Basic Cenozoic geological data for the studied area in the Sakhalin Island such asroute maps showing sampling points, and columnar sections showing sampling horizons were compiled. The area studied are the Makarov and Korsakov regions of southern Sakhalin, and Shmidt Peninsula at northwestern Sakhalin. Micro-fossil biostratigraphy, geo-chemical analysis, and K-Ar dating were studied. As a result, the Oligocene/Miocene boundary, tectonic event and climatic fluctuation are discussed based on the newly established geologic age. The Cenozoic paleoenvironment related to the Pacific Ocean gateways is also discussed from the viewpoint of molluscan fauna.
Pollen and diatom analysis were conducted on sediments in a fossil gully at Schmidt Peninsula, northwest Sakhalin and in a tectonic basin at a western area of central Sakhalin. Between 8, 500 and 7, 400 yBP a taiga dominated with Larix gmelinii and Pinus pumila developed at the Schmidt Peninsula under colder and drier climate than that at present. The paleoenvironment was marine and marine to blackish conditions. After the Betula forest period between 7, 400 and 6, 000 yBP, the present taiga, which was composed of dominant Picea jezoensis, coexisted with Larix and Pinus which developed since 6, 000 yBP under the same climate as that at present. At a western area of central Sakhalin, taiga composed of Picea, Larix and Pinus was distributed under colder and drier climatic conditions than that at present during the interstadial since 37, 000 yBP. In early Holocene, Larix-Pins taiga changed to Picea-dominated taiga due to a recovery of the climate. At around 6, 000 yBP the present taiga was established under the same climate as that at present. Larix, which is a good indicator of a cold and dry climate, formed a taiga together with Picea and Pinus in northern and eastern Hokkaido during LGM. During the migration north since Lateglacial, Larix formed taiga in Sakhalin. However, it decreased in the western area of central Sakhalin during early Holocene, and also decreased in the northwest at around 7, 400 yBP.
Based on foraminiferal biostratigraphy of the Neogene sedimentary sequences of Shmidta Peninsula at the northern tip and the Makarov region in southeastern Sakhalin, Russian Far East, paleoenvironment and ages are discussed. In the Makarov region, the history of environmental changes is roughly delineated, as follows: warm non-marine environment in late Early Miocene, transgression around early Middle Miocene, abrupt deepening into bathyal depth in Middle Miocene, and regression during Late Miocene and Pliocene. This succession is quite similar to those in Hokkaido and the northern Honshu in Japan. In the Pil'vo section at the west coast of Shmidta Peninsula, the similar bathymetric changes are recognized at the same time as in the Makarov region, although this section is regarded as being in the bathyal depths throughout Early to Middle Miocene. The occurrences of some temperate-water species give evidence of the influence of warm-surface water during the Miocene period. Ammonia spp. survived in early Middle Miocene (the transition period between the MNCO and the following cooler stage in early Middle Miocene) in the Makarov region. A planktonic foraminifer, Globorotalia cf. miozea conoidea was found in the middle part of the Pili Formation (early Middle Miocene) of the Pil'vo section. This is the first discovery of keeled Globorotalia in Sakhalin, and is important for the consideration of the geohistory of the Northwest Pacific, in particular the first appearance of subarctic climate and cold Oyashio water.
