Introduction

Because of a large body of theory regarding sediment erosion, transport, and deposition, and the relative ease of flume and settling experiments, the movement of individual shells and bones as sediment particles has probably received more attention than any other aspect of taphonomy. Nevertheless, the extent of transport in natural assemblages may still be difficult to discern. Ideally, fossil assemblages consist of skeletal remains that are autochthonous. Normally, however, shells and bones are moved to a minor extent from their source (parautochthonous) or, in some cases, hardparts are transported considerable distances and mixed with other assemblages and comprise allochthonous components of an assemblage.

The enclosing sediment (matrix) is no less important than fossils in discerning operative taphonomic processes. The sediment may be of two basic types: (1) terrigenous, which is derived from land (often from outside the basin), is almost invariably siliciclastic, and is transported to some degree before deposition; and (2) carbonate, which normally consists of the autochthonous or parautochthonous remains of organisms living within the depositional basin (Maiklem, 1968). Thus, there is typically greater opportunity for hydraulic factors to modify the size distributions and surface features of biogenic and non-biogenic particles of terrigenous deposits than in pure carbonate sediments. Smith (1977) published a dimensionless parameter (H) for physical mixing that represents the ratio of the depth of physical erosion to the frequency of erosion relative to sediment accumulation rate (see also Chapter 4).