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

May 30. Novaculite

Do you remember conodonts, the tiny tooth-like fossils that are often the only remnants of an eel-like animal? We first talked about conodonts in March, during the Ordovician, but they were abundant in Devonian time as well. Like ammonites, conodonts are so specific in nature that they serve as excellent index fossils, and because they are tiny, often no more than a millimeter long, they can be identified from cuttings in oil and gas well drilling. They’re important to the science called biostratigraphy, which helps oil explorationists know exactly where they are as the well drills down.  

We’ve also talked about chert, really fine-grained silica, and how it can preserve even microscopic fossils. Combine chert with conodonts and you’ve got something to hang your hat on, in terms of detailed stratigraphy.

Caballos novaculite ridges (USGS photo).

There are several layers of mostly chert in the United States, including the Arkansas Novaculite and the Caballos Novaculite. Novaculite is the rock name given to a special kind of chert that is hard, tough, and dense. Its broken edges can be sharp, and the name comes from Latin meaning “razor stone.” Native Americans valued novaculite as a resource for making projectile points. Chert is definitely a sedimentary rock, but most geologists would consider novaculite to be a very low-grade metamorphic rock, where heat and pressure have tightened the crystalline structure of the silica even more than in typical chert.

Novaculite such as that from the Devonian of Arkansas has been used for whetstones and abrasives. In West Texas, the Caballos Novaculite serves as a good reservoir for oil and natural gas where it is fractured in the subsurface. These novaculite beds are generally a lot thicker than the chert beds and nodules we talked about earlier this month. Those discontinuous layers might be a few inches thick, typically, while the Arkansas and Caballos Novaculite can be as much as 60 feet of almost nothing but silica. One possible origin for the novaculites is thick accumulations of the shells of diatoms – planktonic or floating algae whose cell walls are made of silica. Even though they are microscopic, these algae in their billions could create quite a layer of silica on the sea floor as they died over many tens and hundreds of thousands of years. Radiolarians, animals with silica shells, also likely contributed to the silica accumulations that became chert and novaculite.

—Richard I. Gibson

USGS Photo from U.S. Geological Survey Professional Paper 187.

May 14. Chert

Earlier this month, we talked about the Rhynie Chert, in Scotland – a hot spring or geyser deposit that preserves a remarkable array of Devonian plants and animals. 

Chert, very fine grained silica, the same as the mineral quartz, is found in lots of places around the world. The Rhynie Chert is special because it was deposited by geysers. Chert forms in other ways, too. Often it’s found as thin discontinuous layers in sedimentary rocks, or as nodules that might range from a few millimeters to maybe as much as a meter, but most nodules are smaller.

Devonian chert (dark bands) from Pennsylvania (photo by Jstuby, public domain)

Since silica, in the form of quartz, is so common in the earth’s crust, it shouldn’t be too much of a surprise that silica is a common material that’s dissolved in water. When that water percolates through rocks or sediments, the silica can precipitate – sometimes into an open space, say one that’s been dissolved in limestone, or sometimes it might precipitate on something that serves as a nucleus, like a fossil. All of this can be part of the process called diagenesis, the change from loose sediment to solid rock, or it might happen later, with the chert precipitating in pore spaces in the solid rock.

Some layers of chert probably develop from concentrations of silica in the sedimentary environment. For example, sponge spicules, which we talked about back in February, are mostly silica. If you had a whole lot of sponges living in an area, when they die their siliceous spicules might accumulate enough to actually make a sediment that is mostly silica. That could lithify into a discontinuous bed of chert.

Likewise, microscopic animals called radiolarians make siliceous shells, and enough of them could also turn into a layer of silica-rich sediment that might become chert.

As with the Rhynie Chert, most chert is extremely fine grained, and often replaces things on a molecular scale. The rich Devonian fossil beds at the Falls of the Ohio, which we talked about on May 5, are mostly in limestones, but there are also interbedded chert layers that contain a wealth of microscopic plant and animal fossils.

There are also thick bedded cherts, especially in the Permian of the western United States. We’ll talk about them in August.

—Richard I. Gibson

Devonian chert (dark bands) from Pennsylvania, Public domain photo by Jstuby via Wikipedia.