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Unusual occurrence of glauconite in a shallow lagoonal environment (Lower Cretaceous, northern Aquitaine Basin, SW France)
Author(s) -
El Albani Abderrazzak,
Meunier Alain,
Fürsich Franz
Publication year - 2005
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/j.1365-3121.2005.00646.x
Subject(s) - glauconite , geology , sedimentary depositional environment , marl , cretaceous , facies , brackish water , sedimentology , geochemistry , foraminifera , paleontology , structural basin , mineralogy , salinity , oceanography , benthic zone
Some minerals are considered to be representative of specific natural environments. Among them glauconite is considered to be formed in marine deep platform conditions. However, the term glauconitic is misused to designate green minerals formed in marine outer‐shelf environments. X‐ray diffraction, scanning electron microscopy and energy‐dispersive X‐ray analyses of individual green materials were carried out leading to the identification of glauconite in the Purbeckian facies. Green glauconite grains predominantly occur as burrow fills and occasionally as faecal pellet replacements. Two depositional environments have been identified from bottom to top of the succession: (1) an argillaceous dolomitic lagoonal sediment formed in saline, shallow water; (2) a marl‐limestone alternation deposited in a brackish water estuary. The crystallochemical properties of the glauconites change abruptly. These findings show that glauconite may form in coastal environments and not only in mid‐shelf to upper deep water platform environments as classically assumed. Moreover, the glauconite composition changes with the chemical conditions imposed by the local environment.