相対的海水準変動に支配されたファンデルタシステムの堆積シーケンス : 北部フォッサマグナ長野県地域の中新統・鮮新統  [in Japanese] Depositional sequences of fan-delta system, controlled by relative sea-level changes, Miocene and Pliocene rocks in Northern Fossa Magna region, Nagano Prefecture, Central Japan  [in Japanese]

北部フォッサマグナ長野県地域の込地向斜・犀川背斜・高府向斜に分布する新第三系は, 相対的海水準変動に支配されたファンデルタシステムによって堆積し, 高府向斜地域はその沖合側, 込地向斜地域は沿岸側を構成していた。これらの地層中には, 低海水準期堆積体・海進期堆積体・高海水準期堆積体と, これら3つの堆積体を挟む約10 Maと約6 Maの2つのシーケンス境界が認められた。これよりファンデルタの第3オーダーの堆積シーケンスモデルが構築された。低海水準期堆積体は海盆底の海底扇状地システムからなり, 海進期堆積体は陸棚上に累重したファンデルタフロント・ファンデルタプレーン堆積物, 海岸平野システムとバリア島システムを含む。そして高海水準期堆積体は海盆側に前進した斜面型ファンデルタシステムからなる。このようなファンデルタの堆積シーケンスの形成には, 卓越した堆積盆の沈降が大きく影響していると考えられる。

A third- order fan-delta depositional sequence is found in the Middle to Upper Miocene Aoki and Ogawa formations, and the Lower Pliocene Shigarami and Ashinojiri formations in the southern part of the Northern Fossa Magna region, central Japan. A sequence of the fan-delta deposits in these formations at back-arc basin setteing is discussed. The Aoki, Ogawa and Shigarami formations are divided into eleven sedimentary facies. The facies analysis suggests that these three formations were deposited in response to progradation and aggradation of a slope-type fan-delta system. The slope-type fan-delta system is composed of fan-delta plain, fan-delta front and fan-delta slope, and associated channel-levee system. Two sequence boundaries representing unconformities or ravinement surfaces were identified in these sedimentary systems. Duration of the deposits between the two sequence boundaries is estimated at about 4 m.y. Therefore, the depositional sequence defined by these two sequence boundaries is interpreted to have developed in response to a third order eustasy superimposed upon the second order regional tectonic subsidence. The depositional sequence is composed of low stand, transgressive and highstand systems tracts. The thickness of the sequence is more than 1000 m and is, in general, thicker than third order depositional sequences in the passive continental margins. The lowstand systems tract consists of the channel-levee system. The main part of this systems tract appear to have developed as submarine fan deposits in a offshore basin. The transgressive systems tract consists of strand-plain and barrier island systems, and is characterized by aggradation of these sedimentary systems of the shelf, resulting from increasing accommodationon the shelf during the transgressive stage. The highstand systems tract is composed of the slope-type fan-delta system and is characterized by aprogradational stacking pattern to the basin ward direction. The fan-delta depositional sequence is controlled not only formed a huge amount of sediment influx but also formed a relative change in sea level controlled by the interaction between sea floor subsidence and eustasy. Sea floor subsidence may have been caused not only formed tectonic subsidence but also sediment load of fan-delta deposits.