Miyazaki Group Research Articles

Spatial variations in the consolidation of sediments in the Neogene Miyazaki forearc basin, Southwest Japan

The Neogene Miyazaki Group comprises forearc basin sediments exposed onland in South Kyushu in Southwest Japan, and is characterized by significant spatial variations in consolidation despite the minor differences in depositional ages. The Miyazaki Group is divided into three lithofacies (the Aoshima, Miyazaki, and Tsuma facies) from south to north. To evaluate the factors controlling lithification in the basin, the physical properties of the sedimentary rocks and the calcareous nannofossil biostratigraphy were investigated. The upper parts of the Aoshima and Miyazaki facies and the lower part of the Tsuma Facies were deposited at the same time (5–6 Ma), and have minor differences in maximum paleo-temperatures (97–116, 85–99, and 80–94 °C, respectively), but major differences in porosity (15.9%, 25.5%–26.9%, and 26.1%–31.6%) and consolidation yield stress (38.2, 13.8–16.2, and 13.6–15.5 MPa). These findings indicate that the porosity reduction associated with consolidation is a more important control on the lithification than the depositional age and maximum temperature. There is a large difference in maximum burial depth between the Aoshima (>3500 m) and Miyazaki facies (<1600 m). It represents variations in sedimentary depositional environments in the Miyazaki Group: the Aoshima Facies were deposited in the deep-marine depocenter and deeply buried, whereas the Miyazaki and Tsuma facies were in the shallower marine facies and shallowly buried. After the deposition, only the southern part of the Miyazaki forearc basin was largely uplifted, making distribution of the highly consolidated sedimentary rocks there. The large amount of uplift in the Aoshima Facies was apparently due to the subduction of the Kyushu–Palau Ridge beneath the forearc basin.

宮崎層群高鍋層(鮮新統)の冷湧水性化石群集に関する新知見

宮崎層群高鍋層(鮮新統)の冷湧水性化石群集に関する新知見

Age of the lowest horizon of the Tsubusagawa Formation based on tephra correlations with the Miyazaki Group, Kyushu Island, Japan

九州地方の北部には，極狭い範囲に分布する陸水成層の津房川層がある．この地層は，ゾウ類をはじめとする多くの重要な脊椎動物化石を有するが，詳細な堆積年代は明らかになっていない．本研究では，同層およびその堆積時期が重なるとされる宮崎層群それぞれのテフラ層について，その層相および性質を明らかにし，広域対比の検討を行った．その結果，津房川層の森1火山灰層と田ノ口テフラ層が，宮崎層群のNKT-2とNKT-13テフラ層にそれぞれ対比された．宮崎層群の堆積年代と対比されたテフラ層から，津房川層最下部の年代は約3.5Maと推定される．

Thermal structure of the Miyazaki Group based on the vitrinite reflectance

Thermal structure of the Miyazaki Group based on the vitrinite reflectance

Mechanism and process of construction of tubes of the trace fossil Schaubcylindrichnus coronus Frey and Howard, 1981

Schaubcylindrichnus coronus is a common trace fossil occurring in post-Cretaceous shallow marine deposits. It consists of a sheaf or bundle of thickly lined tubes, with the tube linings usually composed of colorless minerals (“mineral-lined” type). Recently, many peculiar specimens of S. coronus, whose tube linings are composed mainly of tests of the larger benthic foraminifer Operculina complanata japonica (“Operculina-lined” type), have been found in the inner shelf deposits of the late Miocene Kaichigo Formation in the Miyazaki Group, southwestern Japan. The “Operculina-lined” specimens occur in association with funnel-shaped concentrations of Operculina tests. In some specimens, the end of the tube directly connects to the concentration. We consider these concentrations to be the product of feeding funnels filled with Operculina tests that were delivered from shallower marine settings in association with storm currents. Considering the fact that no Operculina tests are found in the host rocks that bear Operculina-lined tubes, the Operculina tests that line the tubes appear to have originated from Operculina test concentrations. In other words, during feeding, the animal producing S. coronus apparently selected the tests in its feeding funnel by their specific gravity and utilized them for tube construction. This manner of tube construction is highly consistent with that of modern funnel-feeding polychaete worms.

Bed-thickness distributions and their stratigraphic variation in a mudstone dominated turbidite succession: an example of the Aoshima Formation, Miyazaki Group, Kyushu Island

Bed-thickness distributions and their stratigraphic variation in a mudstone dominated turbidite succession: an example of the Aoshima Formation, Miyazaki Group, Kyushu Island

Trace fossil Zoophycos from the Aoshima Formation of the Neogene Miyazaki Group, southeastern Kyushu, Japan

Trace fossil Zoophycos from the Aoshima Formation of the Neogene Miyazaki Group, southeastern Kyushu, Japan

The lowstand fan deposits of Basal Miyazaki Group ( Middle Miocene) along the Aburatsu Bay Coast in Southern Miyazaki Prefecture.

