Morphology and genesis of nodular phosphates in the Cenomanian Glauconitic Marl of south-east England



Phosphatic nodules are abundant in the Glauconitic Marl (Cretaceous, Cenomanian) of south-east England, particularly where the sequence is condensed. Some of the nodules are derived from the underlying Upper Greensand, and are phosphatic fossil fragments, fossil moulds, and calcareous concretions. Concretions in particular show signs of a complex history of multiple phases of boring, encrustation, phosphatisation, and glauconitisation. Phosphate and glauconite are both replacements after fine-grained carbonate sediment and cement. The majority of the phosphates are whole and fragmentary moulds of fossils. The origin of theses nodules involved: (1) infilling of shells, (2) burial, (3) prefossilisation-cementation of fossil infillings. probably by high magnesian calcite, (4) dissolution of aragonitic shell material, (5) disinterment and exposure of moulds on the sea floor, followed by (6) phosphatisation. boring. and enerustation by various organisms, and sometimes glauconitisation. Many nodules bear evidence of several cycles of cementation, exposure, mineralisation, boring and enerustation.

The closest Recent analogues to the Glauconitic Marl phosphates appear to be the phosphatic crust and nodules forming today off the coast of southern California. The features described and processes inferred from the Glauconitic Marl occurrences appear to have been widespread in nodular phosphatic facies.