Current radiometric dates of minerals in marine Neogene strata in California suggest a revision of the correlation between classic biostratigraphic units and the radiometric scale. The base of the lower Miocene corresponds approximately with the base of the Saucesian Stage, and it is dated at about 24.5 m.y.; the base of the middle Miocene is within the lower Relizian Stage, and, by extrapolation, it is slightly older than 19 m.y.; the base of the upper Miocene is equated with the base of the Mohnian Stage (the Globigerina pachyderma datum), and, again by extrapolation, it is no older than about 14 m.y.; the Miocene-Pliocene boundary is no older than about 9 m.y. and coincides approximately with the boundary between the Delmontian and Repettian Stages; the Pliocene-Pleistocene boundary is at least as old as 3 m.y.; and the Pleistocene-Holocene boundary is about 11,000 years B.P.
Significant planktonic foraminiferal species of the California lower Miocene include an abundance of Globigerina concinna Reuss (with 5 chambers in the final whorl), and the presence of Catapsydrax stainforthi Bolli, Loeblich, and Tappan and Globorotaloides suteri Bolli. Abundant and significant planktonic species of the middle Miocene include Globigerina concinna var. (with 4.5 chambers in the final whorl) in the upper Relizian Stage and its replacement by an abundance of G. bulloides in the Luisian Stage. Globorotaloides trema Lipps appears to be restricted to the middle Miocene. Discoaster kugleri is a good index of the Upper Luisian. Primitive orbulines appear initially within the Lower Luisian, which is correlative with the Globorotalia praefohsi Zone.
Upper Miocene planktonic events include the Globigerina pachyderma datum, the first appearance of significant populations of Globigerina pachyderma (Ehrenberg) ; this species continues to be an important and commonly dominant member of planktonic faunas throughout the remainder of the Neogene in California. “Orbulina universa” d’Orbigny, the Globorotalia menardii (d’Orbigny) group, Globigerinella siphonifera (d’Orbigny), and other important planktonic species make their initial appearances near the base of the upper Miocene. Rare specimens of “Sphaeroidinellopsis subdehiscens” (Blow) also occur within the upper Miocene. In addition, the radiolarian Prunopyle titan Campbell and Clark appears initially in the middle Miocene Luisian Stage, but it is most abundant and characteristic of the upper Miocene, disappearing at or below the upper boundary of the Miocene.
The Miocene-Pliocene boundary in California is approximately equivalent to the Delmontian-Repettian boundary and is recognized by the transition from the Miocene forms of “ Sphaeroidinellopsis" to the forms of “Sphaeroidinella,” Pliocene-to-Holocene, which is one way of defining this boundary in tropical latitudes. The California lower Pliocene Repettian Stage is defined by the first appearance of Globorotalia puncticulata (d’Orbigny), Globorotalia inflata (d'Orbigny), and Globorotalia crassaformis (Galloway and Wissler). Upper Pliocene indices of California include the initial appearance of both Globorotalia truncatulinoides tosaensis Takayanagi and Saito and Globorotalia truncatulinoides truncatulinoides (d’Orbigny). Sinistrally coiled populations of Globigerina pachyderma, definitive of cold polar waters, define important cold cycles in the upper Miocene (approximately the Upper Mohnian Stage), the middle Pliocene, and within the Pleistocene.
