About This Item
Share This Item
Wells drilled in the central portion of the Los Angeles basin normally penetrate an unbroken depositional sequence from the upper Pliocene into the Mohnian stage of the upper Miocene. For some twenty years oil company microstratigraphers have placed the Miocene-Pliocene boundary at the base of the continuous occurrence of such typical Repetto foraminifera as Karreriella milleri Natland, Hopkinsina hispida (Schwager), Bulimina rostrata Brady, etc., and at the approximate upward limit of Rotalia garveyensis Natland, Uvigerina hootsi Rankin, and a related costate Uvigerina. Furthermore, there is a pronounced change in the preservation of the forams as the tests of the Miocene forms are generally so badly crushed by compaction that only a small portion of the fauna can be ext acted by washing. Many foraminiferal species are common to both the lower Pliocene Repetto and the upper Miocene Delmontian, and in the Delmontian there are rare intermittent occurrences of some of the more typical lower Pliocene forms.
There is no marked lithologic break at the boundary, but the Miocene shales in general tend to be more laminated, and short intervals of hard, platy, "poker chip" type shale are commonly encountered a short distance below the contact. Platy shales become more abundant with depth, and the color gradually changes from the dominant hair brown of the lower Repetto to the dark chaetura drab of the upper Miocene. In the laminated Miocene shales, the foraminifera normally are concentrated in thin layers, while in the more massive Pliocene shales they are rather evenly distributed throughout the matrix.
End_of_Article - Last_Page 2322------------