This paper aims, on the basis of marine and terrestrial palynology (dinoflagellate cysts and pollen), to discuss ages, paleovegetation, and paleoclimate of Tertiary sections from the Makarov, Aleksandrovsk-Sakhalinskiy and Schmidt Peninsula areas, Sakhalin Island, Far East Russia. The present study of dinoflagellate cysts has drawn good age constraints on most of the sections as follows: upper Lower? to Upper Oligocene (the Gastellov and Kholmsk-Nevel'sk Formations in the Makarov area and most of the Tumi Formation in the Schmidt Peninsula area), lower Lower Miocene (the uppermost part of the Tumi Formation and the lower part of the Pili Formation in the Schmidt Peninsula area) and upper Lower to lower Middle Miocene (the Kurasi Formation and the First Member of the Maruyama Formation in the Makarov area and the Sertunai Formation in the Aleksandrovsk-Sakhalinskiy area). Pollen assemblages from the upper Lower to lower Middle Miocene sediments, the Verkhne Due Formation and the lower part of the Kurasi Formation in the Makarov area, and the Verkhne Due and Sertunai Formations in the Aleksandrovsk-Sakhalinskiy area, are correlative to those from sediments in northern Japan that yielded the Daijima-type flora. Those Sakhalin assemblages indicate a temperate paleovegetation similar to that indicated by coeval assemblages from the northern part of Hokkaido. This suggests that the latitudinal gradient in paleovegetation/paleoclimate was relatively minor between Sakhalin Island and Hokkaido during the late Early to early Middle Miocene.
A diatom biostratigraphic study of the Maruyama Formation in the Makarov and Dolinsk areas of the southern part of Sakhalin Island indicates that its geologic time span ranges from ca. 14 Ma to 3.5 Ma or possibly younger, which is much longer than previous estimates (e.g. 9.5-5.3 Ma by Ingle, 1992). A middle Miocene age of the Kurasi Formation in the Makarov area is also given on the basis of diatoms. These results are presented with correlation to the Neogene sections in the Tenpoku area, northern Hokkaido. The diatom zones of Akiba (1986) and Yanagisawa and Akiba (1998) recognized in the Makarov area include the Denticulopsis simonsenii Subzone of the D. hyalina Zone (NPD4Bb; middle middle Miocene, 14.6/14.5-13.1 Ma) in the Kurasi Formation and the 1st Member of the lower part of the Maruyama Formation, and the D. praedimorpha Zone (NPD5B; upper middle Miocene, 12.9-11.5 Ma) in the 2nd Member of the lower part of the Maruyama Formation. In the middle part of the Maruyama Formation in the Dolinsk area, the type area for the formation, numerous specimens of reworked diatoms were observed. The youngest zone shown by those assemblages is the Thalassiosira oestrupii Subzone of the Neodenticula kamtschatica Zone (NPD7Bb; lower Pliocene, 5.5-3.9/3.5 Ma), thus the middle part of the Maruyama Formation in the Dolinsk area should be of early Pliocene or younger ages. As similar assemblages are known from the Yuchi Formation in the Tenpoku area of northern Hokkaido, which is of Pleistocene age at least in part, the middle part of the Maruyama Formation in the Dolinsk area may be Pleistocene in age, which should be tested by further studies. In addition, our re-evaluation of Sheshukova-Poretzkaya's (1967) data from the Aniva area, south of Yuzhno-Sakhalinsk, suggests that the formation also contains an interval correlative to the Rouxia californica Zone (NPD7A; upper upper Miocene, 7.6-6.4 Ma). The present study geochronologically constrains other geological data such as paleontology, paleomagnetism, and paleoenvironments from the studied sections and their correlative strata. The considerably long geochronologic interval represented by the Maruyama Formation suggests stratigraphic gaps within the formation, thus a further study of the formation may elucidate the history of relative sea-level changes which have localor global implications.
