Abstract
Small-scale transgressive-regressive cycles can be recognized in the lower part of the Tatamigaura Sandstone Member of the early middle Miocene Tougane Formation, Shimane, Japan. Each cycle ranges from 0.9 m to 6 m in thickness. Facies analysis shows that sedimentary environments ranged from shoreface to inner shelf. Transgressive deposits generally consist of shell bed or shell rich-horizon on or above an erosional surface. These shell beds were formed under conditions of shoreface erosion and sediment bypassing in the early transgressive phase. Highstand deposits are poorly fossiliferous and are characterized by physical sedimentary structures such as hummocky cross-stratification. Each cycle probably resulted by high frequency glacio-eustastatic sea-level changes inferred to be less than 50 m.
Eight macroinvertebrate fossil associations or assemblages are recognized in the cycles. Distribution pattern of macroinvertebrate fossils is interpreted to have been controlled by changing sedimentary regime and sedimentation rate. Shoreface erosion and sediment bypassing in early transgressive phase may have prevented colonization by most of the shelly benthos, except for Glycymeris which are adapted to environments where erosion is common. High diversity associations including Anadara, Dosinia and Clementia appeared during the maximum flooding phase, in physically stable environments characterized by low rate of sedimentation due to reduced sediment supply, where risk of being buried alive or being reworked is low. Physical disturbance was also common and rapid deposition occurred in highstand/regressive phase. Only bivalves adapted to this habitat are found here, such as those possessing long siphons (Panopea) or swimming ability (Mizuhopecten).
