The relationship between shellbed type and sequence architecture: examplesfrom Japan and New Zealand

Examples of lithology, fossil content and taphonomic features of shellbeds and intervening less fossiliferous intervals are presented from four Plio-P leistocene successions (Shimosa Group, Bose Peninsula, Omma Formation, Hoku riku area, Japan, and Okehu, Kai-iwi, and Shakespeare groups in Wanganui, a nd the Rangitikei Group along the Rangitikei River in New Zealand). As for pre-Pliocene 3rd- and 4th-order depositional sequences, Plio-Pleistocene 5t h- to 7th-order depositional sequences contain a variety of shellbeds which are often associated with surfaces or intervals that are characterized by sedimentary condensation, omission or erosion (e.g. sequence boundaries, ra vinement surfaces, downlap surfaces and condensed sections). Stratigraphic patterns of shellbed type tend to be similar and repetitive within a basin and a locality. This demonstrates that a specific palaeogeography played an important role in determining the nature of shellbeds. For example, shellb eds formed in the context of toplap are common only in the Shimosa Group, w hich was deposited in a moderately sheltered sea, the palaeo-Tokyo Bay. Top lap shellbeds are rare in other sequences formed in more open conditions. D espite the variability resulting from such basin characteristics, common st yles of shellbeds can be recognized that formed under conditions of marine onlap, backlap, downlap and toplap. Each type of shellbed has a characteris tic fossil composition and taphonomy. Onlap and toplap shellbeds contain lo w-diversity macrobenthic associations including Glycymeris, Mercenaria, Pap hies or other bivalves having robust shells, which are often abraded or fra gmented. Backlap shellbeds, which are equivalent to the condensed section f ormed at the maximum transgression, are characterized by dominance of epifa unal macrobenthos such as bryozoa, brachiopoda, pectinid and ostreid bivalv es, preserved in a slightly cemented, glauconitic muddy matrix. Ln contrast to fossils in such condensed sections, the shell density and species diver sity of downlap shellbed associations are rather low, and in a few examples the macrobenthic association was buried rapidly in a lower unit of the hig hstand systems tract (HST) stratigraphically located above the condensed se ctions. Variations in the stratigraphic distribution of shellbed types are reflected in symmetrical and asymmetrical sequence architectures. Symmetric al sequences have roughly the same thickness of transgressive systems tract s (TST) and highstand systems tracts (HST), and have well segregated shellb eds that were formed during marine onlap and backlap, Asymmetrical cycles h ave very thin TSTs and much thicker HSTs, and are characterized by the amal gamation of condensed onlap and backlap shellbeds. Such contrasting cycle a rchitectures are interpreted to reflect inner (symmetrical) and outer (asym metrical) shelf palaeodepositional settings. The amalgamated onlap/backlap shellbeds appear to be common in Plio-Pleistocene sequences. Owing to the s hort duration of glacio-eustatic sea-level changes with dominant frequencie s of 20,000, 40,000 or 100,000 years, shellbeds in the Plio-Pleistocene are relatively simple and thin compared to those formed in ordinary third-orde r depositional sequences. Infauna-dominated benthic associations are genera lly more common than in third-order cycles, and epifaunal associations faci litated by taphonomic feedback on sediment-starved shell-gravel substrates occur only in the condensed section corresponding to maximum transgression in most Plio-Pleistocene sequences. (C) 1998 Elsevier Science B.V. All righ ts reserved.