The Osaka sedimentary basin is a Quaternary tectonic subsidence basin including the Osaka bay and Osaka Plain. Changes of the sedimentation rate and lithofacies in the middle and upper Pleistocene are examined by major drilling cores in the Kobe area, in the northern part of the Osaka sedimentary basin. As the result, the subsidence of the Kobe area is done by continuing from ca. 1.2Ma with the subsiding rate of 0.7-0.3m/kyr. In the other, the development of the gravel layer is remarkably observed since ca. 0.6Ma in the cores which is close to the basis mountain, since ca. 0.4Ma in the cores which is far from basis mountain. It is presumed that vertical change of lithofacies is not caused by change of the tectonic movement since ca. 0.5Ma which is called Rokko Movements, but it is caused by the accumulation of the upheaval from a past.

View full abstract

Nineteen radiocarbon 14C age data of Shikotsu Pumice Fall (Spfa) and Shikotsu Pumice Flow (Spfl) deposits are newly obtained and ages of volcanic activity of Shikotsu Volcano are re-examined in this paper. Spfa consists of ten pumice and/or scoria fall deposits (Spfa1 to Spfa10) with intercalating loams. Their eruption and sedimentation ages range for about 10,000 years. Spfl is made up of pumice and ash flow deposits. According to field observations, there is no time interval between Spfl and Spfa1. Time intervals are found between Spfa2 and Spfa3-6 and also between Spfa3-6 and Spfa7-10. The eruption and sedimentation ages of Shikotsu Volcano are estimated as follows. Spfl and Spfa1 are 41,000y.B.P., Spfa2 is 42,000y.B.P. to 44,000y.B.P., Spfa3-6 is 43,000y.B.P. to 46,000y.B.P., including a datum of 45,500y.B.P. for Spfa3, and Spfa7-10 is 46,000y.B.P. to 50,000y.B.P. These ages indicate almost 10,000 years older than so far available data. It is necessary to reconsider the geological expressions of Shikotsu volcanic activity.

View full abstract

In the central Sanda Basin, more than nine tuff layers are distributed within the Paleogene Kobe Group, which is composed mainly of non-marine sedimentary rocks. Discrimination of these tuff layers as well as correlation between them are difficult, because of meso- to macroscopic structures, landslides, complication on the lateral variation of the litho-facies in the tuff layers, and numerous layers of tuff to tuffaceous rocks. In this paper, litho-stratigraphic characteristics of the Kamikume and Kusuhara Tuff Layers in the eastern part of the Okutani- and southern part of the Kusuhara-area, Yokawa-cho as well as major chemical composition of pumices and minerals in- and outside the pumices were investigated to discuss the effectiveness of petrographical analysis on tuff layers for discrimination of the tuff layers in the Kobe Group. Quartz, plagioclase, K-feldspar, biotite, zircon and opaques are the common minerals in the pumices of the Kamikume and Kusuhara Tuff Layers, while apatite is observed only in the pumice of the Kusuhara Tuff Layer. The mg# and K/(Na+K) ratios of biotite grains inside pumices in the Kamikume Tuff Layer different from those in the Kusuhara Tuff Layer. Thus, the distinct dissimilarities of the petrographical characteristics can be identified between the Kamikume and Kusuhara Tuff Layers, although the difference related to the major chemical composition of the pumices is not clear. These results emphasize that the petorographical investigation such as description of the mineral assemblages and analysis of chemical composition of biotite in the pumices is possibly effective in the discrimination between the Kamikume and Kusuhara Tuff Layers. It is also considered that the petrographical analysis mentioned above can apply to the discrimination of the other tuff layers in the Kobe Group. Sandy tuff accompanied with epiclastic minerals and granitic fragments was also collected from the Kamikume Tuff Layer in the Kinkai-area to describe the petrographical characteristics of epiclastic minerals included in the tuff layers. Major chemical characteristics of some biotite grains in the matrix part surrounding the pumices of the Kamikume and Kusuhara Tuff Layers, are similar to those of bitotite in the granitic fragment of the sandy tuff and/or the San-in Granite, although major chemical characteristics of biotite grains in the matrix part and the pumices of the Kamikume Tuff Layer are mostly same. These results indicate that the Kamikume and Kusuhara Tuff Layers contain not only pyroclastic but also epiclastic biotite grains, which were possibly derived from the basement rocks such as San-in Granite. Major chemical characteristics of biotite grains in the pumices of the Kamikume and Kusuhara Tuff Layers resemble those of biotite in some igneous rocks related to the Paleogene volcanic front extending in the San-in district, while major chemical composition of biotite in the pumices of the Kamikume Tuff Layer also compares to that of biotite in the Middel Miocene dacite lava of the Nijyo Group.

View full abstract

The Plio-Pleistocene Osaka Group, consisting of gravel, sand and mud with intercalation of several tens of volcanic ash layers, has been considered as fluvial to shallow bay sediments. The volcanic ash layer examined in this study is located at Tondabayashi City, southern Osaka, and can be correlated to Sayama volcanic ash layer by lithofacies and petrographic characteristics including mineral composition, heavy mineral composition, and refractive index of volcanic glass. This ash layer is intercalated in the mud layer interpreted to be bay floor deposits. The volcanic ash layer consists of five units namely unit I, II, III, IV, and V, which range in thickness from 1 to 4 centimeters. The unit I is composed of coarse sand to medium sand sized volcanic materials, and shows normal grading. The basal boundry is comformable. The depositional process of the unit I is derived from direct pyroclastic air fall deposition. The unit II, III and IV are composed of very coarse sand to silt sized volcanic materials, show normal grading with parallel- and cross-lamination and form incomplete Bouma Sequence (e.g. Ta, Tb, Tc, Td). These units show laterally discontinuation and changes of thickness and basal boundaries are erosive. Mud clasts included in unit II and III are divided into two types. One is white and is composed of silt sized volcaniclastics, and the other is black to dark gray and consists of non-volcaniclastic materials. Both mud clasts are interpreted as rip up clasts. These characteristics are suggestive of a turbidite, and therefore, the depositional process of the unit II, III and IV is derived from resedimentation by turbidity current on the bay floor rather than from direct pyroclastic air-fall deposition. The unit V consists of alternating beds of thin volcanic materials and mud layers. This unit is the distal facies of turbidites.

View full abstract

Nautiloid specimens identifiable as Aturia cubaensis were collected from the earliest Middle Miocene Moniwa Formation distributed in Sendai City, Northeast Honshu, Japan. This is the first record of the genus Aturia from Northeast Honshu and the most northern distribution of the Miocene Aturia in the Japanese Islands. It indicates a warm current inflow in the Sendai district during the early Middle Miocene time.

View full abstract