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ABSTRACT: Geochemical Indicators of Subsidence in Sediment, Terrebonne Coastal Plain, Louisiana

Alan M. Bailey, Harry H. Roberts

Sediments comprising the Terrebonne Coastal Plain consist primarily of clays, silts, and peats that fill the depressions between alluvial ridges created by former Mississippi River courses. These sediments are subsiding at variable rates. Depositional history, sedimentation rates, and environment of deposition affect both the types and abundances of diagenetic products found in deltaic sediments. Early diagenesis influences the geotechnical properties of these coastal plain deposits and their local subsidence rates. Diagenetic mineralogy and elemental geochemistry therefore offer clues to understanding some of the variability associated with subsidence in the lower deltaic plain.

To determine relationships between geochemical features and subsidence rates, samples were collected from undisturbed 12.5 cm diameter borings where subsidence rates had been determined by radiocarbon dating. Diagenetic features in these samples were identified using X-ray radiography and later examined more closely using light and electron microscopy, X-ray diffraction, and various chemical methods of testing.

Results show that diagenetic minerals consist of carbonates (siderite, calcite, dolomite, and rhodochrosite), iron sulfides, vivianite, and iron oxides. In general, siderite and other carbonates as well as pyrite are higher in the fine-grained swamp and lacustrine clays of the upper/middle deltaic plain environments where sediment compaction and subsidence rates are considerably lower than rapidly deposited lower delta and marine deposits. Prodelta clays, for example, contain fewer varieties and lower abundances of diagenetic inclusions than their freshwater free-grained counterparts. Oxidized sediments of well-drained swamp and natural levee deposits that contain goethite, iron oxides, manganese oxides, and carbonates are stabilized early in their depositional history and therefore d not lend themselves to rapid volume reduction associated with dewatering and compaction. Elemental concentrations can also be interpreted to reflect relative diagenetic activity in the sediments. These preliminary results indicate that close examination of diagenetic mineralogical and geochemical features can provide valuable information concerning the subsidence history of delta plain areas.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990