Abstract

Despite conventional wisdom that fossil preservation requires rapid burial, stratigraphic and taphonomic analysis of Neogene mollusc-dominated shell beds indicates that most macroinvertebrate concentrations in shelf siliciclastics record conditions of low net sediment accumulation. These shell beds thus can mark stratigraphically significant surfaces and usually provide a condensed record of the depositional hiatus. Moreover, the dynamics of sediment accumulation can be reconstructed from single outcrops or cores on the relatively objective basis of shell bed contacts, which can be sharp or gradational. Hiatal shell concentrations can be: END-CYCLE, capped by an omissional (type I) or erosional surface (type II) and recording a deceleration in sediment accumulation (e.g., progressive starvation, or bypassing such as during toplap); BASE-OF-CYCLE, resting on an omissional (type III) or erosional surface (Type IV) and reflecting acceleration in siliciclastic accumulation (e.g., onlap, downlap, post-ravinement deposition); MID-CYCLE (composite I-III; slowdown then gradual resumption of sedimentation); or INTER-CYCLE (composite II-IV; slowdown, partial truncation, then resumption). These shell bed types are well represented in 3rd and 4th order sequences from passive (Maryland) and active continental margins (California), where they record hiatuses of (?)3 m.y. duration. Whether the model applies to hiatuses of greater duration is as yet unknown; the model may fail in Paleozoic sequences owing to very rapid postmortem disintegration of brachiopod and trilobite hardparts.