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, iRocks.com
CC-BY-SA-3.0

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

Resources:
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, iRocks.com – CC-BY-SA-3.0

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.

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

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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)