--> ABSTRACT: Early Diagenesis of Interdistributary Basin Sediments--Atchafalaya Basin, Mississippi Delta Complex, by Alan M. Bailey, Harry H. Roberts, and Elizabeth Zanghi; #91038 (2010)

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Early Diagenesis of Interdistributary Basin Sediments--Atchafalaya Basin, Mississippi Delta Complex

Alan M. Bailey, Harry H. Roberts, Elizabeth Zanghi

Terrigenous clays, silts, and organic sediments are filling the topographic depressions between alluvial ridges that partition the Mississippi River's deltaic plain. As compared to many other deltaic deposits, these sediments are accumulating slowly and are therefore impacted early in their depositional history by organisms and diagenesis. Early emplacement of numerous diagenetic products suggest the possibility that minerals that survive later diagenesis may be used as indicators of early depositional conditions, a useful approach for studying ancient sequences.

To better understand these relationships, samples from six undisturbed 5-in. (12.7-cm) diameter borings [to 152 ft (96 m) long] have been examined using x-ray radiography, light and electron microscopy, and x-ray diffraction analysis. The maximum C14 age of these sediments is 8,200 y.B.P. Early diagenetic minerals identified using these methods include iron oxides, pyrite, siderite, a variety of other carbonates, vivianite, and gypsum. Iron oxides occur primarily in well-drained swamp and levee deposits, associated with permeable pathways in the sediments. Pyrite is found throughout most borings and is associated with organic-rich zones and organic fragments. It occurs as individual octahedral crystals, framboidal aggregates, and mats. Crystals commonly line rootlets and fi l cells. Siderite occurs primarily in lake sediments and lake deltas as diffuse lenticular masses. The shapes of siderite masses appear to be controlled by bedding characteristics. SEM examination indicates that manganese is present and that there are numerous inclusions of CaCO3 and quartz. The CaCO3 contains various amounts of magnesium, and diffractograms indicate dolomite in some places. Vivianite is common as radial aggregates in organic-rich, poorly drained swamp deposits. Finally, gypsum occurs as radial aggregates associated with very organic-rich zones.

The abundance of these early diagenetic minerals varies greatly with depositional environment. In some places, products of early diagenesis reach levels that may significantly influence permeability and the course of future diagenesis.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.