Trace Fossils at Omission Surfaces

Trace fossils at discontinuity surfaces may be classified as three distinct ’ assemblages, according to their time relationship with the depositional hiatus. These assemblages may be called the “preomission,” “omission “and “postomission” suites. Discrimination of these suites is essential for the stratinomic interpretation of the discontinuity surface. The three suites may represent the same ichnocoenose in different modes of preservation, or the appearance of a new ichnocoenose may suggest the environmental conditions responsible for the omission of sediments and the resumption of deposition. Synsedimentary lithification of the discontinuity surface to produce a hardground brings about profound changes in the trace fossil assemblage. The omission suite is then sub divided into two distinct ichnocoenoses: prelithification burrows and postlithification borings. In certain carbonate sequences, the processes of lithification and bioturbation show mutual interference, causing irregular distribution of the cement and stenomorphism of the burrows. Mature hardgrounds commonly contained a “fossilized” system of empty burrows, the hardened walls of which provided substrates for postlithification borings, so that the two distinct ichnocoenoses form an intimately intermingled assemblage. In many cases the omission suite reveals temporary periods of sedimentary cover on the hardground, in the form of multiple excavation of the fill of the burrows and super-imposition of several suites of borings.