Takuo Yokoyama

Preliminary results of the first scientific drilling on Lake Baikal, Buguldeika Site, Southeastern Siberia

The Baikal Drilling Project (BDP) is a multinational effort to investigate the paleoclimatic history and tectonic evolution of the Baikal sedimentary basin during the Late Neogene. In March 1993 the Baikal drilling system was successfuly deployed from a barge frozen into position over a topographic high, termed the Buguldeika saddle, in the southern basin of Lake Baikal. The BDP-93 scientific team, made up of Russian, American and Japanese scientists, successfully recovered the first long (>100 m) hydraulic piston cores from two holes in 354 m of water. High quality cores of 98 m (Hole 1) and 102 m (Hole 2), representing sedimentation over the last 500,000 years, were collected in 78 mm diameter plastic liners with an average recovery of 72% and 90%, respectively. Magnetic susceptibility logging reveals an excellent hole-to-hole correlation. In this report the scientific team describes the preliminary analytical results from BDP-93 hole 1 cores. Radiocarbon dating by accelerator mass spectrometry provides an accurate chronology for the upper portion of Hole 1. Detailed lithologic characteristics, rock magnetic properties and inorganic element distributions show a significant change to the depositional environment occuring at 50 m subbottom depth, approximately 250,000 BP. This change may be due to uplift and rotation of the horst block in the Buguldeika saddle. The sedimentary section above 50 m is pelitic with varve-like laminae, whereas the section below 50 m contains a high proportion of sand and gravel horizons often organized into turbidite sequences. Accordingly, high resolution seismic records reveal a change in sonic velocity at this depth. It is inferred that sedimentation prior to 250 ka BP was from the west via the Buguldeika river system. After 250 ka BP the Bugnldeika saddle reflects an increase in hemipelagic sediments admixed with free-grained material from the Selenga River drainage basin, east of Lake Baikal. Variations in the spore-pollen assemblage, diatoms, biogenic silica content, rock magnetic properties, clay mineralogy and organic carbon in the upper 50 m of BDP-93-1 reveal a detailed record of climate change over approximately the last 250,000 years. These variables alternate in a pattern characteristic of glacial/interglacial climatic fluctuations. The present age model suggests that the climate signal recorded in Lake Baikal sediments is similar to Late Quaternary signals recorded in Chinese loess sections and in marine sediments. Copyright

A third volcanic chain in Kamchatka : thermal anomaly at transform/convergence plate boundary

The Kamchatka volcanic arc, which is located at the northern edge of the Kurile arc, consists of three volcanic chains, all parallel to the trench axis. In contrast, most subduction zones have only two subparallel volcanic chains. The third chain in Kamchatka, which is farthest from the trench, is characterized by the occurrence of voluminous plateau lavas; volcanoes in the two chains closer to the trench are stratovolcanoes typical in arc magmatism. The third chain magmatism is also unusual in that lavas show concentrations of incompatible elements intermediate between those in the two trenchward chains. Both the unusual occurrence of the third volcanic chain and the unusual lava chemistry could be caused by partial melting of K-amphibole bearing peridotites in the downdragged hydrous layer at the base of the mantle wedge under anomalously high-temperature conditions associated with the characteristic tectonic setting of transform/convergence transition in the region.

