Obsidian artifact assemblages and chipping wastes have been commonly found in various prehistoric sites in the Ishikari-Tomakomai Low-land area of Hokkaido. On the other hand, obsidian lavas have been known to crop out in the following four widely scattering localities in Hokkaido; Akaigawa (A), Shirataki (S), Oketo (O) and Tokachi (T) as shown in Fig. 1. In order to seek correlation between obsidian artifacts and naturally cropping out obsidian, the author tried fission track age determination (Tables 1 to 3) and neutron activation analysis on them (Tables 4 to 6). They are meant to find the source areas for obsidian artifacts from each site. Obsidian chips found in Sankakuyama site of Shukubai, Chitose, which is of the Late Paleolithic age, reveal fission track ages and chemical contents well coresponding to those measured on obsidian of Akaigawa (A) and Shirataki (S) (Figs 4 and 8). Thus they indicate that the source areas of obsidian artifacts of Sankakuyama site must be sought in Akaigawa as well as Shirataki obsidian. On the contrary, obsidian chips obtained from the Kashiwagi-B site of Eniwa, which is from the latter part of the Jomon age to the end of it, show slight divergent ages and chemical contents. Yet when carefully analysized these values reveal several concentration areas (Figs 5 and 9). They nicely correspond to those figures measured on the obsidian of Akaigawa, Oketo, Shirataki and Tokachi respectively. Therefore obsidian artifacts from the Kashiwagi-B site are thus inferred to have come from all of these four localities. The result mentioned above may be rather interesting, because one may consider that man could only get obsidian as material for artifacts from relatively nearby localities in the old days, while in later ages they could utilize obsidian originally cropped out in far distant localities, either through trade, and/or transportation.

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The Shibata-Koide tectonic line has been proposed to occur along the boundary between the preTertiary basement region and the Niigata Oil Fields, although its nature has not been clarified. The authors carried out a gravity exploration in order to make its character clear in the eastern margin of the Niigata Plain, where the line is topographically distinct, and a detail geological survey was performed by the Collaborative Research Group for the Sasagami Hills (1980). The following results were obtained through the present investigation. 1. The Shibata-Koide Tectonic Line which has been assumed through geological survey was certified to occur as a large normal fault. The total throw is estimated to be about 1,500 m vertical in the southern part of the investigated region, and about 4,000 m in the northern part. 2. This line, however, does not consist of a single fault, but of four to five sets of normal faults, namely a step-fault system with westerly throwdown sence. The cross section drawn from the gravitational exploration is consistent with those presumed by the Collabarative Research Group for the Sasagami Hills (1980). 3. On the west side of the line, the depression zone parallel to the line was found in the basement underlaid with the Tertiary sediments. This depression zone may have been existed at the stage of sedimentation of the Nishiyama Formation and is probably related to the Shibata-Koide Tectonic Line. 4. The Shibata-Koide Tectonic Line is thought to be an significant tectonic boundary between the pre-Tertiary basement region and the Tertiary to Quaternary sedimentary basin as proposed by YAMASHITA (1970).

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The Loam formations, weathered volcanic ash deposits, is ditributed on the terraces and the hills along the River Shinano-gawa from Tsunan to Nagaoka in Niigata prefecture, central Japan. The formations have been subdivided into three formations according to the respective terraces that are overlain by each formations, namely Taniage, Maibara, and Kaisaka Loam formations in ascending order. The present authers propose here to name these formations comprehensively as the Shinano-gawa Loam formations. They have surveyed these formations, described it in detail, traced the intercalated nineteen key beds widely to the adjacent area, analyzed the mineral assemblages contained, and estimated roughly origin of materials of several key beds. The results are as follows: 1) The Taniage Loam formation is subdivided into three parts. The lower part is brown colored clayey loam that contains grains of biotite and high quartz, and are intercalated with three key beds of scoria and pumice. The middle part is brown colored viscid and clayey loam with two key beds. The upper part consists of yellowish brown colored clayey loam sporadically including orange colored scoria, and are intercalated with four key beds. The Maibara Loam formation is subdivided into two parts. The lower part is brown colored viscid and clayey loam and are intercalated with two key beds. The upper part is brown colored clayey loam that stippled with scoria and are intercalated with three key beds. The Kaisaka Loam formation with intercalations of three key beds is brown colored soft loam with fragments of volcanic glass in the upper part. Some archaeological remains have been found in the upper half of the formation. 2) On the bases of several key beds and contained mineral assemblages, the Kaisaka Loam formation is correlated with the Nojiri Loam formation in the northern part of Nagano prefecture, likewise, the Maibara and Kaisaka Loam formations are correlated with the Kubiki Loam formations in the southwestern part of Niigata prefecture. 3) Isopach maps and mineral assemblages of some key beds show the following facts which suggest the origins of main components of the Shinano-gawa Loam formations: Volcanic ejecta inferred to be derived from the Iiji and, Iizuna or Kurohime volcanos are found in the lower and upper part of the Taniage Loam formation, respectively. The thicknesses of some scoria beds in the lower part of Maibara Loam formation tend to increase toward the Kurohime volcano. In the upper part of the Maibara Loam formation and the lower part of the Kaisaka Loam formation, volcanic ejecta inferred to be derived from both the Kurohime and Myoko volcanos are recognized, respectively, the latter ejecta are also found in the upper part of Kaisaka Loam formation.

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The uppermost Cretaceous Izumi Group is distributed along the Median Tectonic Line from west Shikoku to west Kinki, except for the sporadical distribution in east Kinki and farther east. The eastern part of the Izumi Mountainrange, west Kinki, is regarded as the eastern margin of the main Izumi sedimentary basin. The Izumi Group in this area covers, on the north, the Ryoke granitic rocks and the Sennan Group unconformably or is faulted against them, while on the south, it is in tectonic contact with the Sambagawa metamorphic rocks along the Median Tectonic Line. (1) Three facies are discriminated in the Izumi Group of this area, i. e. Northern marginal facies (Kasayama conglomerate, Takihata Formation), Main facies (a part of Iwade Formation, Kokawa Formation), and Southern facies (Nate Formation). These facies change laterally westsouthwestwards along the strike of strata. (2) The Kokawa Formation, which is most widely distributed in this area, consists of the sandstone-rich member (facies I), mudstone-rich member (facies II) and conglomerate〜sandstone member (facies III). Cyclic sedimentation in a mappable scale is recognized in the facies I, but not in the facies II and III. The Kokawa Formation shows a lateral variation (facies III→II→I) westsouthwestwards. The facies boundaries migrate eastwards through the sedimentation of the Kokawa Formation. (3) The Izumi Group forms homoclinal structure dipping toward SSE and is deformed by the Median Tectonic Line fault system. The faults of the ENE-WSW direction (e. g. faults of S0a〜S5) are well developed. (4) The mode of the lateral variation of lithofacies as shown in (1) and (2) is different from that of the Izumi Group in the western part of the Izumi Mountainrange. The homoclinal structure of this area contrasts to the major synclinal structure of the Izumi Group in the western part of the Izumi Mountainrange.

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