Tertiary sedimentary sequences containing the Paleogene/Neogene boundary are well exposed in Sakhalin, Russia. Sequences of Oligocene to Miocene mudstones from Korsakov, Makarov, and Shmidt Peninsula areas were studied to reveal the palaeoceanographic environment at the time of the Paleogene/Neogene transition. Late Oligocene rocksare characterized by siliceous mudstones. The SiO2 contents and total organic carbon concentra tions range from 70 to 95% and from 0.5 to 2.7%, respectively. Siliceous rocks from the Shmidt Peninsula are rich in organic carbon, and were deposited in an anoxic environment as indicated by the abundance of C35 homohopane, and low C/S ratio. Open system pyrolysis and pyrolysis gas chromatography show that siliceous rocks from the Shmidt Peninsula are rich in typical marine Type II kerogen with hydrogen indices ranging from 250 to 400mgHC/gC. The siliceous rocks from the Makarov section are characterized by low hydrogen indices of less than 200mgHC/gC. Their maximum temperatures experienced during burial diagenesis are estimated to be less than 100°C based on silica mineral transformation and biomarker isomer ratios. Late Oligocene siliceous mudstones are rich in biomarkers derived from diatoms and dinoflagellates, showing enhanced primary productivity of the Late Oligocene ocean in the Sakhalin region. Invasion of nutrient-rich proto-AABW (Antarctic Bottom Water) and its upwelling might be responsible for the high primary production in the Late Oligocene. Benthic water in the Paleogene / Neogene transition was abruptly oxidized, which is attributable to a decline of proto-AABW. This palaeoceanographic event seems to be related to global warming in the Paleogene/Neogene transition.
The region from Sakhalin in Russia to the eastern margin of the Japan Sea has been regarded as a convergent plate boundary zone between the Eurasia and the North America Plates because large earthquakes and active crustal movements are prominent in this zone. These activities along the eastern margin of the Japan Sea to southern Sakhalin are almost consistent with the expected relative plate motion deduced from the Euler pole which is estimated from the magnetic anomaly lineations in the Atlantic Ocean, but the modern tectonic aspect in northern Sakhalin is inconsistent; e.g., 1995 Neftegorsk Earthquake is one of the typical events. As a result of a structural and tectonic study in northern Sakhalin, the NE-SW compressive tectonic feature since the Late Miocene was clarified. In the northernmost area of the Schmidt Peninsula, the early Cretaceous ophiolite thrusts upon the Late Cretaceous sedimentary rocks. The ophiolite has an overturned sequence: serpentinites, gabbros, basaltic rocks, and hemipelagic sediments in descending order. The Cretaceous sediments form a map-scale and NW vergent synclinorium. All the deformation structures in outcrop scale, such as micro-folds, minor reverse and normal faults, axial plane cleavages and bedding slips, are consistent with a large scale folding. These structural relationships suggest only one event of deformation, which appears to be linked to overthrusting of the ophiolitic rocks. A structural investigation of the southeastern part of the Schmidt Peninsula revealed that the Middle Miocene sediments composed of siltstone and sandstone are folded with wavelength of several tens to several hundred meters. Their fold axes trend in the NW direction, which indicates that deformation under the NE compression is the same as the deformation in the northernmost area. The thickness of sedimentary layers are constant everywhere in folding. This fact indicates that the timing of the deformation is after sedimentation, that is, after the Middle Miocene. This deformation event is consistent with modern activity in northern Sakhalin. Therefore, the modern tectonic framework might have started in the Late Miocene time. A plate tectonic model indicates that the modern relative motion between the Eurasia and the North America Plate started at about 11 m. y. ago. The Late Miocene onset of the modern tectonic framework in northern Sakhalin occurred at almost the same time. The most reliable model to explain the discrepancy of sense of movement from the Eurasia-North America retative motion, may be the “extrusion” of the Okhotsk Block toward the Pacific Ocean. The dextral extrusion boundary in northern Sakhalin may be traced along theeastern coast of Sakhalin to the northern edge of the Kuril Basin, where active seismicityhas been observed although a detailed study of focal mechanisms and other tectonic aspects is needed in the future.
