SEM analysis of siderite cements in intertidal marsh sediments, Norfolk, England

Abstract

Siderite concretions and more continuous cemented layers are fairly common in intertidal marsh sediments on the coast of East Anglia. Concretions from the Norfolk coast at Warham have been examined by scanning electron microscopy in the secondary electron and backscattered electron modes, and by microprobe analysis. Siderite occurs primarily as passive void-filling cement and as coatings on quartz grains. No evidence of quartz replacement or displacement was found in the sections examined. The siderite is somewhat variable in composition, but is predominantly relatively pure with less than 6% ionic magnesium substitution. X-ray powder diffraction and SEM analyses have shown that small amounts of calcite, greigite and iron monosulphides are also present. Siderite is currently forming at very shallow depths in rapidly accreting marsh sediments which have large amounts of decaying organic matter, high bacterial activity, and low interstitial concentrations of dissolved sulphate and H 2S.