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 (almost-equal-to 1 My duration), supporting the use of taphonomic criteria in establishi ng the relative magnitudes of sedimentary hiatuses. Comparison of reco rds across a spectrum of subsidence rates, however (from less-than-or- equal-to 10 m to > 1 km/My), shows that, along with appreciable change s in sequence anatomy, the nature of surface-mantling bioclastic conce ntrations also changes. The most significant surfaces (second- and thi rd-order sequence boundaries, surfaces or intervals of maximum transgr ession, transgessive 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 m antled by composite or event concentrations, if they were bioclastic a t all. In all subsidence settings, comparatively minor surfaces (paras equence boundaries, bed set boundaries and bedding planes) were bare o r mantled with relatively simple event and composite concentrations. A lthough all fossil assemblages are biased taphonomically to some degre e, relative degrees of bias should almost certainly vary among discont inuities as a general rule, suggesting specific adjustments in samplin g strategies for evolutionary studies.