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Mid-Cretaceous ammonite zones, Western Interior, United States
| Citations: | 5 - 0 self |
BibTeX
@MISC{Cobban_mid-cretaceousammonite,
author = {William A. Cobban},
title = {Mid-Cretaceous ammonite zones, Western Interior, United States},
year = {}
}
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Abstract
The Western Interior of the United States contains an important sequence of middle Cenomanian-late Turonian ammonites. Ammonites from the northern part of the Western Interior are mostly North Temperate forms, whereas those from the southern part are a mixture of Tethyan and North Temperate forms. The ammonite succession in the southern part can be more finely zoned than that of the northern part, and offers better possibilities for international correlation. In the zonation, the boundary between the middle and upper Cenomanian is placed at the top of the zone of Plesiacanthoceras aff. P. wyomingense. The top of the Cenomanian is drawn at the top of the zone of Neocardioceras juddii as recommended in a 1981 study by C. W. Wright and W. J. Kennedy. However, Mammites nodosoides, which occurs three zones higher, is usually regarded in Europe as basal Turonian, although Wright and Kennedy have now recognized a zone of Watinoceras coloradoense at the base, which lies below the zone of Mammites nodosoides. In the Western Interior, W. coloradoense is restricted to the zone of Vascoceras (Greenhornoceras) birchbyi, which is separated from the zone of Neocardioceras juddii by a zone of Pseudaspidoceras flexuosum. Inasmuch as the ammonites in the latter zone are closely related to those in the zone of V. birchbyi, a Turonian assignment seems best for the zone of P. flexuosum. Collignoniceras woollgari, which is widely distributed in Europe, Asia, and North America, is usually
Keyphrases
western interior united state mid-cretaceous ammonite zone northern part southern part neocardioceras juddii mammites nodosoides north temperate form latter zone important sequence ammonite succession international correlation collignoniceras woollgari north america watinoceras coloradoense turonian assignment pseudaspidoceras flexuosum basal turonian middle cenomanian-late turonian plesiacanthoceras aff upper cenomanian