The Cenozoic volcanic rocks in Sakhalin Island record a tectonic transition from convergence to divergence in connection with the formation of the Japan Basin and the Kuril Basin. The magmatism of this region comprises the following distinct stages of tectonomagmatic evolution: 1) subduction-related calc-alkaline volcanism in the Oligocene along the northeastern edge of Eurasian continent. 2) early Miocene volcanism characterized by island arc tholeiites surrounding the developing back-arc basins. 3) late Miocene to Pliocene volcanism, yielding within-plate geochemical signatures. The temporal geochemical trends over 55 m. y. in volcanic rocks from Sakhalin and nearby Sikhote-Alin suggest that there was a change in magma source from the lithosphere to the depleted asthenosphere as both the Japan Sea and the Okhotsk Sea opening progressed. The early Miocene volcanism along the western edge of central Hokkaido from Rebun Island to off the coast of Tomakomai is characterized by bimodal volcanic rocks. The lack of correlation between relatively enriched basalts from central Hokkaido and depleted basalts from Sakhalin and Sikhote-Alin suggests that the source of the former basalts does not have a significant contribution from the asthenosphere.
This paper reviews the Tertiary chronostratigraphy and paleoenvironments in the Makarov and Chekhov areas, southern Sakhalin Island, Russia, emphasizinsubsidence and uplifting history. After the late Cretaceous to Paleocene accretion of fore-arsediments and trench-fills, subsidence started in the middle Eocene in the Chekhov area, on the west coast of Sakhalin, and in the late Oligocene in the Makarov area, on the east coast of Sakhalin. This subsidence culminated perhaps during the late Oligocene, followed by uplifting to the surface with increasing eruptions of tholeiitic or calcalkaline basalt to andesite. Subsidence in the two areas started again in the late early Miocene at a very rapid rate, followed by slow subsidence or uplifting after the adjacent Chishima (Kuril) and the Japan Sea Basins ceased opening at about 15 Ma. The first subsidence perhaps implies the rifting of the back-arc or intra-arc area of Sakhalin Island, and the second subsidence implies the opening the Chishima and Japan Sea Basins. The early Miocene unconformity, which separates two cycles of Tertiary subsidence and uplifting, also occurs along the Japan Sea side areas of Hokkaido and Tohoku Provinces, northeast Japan, and is thought to record a tectonic uplifting with the upwelling of the athenospheric mantle, or by other unknown processes related to the opening of the back-arc basins.
The Upper Oligocene, distributed in the Makarov, Korsakov and Shmidt areas of Sakhalin, yields detrital chromian spinels. These detrital spinels are characterized by extremely low TiO2 and Fe3+ contents, and they were probably derived from highly to moderately depleted mantle peridotite and/or serpentinite (>0.4 in Cr#). The source peridotites are very similar to those now exposed at the Kamuikotan metamorphic belt, Hokkaido, Japan, a possible southern extension of the N-S trending serpentinite melange belts of Sakhalin. It is highly probable that some of their serpentinite bodies had provided detritus of serpentinite and chromian spinel to the sediments. The occurrence of detrital chromian spinels has been reported from the Cretaceous, Paleogene, and Miocene in the axial zone of Hokkaido, indicating that serpentinites were emplaced into the erosional level during these periods. This suggests a protrusion of serpentinite along the N-S trending fault line, probably the extension of some faults in Hokkaido, occurred in late Oligocene as a presage of Miocene transcurrent tectonics in Sakhalin and Hokkaido.
We have mapped a series of north-trending faults along the northeastern coast of Sakhalin, Russia, based on aerial photograph interpretation and field observations. These faults include the Upper Pil'tun fault which ruptured during the 1995, Mw 7.0 Neftegorsk earthquake, and the Ekhabi-Pil'tun and Garomay faults east of the surface ruptures. The coseismic displacement associated with the 1995 earthquake shows that the Upper Pil'tun fault is a right-lateral strike-slip fault. Right-lateral stream offsets as much as 80m demonstrate that similar earthquakes have occurred repeatedly along the fault in the late Quaternary. The fault extends for 10 km further to the south beyond the southern termination of the 1995 surface breaks, which is consistent with the coseismic ground deformation detected by SAR interferometry. The Ekhabi-Pil'tun fault trends N10° W to N10°E with a series of fault scarps down-to-the-west across low-relief hilly terrain eroded byperiglacial processes. Right-laterally offset streams and lake shores suggest thatthe Ekhabi-Pil'tun fault is also a right-lateral strike-slip fault. A 3-m-deep trench across the fault north of the Sabo River contains geologic evidence of two episodes of pre -historic surface faulting. The most recent event occurred after 4000 yr B.P. and the penultimate earthquake occurred at about 6200 yr B.P. The interval between the events is greater than 2000 years, which is significantly longer than that obtained for the Upper Pil'tunfault by previous trench excavations.
