An image-analysis technique to measure grain-size variation in thin sections of soft elastic sediments

This paper presents a simple and fast image-analysis technique to measure grain-size variation in thin sections of soft elastic sediments. Optical and backscattered electron microscope photographs are digitised from thin sections, cut by freeze-drying to avoid disturbing the sediments. Processing of the 256 grey-scale images produces binary (black and white) images, where white pixels represent the clay-rich sedimentary matrix and black pixels represent elastic grains. Data measured include grain surface, grain perimeter, and centre of gravity. Two indices are defined to quantify grain-size: D-0 = equivalent disk diameter; and P% = phase percentages. The technique is applied to silt and clay laminations attributed to the D-division of a distal turbidite from Lake Baikal. The data obtained using image analysis are similar to the conventional descriptions of such deposits. Indices calculated on very small samples, e.g. on a single lamina, allow for greater precision in interpretation of sedimentary processes. Compared to photomicrographs, backscattered electron microscope images have better qualities and allow to quantify variations in the grain-size along a sediment core. The technique produces objective and quantified grain-size observations in less time than conventional petrographic methods. (C) 1998 Elsevier Science B.V. All rights reserved.