- Usage in publication:
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- rhyolite of Johnson Shut-ins
- Modifications:
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- Geochronologic dating
- AAPG geologic province:
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- Midcontinent region
Summary:
Whole rock Sb-Sr age determined to be 1420 +/-30 m.y. Study area in St. Francois Mountains, southeast MO, Midcontinent region. Other units having same Rb-Sr age as rhyolite of Johnson Shut-ins include Hogan Mountain and Royal Gorge rhyolites and Pilot Knob felsite. Older than Munger granite porphyry and Magee granite (1260 +/-30 m.y.) and Stout's Creek rhyolite (1315 +/-35 m.y.). Age similar to Silvermine and Butler Hill granites (1400-1450 m.y.).
Source: GNU records (USGS DDS-6; Denver GNULEX).
- Usage in publication:
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- Johnson Shut-ins Rhyolite
- Modifications:
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- Named
- Dominant lithology:
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- Rhyolite
- Tuff
- AAPG geologic province:
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- Midcontinent region
Summary:
Johnson Shut-ins Rhyolite. Unit is part of sequence of Precambrian volcanic rocks associated with Taum Sauk caldera in western part of St. Francois Mountains, southeast Missouri (Midcontinent region). Exposed in swimming area of Johnson Shut-ins State Park. Divided into (ascending): (1) lower part, maroon ash-flow [rhyolite?] tuff with 15 to 20 percent quartz and feldspar phenocrysts and abundant lithophysae, 23 m thick; (2) middle part, cross-bedded water-deposited [rhyolite?] tuff filling former stream channel in lower member, 0 to 5 m thick; (3) upper part, gray ash-flow [rhyolite?] tuff with 15 to 20 percent quartz and feldspar phenocrysts and abundant lithophysae, 27 m thick. Overlies Proffit Mountain Formation (new); underlies Cope Hollow Formation (new). Age is Precambrian. Report includes geologic map. Unit named and described on stratigraphic column (table 1).
Johnson Shut-ins Rhyolite replaces: Hogan Mountain Rhyolite (swimming area of Johnson Shut-ins State Park) as mapped by Tolman and Robertson (1969, Missouri Geol. Survey Rpt. Inv., no. 44); and middle part of tuff of Johnson Shut-ins of Anderson (1970, Missouri Geol. Survey Rpt. Inv., no. 46).
Type section: in NW/4 SW/4 sec. 16, T. 33 N., R. 2 E., Johnson Shut-ins quadrangle, MO. Origin of name not stated by author, but probably named from Johnson Shut-ins State Park, Reynolds Co., southeast MO.
Source: Modified from GNU records (USGS DDS-6; Denver GNULEX).
- Usage in publication:
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- Johnson Shut-ins Rhyolite
- Modifications:
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- Revised
- AAPG geologic province:
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- Midcontinent region
Summary:
Johnson Shut-ins Rhyolite, uppermost formation (of 12) of /Taum Sauk Group (new) of /St. Francois Mountains Volcanic Supergroup (revised). Study area is St. Francois Mountains, southeast Missouri (Midcontinent region). Overlies /Taum Sauk Rhyolite (revised) of Taum Sauk Group; older than /St. Francois Mountains Intrusive Suite (revised). Age is Precambrian Y. Nomenclature listed in table 1.
[Conflicts with nomenclature guidelines (ACSN, 1970; NACSN, 1983, 2005, 2021): name Taum Sauk used for both group and formation within it (Taum Sauk Rhyolite has priority); name St. Francois Mountains applied concurrently to two different units in same area; a [supergroup] name combines a geographic name with the term ["supergroup,"] and no lithic designation is included.]
Source: Modified from GNU records (USGS DDS-6; Denver GNULEX).
For more information, please contact Nancy Stamm, Geologic Names Committee Secretary.
Asterisk (*) indicates published by U.S. Geological Survey authors.
"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 conflicts with nomenclatural guidelines (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005, 2021). May be explained within brackets ([ ]).