Neo-tectonics of the Strait of Sunda, Indonesia

Abstract Geological survey, gravity measurements, paleomagnetic and geochemical studies have been carried out in the area of the Strait of Sunda. The Krakatau complex lies at the intersection of two graben zones and a north-south active, shallow seismic belt, which coincides with a volcanic lineament about 200 km long. The activity of this lineament coincides with a fracture zone along this seismic belt and commenced with extrusion of alkali basaltic rocks at Sukadana and shifted southward through Rajabasa, Sebsi, Sebuku and Krakatau. A 3000 m deep well was drilled in the Strait of Sunda portion of the Lampung Banten contract area. The well penetrated an apparently continuous sedimentary sequence ranging in age from Quaternary to Upper Pliocene. This area of the Strait of Sunda must have formed a rapidly subsiding trough, in which a vast thickness of clastic sediments accumulated, extending from Lampung to the Krakatau fracture zone. The gravity anomalies were observed (i) in the area of north of Ujung Kulon, indicating the existence of a low gravity caldera, from which the Malingping and Banten tuffs were ejected 0.1 Ma ago, and (ii) at the area of Kotaagung, where a graben structure was observed and an ignimbrite eruption occurred at 1 Ma. From the chemical analyses of volcanic materials, we deduced that the ignimbrite magma was generated by remelting of crustal materials. In the volcanic rocks along the volcanic lineament in the Strait of Sunda, no zonal arrangement comparable to the volcanic rocks in Java is recognized, but the chemical composition of Krakatau has a wider range than normal island arc volcanoes. Paleomagnetic studies suggest that the island of Sumatra has been rotating clockwise relative to Java from at least 2.0 Ma to the present at a rate of 5–10°/Ma. As the difference in strike of Java and Sumatra exceeds 20°, the rotation of Sumatra, and therefore the opening of the Strait of Sunda might have started before 2 Ma. The Strait of Sunda appears to be situated at a submerged volcano-tectonic depression in southernmost Sumatra which produced a large quantity of acid pyroclastics in the Late Quaternary.

A gravity and volcanostratigraphic interpretation of the Lake Toba region, North Sumatra, Indonesia

Abstract In northern and central Sumatra a wide distribution of ignimbrite and volcanic sediments is observed. Some geologists consider that these volcanic products were erupted from Lake Toba. The subduction of the Indian Ocean plate along the trench of central Sumatra started at least as long ago as the Late Miocene. Fitchs model (1972) obtained the plate convergence vector (67 mm/yr), decomposed into its components perpendicular (58 mm/yr) and parallel (34 mm/yr) to the trench axis. Tears seaward of the mountain range created the Sumatran fault about 2 Ma ago. At that time, the edge of the plate reached a depth of almost 100 km, and a large amount of dacite magma was generated. The Tuk-Tuk dacite, a kind of ignimbrite, was erupted along the Sumatran fault and covered a broad area. The age of this ignimbrite was obtained as 1.9 ± 0.4 Ma (K-Arage) and 2.0 ± 0.3 Ma (Fission Track age). Since this eruption, the hinterland of this fault has been changed to a tensional field forming a graben structure. At the northern end of this graben, west of Harranggaoul, the ignimbrite (Haranggaoul formation) was erupted (1.3-0.3 Ma ago) forming a caldera. The lacustrine sediments of Samosir Island have a normal magnetization and overlie the Tuk-Tuk dacite unconformably. From this we can conclude that the graben structure has been covered with water (Lake Toba) at 0.7-0.6 Ma ago. This tectonic movement has continued, so that it is difficult to distinguish the caldera structure on the gravity anomaly map of the northern part of Lake Toba. From the results of the gravity survey it can be concluded that at 0.1 Ma ignimbrite flowed down along the Asahan river and filled the old river along the lowland between Parapat and Porsea, forming a caldera. 30,000 years ago, dacite and and esite eruptions have occurred around Lake Toba, forming the volcanic cones.

Paleomagnetic stratigraphy and biostratigraphy of the Plio-Pleistocene in the Kinki district, Japan

The magnetostratigraphic subdivision of the Plio-Pleistocene series in the Kinki district around Osaka in southwest Japan, based on NRM measurements of volcanic ash beds, is correlated with biostratigraphic and paleoclimatic marker beds as well as past sea level oscillations. At least 10 high sea level stands occurred in the study area between about 1.5 and 0.27 million years ago, all associated with relative warming. A significant change in the composition of flora, from Metasequoia to Fagus dominated assemblages, was observed around the Jaramillo event.

