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 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.

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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.

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.

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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.

Radiolaria

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