The Oligocene-Miocene San Gregorio Formation of Baja California Sur, Mexico: An early record of coastal upwelling along the eastern Pacific margin

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

OSTI ID:5141893

Grimm, K A; Ledesma, M; Garrison, R E ^[1]; Fonseca, C ^[2]

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1. Univ. of California, Santa Cruz (United States)
2. Univ. Autonoma de Baja California Sur, La Paz (Mexico)

The Oligocene-Miocene San Gregorio Formation (Tsg) is a lithologically diverse package of siltstones, biosiliceous sediments, tuffs, and phosphorites. In the northern part of the study area (near La Purisima), the Tsg is richly biosiliceous, organic rich, and finely laminated; in many respects, the Tsg resembles the Hames member of the Monterey formation in the Salinas basin of California. In the south, the Tsg is thinner, detritus rich, and contains minable phosphorite reserves. Allochthonous phosphatic beds are prominent throughout the formation; many of these deposits apparently imported burrowing crustacea into anoxic paleoenvironments. The authors conclude that iterative episodes of turbulent transport of well-sorted (peloidal and coated grain) phosphatic sediment commonly resulted in depositional amalgamation of discrete event beds; varying degrees of postdepositional homogenization by infauna resulted in crudely stratified to massive (and economically minable) phosphorite beds. The authors regard the association of organic-rich and finely laminated biosiliceous sediments with phosphorites as a primary index of sedimentation in an upwelling environment. In addition, they observed thick, cross-stratified accumulations of barnacle conquina; they attribute this low diversity deposit to cool-water deposition on a carbonate shoal. The distinctive occurrence of cool water carbonates at near tropical paleolatitudes suggests the ecologically limiting effect of cold upwelled waters.

• OSTI ID: 5141893
• 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

58 GEOSCIENCES
580000* -- Geosciences
CENOZOIC ERA
DEPOSITION
DEVELOPING COUNTRIES
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGIC HISTORY
GEOLOGY
LATIN AMERICA
MATTER
MEXICO
NORTH AMERICA
OCEANIC CIRCULATION
ORGANIC MATTER
PACIFIC OCEAN
PALEONTOLOGY
PETROLOGY
PHOSPHATE ROCKS
PHOSPHORITES
ROCKS
SEAS
SEDIMENTARY ROCKS
SILTSTONES
SURFACE WATERS
TERTIARY PERIOD
UPWELLING