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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 10. (October)

First Page: 1691

Last Page: 1691

Title: Distribution and Depositional History of Neogene Phosphorites Along Pacific Coast of North America: ABSTRACT

Author(s): James C. Ingle, Martin B. Lagoe, Javier Helenes-Escamilla

Article Type: Meeting abstract

Abstract:

Pelletal and nodular phosphorites occur commonly to abundantly in neritic (shelf) and bathyal (slope) deposits of uppermost Oligocene (25 Ma) to upper Miocene (10 to 7 Ma) age from 23°N on the peninsula of Baja California, Mexico, to 39°N near Point Arena, northern California, encompassing a belt of deposition about 1,430 mi (2,300 km) in length. In addition, pelletal phosphorite sands are commonly present within adjacent middle and lower bathyal deposits of similar age range representing redeposited material in conduits, feeding basins, and submarine fans. In some areas, pelletal phosphorites reach 200 ft (60 m) in thickness. Mining of Miocene phosphorites is now well under way in Baja California. The age of peak formation and accumulation of Pacific Coast phos horites appears to become younger from south to north implying variations in patterns of upwelling and/or eustatic, climatic, and tectonic control of shelf character and flux of terrigenous clastics to the margin. Later reworking of the Miocene deposits has allowed reconcentration of the phosphorites in adjacent Pliocene, Pleistocene, and Holocene neritic units.

Paleontologic, isotopic, and sedimentologic evidence indicate that the widespread Neogene phosphorites formed under a special set of climatic, oceanographic, eustatic, and tectonic conditions associated or coincident with a major climatic threshold occurring in mid-Miocene (15 Ma) time and commencement of a glacial climatic state. Three key factors were apparently responsible for allowing the unusually prolific formation of Miocene phosphorites as well as simultaneous widespread deposition of diatomaceous sediments in this region including (1) vigorous upwelling of nutrient-rich water and accelerated productivity as a function of deteriorating Neogene climate, (2) associated development of intense oxygen minima impinging against the various slope and shelf areas creating appropriate b ogeochemical conditions for phosphorite precipitation, and (3) the absence or severe reduction in delivery of terrigenous clastics to sites of phosphorite precipitation as a function of Neogene eustatic, climatic, and tectonic events.

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