The dolomitized reefs in the Middle and Upper Silurian carbonate rocks of the upper midwest of the United States have a low silica content and are nearly devoid of chert. The surrounding interreef is highly siliceous and cherty.

Two periods of chert formation occurred in the interreef. The first period took place early in the history of the sediment, as indicated by the presence of well-preserved soft-bodied microorganisms in chert nodules, by high strontium contents (as high as 1,900 ppm), and by high CaCO3/MgCO3 ratios (up to 5.8) of carbonate remnants in the chert. Strontium contents decrease to about 90 ppm, and the CaCO3/MgCO3 ratio decreases to 1.2 toward the edges of the chert, suggesting that the chert grew during diagenesis and dolomitization of the surrounding sediment.

The second episode of chert formation in the interreef formed a fringe of partial chert, around the nodules of the first chert episode. Because the partial chert has a lower strontium content (85 to 230 ppm) and contains very little organic material and virtually no fossils, it is concluded that diagenesis was much more advanced when the second chert episode occurred.

The strontium content of the dolomite of the interreef is about 55 to 100 ppm. The strontium content of both the dolomite and the chert of the reef flank is lower than in the interreef. This is probably caused by introduction of fresh water through the reef into the interreef during the late Niagaran emergence.

Siliceous organisms provided an adequate supply for the deposition of chert between the Silurian reefs. It is likely that fresh meteoric water filtered down through the reef at the time of chertification and kept chert from forming in most of the Thornton reef while the surrounding interreef was partly chertified.

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