THE STUDY ON PALEOGEOGRAPHIC CHANGE OF THE JAPAN SEA BASED ON THE DIATOM ASSEMBLAGES AND SEDIMENTARY FACIES ACCOMPLISHED THE OPENING OF THE JAPAN SEA

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THE SEDIMENTS INCLUDING FRESH-WATER DIATOMS ACCOMPANIED WITH THE ANIAI-TYPE FLORAS ARE OLDER THAN 20MA, AND ANOTHER ONES ACCOMPANIED WITH THE DAIJIMA-TYPE FLORAS FROM 18 TO 16MA ARE ESTIMATED TO BE OF A BRACKISH-WATER ORIGIN.THESE EARLY MIOCENE SEDIMENTS ARE RECOGNIZED IN AREAS ALONG THE PERIPHERY OF THE JAPAN SEA AND IN THE SAMPLES FROM THE YAMATO RISE.THE DIATOM ASSEMBLAGES ACCOMPANIED WITH THE ANIAI-TYPE FLORAS CHARACTERISTICALLY CONSIST OF PREDOMINANT MELOSIRA DISTANS, MELOSIRA GRANULATA, MELOSIRA ISLANDICA, AND ASSOCIATED FORMS OF FRESH-WATER DIATOMS.AN ADMIXTURE OF BRACKISH AND MARINE DIATOMS IS NOT OBSERVED.THE DIATOM ASSEMBLAGES ACCOMPANIED WITH THE DAIJIMA-TYPE FLORAS, ON THE OTHER HAND, ARE CHARACTERIZED BY THE MIXING OF SEA-WATER DIATOM ACTINOCYCLUS OCTONARIUS AND A FRESH-WATER DIATOM ASSOCIATION PREDOMINATING MELOSIRA DISTANS, MELOSIRA GRANULATA, AND MELOSIRA ISLANDICA.THIS TYPE OF ADMIXTURED DIATOM ASSOCIATION IS ALSO RECOGNIZED IN THE SAMPLES FROM THE YAMATO RISE.A LARGE FRESH-WATER LAKE HAD BEEN EXISTED BETWEEN THE ASIAN CONTINENT AND JAPAN ISLANDS DURING THE PERIOD OLDER THAN 20MA BEFORE THE BACK-ARC SPREADING OF THE JAPAN SEA.ENVIRONMENTAL CHANGES OF THE FRESH-WATER LAKE BY INVADED SEA-WATER CAN BE TRACED SEQUENTIALLY AT SEVERAL SITES IN THE JAPAN SEA AND ALSO IN THE PERIPHERY OF THE JAPAN SEA.THE DIATOMACEOUS SEDIMENTS OF THE JAPAN SEA COAST ORIGINATE IN A SEDIMENTARY ENVIRONMENT CHARACTERIZED BY SUBSIDING, SILLED SMALL BASINS DURING THE ROTATION OF THE JAPANESE ISLANDS AND THE RISING EUSTATIC SEA LEVEL DURING THE CLIMATIC OPTIMUM IN THE EARLY MIDDLE MIOCENE.THE DEPOSITION OF PURE DIATOMACEOUS SEDIMENTS IS MARKED BY THE FIRST APPEARANCE OF THE COLD-WATER DIATOM DENTICULOPSIS PRAEDIMORPHA.IT IS CONSIDERED THAT MOST OF THE YOUNGER DIATOMACEOUS SEDIMENTS RESULTED FROM THE CONVERGENCE OF COLD AND WARM CURRENTS OVER THE CONTINENTAL SLOPE.