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Reevaluation of Early Eocene, Penutian Stage

Kristin McDougall

The Penutian Stage was originally described as an early Eocene benthic foraminiferal stage. The type area of the stage and its two zones is in the Media Aqua Creek section of California. Faunas indicative of this stage were recognized throughout California. Planktonic foraminifers and calcareous nannoplankton studies suggest this stage is time-transgressive, ranging from planktonic foraminiferal zones P7 to P9 and nannofossil zones Tribrachiatus orthostylus to Discoaster sublodoensis. The present study reexamines the type section of the Penutian Stage and several critical sections and concludes that the Penutian Stage, with modification, represents a biochronostratigraphic stage equivalent to planktonic zones latest P7 to early P9, nannofossil zones Tribrachiatus orthosty us to early Discoaster lodoensis, and the latter part of the supercycle TA2. The time-transgressive nature of this stage was due to ecologic control of benthic foraminiferal ranges and to problems that became apparent when reexamining the type area, and faunas considered coeval. These problems include: (1) differences in taxonomy, (2) discrepancies between the species ranges in the original description of the stage and their actual range in the type area, California, and worldwide, and (3) the assumption that deposition was continuous in the type area and in other critical sections.

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