Scarplets and east-dipping steep slopes, both of which form a marked lineament N20°E in direction and 16 km in length, develop on the Pleistocene fluvial surfaces along the geomorphic boundary between the Yuzhno-Kamyshovvy Mts. and the Susunayskaya Lowland in South Sakhalin. These landforms have been created by Quaternary activities of the Taranay fault. Our recent trenching surveys in Taranay (46° 39' N, 142° 24' E) revealed west-dipping thrusts truncating a fluvial gravel layer. Flexure of the fluvial gravel layer with >2 m vertical displacement was also identified. The latest event took place in the period between 42 and 0.1 ka. An average slip rate is estimated geomorphologically to be 0.05 to 0.08 m/ky, or faster. Whereas the Taranay fault can be regarded as a C-class active fault, further investigations to discover the main fault and to obtain other kinds of fault parameter are required.
We have prepared a preliminary active fault map of Sakhalin, Russia, based on an interpretation of aerial photographs and satellite images. Major active structures include 110-km-long active faults along the western margin of the Yuzhno-Sakhalinsk Lowland in southern Sakhalin and 120-km-long active faults along the western margin of the Poronaysk Lowland in central Sakhalin. These active faults are parallel to but are located as far as 10 km east of the Tym-Poronaysk fault. Geomorphic surfaces on the upthrown side of the fault are tilting westward, therefore, the faults are considered to be west-dipping low-angle reverse faults. The vertical component of slip rates of these faults are >0.3 mm/yr in southern Sakhalin and 1.0-1.5 mm/yr in central Sakhalin. The net-slip rates could be much greater because the faults are low-angle reverse faults. If these faults rupture along their entire length during individual earthquakes, the earthquakes could be as great as M7.6-7.7. In northern Sakhalin, we have identified a series of right-lateral strike-slip faults, including the 1995 Neftegorsk earthquake fault. The slip rates for these faults are estimated at 4 mm/yr. The right-lateral shear in northern Sakhalin and east-west compression in central and southern Sakhalin may reflect relative plate motion in far-east Asian region.
Late Quaternary marine terraces are well-developed around the Yelizavety Cape, in northern Sakhalin. There are two levels of Pleistocene terraces (H and M surfaces) and four levels of Holocene terraces (F and L1-3 surfaces). The M surface is either an abrasion platform covered by talus deposits several ten meters thick, or constructional surface of littoral sand and gravel overlain by periglacial deposits less than two meters thick. These observations strongly indicate that the M surface was formed in the Last Interglacial age. The height of the M surface is 11 to 24 m above sea level, so that the average uplift rate since the Last Interglacial age is estimated to 0.1 to 0.2 mm/yr. The Tua fault to the west of the Yelizavety Cape has been inactive during the Late Quaternary, because of the scarcity of tectonic geomorphologic features along the fault. The formation age and the height of the M surface indicate that during the Late Quaternary, the uplift rate around the Yelizavety Cape is comparable with that along the eastern coast of south Sakhalin. The L surfaces are composed of storm-ridge deposits, in which well-sorted sands and granules are intercalated with peaty layers. The landward limit of the backshore of the Li Surface is 5.5 to 7.2 mabove sea level. The L1 surface was formed at Subatlantic Period (1600-520 14C yrBP).