Cooccurrence of Fe-, Fe-Ca-, and Ca-phosphate minerals in concretions within the Monterey Formation : a record of uplift of the Santa Maria Basin, California /

Phosphatic concretions occur within diatomite in the upper part of the Miocene Monterey Formation near Lompoc, Calif. Absence of disruption of fine laminar bedding in the associated sediment by the concretions shows that they formed after complete compaction of the enclosing sediment. The concretion...

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Bibliographic Details

Main Authors:	Piper, David Z. (Author), Isaacs, Caroline M. (Author), Medrano, M. D. (Author)
Corporate Author:	Geological Survey (U.S.)
Format:	Government Document Book
Language:	English
Published:	Washington, D.C. : Denver, CO : U.S. G.P.O. ; For sale by Book and Open-File Report Sales, U.S. Geological Survey, 1993.
Series:	Evolution of sedimentary basins/onshore oil and gas investigations--Santa Maria Province ; ch. C. U.S. Geological Survey bulletin ; 1995-C.
Subjects:	Phosphate minerals > California > Santa Maria Basin. Geology, Stratigraphic > Miocene. Geology > California > Santa Maria Basin. Monterey Formation (Calif.) Phosphate minerals. Geology, Stratigraphic. Miocene Geologic Epoch. Geology. California > Santa Maria Basin. California > Monterey Formation. California.
Summary:	Phosphatic concretions occur within diatomite in the upper part of the Miocene Monterey Formation near Lompoc, Calif. Absence of disruption of fine laminar bedding in the associated sediment by the concretions shows that they formed after complete compaction of the enclosing sediment. The concretions exhibit a strongly concentric color, chemical, and mineralogic zonation. Many of them are composed of a nucleus in which vivianite is the dominant mineral. Amorphous ferric phosphate, mitridatite, and francolite are the dominant phosphatic phases in successive layers toward the surface of the concretions. Cd and As contents increase tenfold from the nucleus outward, reaching a maximum of 2,000 ppm, whereas Ni content, with a maximum of 720 ppm, and Co content show the opposite trend. This mineralogy and elemental composition favor accretion under conditions of continuously increasing Eh and pH, during uplift into the fresh-ground-water zone of the terrestrial environment. Shale-normalized rare-earth-element patterns, however, suggest a marine source for the elements biogenic debris consisting of opal-A, organic matter, and carbonates of the enclosing sediment.