Neogene Planktonic Events and Radiometric Scale, California: ABSTRACT

Abstract

More than seven significant events are recorded by planktonic microfossils of the Neogene for the eastern Pacific and the Pacific Coast of North America, correlated with provincial microfaunal stages and a radiometric scale based on available K-Ar dates. (1) There was a significant evolution from Globigerina concinna to Globigerina bulloides about 17 m. y. ago marking the approximate Relizian-Luisian boundary of California. (2) The youngest known dextral specimens of Globoquadrina altispira occur about 12 m. y. ago correlating with part of the upper lower Mohnian. (3) The youngest known sinistral specimens of Globorotalia maveri occur about 12 m. y. ago, correlating with part of the lower Mohnian. (4) The radiolarian, Prunopyle titan, spans the interval of about 15-10 m. . B.P., becoming extinct before the end of the Miocene (Delmontian). (5) The transition from Sphaeroidinellopsis to Sphaeroidinella, the SPHAEROIDINELLA DEHISCENS DATUM, and the introduction of Globorotalia inflata marks the Miocene-Pliocene boundary (approximate Delmontian-Repetto boundary) and is about 9 m. y. B.P. (6) Globorotalia (Truncorotalia) truncatulinoides ranges from basal Pliocene to Recent; however, it is most characteristic End_Page 453------------------------------ of the 5-3 m. y.-interval or upper Pliocene of southern California. (7) Coiling preferences of Globigerina pachyderma are predominantly dextral during the past 11,000 years, mostly sinistral from this point back to 3 m. y. B.P. (Pleistocene), dextral although somewhat variable in the 3-5 m. y. interval (upper Pliocene), sinistral in the 5-7 m. y. interval (middle Pliocene), dextral in the 7-10 m. y. interval (lower Pliocene and uppermost Miocene), sinistral in the 10-11 m. y. interval (upper Mohnian-Delmontian transition), and dextral in its earliest occurrences of about 11-12 m. y. B.P. (within the Mohnian). End_of_Article - Last_Page 454------------