Paleoecological and paleoenvironmental information decorded from hard substrates ichnofacies

钻岩痕迹化石以双壳类、腹足类、腕足类、藤壶类、多毛纲环节动物、海绵、苔藓虫等钻岩生物为特征。一般来说，这些痕迹化石可作为古岩岸的指示物，对重建古环境具有重要意义。北海道浅海相沉积于中新世，盛产钻岩双壳类。我在北海道北部的两个地方研究了岩石钻孔双壳类和它们的棒状钻孔。在阿克波诺地区，三克别苏组基底部不整合覆盖基底，由细粒砂岩组成。在基底中可识别出钻岩双壳类动物，如：kotakae Penitella和Platyodon nipponica，而在基底中以Gastrochaenolites turbinatus的棒状钻岩为主。该产状为原生产状，推测为古岩岸的浪蚀滩。在大和地区，大和组基底部不整合覆盖基底，由基底砾岩组成。深孔双壳类在砾岩的碎屑中占多数，在地下室中没有。钻岩双壳类（P . sp.）和它们的棒状钻岩类（G. turbinatus）在碎屑中占主导地位。该产状为半原生产状，推测为古岩岸的巨石滩。这些地区的软体动物和鱼化石组合的古生态特征反映了海平面海侵期间沉积环境的变化。岩石海岸形成于中新世海侵事件。海洋岩石表面的生物侵蚀被证明是揭示地质事件的一个很好的工具，如海平面的相对振荡、侵蚀/沉积速率和物理扰动的强度。

Rock-boring trace fossils are characterized by boring organisms such as bivalves, gastropods, brachiopods, barnacles, polychaete annelids, sponges, bryozoans, etc. In general, these trace fossils are used as indicators of ancient rocky shores, and are useful for reconstruction of pal eoenvironments.The shallow-marine facies of Hokkaido rich in rock-boring bivalves was deposited during the Miocene. I studied rock-boring bivalves and their clavate borings at two localities in northern Hokkaido. In locality Akebono, basal part of the Sankebetsu Formation unconformably overlies the basement and is composed of fine-grained sandstone. Rock-boring bivalves such as Penitella kotakae and Platyodon nipponica are recognized, and the clavate boring of Penitella, Gastrochaenolites turbinatus, dominated in the basements. This occurrence represent autochthonous, and is presumed to wave-cut bench of ancient rocky shore. In locality Yamato, basal part of the Yamato Formation unconformably overlies the basement and is composed of basal conglomerate. Boring bivalves dominated in clasts of conglomerates, and no occurrence on the basements. Rock-boring bivalves, P sp., and their clavate borings, G. turbinatus, dominated in the clasts. This occurrence represent semi-autochthonous, and is presumed to boulder beach of ancient rocky shore.The paleoecological characteristics of the molluscan and ichnofossil assembleges in these localities reflect changes in the sedimentary environment during a sea-level transgression. The rocky shores were formed during the transgression events in the Miocene time. Bioerosion on marine rocky surfaces demonstrates to be a very good tool to reveal geological episodes such as relative oscillations of the sea-level, erosion/sedimentation rates and intensity of physical disturbance.