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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 26 (1956)No. 4. (December), Pages 289-300

Thickness of Strata

Vincent C. Kelley


"Thick" and "thin" applied to bedding are terms that are relative to one's stratigraphic training and experience and relative to the associated strata. The terms, "thick bedded," "medium bedded," and "thin bedded" should not be used without definition, although undefined usage is common in the current literature. Recent efforts to standardize the limits of "thick," "medium," and "thin" for bedding are in considerable disagreement.

Standardization or pigeon-holing of thicknesses may be necessary only for statistical treatment of stratification. However, in such studies mean bed thickness, thickness frequencies, and other statistical indices may serve to better describe and understand the origin of the assemblage. Many formations possess stratification facies (phyllofacies) as well as biofacies, and lithofacies. It may be just as productive to determine regionally the stratification indices or ratios as it is to study sand-shale ratios. It may be possible and useful to construct regional iso-stratification (isobed) maps just as it is to construct isolith maps.

It is proposed that the beddedness of sedimentary units be expressed by the following index.

Stratification index =[no. of beds^times100]/[thickness]

The task of determining, measuring, and counting individual beds is large and subjective. However, it may consist of a partial sampling just as sampling for lithology or chemical composition commonly is. I am convinced from my consideration of the quantitative and statistical approach to bed thickness that the additional effort involved yields a much better awareness and appreciation of the nature and significance of bedding than without such an approach.

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