Publication Date:
2017-01-26
Description:
In tropical areas four important planktonic foraminiferal datum planes serve
to delimit and subdivide the Miocene. The Globigerinoides trilobus datum defines
the base of the Lower Miocene, the Orbulina datum defines the base of the
Middle Miocene, the Globorotalia menardii datum defines the base of the Upper
Miocene, and the Sphaeroidinella dehiscens datum defines the top of the Miocene
(Bandy, 1963, 1964). In tropical areas, Praeorbulina and orbulines appear initially
at the base of tlie four Globorotalia fohsi zones whereas the Globorotalia
menardii datum is at or near the upper limit of the G. fohsi zones. By contrast
in temperate areas of the eastern Pacific, off northern Mexico and in California,
Praeorbulina and orbulimes appear initially in the upper p a ri of the G. fohsi
zones (G. praefohsi zone); Orbulina universa usually appears at the base of the
Upper Miocene or with the Globorotalia menardii group.
In the Aquitaine Basin of France, orbulines first appear at a midpoint in the
Globorotalia fohsi lineage. In the Western Garpathians, orbulines and Praeorbulina
appear first just below the Upper Miocene Globigerina nepenthes zone of
the Tortonian. It is suggested th a t the Karpatian and Sallomacian Stages of
the Garpathians are approximately equivalent to the Middle Miocene zones of
the G. fohsi lineage.
The later appearance of orbulines in higher latitudes of some areas than in
the tropics is attributed to a polyphyletic origin. Differences in chronology of
oppearance is supported by comparing appearances in different areas with
other planktonic foraminiferal lineages. A polyphyletic origin of orbulines is
supported by the development of different species of Globigerina and Globigerinoides
inside of orbuline chambers, by the differences in wall structure of
orbulines, and by laboratory cultures and the biology of the living forms of
orbulines.
Most temperate orbulines probably arose from Globigerina bulloides, Globigerina
praebulloides, or Globigerina quinqueloba in the later Middle Miocene.
Most tropical orbulines probably arose from Globigerinoides sicanus originally
at the base of the Middle Miocene and from still ottier species of Globigerinoides
subsequently.
Type:
Article
,
PeerReviewed
Format:
text
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