Akihisa Kitamura

THE RELATIONSHIP BETWEEN SHELLBED TYPE AND SEQUENCE ARCHITECTURE : EXAMPLES FROM JAPAN AND NEW ZEALAND

Abstract Examples of lithology, fossil content and taphonomic features of shellbeds and intervening less fossiliferous intervals are presented from four Plio–Pleistocene successions (Shimosa Group, Boso Peninsula, Omma Formation, Hokuriku area, Japan, and Okehu, Kai-iwi, and Shakespeare groups in Wanganui, and the Rangitikei Group along the Rangitikei River in New Zealand). As for pre-Pliocene 3rd- and 4th-order depositional sequences, Plio–Pleistocene 5th- 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, ravinement 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, shellbeds formed in the context of toplap are common only in the Shimosa Group, which was deposited in a moderately sheltered sea, the palaeo-Tokyo Bay. Toplap shellbeds are rare in other sequences formed in more open conditions. Despite the variability resulting from such basin characteristics, common styles of shellbeds can be recognized that formed under conditions of marine onlap, backlap, downlap and toplap. Each type of shellbed has a characteristic fossil composition and taphonomy. Onlap and toplap shellbeds contain low-diversity macrobenthic associations including Glycymeris, Mercenaria, Paphies or other bivalves having robust shells, which are often abraded or fragmented. Backlap shellbeds, which are equivalent to the condensed section formed at the maximum transgression, are characterized by dominance of epifaunal macrobenthos such as bryozoa, brachiopoda, pectinid and ostreid bivalves, preserved in a slightly cemented, glauconitic muddy matrix. In contrast to fossils in such condensed sections, the shell density and species diversity of downlap shellbed associations are rather low, and in a few examples the macrobenthic association was buried rapidly in a lower unit of the highstand systems tract (HST) stratigraphically located above the condensed sections. Variations in the stratigraphic distribution of shellbed types are reflected in symmetrical and asymmetrical sequence architectures. Symmetrical sequences have roughly the same thickness of transgressive systems tracts (TST) and highstand systems tracts (HST), and have well segregated shellbeds that were formed during marine onlap and backlap. Asymmetrical cycles have very thin TSTs and much thicker HSTs, and are characterized by the amalgamation of condensed onlap and backlap shellbeds. Such contrasting cycle architectures are interpreted to reflect inner (symmetrical) and outer (asymmetrical) shelf palaeodepositional settings. The amalgamated onlap/backlap shellbeds appear to be common in Plio–Pleistocene sequences. Owing to the short duration of glacio-eustatic sea-level changes with dominant frequencies 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-order depositional sequences. Infauna-dominated benthic associations are generally more common than in third-order cycles, and epifaunal associations facilitated by taphonomic feedback on sediment-starved shell-gravel substrates occur only in the condensed section corresponding to maximum transgression in most Plio–Pleistocene sequences.

Cyclic changes in lithofacies and molluscan content in the early Pleistocene Omma Formation, central Japan related to the 41,000-year orbital obliquity

Abstract Eleven cyclothems based on change in sedimentary facies and molluscan fossil associations are identified in the middle part of the Omma Formation (early Pleistocene), in Kanazawa, Central Japan. The systematic change in the molluscan fossil associations within individual cycles indicates that the oceanic conditions changed in parallel with water depth fluctuations, such that an increase in water depth corresponds to a marine climatic warming. These changes in water depth and oceanic conditions exactly coincides with the oxygen isotope fluctuations reported for deep-sea sediments. The cyclothems identified here in the Omma Formation can be explained best as the result of glacio-eustasy with a period of the 41,000-years, which corresponds to the period of orbital obliquity.

Early Holocene mud-ridge formation in the Yangtze offshore, China: a tidal-controlled estuarine pattern and sea-level implications

Abstract The purpose of the present study is to examine the topographic evolution and sedimentary characteristics of a mud-ridge system in the Yangtze offshore region. As revealed by seismic profiling, 2–3-km-wide, 5–8-km-long, up-to-15-m-thick mud ridges, situated 5–10 km apart, extend here in a southeasterly direction, approximately parallel to the direction of the ebb and flood flow of the modern tide. Directly resting upon the latest Pleistocene terrestrial sediments, some mud ridges are exposed on the present seafloor, while others are buried by landward-thickening Holocene deltaic sediments. An 11-m-long vibrocore recovered from one mud ridge reveals homogenous and mottled silty clay and clayey silt, rich in foraminifera, suggesting an estuarine environment of deposition. Chronostratigraphic correlation in association with acoustic facies indicate that the mud ridge was formed primarily during ∼10–7 ka, corresponding to a period of global decelerating rise in sea level. This mud ridge is underlain by thick, late Pleistocene fluvial to coastal fine to medium sands with an unconformity surface on top, and it is overlain by modern deltaic sediments with an erosional surface in between. We propose that the mud ridges were deposited under tidal-controlled estuarine conditions during the early Holocene deceleration of the rise in sea level. The Holocene transgression in the study area was nearly coeval with the mud ridge deposition, and the deltaic sedimentation occurred after the mud ridges formed. The ridges were subsequently reworked, largely by strong tidal currents.

