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Eight submarine cores, obtained by the Lamont Geological Observatory from Ampere Bank (a sunken island located at 35°00^prime N. Lat. and 13°00^prime W. Long. about 630 km. WSW of Gibraltar), were studied with the purpose of long-distance correlation of mid-Tertiary stratigraphic sequences by means of planktonic Foraminifera. Ampere Bank rises from the floor of the ocean at a depth of about 4,000 m. to a minimum depth of 53 m. from the sea-level.
The core sediments consist of white or light brown calcareous sand and lutite, except for one core taken from the western slope of Ampere Bank where the coarse fraction includes particles of volcanic rock and pyroxene. Planktonic foraminiferal tests are a major component of these calcareous sediments with a few benthonic species representing various habitats from shallow to deep water.
Miocene planktonic foraminiferal faunas occur in 4 out of 8 cores. The Miocene sediments are usually very thinly covered (10-20 cm.) by the younger sediments. Based on the stratigraphic distribution and species composition of planktonic Foraminifera, three concurrent-range zones were recognized. These are in ascending order: Globorotalia mayeri/Globigerina nepenthes, Globorotalia menardii/Globigerina nepenthes, and Sphaeroidinellopsis seminulina zones. However, no single core contains more than two of these zones. The boundary between the G. mayeri/G. nepenthes zone and G. menardii/G. nepenthes zone indicates that of the Helvetian-Tortonian stages. The planktonic faunas as found in Ampere Bank are very similar to those of the Donni sandstone in Saipan, the Nobori formation in SW Japan and the Pozon formation in Venezuela. This evidence confirms the supposed value of planktonic Foraminifera for long-distance stratigraphic correlation.
In this region Globorotalia hirsuta, which has not yet been reported from equivalent zones of the Pacific regions, makes its first appearance at the top of the G. mayeri/G. nepenthes zone and occurs abundantly in the overlying two zones.
The post-Miocene sediments are distinguished by the predominance of Globorotalia truncatulinoides, sparsity of G. menardii, and dominant dextral coiling of G. hirsuta. Several phylogenetic trends of Recent planktonic foraminiferal species are found within these three zones. Since many Recent species appear near the end of the Miocene, more refined biostratigraphic subdivision of the post-Miocene sediments is a difficult task and requires further studies.
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