Techniques for the Study of Fossil and Recent Traces

Synopsis

The study of trace fossils demands an acute observational technique because the structures must be visualized in three dimensions. In lithified rocks, a massive “structureless” bed should be regarded with suspicion; the homogenization created by intense burrowing can sometimes be revealed only in the laboratory. Techniques for enhancing obscure bioturbation structures differ but little from those used for physically formed sedimentary structures: initial sandblasting or staining may be followed by x-ray radiography and infrared photography. Thin sections of burrowed sediments should be slightly thicker than the normal 0.03 mm, to produce greater contrast; if of suitable areal dimensions (5×5 cm), they may be projected directly onto a screen by a slide projector.

Great advances have been made in the replication of burrows and trails from unconsolidated sediments through the use of resins. Whereas lacquer peeling is limited by lithology and wetness, resins harden under water and can be used both to cast open burrow systems (divers have used polyesters at depths of 40 m) and to peel a wide range of bioturbated sediments. Epoxy relief peels produce outstanding results, and when taken in block form, they enable the shape, configuration, and density of burrows to be comprehended fully.

The Senckenberg box and the can corer are the most satisfactory devices for sampling both intertidal environments and—in modified form—offshore regions. These corers collect rectangularo chunks of undisturbed sediment that are ideal for serial slicing, peeling, and impregnation; vacuum impregnation with resin permits casts to be taken of burrows less than 1 mm in diameter. Thus, highly detailed block diagrams may be constructed for a variety of sedimentary fades, regardless of depth, and then compared in the closest detail with their ancient counterparts.