Glaucony and carbonate grains as indicators of the condensed section: Omma Formation, Japan

The fifth-order depositional sequences of the Early Pleistocene Omma Formation exposed along the Japan Sea coast of central Japan were formed by glacial-eustasy during oxygen isotope stages 50 to 28. In each depositional sequence, two ecostratigraphic datums are always present: the appearance and disappearance datums of warm-water molluscan species. These datums are independent of sequence stratigraphic concepts, because the establishment of them is based on immigration events of molluscan species associated with glacio-eustatic sea-level changes. Determination of time planes shows that the appearance datum seems to occur near the midpoint of sea-level rise on the glacial to interglacial shift in deep-sea d 18 O records. In order to evaluate the significance of the condensed section in sequence stratigraphy and also to facilitate its recognition, this study examines the stratigraphic relationship of the condensed section indicators, glaucony and carbonate grains, with respect to the position of the ecostratigraphic datums in depositional sequences of the Omma Formation. The results show that the maximum concentration of carbonate grains is a more reliable maximum flooding surface indicator than the concentration of glaucony. The combination of indicators of condensed section and ecostratigraphic datums represented by incursion epiboles enables the boundary between transgressive and highstand systems tracts to be recognized in the inner shelf parts of depositional sequences. Moreover, truncation of ecostratigraphic datums during sea-level falls demonstrates significant erosion at the sequence boundaries. Integration of climatic palaeoecology and sequence stratigraphy permits a level of correlational precision of the order of a few thousands of years.

The Pliocene to recent history of the Kuroshio and Tsushima Currents: a multi-proxy approach

The Kuroshio Current is a major western boundary current controlled by the North Pacific Gyre. It brings warm subtropical waters from the Indo-Pacific Warm Pool to Japan exerting a major control on Asian climate. The Tsushima Current is a Kuroshio offshoot transporting warm water into the Japan Sea. Various proxies are used to determine the paleohistory of these currents. Sedimentological proxies such as reefs, bedforms, sediment source and sorting reveal paleocurrent strength and latitude. Proxies such as coral and mollusc assemblages reveal past shelfal current activity. Microfossil assemblages and organic/inorganic geochemical analyses determine paleo- sea surface temperature and salinity histories. Transportation of tropical palynomorphs and migrations of Indo-Pacific species to Japanese waters also reveal paleocurrent activity. The stratigraphic distribution of these proxies suggests the Kuroshio Current reached its present latitude (35 °N) by ~3 Ma when temperatures were 1 to 2 °C lower than present. At this time a weak Tsushima Current broke through Tsushima Strait entering the Japan Sea. Similar oceanic conditions persisted until ~2 Ma when crustal stretching deepened the Tsushima Strait allowing inflow during every interglacial. The onset of stronger interglacial/glacial cycles ~1 Ma was associated with increased North Pacific Gyre and Kuroshio Current intensity. This triggered Ryukyu Reef expansion when reefs reached their present latitude (~31 °N), thereafter the reef front advanced (~31 °N) and retreated (~25 °N) with each cycle. Foraminiferal proxy data suggests eastward deflection of the Kuroshio Current from its present path at 24 °N into the Pacific Ocean due to East Taiwan Channel restriction during the Last Glacial Maximum. Subsequently Kuroshio flow resumed its present trajectory during the Holocene. Ocean modeling and geochemical proxies show that the Kuroshio Current path may have been similar during glacials and interglacials, however the glacial mode of this current remains controversial. Paleohistorical studies form important analogues for current behavior with future climate change, however, there are insufficient studies at present in the region that may be used for this purpose. Modeling of the response of the Kuroshio Current to future global warming reveals that current velocity may increase by up to 0.3 m/sec associated with a northward migration of the Kuroshio Extension.

Molluscan response to early Pleistocene rapid warming in the Sea of Japan

Major changes occurred in the benthic molluscan fauna of the Sea of Japan continental shelf during the transition from Pleistocene glacial to interglacial stages, owing to rapid warming associated with the inflow of the warm Tsushima Current. Molluscan associations representing this transition occur in the lower Pleistocene Omma Formation in central Japan and suggest that there were two patterns of faunal change. The first was when warm-water species migrated into the Sea of Japan and lived along with cold-water species, accompanied by a northward shift in species ranges. The second pattern involved the migration of warm-water mollusks shortly after the local extinction of cold-water species. In the latter, it is possible that benthic molluscan communities with very low diversity and density existed temporarily and locally at inner shelf depths (<100 m) during the warming phase. Such a community has no modern analogue, but may have resulted from a marine climate with a higher seasonality than occurs today. These findings show that episodes of rapid warming have severe impacts on offshore benthic communities and molluscan species that include patterns not represented by modern faunal distributions.

