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National Geologic Map Database
Map showing publication footprint
  • Usage in publication:
    • New Chicago Member*
  • Modifications:
    • Named
  • Dominant lithology:
    • Greenstone
  • AAPG geologic province:
    • Sierra Nevada province
Publication:

Duffield, W.A., and Sharp, R.V., 1975, Geology of the Sierra foothills melange and adjacent areas, Amador County, California: U.S. Geological Survey Professional Paper, 827, 30 p., (incl. geologic map, scale 1:24,000)


Summary:

Named for town of New Chicago. Type locality: exposures in and near roadcut of HWY 49 sec 36 T7N R10E, Amador Co, CA. Adopted as uppermost member of Logtown Ridge Formation. Consists of volcanic-breccia greenstone with chiefly fine-grained clasts; forms thin, discontinuous, elongate bodies. Is 150 m thick. Conformably overlies Pokerville Member (new). Age is Late Jurassic (Callovian) and Middle(?) Jurassic (Oxfordian(?) and Kimmeridgian(?)) based on stratigraphic relations.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • New Chicago Member*
  • Modifications:
    • Age modified
  • AAPG geologic province:
    • Sierra Nevada province
Publication:

Imlay, R.W., 1976, Paleobiogeography of Jurassic deposits in the conterminous United States, IN Cohee, G.V., and Wright, W.B., Changes in stratigraphic nomenclature by the U.S. Geological Survey, 1975: U.S. Geological Survey Bulletin, 1422-A, p. A55-A63.


Summary:

Age changed based on Middle-Upper Jurassic boundary change based on top of Callovian-bottom of Oxfordian. Thus New Chicago Member is Middle Jurassic and Late(?) Jurassic age.

Source: GNU records (USGS DDS-6; Menlo 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 ([ ]).

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