- Usage in publication:
-
- Orellan age
- Modifications:
-
- Original reference
- AAPG geologic province:
-
- Chadron arch
Summary:
Pg. 11, pl. 1. Orellan age. Provincial time term, based on Orella member of Brule formation, type locality, Orella, Nebraska. Includes the old term "OREODON beds" used in the most extended sense. Covers interval between Oligocene Chadronian (older) and Whitney an ages. [In middle part of Oligocene; = OREODON faunal zone.] Report defines 18 provincial time terms, based on mammal-bearing units, for North American continental Tertiary. [For sequence see under Puercan.]
Type area: northwestern NE, southwestern SD, and eastern WY.
Source: US geologic names lexicon (USGS Bull. 1200, p. 2849).
- Usage in publication:
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- Orellan North American Land Mammal "Age"
- Modifications:
-
- Age modified
- Geochronologic dating
- AAPG geologic province:
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- North America
Summary:
40Ar/39Ar dates on biotite from the "Purple White Layer" (PWL) yielded a mean age of 33.91 +/-0.06 Ma (latest Eocene). PWL is an ash that marks boundary between Chadron Formation and overlying Orella Member of Brule Formation and marks the Chadronian-Orellan North American Land Mammal "Age" boundary and the Eocene-Oligocene boundary. Thus, the Chadronian and Chadron are now latest Eocene age and the Orellan and lower part of Brule are earliest Oligocene age. Ash samples taken from Flagstaff Rim section, Natrona Co, south-central WY, Wind River basin. Tables of isotopic dates.
Source: 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 ([ ]).