Facies Architecture in Clastic Sedimentary Basins

Abstract

Clastic deposits can be subdivided into microforms, mesoforms, and macroforms [using Jackson’s (1975) terminology], reflecting a range of physical scales of deposit and the time scale during which they form. A formal three-dimensional subdivision of some types of deposit, which clarifies these physical and temporal scales, can now be attempted using the concept of a hierarchy of internal bounding surfaces. Brookfield’s (1977) three-fold classification of eolian bounding surfaces is widely accepted. Allen’s (1983) comparable attempt to classify fluvial bounding surfaces is also useful but, it is suggested here, should be expanded to a six-fold hierarchy, mainly to facilitate the definition of macroforms. The three-dimensional depositional units so identified can be classified using standard facies-analysis techniques. To emphasize their three-dimensional nature they are termed “architectural elements.” The lithofacies composition, external shape, and internal geometry of these elements are characteristic of various suites of processes in depositional systems; for example, the well known lateral-accretion deposit represents the accretion of bank-attached bars inside meander bends of channels in rivers, deltas, tidal creeks, and submarine fans. Analysis of vertical profiles, in particular their cyclicity, was formerly a vital component of facies analysis methodology. However, in many situations, especially where the architectural elements have complex three-dimensional shapes, such analyses have become much less important. For example, subsurface mapping techniques can make use of petrophysical log correlations in well developed oil and gas fields, and recent advances in three-dimensional seismic data-display techniques permit many elements to be mapped using horizontal seismic sections. Most of the ideas discussed in this paper evolved from studies of nonmarine deposits, but it is suggested that there is considerable potential for further development in the field of facies analysis methodology by the application of these concepts to marine deposits, including those in which chemical sediments are deposited by clastic processes.