Variations in phosphatic grain types and facies variations in the Oliog-Miocene San Gregoria Formation of Baja California Sur, Mexico

Conference · Fri Feb 01 04:00:00 EST 1991 · AAPG Bulletin (American Association of Petroleum Geologists); (United States)

OSTI ID:5140901

Ledesma, M; Grimm, K; Garrison, B ^[1]

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1. Univ. of California, Santa Cruz (United States)

Sediments from the San Gregorian Formation, including phosphorites,were deposited during a period of active volcanism in a fore-arc basinal setting. Three major types of phosphatic grains are present in the phosphorites: multiple coated grains, concentrically coated grains with multi-generations of phosphate accumulation around a nucleus and commonly bored, possibly by bacteria, single-coated grains with one generation of phosphatization surrounding a detrital nucleus, and non-nucleated peloidal grains that also represent single events of phosphatization. Phosphatized microstructures of probably microbial origin have been identified by electron microscopy in the multiple coated grains, suggesting possible involvement in the phosphatization process. In the northern sections of the study area, transport and deposition of phosphatic grainstones into an oxygen-poor basin occurred via turbidity currents. Here, phosphorites occur interbedded with deep-water porcelanites, cherts, and thinly laminated organic-rich mudrocks; the dominant type of phosphatic grain is the multiple coated variety, suggesting original formation on a sediment-starved outer shelf or isolated bank prior to redeposition. In the south, phosphorites are also allochthonous, but here they are associated with more detrital-rich, shallow-water lithologies (including coquinas and tidal laminities). Dominant phosphatic grains are single-coated and non-nucleated peloids, suggesting fewer episodes of phosphatization. In contrast to the northern occurrences, these southern phosphorites appear to have been redeposited on a shallow shelf.

• OSTI ID: 5140901
• Report Number(s): CONF-9103128--
• Conference Information: Journal Name: AAPG Bulletin (American Association of Petroleum Geologists); (United States) Journal Volume: 75:2
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360602 -- Other Materials-- Structure & Phase Studies
58 GEOSCIENCES
580000* -- Geosciences
CENOZOIC ERA
CRYSTAL STRUCTURE
DEPOSITION
DEVELOPING COUNTRIES
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGIC STRUCTURES
GEOLOGY
GULF OF CALIFORNIA
LATIN AMERICA
MEXICO
MICROSTRUCTURE
MINERALIZATION
NORTH AMERICA
NUCLEATION
PACIFIC OCEAN
PETROLOGY
PHOSPHATE ROCKS
PHOSPHORITES
ROCKS
SEAS
SEDIMENTARY BASINS
SEDIMENTARY ROCKS
SEDIMENTS
SURFACE WATERS
TERTIARY PERIOD
VOLCANISM