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ABSTRACT: Compaction of Alluvial Overbank Mudstones

Stephen H. Anderson

The quantification of compaction of alluvial overbank mudstones is integral to the understanding of alluvial sequences and, in particular, their architectural modeling. Therefore, the identification of the major parameters controlling overbank mudstone compaction is important to all studies of alluvial sediments.

An accurate estimate of the initial porosity of the mudstones is fundamentally important to architectural modeling. Variations in initial porosity estimates will result in different interpretations of the influence of compaction. Initial porosity of the overbank mudstones is dependent on grain size, grain shape, mineralogy, and most significantly, the depositional environment. The degree of subaerial exposure of the newly deposited muds and silts will determine the porosity of the sediment at the onset of compaction. The difference in water content accounts for the variation in compaction of mudstones between semiarid and temperate sequences. In effect, climate has a direct influence on mudstone compaction.

Grain size of alluvial mudstones constitutes particle sizes ranging from clay to silt, and compaction varies according to the proportion of each grain size within the sediment. Mineralogy is an important parameter because it controls the early diagenesis within the mudstones, which may promote the growth of compaction thereby inhibiting cements.

The analysis of fine-grained sediments allows the establishment of individual compaction curves for varying grain sizes, grain shapes, and depositional environments. Therefore, when quantifying compaction in alluvial sequences it is necessary to assess the effects of these various parameters, along with depth of burial. In effect, every alluvial sequence modeled should have an individual compaction curve.

On this basis, a study of fine-grained sediments from modern alluvial environments was undertaken. Sediment samples were taken from three localities: (1) River Endrick, Scotland, (2) Solway Firth, Scotland, and (3) ephemeral streams in southern California, United States. These samples illustrate the complex relationships between the major parameters and the degree of influence these parameters exert on compaction.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990