book and for these podcasts, I’m using U.S. nomenclature, so the month of June is the Mississippian and July will be the Pennsylvanian. But technically, they are parts of the Carboniferous Period. Sometimes you’ll see the Mississippian equated with early Carboniferous and the Pennsylvanian with the late Carboniferous. It’s all a matter of human convenience, and in this case, a matter of some notable differences between the rocks of this time period in the U.S. versus those of Europe.
—Richard I. Gibson