--> --> Abstract: Possible Milankovitch Cyclicity Recorded in Rythmically Bedded Sections of Neogene Biosiliceous Sediments (Monterey and Pismo Formations), California, by Ivano W. Aiello, Richard J. Behl, and Robert E. Garrison; #90914(2000)

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Ivano W. Aiello1, Richard J. Behl2, Robert E. Garrison1
(1) University of California, Santa Cruz, CA
(2) California State University, Long Beach, CA

Abstract: Possible Milankovitch cyclicity recorded in rythmically bedded sections of Neogene biosiliceous sediments (Monterey and Pismo Formations), California

Neogene biosiliceous sediments of California show prominent rhythmic bedding patterns on several scales. In Monterey sections of the Santa Barbara area, which were probably deposited on a bank-top, condensation and depositional hiatuses are concentrated at the Luisian-Mohnian boundary. At Arroyo Burro Beach (sedimentation rate ~2.7 cm/ky) rhythmic bedding is due to alternations of organic-rich phosphatic shale (C beds) and calcareous siliceous shale beds (S beds).

For the Upper Mohnian Pismo Formation section at Montaña de Oro near Morro Bay, the sedimentation rate was higher (~20 cm/ky) in a probable slope setting. The most prominent rhythmicity occurs as alternations between laminated beds of porcelanite and shale (S and C beds respectively). A few phosphatic hardgrounds and dolomitized horizons form condensed intervals. The basic couplet of alternating S and C beds reflects either rhythmic pulses of biogenic productivity, cyclic dilution from terrigenous sources, or both.

We plotted the ratio of the thickness of the two beds in the couplet (S/C ratio) to quantitatively analyze cyclicity by spectral analysis (FFT and Blackman-Tukey). In the Arroyo Burro section four orders of cycles are recognized: 210 ky (?), 109 ky (eccentricity), 23 ky and 19 ky (precession). In the Montaña de Oro section two cycles are recognized, 90 and 120 ky (eccentricity), and 43 ky (obliquity). Dolomite horizons appear also to occur within the Milankovitch band. In the Arroyo Burro section they are present at the 210 ky cycle peaks, while in the Montaña de Oro section their distribution roughly coincides with the eccentricity cycle.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana