Concretions and diagenetic zones in Mississippi and Mahakam deltaic sediments are being examined to characterize diagenetic minerals and to determine possible controls on their distributions in these modern settings. Data from structurally intact samples are generated by X-ray radiography, petrography, and electron microprobe analysis. These data are supported by powder X-ray diffraction analysis of matrix and diagenetic minerals and porewater and isotope data of selected samples.
The fine fraction of Mississippi River sediment is high in Fe and composed primarily of smectites, with smaller amounts of kaolinite, illite and chlorite. Major diagenetic minerals include: siderite > pyrite > dolomite > calcite. Siderite is dominant in more freshwater samples and occurs as diffuse bodies elongated parallel to bedding planes. Two types are present: matrix siderite filling interstices between siliciclastic grains, and rims around accessory dolomite grains included in concretions. Both types of siderite are high-Ca varieties. Rim siderite appears to contain less Mn. Pyrite is dominant in marine-influenced, organic-rich samples, though pyrite-rich, peaty horizons from inland locations are found. Pyrite occurs as individual octahedral crystals and framboidal aggregates associated with organic fragments and filling rootlets. Dolomite is ubiquitous and occurs as subhedral grains which are 10-30 mM in diameter and which are usually coarser than associated siderite. Dolomite from less marine-influenced samples appears to contain more Fe than dolomite in more marine-influenced samples. Calcite is present primarily as fossil fragments and occasional concretions.
In the Mahakam Delta, fine fractions of sediments from platform and delta front sediments are composed primarily of chlorite/vermiculite, illite, and kaolinite. Major diagenetic minerals include: high-Mg calcite > pyrite > siderite. From initial examinations, carbonate concretions occur as discrete bodies that are about 1 cm in diameter and are composed primarily of high-Mg calcite and high-Ca siderite in varying proportions. Pyrite is ubiquitous in these facies and higher concentrations are associated with organic-rich horizons.
Differences between Mississippi and Mahakam diagenetic minerals may be related to differences in matrices and to differences in marine influence. In Mahakam sediments, the system is tide-dominated, resulting in higher marine influence, more sulphate, and more pyrite. These factors may result in formation of significant high-Mg calcite from essentially marine porewaters. Siderite then forms in various proportions. In Mississippi sediments, which are river-dominated, lower sulphate concentrations permit higher dissolved Fe concentrations in most of the sediments upon burial, resulting in abundant siderite formation. Exceptions occur in oxidized levee deposits and a more marine-influenced coastal boring. In addition, non-smectitic, more permeable sediments in the Mahakam Delta seem to result in formation of equidimensional concretions, versus laminar bodies more common in the Mississippi Delta.