BIGFORK CHERT/FORMATION

Age: Middle and Late Ordovician Period
Distribution: West-central Arkansas, Ouachita Mountains; southeastern Oklahoma
Geology: The Bigfork Chert consists of thin-bedded, dark-gray, cryptocrystalline chert interbedded with varying amounts of black siliceous shale, calcareous siltstone, and dense, bluish-gray limestone. The cherts normally occur in thin to medium beds and are usually highly fractured. The interbedded siliceous shales occur in thin to thick sequences and are often pyritic. Limestones occur mostly as interbeds in the chert and typically weather to soft brown layers. The limestones are more common in the northwestern exposures. Fossils are rare, but fragments of brachiopods, crinoids, sponges, conodonts, and graptolites have been reported. The contact between the Bigfork Chert and the underlying Womble Shale is conformable. The Bigfork in Arkansas ranges in thickness from about 450 feet in the northern Ouachitas to about 750 feet in the southern Ouachitas.
Original reference: A. H. Purdue, 1909, Geological Society of America Bulletin v. 19, p. 557; A. H. Purdue, 1909, Slates of Arkansas: Arkansas Geological Survey, p. 30, 35.
Type locality: Named for exposures near the Bigfork Post Office, Montgomery County, Arkansas

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