The lowstand fan deposits of Basal Miyazaki Group ( Middle Miocene) along the Aburatsu Bay Coast in Southern Miyazaki Prefecture.

Sedimentary processes of sediment-gravity flows run over Sediment wave: an example of Aoshima Formation, Miyazaki Group

Sedimentary processes of sediment-gravity flows run over Sediment wave: an example of Aoshima Formation, Miyazaki Group

The Pliocene/Pleistocene boundary of the upper Takanabe Formation of the Miyazaki Group, southern Kyushu, Southwest Japan

The Pliocene/Pleistocene boundary of the upper Takanabe Formation of the Miyazaki Group, southern Kyushu, Southwest Japan

宮崎層群上部の石灰質微化石に基づく，鮮新世／更新世境界付近の海洋環境変遷

九州宮崎地域に分布する宮崎層群高鍋層上部は，鮮新世／更新世境界を含むことから，後期鮮新世から前期更新世の北西太平洋の海洋環境変遷を明らかにするのに適した地層である．同時代の黒潮変動を明らかにするために，宮崎県高鍋町永谷川沿いに露出する，高鍋層最上部から産出する石灰質ナンノ化石と浮遊性有孔虫化石群集を解析した．特に，古環境代替指標として用いた石灰質ナンノ化石のReticulofenestra属，Coccolithus pelagicusおよび浮遊性有孔虫化石のGlobigerinoides ruber, Globigerinoides quadrilobatus, Globigerinoides sacculifer, Neogloboquadrina incomptaが認められるほかに，石灰質ナンノ化石のDiscoaster属が特徴的に産出する．これらの種から，鮮新世末まで黒潮の影響下にあり海洋が成層化する環境から，漸移的な移行期を経て，更新世の冷水塊などによる湧昇流が卓越する海域へと変化したことを明らかにした．

Temporal Changes of Plio—Pleistocene Ostracoda from the Takanabe Formation, Miyazaki Group, Southwest Japan

A total of 145 ostracode taxa were obtained from the upper part of the Plio—Pleistocene Takanabe Formation, Miyazaki Group, in the northern part of the Miyazaki Plain, Southwest Japan. Most of the assemblages are extant temperate and subtropical water species that have been reported from the recent western North Pacific Ocean around Southwest Japan, the Tsushima Strait, and the shelves off China. Four ostracode bioassociations (UA, MA, LA, and DA) and two biofacies (SF and DF) were discriminated on the basis of Rand Q-mode cluster analyses. Biofacies SF is divided into six sub-biofacies (SF1–6). The results revealed at least two transgressions and one regression during the deposition of the upper part of the Takanabe Formation. The sea-level fluctuation at approximately 2.6 Ma inferred in this study is extremely large compared with the glacio-eustatic sea-level fluctuations during the late Pliocene to early Pleistocene, suggesting that local tectonic movement in the study site during the deposition of the upper part of the studied interval at ∼2.6 Ma may have led to the large sea-level fluctuation. This paper also includes descriptions of two new species, Acanthocythereis abei sp. nov. and Acanthocythereis takanabensis sp. nov.

軟岩・宮崎層群の続成作用と一軸圧縮強度の関係について

軟岩・宮崎層群の続成作用と一軸圧縮強度の関係について

Integrated stratigraphy of the Middle to Late Pliocene upper Miyazaki Group, southern Kyushu, Southwest Japan

Neogene sedimentary deposits are present in the Miyazaki region, southern Kyushu, on the Pacific side of southwestern Japan. The upper part of the Miyazaki Group, exposed in the Tsuma–Takanabe region of northern Miyazaki, is divided into the Tsuma, Sadowara, and Takanabe formations, in ascending order, and contains a planktic foraminiferal and calcareous nannofossil record of Pliocene events of the mid-latitude northwestern Pacific. The geological age of these units is based on magneto- and biostratigraphy. Five calcareous nannofossil datum horizons are recognized: the last occurrence (LO) of Reticulofenestra pseudoumbilicus, the termination of acme zone II of Reticulofenestra minutula var. C, the LO of R. minutula var. B, calcareous nannofossil datum plane A, and the termination of acme zone I of R. minutula var. C. Six planktic foraminiferal bioevents are also recognized: a coiling direction change, from sinistral to dextral, in the genus Pulleniatina; the first occurrence (FO) of Globorotalia crassaformis; the LOs of Globorotalia margaritae, Sphaeroidinellopsis seminulina, and Dentoglobigerina altispira; and the FO of Neogloboquadrina asanoi. The presence of two calcareous nannofossil bioevents, Datum A and the termination of acme zone I of R. minitula var. C, suggests that the normal and reversed polarity intervals of the upper part of the Takanabe Formation correspond to Chron C2An.1n (latest Gauss Normal Epoch) and Chron C2r.2r (earliest Matuyama Reversed Epoch), respectively. Based on combined floral and faunal data and magnetostratigraphic chron boundaries from the ATNTS2004, the upper part of the Miyazaki Group is Middle to Late Pliocene in age.

