Lithostratigraphy and geological structure of Mesozoic successions were re-examined in the Kurosegawa Tectonic Belt in the Uminoura area, western Kyushu. The conventional Permian to Cretaceous stratigraphy in the study area is revised, and a new late Early Jurassic to Late Jurassic stratigraphic unit, the Ashikita Group is proposed. The Ashikita Group consists of the Idenohana (newly defined), Kyodomari (newly defined) and Sakamoto (redefined) Formations, which represent discrete fining-upward sequences. The fossils newly found are Late Toarcian ammonite Haugia cf. variabilis and Middle to Late Jurassic radiolarians indicative of the Tricolocapsa plicarum to Pseudodictyomitra primitiva zones. An assessment of the previously reported Pemian and Triassic fossils advocated that these fossils were derived fossils contained in the mass-wasting deposits. Contrary, hemipelagic drape deposits have exclusively yielded Jurassic fossils, although some of which were also suspicious of reworked origin. Furthermore, the newly reported fossil data are concordant with the geological structures characterized by the northwesterly verging overturned to recumbent folds. All facts demonstrated in the present contribution indicate that the previous stratigraphy that had been assigned to the Permian to Cretaceous should be revised.
A granitic enclave, ca. lm in diameter, was found in a basalt/rhyolite composite dike along the Yoshino River at Ifashio in the central part of Nara Prefecture, Southwest Japan. The dike intruded into Mesozoic accretionary complex, and there are no outcrop of granitic rocks in the vicinity. To clarify the origin of the granitic enclave, we carried out whole rock chemical analysis with XRF and U-Pb zircon dating using laser ablation ICP-MS. The whole rock composition of the granitic enclave is within the range of the Ohmine granitic rocks, one of the member of the Middle Miocene granitic rocks of Outer Zone of Southwest Japan. The time of the solidffication of the granitic enclave is estimated to be 13.2 ± 1.0 Ma based on the youngest zircon U-Pb results. These facts suggest the existence of concealed middle Miocene granitic body beneath the Kashio region.
In-situ measurements of initial magnetic susceptibility were carried out on sediments of the Pleistocene Sahama Mud Member in order to examine the applicability of susceptibility measurements to correlation of poorly stratified or unstratified sections onshore. The measurements were done in the field at five sites with a handy susceptibility meter (Kappameter KT-9). Our measurements show that there is a linear relationship between values displayed on a KT-9 and volume susceptibilities determined with a Bartington MS2 meter. Despite lithologically homogeneous silty sediments, significant variations in susceptibility were observed at almost all the sites. At one site where four sections 4-5.5 m apart were measured, susceptibility profiles of individual sections were very similar and definitely correlated to each other. Two sites about 150 m apart also showed similar, correlative susceptibility variations. This result suggests that the simple and rapid technique of susceptibility measurements is useful for correlating poorly-stratified or unstratified sedimentary sections on land. To decipher the cause of the observed susceptibility variations, rock magnetic analyses were performed, drawing followings: (1) the section investigated has an admixture of several magnetic constituents, among which the principal contributor to the susceptibility is partly oxidized magnetite or maghemite; (2) the observed variation of susceptibility basically depends on the concentration of such ferrimagnetic minerals.
Well preserved specimens of Lockeia siliquaria James 1879 are found in the Miocene Tatsukushi Formation, southwestern Japan. This bivalve escape traces are subvertical structures, filled with convexdown, chevron-like laminae, having an almond-shaped or elliptical cross-sections. Based on depositional facies analysis of the trace fossil-bearing interval, the trace fossils are interpreted to have formed in laterally accreted point-bar deposits. Although laterally accreted point-bar deposits are uncommon in braided fluvial systems, they do occur in such environment, and sedimentologic evidence indicates that the trace fossil-bearing unit represents a sandy braided-river system. In response to the aggradation of the river bed, the trace-making bivalves probably escaped upward in the sediment leaving chevron-like laminae. Many individuals of L. siliquaria are found on a current-rippled surface of a sandstone block. Most of the specimens show preferred orientation with their long axes arranged parallel to the paleocurrent direction, which is inferred from the dipping directon of foreset laminae of the ripples. Rounded ends of the almond-shaped traces point in the down-stream direction. Such a mode of trace fossil occurrence is interpreted as the result of the trace-maker's response to unidirectional water flow. The bivalves oriented their commissure plane parallel to the current direction and pointed the exhalent (siphonal?) area on the down-stream side in order to avoid recycling of water that was expelled from the shell of the bivalve. Thus, this trace fossil can be used as a reliable indicator of the paleocurrent direction.
Radiolarian fossils were discovered from a mudstone of the Seki Formation at the Rikuchu-Seki district in the North Kitakami Belt. The mudstone containing the radiolarians comprises a part of 'chert-clastics sequences' together with stratigraphically underlying cherts. The radiolarian fauna consists of several genera including Archaeodictyomitra, Parvicingula (?), Setho-capsa, Transhsuum, Xitus (?), Zhamoidellum and others. They are generally regarded as Jurassic radiolarians, but state of their preservation is not good enough to determine preciser age of this fauna. Nevertheless, the age is considered to be middle Late Jurassic (Kimmeridgian) on the basis of the presence of Transhsuum sp. cf. Tr. maxwelli (Pessagno), Sethocapsa hexagona Hori and Zhamoidellum ovum Dumitrica. The above proves that a Late Jurassic accretionary complex (Seki Formation) is widely exposed in this district.