Determination of Depositional Environments of Sand Bodies Using Vertical Grain-Size Progressions

ABSTRACT An attempt to distinguish modern fluvial, coastal barrier and turbidite sands, using grain-size (acquired from an automated settling tube) analysis yields poor results when two dimensional scatter diagrams and log normal plots are used to examine samples collected at random from these environments. This is not to suggest that grain-size analysis is not a valuable "tool", but only that the environmental subdivisions used were too general. In reality, each of the environments examined consists of several different subenvironments, characterized by its own unique processes, and often by its own unique grain-size populations. Through time, these subenvironments migrate laterally, occasionally depositing an orderly, unique, stratigraphic sequence. Much previous work has been done in describing such sequences but emphasizing those features that are most easily recognized in the field. Our results demonstrate that unique grain-size progressions also exist within these sequences. By examining vertical progressions in grain-size data, it was possible to distinguish sand bodies deposited in braided rivers and alluvial fans, meandering rivers, coastal barriers and deep-sea fans with far better results than obtained by examining samples taken at random from these different environments. This approach has been used to investigate ancient sandstone bodies whose depositional origins have been previously established from stratigraphic and facies relationships and from detailed examination of sedimentary structures. Our diagnosis, based on grain-size progressions along, is the same as those of previous investigations. Hence, grain-size vertical progressions should prove to be a useful "tool" in subsurface geology in providing a means of reconstructing the depositional environment of sandstones, and thus, in predicting the lateral extent and facies relationships, and in the correlation of sand units.