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Recognition of Foraminifers in Petrographic Thin Section, Monterey Formation, Santa Maria Coast

Kenneth L. Finger

Foraminifera are often found on petrographic slides of mudstones from the California Neogene, but the lack of a reference for identifying sectioned tests has made it impossible for these observations to yield much information. In a study of coastal outcrops of the Monterey Formation in the Santa Maria basin, this shortcoming was particularly evident, as many of the foraminiferal rocks are highly indurated and the specimens commonly are too eroded and fragile to survive conventional disaggregation procedures. Also, foraminifers encased as nuclei of phosphatic pellets remain obscured in washed residues. In order to overcome these problems, well-preserved specimens from coeval strata elsewhere (e.g., upper Newport Bay, Naples Beach) were isolated, identified, and thin sectio ed for comparison with the specimens seen in petrographic slides derived from the Mussel Rock, Lions Head, and Boathouse Beach (Rodeo Canyon) sections. An innovative thin-sectioning technique devised for this analysis has been useful in interpreting foraminiferal assemblages that are not readily extracted from the rocks.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.