Ichnofossil assemblage and sedimentary facies of the middle Oxfordian Arimine Formation, southeastern Toyama Prefecture (The Upper Jurassic Kuzuryu Group in the Toyama area, Part 1)
生痕化石群集に注目して,富山県南東部に分布する上部ジュラ系九頭竜層群有峰層の堆積相について考察した.本報告の要約は以下の4点である.(1)有峰層の中部に,Ophiomorphaを主体とする生痕化石群集の産出区間を発見した.(2)産出区間の岩相は,スウェール状斜交層理の発達した細粒砂岩と生物攪拌の顕著な泥質細粒砂岩の互層である.(3)生痕化石群集は,暴浪イベント直後の日和見種からなるSkolithos生痕相と,晴天時に平衡種の形成したCruziana生痕相との混合相を示す.(4)産出区間の堆積相は,しばしば暴浪イベントの影響を受けた中部~下部外浜である.
The shallow marine Upper Jurassic Kuzuryu Group, distributed in north-central Japan, is important in paleontological studies of East Asia during late Mesozoic time. However, sedimentological and ichnological information on the group is still insufficient. This is a preliminary report on the sedimentary facies and ichnofossil assemblage of the middle Oxfordian Arimine Formation (upper part of the Kuzuryu Group), southeastern Toyama Prefecture. Results of the study are summarized as follows:
(1) A mixed Skolithos-Cruziana ichnofacies dominated by Ophiomorpha, is recognized in the middle part of the Arimine Formation.
(2) The ichnofossil-bearing interval consists of alternating beds of swaley cross-stratificated fine-grained sandstone and intensely bioturbated muddy fine-grained sandstone.
(3) Sedimentary facies of the interval is suggested to be a moderately storm-dominated, middle to lower shoreface.