Paleomagnetic evidence for the paleoposition of Sumba island, Indonesia

Abstract Jurassic paleomagnetic directions have been obtained for the sedimentary rocks in Sumba. The mean direction is 59.2° in declination and −44.2° in inclination. The reliability of the direction is ascertained through thermal demagnetization and the presence of normal and reversed polarities. Comparison with the Permian paleomagnetic direction of Timor indicates that Sumba was subjected to a clockwise rotation through 79.4° relative to Timor since the Jurassic. When Sumba is restored by a counter-clockwise rotation, the Sumba pole is gradually approaching the Jurassic pole of Australia as well as the Timor Permian pole. This implies that, at least until the Jurassic, Sumba and Timor were situated at the Australian continental margin and that Sumba rotated clockwise during or after the Jurassic.

Brunhes/Matuyama polarity epoch boundary in the Osaka Group of the Senriyama Hills, southwest Japan

Abstract Magnetic measurements were carried out on core samples of the Osaka Group distributed in the Senriyama Hills, north of Osaka Bay. The studied section, about 53 m thick, includes four layers of clay or silts, and intercalated sand and gravel layers. The four muddy layers, which have been known as the Ma 3, Ma 4, Ma 5 and Ma 6 “marine clay” beds (in ascending order), seem to have been deposited in shallow marine environments, possibly associated with transgression during interglacial periods. The mud layers were sampled using a new type of core sampler, designed to mark the north direction on the cores. The magnetic analysis revealed that the upper part of the section (Ma 5 and Ma 6) has normal polarity, and that the lower part (Ma 3) has reversed polarity. The Ma 4 bed, which was formerly assigned to the Brunhes Chron, showed reversed polarity in the lower part, while its upper part had downward inclinations. This result shows that the Brunhes/Matuyama polarity transition occurred in an interglacial period. The Ma 4 layer is assigned to the period of marine transgression corresponding to the oxygen isotope stages 19-18.

Marine transgressions on the Pleistocene Pecangan Formation in the Sangiran area, central Java, Indonesia

Abstract Sangiran in central Jawa, Indonesia, is very famous for the occurrence of the fossil bones of Pithecanthropus erectus. The Pleistocene sediment at Sangiran area is divided into two formations; Kabuh Formation of volcanic material and Pecangan Formation of mainly clays with some diatomites, thin volcanic ashes and thick pyroclastic flow-deposits. About a hundred samples were taken from muddy beds of the Pecangan Formation for paleomagnetic measurements, diatom analysis and electric conductivity measurement. Nineteeen of them contain fossil diatoms. As the results of diatom analysis and electric conductivity measurements, four marine transgressions are found in the Pecangan Formation. The first major transgression took place after the Olduvai Event and the next one just before the Jaramillo Event. The latter can be correlated with that of the Ma0 Marine Clay Bed in the Osaka area in Japan. The results of the work indicate that the age of the marine transgression in the east Pacific areas reflects the glacial eustasy.

Radiometric ages and paleomagnetism of the Sigura-gura formation, upper part of the “Toba Tuffs” in Sumatra, Indonesia

Abstract A radiometric age determination by means of KAr and fission-track method was carried out on the Sigura-gura Formation, upper part of the “Toba Tuffs” in Sumatra, Indonesia. The results are as follows: (1) KAr age: 0.96±0.15 Ma. (2) Fission-track ages by re-etched method of external crystal surface are 1.11±0.12 and 1.11±0.14 Ma. (3) Fission-track ages by external detector method of crystal surface are 0.87±0.09 and 0.86±0.10 Ma. (4) The Paleomagnetic polarity of the Sigura-gura Formation is reversed. So this reversed polarity surely belongs to the Matuyama Reversed Epoch though it was previously correlated with the Blake Event in the Brunhes Normal Epoch. (5) The ash-flow deposit, which was the uppermost part of the Sigura-gura Formation, shows the youngest fission-track age, 0.03±0.01 Ma in previous and this works. So, this must be a different unit, called the Porsea Formation, which overlies the Sigura-gura Formation. (6) The Porsea Formation may be a product of the same volcanic activity which erupted the Toba ash layer identified in the deep sea sediments at about 0.075 Ma ago. (7) In addition, six fission-track age values were obtained from the Younger Toba Tuffs of “Toba Volcanic Complex”. They are dated at 0.62-0.1 Ma.