Paleoecological and paleoenvironmental information decorded from hard substrates ichnofacies

钻岩遗迹化石以钻探生物为特征，如双壳类、腹足类、腕足类、藤壶类、多毛类环节动物、海绵、苔藓动物等。这些遗迹化石通常被用作古岩质海岸的指示物，有助于重建古生态环境。北海道浅海相富含钻岩双壳类，沉积于中新世。我在北海道北部的两个地方研究了钻石式双壳类和它们的棍棒类钻探。在Akebono局部，Sankebetsu组的底部不整合地覆盖在基底上，由细粒砂岩组成。识别出的岩石钻孔双壳类有Penitella kotakae和Platyodon nipponica，而Penitella的碎屑钻孔在基底中占主导地位。该产状代表原地，推测为古岩质海岸的波浪切割台地。在Yamato局部，Yamato组的底部不整合地覆盖在基底上，由底部砾岩组成。沉闷的双壳类主要分布在砾岩的碎屑岩中，地下室中没有发现。碎屑中主要分布着钻石型双壳类P sp.和它们的棍棒类钻孔虫G.turbinatus。本区软体动物和遗迹化石组合的古生态特征反映了海侵过程中沉积环境的变化。岩质海岸形成于中新世的海侵事件。海洋岩石表面的生物侵蚀是揭示海平面相对波动、侵蚀/沉积速率和物理干扰强度等地质事件的一种非常好的工具。

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.