宮崎県日南海岸沿いに分布する新第三系宮崎層群青島層の重力流堆積物とその層序パターンの特異性

The Neogene Aoshima Formation of the Miyazaki Group, well exposed along the Nichinan coast, has been considered to be a turbidite succession deposited at a forearc basin, although the thickness-frequency distributions and the sedimentary facies of the formation are in disagreement with those of typical turbidite successions. In this study, we investigated the depositional system and the sedimentary environments of the formation, on the basis of the facies of sediment-gravity flow deposits and stratigraphic variations of bed-thickness distributions. From the results, we can classify the sediment-gravity flow deposits of the formation into the following types; Type G: sedimentary structures containing grading beds, whose wavelength is in the range of 0.5-5m; Type I: inverse grading structures composed of a finer lower unit with climbing-ripple cross lamination or HCS-mimics, and a coarser upper unit with massive sandstones; and Type M: massive structureless sandstones. Individual sediment-gravity flow deposits can be correlated over 10 km using key-tuff markers along palaeocurrent directions. Stratigraphic fluctuations of the hemipelagic-mudstone thickness and sediment-gravity flow thickness show that the formation along the Aoshima Island consists of a lower progradational system indicated by a thinning-upward trend of hemipelagic mudstones and an upper system dominated by lobate sand bodies characterized by the cyclic sedimentation of thick-sediment-gravity flow deposits. These results indicate that the depositional setting of the formation is a deep-offshore basin of a fandelta along the south-east coast of the Kyushu Island.

宮崎県鮮新統中部佐土原層から発見された国内最古の現生種タコブネ化石とその堆積年代

A fossil egg case of Argonauta hians Lightfoot, 1786 was collected from the middle Pliocene Sado-wara Formation of the Miyazaki Group in Shintomi Town, Miyazaki Prefecture, southwest Japan. This is the second report of argonautid fossils from Pliocene formations in Japan and marks the earliest occurrence of living species of Argonautidae in the North Pacific region. Fossil planktonic foraminifers and calcareous nannofossils were examined to establish the biostratigraphy of the fossil-bearing sequence. The horizon yielding the specimen of A. hians corresponds to the planktonic foraminiferal zone N.21 and calcareous nannofossil zone CN12a (ca. 3.25–3.12 Ma).

Geology and fossil ostracode assemblages from the Pliocene Miyazaki Group in the northern part of the Miyazaki Plain, Southwest Japan

Geology and fossil ostracode assemblages from the Pliocene Miyazaki Group in the northern part of the Miyazaki Plain, Southwest Japan

O-80 宮崎県南部油津湾沿岸に分布する宮崎層群の岩相と堆積環境(9.堆積相と堆積システム・シーケンス,口頭およびポスター発表,一般講演)

O-80 宮崎県南部油津湾沿岸に分布する宮崎層群の岩相と堆積環境(9.堆積相と堆積システム・シーケンス,口頭およびポスター発表,一般講演)

宮崎層群高鍋層(鮮新統)の冷湧水性化石群集の新露頭

宮崎層群高鍋層(鮮新統)の冷湧水性化石群集の新露頭

An outer-shelf cold-seep assemblage in forearc basin fill, Pliocene Takanabe Formation, Kyushu Island, Japan

A cold-seep invertebrate assemblage consisting mainly of the large bivalve Lucinoma sp. is exposed at the Kuge Shrine and surrounding area in Shintomi Town, Koyu County, Miyazaki Prefecture, Kyushu Island, southwestern Japan. The outcrops with this assemblage are in the upper part of the Upper Pliocene Takanabe Formation of the Miyazaki Group. The Takanabe Formation exposed in Shintomi Town is a cyclical sequence of upward-coarsening and upward-fining sediments, which reflects sea-level change: the paleobathymetry inferred from molluscs implies a shallowing upward from 160 m–200 m to 50 m in water depth, followed by a deepening back to 160 m–200 m. The cold-seep assemblage occurs in the coarsest (muddy sandstone) and bathymetrically shallowest (50 m in water depth) horizon, an outer shelf environment near the average storm-wave base, in this shallowing and deepening cycle. The cold-seep assemblage around the Kuge Shrine occurs sporadically in area of about 100 m in a north-south direction and about 60 m in an east-west direction, and is about 12 m in stratigraphic thickness. Many articulated and disarticulated bivalves are preserved with their commissure planes parallel to bedding, but some articulated bivalves are oriented normal to it, in the characteristic life position of lucinids. The muddy sandstone containing this assemblage is cemented strongly to weakly by authigenic carbonate depleted greatly in 13C (δ13C = −30.69‰ to −53.21‰ vs. PDB), which suggests the influence of methane seepage. The preserved life positions of the lucinid bivalves and the carbon isotope ratios of the associated carbonates suggest that this assemblage is autochthonous. The bedding-parallel orientation of many shells is inferred to have resulted from in-situ reworking by storm-induced waves and currents that are prevalent in such a shallow environment.