October 20. The Solnhofen Limestone

In several of this month’s episodes I’ve mentioned the Solnhofen Limestone, the rocks in Bavaria where spectacular fossils are found – from the lobster-like Eryon to the flying pterosaur Rhamphorhynchus to Archaeopteryx that we talked about yesterday. Why is this rock so special?

Crinoid from Jurassic Solnhofen limestone (source)

During Late Jurassic time, about 150 million years ago, central Europe was a string of islands, the high-standing areas along seaways defined by rifts that formed as Pangaea began to break apart. The area was close to the Jurassic equator. The array of islands made for some restricted lagoons between them, and in some locations, arid conditions together with restricted circulation made for anoxic conditions where life could not survive. No scavengers, no oxygen to decompose bodies. We’ve heard this story before.

But wait, you say – when we’ve heard this in previous episodes, those stagnant lagoons accumulated organic-rich mud that became black shale. You said this was limestone. What’s the deal? We have to infer that the islands were low-lying, and not shedding much in the way of clastic sediment into the lagoons. Clastics – sand, silt, and mud – are deposited typically in settings where the topographic relief is at least moderately high, so erosion can remove those materials from outcopping rocks on land and dump them into adjacent marine settings. Here in Bavaria during the Jurassic, I think we have to see the setting as something like the modern Bahamas – low islands, and even there the rocks on the surface were probably limestone, not granite or other rocks that would yield quartz sand, silt, and mud. The lagoons were carbonate-rich, so it was fine grained calcite, calcium carbonate, that precipitated out.

The resulting extremely fine-grained rock was also remarkably uniform, so much so that the Solnhofen is called a lithographic limestone – ideal for making lithographic plates – stones carved in fine detail to use in printing illustrations, including multi-color lithographs. It was the quarrying operation in the 19th century for lithographic uses that revealed the spectacular fossils of the Solnhofen.

Fossils are actually not all that common in the Solnhofen limestone, but when they are found, they are preserved in exquisite detail, some of the finest fossils ever found anywhere. In addition to the animals we have talked about, the fauna includes jellyfish with soft parts preserved, free-floating crinoids, beetles, cephalopods, horseshoe crabs, turtles, fish, dragonflies, crocodiles, and more. It is truly a world-class lagerstätte, one of those rare natural collections of spectacularly preserved fossils.

—Richard I. Gibson

Link:
UC Berkeley on Solnhofen 

Photo by Ushakaron, used under Creative Commons license. 

August 24. Capitan Reef

El Capitan. Photo by Richard Gibson (1980)

That huge reef that developed around the margins of the Delaware Basin portion of the Permian Basin is called the Capitan Reef. It’s named that for outcrops in the Guadalupe Mountains, specifically for El Capitan, a prominent mountain there which is the core of the Permian reef. El Capitan stands about 2,000 feet above the adjacent valley floor today – just about the same elevation that the ancient reef stood above the adjacent deep sea basin. The overall geometry of the reef was a long, narrow ribbon encircling the oval Delaware Basin. There were some breaks in it, but its total length was about 450 miles.

Source: National Park Service

The reef was constructed in the standard way, by the skeletons of tiny calcite-secreting organisms including algae, sponges, bryozoans, and some corals, but not mostly corals. Other life, including brachiopods and fusulinids, contributed their shells to the reef complex as well, so that it became a huge edifice nearly a half-mile high. The top would have been just about at the water line, the warm, aerated shallow zone where life lived, as in the modern Great Barrier Reef of Australia or the reefs that make atolls around tropical islands.

The critters were sustained by an influx of nutrients from all directions. The back reef, the shallow lagoonal area on the shelf, was much like the shallow seas we heard so much about throughout the Paleozoic. Lots of life – crinoids, even more fusulinids, the large single-celled organisms we talked about a few days ago, plus gastropods (snails) and more. The forereef area, the deep ocean in front of the high reef itself, also contained abundant nutrients that would flow up the slope to feed the reef organisms. Organic matter that settled into the deep forereef basin also helped create some of the rich hydrocarbon source rocks that have matured and migrated into the oil and gas fields we exploit in this area today.

Source: Texas Water Development Board

The area was tropical to subtropical, with the Permian equator running approximately from what is now northern California to Newfoundland, making the region similar to the Great Barrier Reef in terms of latitude.

The modern El Capitan and Guadalupe Mountains are exposed and high-standing because of much later tectonics and erosion that exposed the reef core. In arid country like West Texas, carbonates tend to be resistant. They dissolve better in rainy country because of the weak carbonic acid produced by the interaction of rainwater and carbon dioxide in the atmosphere. The shales and sands in the forereef area, a broad plain today, are less resistant and end up eroding away more easily. This is not to say that there was no dissolution of these limestones. There was, in spectacular fashion – and we’ll talk more about that tomorrow. 

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The eruption of Mt. Vesuvius in Italy on this date, August 24, 79 A.D., destroyed the cities of Pompeii and Herculaneum. Most of the estimated death toll of 16,000 were buried under ash falls or pyroclastic flows. Vesuvius has erupted many times, including 17 since 1700. The last major eruption was in 1944. The volcanism in southern Italy and Sicily is ultimately related to subduction caused by the collision of Africa with the Italian Peninsula, but it’s pretty complicated because there are multiple small blocks involved in the collision. Italy itself is pushing like a finger into Europe, raising up the Alps. Oceanic crust beneath the Tyrrhenian Sea north of Sicily and west of southern Italy is probably subducting or being overridden by small continental blocks, resulting in the volcanism.

—Richard I. Gibson

Links and references
Guadalupe Mountains 
Permian Reef 
Capitan Reef  (TX Water Development Board – source of cross-section)

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.

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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).

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.

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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.