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To better understand the sedimentary facies of tide‐dominated deltas, a core dataset from the Ba Lai palaeochannel in the Mekong River Delta was obtained and studied. Nine sedimentary facies were identified and interpreted as representing the Late Holocene evolution of the Ba Lai palaeochannel, including its pre‐abandonment and post‐abandonment phases, as well as pre‐channel phases. The channel formed at 2.6 ka as a distributary channel connected to the deltaic network and was abruptly abandoned and rapidly infilled with sediment at 0.7 ka. The channel deposits are up to ca 11 m thick and overlie shelfal shell layers, which, in turn, overlie Mid‐Holocene and Pleistocene deposits. The active‐channel fill and mouth‐bar deposits consist of sand and mud with cyclical patterns, bidirectional lenses and abundant mud layers, suggesting deposition mainly driven by river and tidal processes. The abandoned‐channel fill consists entirely of organic‐rich mud, suggesting a predominance of tidal processes. Other sedimentary facies include tidal‐flat and marsh deposits; they mostly consist of mud and formed in shallow to subaerial areas near the channel margins or on barforms. Depending on the exact location of the core in this depositional setting, three possible stratigraphic successions and facies models are presented herein. Near channel margins, the deposits show an upward gradual change from heterolithic to mud with a well‐developed fining‐upward trend. Away from the channel margins, the deposits show an upward sharp change from heterolithic to mud due to the channel abandonment. The mouth‐bar‐area facies model shows an upward gradual change from heterolithic to heterolithic/sand to mud deposits with a coarsening‐upward to fining‐upward trend. Although differences among systems likely exist, the authors suggest that the sedimentary facies described in this study and the resulting facies models should be used to better understand tide‐dominated deltaic systems and to improve their interpretation in the geological record. [ABSTRACT FROM AUTHOR]

Abundant fossil and modern specimens of the bivalve Panomya collected from Hokkaido, northern Japan, belong to five extant species, including P. norvegica and P. trapezoidis (both no longer present in Japan), and P. beringiana, P. nipponica, and P. ampla for modern specimens. I examine the taxonomy lot these species around Japan and also for the extinct P. simotomensis and P. izumo from Japan and discuss the geographic history of Panomya. Panomya originated during the latest Oligocene to middle Miocene in the northeastern Gulf of Alaska. Subsequently, P. simotomensis and P. beringiana appeared in the early to late Miocene of northeastern Asia, while P. izumo and P. trapezoidis appeared in the middle to late Miocene of northeastern Asia and Alaska. Panomya nipponica, P. ampla, and P. norvegica appeared during the Plio-Pleistocene in northern Japan and Bering regions. All extant species evolved in the North Pacific: Panomya trapezoidis and P. norvegica extended their ranges to the North Atlantic in the middle Pliocene and early Pleistocene, respectively, and P. beringiana and P. ampla migrated into the Arctic Ocean in the late Pleistocene to Holocene.

Sandy shelf sediments are important elements of clastic sedimentary systems because of their wide distribution in the geological record and their significance as hydrocarbon reservoirs. Although many studies have investigated shelf sediments influenced by waves or tidal currents, little is known about shelf sediments influenced by oceanic currents, particularly their lithofacies characteristics and stratigraphic evolution. This study investigated the stratigraphic evolution of shelf sediments off the Kujukuri strandplain facing the Pacific Ocean, which is influenced by the strong Kuroshio Current. Sediment cores were obtained from six locations on the Kujukuri shelf (34 to 124 m water depth) using a vibrocorer. The dominant lithofacies is mud‐free sand with low‐angle cross‐lamination associated with alternating beds of finer and coarser sand with cross‐lamination. These display depositional processes influenced by storm waves and the Kuroshio Current, respectively. This finding is consistent with the previously presented modern and historical observations of the Kuroshio Current and estimates of the storm‐wave base. Radiocarbon dates show that the sediment succession formed during the last transgressive and highstand stages after 13·1 ka. The depositional processes during the stages represent a transition from storm waves with abundant sediment supply to both storm waves and the Kuroshio Current with sediment starvation mainly due to its trapping in the strandplain. Comparison to other Holocene–Modern shelf systems suggests that the sandy shelf successions are strongly influenced by oceanic currents under conditions of limited riverine input and open coastal geometry. The resultant sand‐dominated succession is characterized by reversal of the proximal to distal grain‐size trend compared to the fining for most other recognized wave/storm‐dominated shelf successions. This is because of seaward increase in the influence of the Kuroshio Current. Thus, shelf deposits are naturally complex, and these may be further complicated by the additional influence of oceanic currents above the usual wave‐dominated and tide‐dominated end members. [ABSTRACT FROM AUTHOR]