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Milankovitch Cyclicity in the Miocene-Pliocene Monterey, Pismo, and Sisquoc Formations of California

Hill Pamela S.; Behl, Richard J.; and Strickland, Heather
[email protected]

The Miocene Monterey Formation and related hemipelagic sediments of the California Margin display pronounced lithologic alternations on scales from millimeters to tens of meters in thickness. For decades workers have hypothesized the influence of the earth’s orbital parameters on the complex cyclicities recorded in these important stratigraphic sections. In this study, we demonstrate the profound influence of Milankovitch cyclicities, from eccentricity to precession time scales, on the Miocene/Pliocene stratigraphy of the offshore central California margin (~3.5 – 5.5 Ma) and extend the analysis at Montana de Oro to ~8 Ma. We apply astrochronology to the dating of the fine-grained, largely diagenetically altered sediments of the Upper Neogene Monterey and Sisquoc formations of the petroliferous Santa Maria and Pismo basins to develop refined age-depth scales and to understand the relationship between lithologic cycles and astronomical/climate forcing.

We used gamma-ray logs from Ocean Drilling Project (ODP) Hole 1016A, 150 km offshore, to create a high-resolution dating and correlation tool which could then be applied to proximal wells in the offshore basin. Temporal resolution was improved about five-fold from that capable with benthic foraminiferal zones to ~100 kyr age increments. The new age scale was then used to calculate refined sedimentation and mass accumulation rates, to identify intervals of condensed section, to show geographic trends in sedimentation rates along the paleomargin, and to suggest refinements to dating of Late Neogene radiolaria of the central California margin.

Spectral gamma-ray data from outcrops in the upper Miocene Miguelito Member of the Pismo Formation near Montaña de Oro show strong cyclicity at several frequencies and provide a tantilizing suggestion of strong cyclicity at ~100 kyr and possibly other Milankovitch cyclicities. Pending additional age control, this section would extend the Californian Neogene record of orbital cyclicity well into the Miocene.

 

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013