The formation of plant compression fossils: Experimental and sedimentological investigations

Abstract

The mechanisms and processes that lead to the formation of a plant compression fossil have been experimentally reproduced and studied in the present investigation. This research has used two main lines of investigation: firstly, experimental modelling of the fossilisation process; and secondly, a detailed examination of plant compression fossils. Early experimental modelling was based on the simplest system possible. A dry compression apparatus was used in which artificial materials, representing plant and matrix, were subjected to vertical deformation. Forms were produced which closely resembled vertical sections of Sawdonia, Stigmaria, Calamites and Lepidodendron.

Further apparatus was subsequently constructed, in which wet sediment and actual plant material were compressed, thus more closely reproducing the natural system of compacting sediment. Using this apparatus, factors, such as the role of grain size, change in dimensions of the plant organ, and rigidity of plant tissues were investigated. A recirculating laboratory flume tank was used to examine the flow conditions, and other parameters that were required to produce burial of plant material and in-fill of hollow stems. The results obtained formed a basis for interpreting the conditions of formation of observed fossil stem infills. Experimental compression of the stems buried in the flume indicated the effect of an infill on the eventual form of the fossil.

Examination of plant compression fossils in the present investigation has been mainly based on a study of vertical sections through the matrix. These reveal the overall structure of the compression, and the processes leading to its formation. The Carboniferous plants, Cyperites, Alethopteris, Lepidostrobus and Trigonocarpus, have been investigated in this way.

An understanding of the factors that dictate the final form of the fossil, using the techniques described above, gives new insight into the fossilisation process. This is of potential value in reconstructing plants from fossils only known in the compression state of preservation. This is illustrated with reference to Permian ovulate glossopterid fructifications from Gondwanaland.

Computer modelling provides an alternative method for studying the compression process, and an account is given of a computer-based investigation of the deformation process.