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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 999

Last Page: 999

Title: Oceanographic Controls on Organic Matter in Miocene Monterey Formation, Offshore California: ABSTRACT

Author(s): Colin P. Summerhayes

Article Type: Meeting abstract


Analyses of the type and amount of organic matter in Tertiary through Quaternary sediments drilled during DSDP Leg 63, off the coast of California, can be used to provide insights into the controls of deposition of the Monterey Shale. The regional oceanography, rather than basin silling, controls the accumulation of organic matter in these sediments. The laminated, siliceous, and organic-rich Monterey Shales were deposited under anoxic conditions within a well-developed oxygen minimum zone like that in today's Gulf of California. The oxygen minimum zone became strongly developed in response to increased upwelling and productivity caused by global cooling following development of an Antarctic ice sheet 13 to 14 m.y. ago. A drop in sea level 10 to 11 m.y. ago lowered the ba e of the anoxic oxygen minimum zone to water 2,500 to 3,000 m deep permitting substantial accumulation of organic matter in the late Miocene and early Pliocene of the California borderland at DSDP Site 467. The base of the minimum stayed near this level until the Quaternary, then rose to 1,500 to 2,000 m, where it remains today. Phosphorite, indicative of a high rate of supply of nutrients, formed at the same time as the Monterey Shale, during the period of intensive upwelling from 13 to 14 m.y. into the Pliocene. A lessening of upwelling and supply of nutrients, and development of the oxygen minimum zone from the early Pliocene into the Quaternary, is implied by these data. The changes are associated with a warming trend and a rise in sea level.

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Copyright 1997 American Association of Petroleum Geologists