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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 10. (October)

First Page: 1688

Last Page: 1688

Title: Significance of Neogene Phosphorites in Capistrano Embayment, Southern California: ABSTRACT

Author(s): Donald L. Fife, Mark E. Bryant

Article Type: Meeting abstract

Abstract:

The post-Relizian Monterey, Capistrano, and Niguel Formations comprise about 3,280 ft (1,000 m) of phosphorite-bearing marine sediments. Primary deposition appears to have been as pelletal and nodular phosphorite forming phosphoritic shales with occasional high-grade zones ranging up to 20 to 40% P2O5 and individual beds ranging up to 7 ft (2 m) or more in thickness. Most high-grade shale beds, however, are less than 3 ft (1 m) thick and average 20 to 25% P2O5. Some of the best exposures are in Aliso and Oso Canyons within the San Juan Capistrano quadrangle. Excavations for construction have exhumed weathered phosphorites at and below the water table exposing remobilized phosphate which is readily recognizable as vivianite [Fe3(PO4)2).8H2O]. The purple (azurite colored) vivianite oxidizes on drying to a brownish color within a few weeks, thus it is rarely identified during conventional field mapping.

Large-scale landsliding within the Monterey and Capistrano Formations has commonly fractured phosphorite-bearing beds and allowed mobilization and redeposition of the original phosphorite as vivianite above the basal shear plane of block-glide landslides and downstream. Water in downstream drainages has the potential for being misidentified as a pollutant from septic systems. This suggests that geochemical techniques might provide valuable methods of exploration for certain phosphate occurrences.

Of several phosphorite basal conglomerates within or between the Monterey, Capistrano, and Niguel Formations, the best exposed and one of the best developed lies along the angular unconformity between the Monterey Formation and the overlying Niguel Formation on the east flank of the San Joaquin Hills, immediately north of the U.S. Geological Survey office at Laguna Niguel. This resistant phosphorite bed is composed of what appears to be nodules derived from the underlying Monterey Formation. They have been redeposited in and just below the littoral zone. Some of the phosphorite is concentrated in downslope channels within the neritic zone.

Additional exploration within the Capistrano embayment would probably yield economic amounts of phosphate rock. Rapid urban expansion in this part of southern California will probably preclude further exploration and development. However, as the Neogene phosphorite-bearing formations of the Pacific slope become better known, there may be substantial incentive for exploration and development in other areas. California is one of the largest consumers of phosphate in the United States and imports almost 100% of its supply from Florida, Idaho, and other areas. Potentially, millions of dollars could be saved annually in transportation costs and a strategic commodity (petroleum) would be conserved, if local deposits could supply the California market.

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Copyright 1997 American Association of Petroleum Geologists