Early diagenetic phosphate cements in a turbidite basin

Abstract

Summary Early diagenetic phosphate cements of late Llandovery age developed in fine-grained turbidites deposited in the Welsh Basin. Bottom water conditions fluctuated between dysaerobic and anaerobic. Phosphate enrichment of layers close to the tops of fine-grained turbidites occurred very rapidly after turbidite deposition. Early phosphate cement precipitation caused lithification of these layers prior to significant sediment compaction. The cements consist of apatite crystallites (0.5–5 µm in diameter) which filled pore spaces and also grew displacively in the cleavages of detrital phyllosilicates. Two possible mechanisms for the localization of cements close to turbidite tops are (a) bacterial concentration of phosphate in near-surface layers of transition between oxic and sulphidic environments, and (b) adsorption of upward-diffusing dissolved phosphate from underlying anoxic porewaters by ferric and manganese oxyhydroxides present above levels of oxygen and nitrate exhaustion. Phosphate cements are absent from adjacent Lower Silurian shelf facies deposited in fully oxic bottom water environments, and are also absent from turbidites deposited in totally anoxic bottom waters. In the former case oxidation of labile organic matter close to the sediment-water interface prevented a source of dissolved phosphate from being buried, whereas in the latter case phosphate would have been free to diffuse out into bottom waters.