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Geologic Unit: Bigfork
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Map showing publication footprint
  • Usage in publication:
    • Bigfork chert
  • Modifications:
    • Original reference
  • Dominant lithology:
    • Chert
  • AAPG geologic province:
    • Ouachita folded belt
Publication:

Purdue, A.H., 1909, The slates of Arkansas: Arkansas Geological Survey, p. 1-95, (incl. geologic map)


Summary:

Pg. 30, 35; GSA Bull., v. 19, p. 557, 1909 [abs.]. Bigfork chert. Close-textured, even-bedded, siliceous rock in layers 1 to 18 inches thick; of slate to dark-gray color; very friable. In places thickly set with network of fine quartz veins. Weathered portions have appearance of fine-grained, gray weathered sandstone. Usually layers are crumpled to astonishing degree. Thickness 700 feet. Overlies [unconformably] Stringtown shale and underlies Polk Creek shale. [Stringtown shale now abandoned, for Womble shale.] Age is Middle Ordovician.
[Recognized in southwestern AR and southeastern OK. Named from development over large area around Bigfork P.O., Montgomery Co., southwestern AR.]

Source: US geologic names lexicon (USGS Bull. 896, p. 184).


Map showing publication footprint
  • Usage in publication:
    • Bigfork chert*
  • Modifications:
    • Areal extent
  • AAPG geologic province:
    • Ouachita folded belt
Publication:

Harlton, B.H., 1953, Ouachita chert facies, southeastern Oklahoma: American Association of Petroleum Geologists Bulletin, v. 37, no. 4, p. 778-796.


Summary:

Pg. 781 (fig. 2), 782, 783-787. Bigfork chert. Geographically extended into Ouachita Mountains, southeastern Oklahoma, where it is well exposed at Black Knob Ridge and in the Potato Hills. At Black Knob Ridge, light- to dark-gray hard cryptocrystalline well-bedded cherty highly siliceous limestone with thin layers of coarsely crystalline limestone; upper part consists of alternating chert and marly shale or shale beds ranging from 1 inch to 2 feet in thickness. About 600 feet thick. Conformably underlies Polk Creek shale; overlies Womble shale. Upper Ordovician.

Source: US geologic names lexicon (USGS Bull. 1200, p. 333).


Map showing publication footprint
  • Usage in publication:
    • Bigfork Chert*
    • Bigfork Formation*
  • Modifications:
    • Overview
  • AAPG geologic province:
    • Ouachita folded belt
Publication:

McFarland, J.D. (compiler), 2004, Stratigraphic summary of Arkansas: Arkansas Geological Commission Information Circular, no. 36 (Revised), 39 p. [Available online from the Arkansas Geological Survey.]


Summary:

Pg. 19-20. Bigfork Chert; Bigfork Formation. 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. The limestones occur mostly as interbeds in the chert and typically weather to a soft brown layer. The limestones are more common in northwestern exposures. Thickness from about 450 feet thick in the northern Ouachitas to about 750 feet thick in the southern Ouachitas. Conformably overlies Womble Shale. Fossils are rare but fragments of brachiopods, crinoids, sponges, conodonts, and graptolites have been reported. Age is Middle and Late Ordovician.
Recognized in Ouachita Mountains, west-central AR, and in southeastern OK.

Source: Publication.


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For more information, please contact Nancy Stamm, Geologic Names Committee Secretary.

Asterisk (*) indicates usage by the U.S. Geological Survey. Other usages by state geological surveys.

"No current usage" (†) implies that a name has been abandoned or has fallen into disuse. Former usage and, if known, replacement name given in parentheses ( ).

Slash (/) indicates name does not conform with nomenclatural guidelines (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005). This may be explained within brackets ([ ]).