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.