Effects of seasonality, forced by orbital-insolation cycles, on offshore molluscan faunal change during rapid warming in the Sea of Japan ☆

During oxygen isotope stages 50 to 26, an offshore environment that was unsuitable for both cold- and warm-water molluscs may have temporarily prevailed in the southern Sea of Japan. This inner- to outer-shelf environment existed with a lateral scale of a few kilometers and a vertical scale of a few tens of meters during at least three transitions from oxygen isotope stages 48 to 47, 44 to 43 and 32 to 31. These deglaciation periods coincided with the three highest peaks of July solar insolation at 65°N (495, 493 and 500 W/m2) between oxygen isotope stages 50 and 26. This implies that anomalously high seasonality induced by orbital-insolation cycles is likely to have played an important role in establishing non-analog benthic communities with a very low density and diversity of molluscs in the early Pleistocene Sea of Japan.

The sequence of local recolonization of warm-water marine molluscan species during a deglacial warming climate phase: a case study from the early Pleistocene of the Sea of Japan

Abstract Changes in the fossil assemblage of offshore molluscs within each sixth-order (41-kyr) depositional sequence of the early Pleistocene Omma Formation document repeated recolonizations of warm-water species into the Sea of Japan. Each recolonization started as soon as the warm Tsushima Current began to flow in the Sea of Japan during a deglaciation and subsequent interglacial period. Here we examined in detail the stratigraphic distribution of six selected significant warm-water species and calculated confidence intervals for their first (local) appearances in a total of 10 deglaciations using continuously sampled data. They were subdivided into two groups, based on the chronological order of their first appearance during warming phases. Comparison of geological with biogeographic data reveals that these migration patterns are not always consistent with the modern geographic distributions of the species. These results show that fossil records may anticipate how the assemblage of local species would change in response to climate warming.

EXPOSED COASTS VS SHELTERED BAYS : CONTRAST BETWEEN NEW ZEALAND AND JAPAN IN THE MOLLUSCAN RECORD OF TEMPERATURE CHANGE IN PLIO-PLEISTOCENE CYCLOTHEMS

Abstract Cyclothems in the Omma Formation, on the Japan Sea coast of central Honshu, each contains two ecostratigraphic datums: the appearance and disappearance datums of warm-water molluscs. Cold-water molluscs occur both below (in early TST) and above (in late HST and possibly RST) warm-water ones (which occur in late TST and early HST). In contrast, no New Zealand cyclothems in the main Plio–Pleistocene basins contain both cold- and warm-water faunas in one cycle. Cyclothems in the Petane Group, central Hawkes Bay and in the Castlecliff section, Wanganui Basin, contain entirely eurythermal and warm-water molluscs throughout. Cyclothems in South Wairarapa have entirely eurythermal and cold-water molluscs throughout (the subantarctic Zygochlamys delicatula fauna), even in HSTs of some cycles. This contrast demonstrates the importance of exposure of the deposition site to strong coastal currents for the maximum faunal expression of temperature change. The Omma Formation was deposited in an open site on the Japan Sea coast, where currents transported larvae of a wide variety of molluscs northwards and southwards in response to temperature oscillations. The main New Zealand cyclothemic successions were deposited in large embayments, sheltered from the influence of coastal currents. The maximum faunal expression within one cyclothem of Plio–Pleistocene temperature change can be expected in only those few deposition sites that satisfy the optimum requirements: deposition of reasonably complete sequences (at least TSTs, HSTs and RSTs preserved) in an exposed coastal site influenced by strong currents from both the north and south, on the inner shelf, in the temperate regions (between ca. 30° and 60°, north and south), near the north–south oriented coast of a large continent.

Identifying possible tsunami deposits on the Shizuoka Plain, Japan and their correlation with earthquake activity over the past 4000 years:

Three washover sand beds, ranging from 15 to 34 cm in thickness, were recorded in a lagoonal mud sequence on the Shizuoka coast of central Japan, which faces the Suruga Trough. The sand beds were composed of well-sorted and well-rounded beach sand derived by a marine inundation. The basal erosion surface, mud clasts, and presence of both inverse and normal grading suggest that the sand beds formed as a result of high-energy deposition. The sand beds are multilayered, with fine alternations of sand sheets and mud drapes, which are consistent with deposition from a long-period wave train. Radiocarbon dates obtained from the three washover deposits are around ad 1000, 3565–3486 cal. yr BP, and 4000 cal. yr BP. Vertical changes in the diatom assemblages suggest a gradually increasing inflow of seawater up to the second sand bed, followed by a rapid change to freshwater conditions just above the bed. The gradual increase of seawater inflow begins again just above the second sand bed. We conclude that this series of sequential environmental changes indicates interseismic subsidence and coseismic uplift. The study area experienced around 1 m of uplift during the ad 1854 Ansei Tokai earthquake (Mw 8.4). In contrast, the area has subsided gradually (c. 6 mm/yr) during the present interseismic period. The youngest sand bed may be correlated with the ad 1096 Eicho earthquake, which caused severe damage along the Shizuoka coast.