Taphonomic expressions of sedimentary hiatuses: field observations on bioclastic concentrations and sequence anatomy in low,moderate and high subsidence settings

Field studies of post-Palaeozoic siliciclastic records reveal a strong concordance between different types of bioclastic concentrations and discontinuity surfaces within third-order sequences ( 1 My duration), supporting the use of taphonomic criteria in establishing the relative magnitudes of sedimentary hiatuses. Comparison of records across a spectrum of subsidence rates, however (from 10 m to > 1 km/My), shows that, along with appreciable changes in sequence anatomy, the nature of surface-mantling bioclastic concentrations also changes. The most significant surfaces (second- and third-order sequence boundaries, surfaces or intervals of maximum transgression, transgressive surfaces) tend to be either bare or mantled with taphonomically complex hiatal and lag concentrations. These were more consistently encountered in low subsidence than in moderate subsidence records. In high subsidence records, major surfaces were more often mantled by composite or event concentrations, if they were bioclastic at all. In all subsidence settings, comparatively minor surfaces (parasequence boundaries, bed set boundaries and bedding planes) were bare or mantled with relatively simple event and composite concentrations. Although all fossil assemblages are biased taphonomically to some degree, relative degrees of bias should almost certainly vary among discontinuities as a general rule, suggesting specific adjustments in sampling strategies for evolutionary studies.