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Volume 106, Issue 7
Displaying 1-7 of 7 articles from this issue
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Hiroko Okazaki, Shinji Isaji, Hiroomi Nakazato2000 Volume 106 Issue 7 Pages 461-471
Published: July 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Kamiizumi Formation, Shimosa Group, which is exposed in the Kanto area, Northeast Japan, was deposited in middle Pleistocene Paleo-Tokyo Bay (ca. 250-200 KaBP). Paralic and shallow marine deposits (river, estuary, delta, shoreface-beach) are recognized in the formation. The delta deposits in Hatazawa sand pit, Kisarazu-city, are composed of gravel and sands with abundant molluscan shells. The delta beds occur as a steeply dipping, large-scale foreset with a variety of facies, mainly products of sediment gravity flows on subaqueous slope of a Gilbert-type coarse-grained delta. The foreset beds are divided into two units, I and II in ascending order. The unit I is composed of alternation of two facies, parallel beds with ripple lamination (facies A) and scour-filling backset beds (facies B). The latter is thought to reflect development of chutes and pools with sedimentation related to upstream-migrating hydraulic jumps. In the top of unit I, there are abundant convolute structures and sand volcanoes which might be induced by earthquake. The unit II is composed of finer sands than those of lower unit and has slightly gentle delta slope. Parallel beds, trough and tabular cross-beds in the foreset beds of unit II might be formed by tidal currents. The direction of paleocurrents of unit II differs from that of the lower units. This difference between unit I and II is caused by shoaling upward and effect of the event of the top of unit I.This Gilbert-type coarse-grained delta developed in the margin of Paleo-Tokyo Bay where stable supply of coarse-sediment, relatively steep channel/basin ratio is present.View full abstractDownload PDF (2367K) -
Asako Ishigaki, Makoto Ito2000 Volume 106 Issue 7 Pages 472-481
Published: July 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Lower Cretaceous Choshi Group in the northeastern Boso Peninsula of Japan is characterized by shoreface and offshore deposits up to more than 620 m thick and is classified into four depositional sequences. Lithofacies assemblages of the group are interpreted to have been deposited in an open and stormdominated shallow marine environment.Size population of hummocky bedforms of shoreface and offshore deposits, in particular, those of the Inubozaki and lowest Toriakeura Formations in the middle part of the group, was analyzed based upon detailed field measurements of ripple spacing of hummocky bedforms. The ripple spacing of hummocky bedforms shows a log-normal distribution pattern with a mode about 300 cm and this distribution pattern is similar to that of symmetrical ripples, suggesting oscillatory-dominated combined flow condition. The ripple spacing of hummocky bedforms is also inversely proportional to supposed paleo-water depth. This relationship is interpreted that the orbital diameter possibly responsible for determining the ripple spacing of hummocky bedform decreased in response to the increase in paleo-water depth. Bed thickness of hummocky cross-stratified sandstones, in particular, of those of offshore deposits has been supposed to be proportional to the magnitude of storm waves. The ripple spacing of hummocky bedform of offshore sandstones, however, does not show any distinct relationship with bed thickness. In general, thicker sandstone interbeds of offshore deposits contain planar stratification in the lower part and such a sedimentary structure probably developed under an intense storm wave condition superimposed by an offshoredirected current than for hummocky cross-stratification in an offshore environment.View full abstractDownload PDF (2163K) -
Fujio Masuda, Basara Miyahara, Jyunji Hirotsu, Toshiaki Irizuki, Yo Iw ...2000 Volume 106 Issue 7 Pages 482-488
Published: July 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSTemporal variation of Holocene Osaka Bay environment was inferred from facies analysis and many highly dense 14C age determination of a core collected from the bay floor off-Kobe. The results indiate the following detailed variation of depositional environments and conditions of Osaka Bay in Holocene time.The depositional environments inferred from the core succession are as follows : The lower muds (8.5 m thick) are estuary and inner bay deposits and deposited rapidly (8 mm/yr) during rapid rising stage of sea-level ; the middle sand and mud (8 m thick) are tidal delta deposits showing rapid deposition (about 7 mm/yr) ; the upper sand and mud (9 m thick) are bay-floor deposits of slow deposition (0.9 to 1.8 mm/yr)during a marine flood and highstand in sea-level ; and the uppermost clay (4 m thick) is bottomset deposits of the modern Yodo River and shows increasing depositional rates (2.0 to 3.7 mm/yr).The following bay environments also are given from the core analysis : the sea-level at 11000 B.P., was -51 m below the modern sea-level in Osaka basin ; in 9700 B.P., -31 m in sea-level high, the Akashi Strait was formed ; in 8000 B.P., the sea-level of -12 m high, the Setouchi Inland Sea (Setonaikai) was formed when the seas were linked together at the Bisan-Seto ; during 5300 to 5000 B.P., the maximum highstand of sea level in the Osaka basin was +1 to 2 m in elevation ; during 8000 to 3000 B.P., coarser deposits were distributed in all Osaka Bay with slow deposition by strong tidal-currents in the Akashi Strait ; restarting of inflow of much flood clay from the Yodo River into Osaka Bay in about 1700 B.P. and spread the distribution until off-Kobe in 1000 B.P.View full abstractDownload PDF (1118K) -
Mahito Watanabe, Masaki Takahashi2000 Volume 106 Issue 7 Pages 489-500
Published: July 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSDiatom biostratigraphy is established for the Middle Miocene Kinone and lower Amatsu formations in the Kawadani section, Boso Peninsula, central Japan. Nine out of 31 collected calcareous nodules yield age diagnostic diatom fossils. The Denticulopsis hyalina Zone (NPD 4B), Crucidenticula nicobarica Zone (NPD 5A) and Denticulopsis praedimorpha Zone (NPD 5B) are recognized from the upper Kinone Formation to lower Amatsu Formation. A diatom assemblage correlated with the Thalassiosira fraga Zone (NPD 2A) is found in a reworked nodule in the Kanigawa Formation. Useful biohorizons such as the fisrt occurrence (FO) of D. praedimorpha var. minor (D51), the last occurrence (LO) of C. nicobarica (D52), the first acme (AC1) of Denticulopsis crassa (D52.5) and the FO of D. praedimorpha var. praedimorpha (D53) are recognized in the lowest Amatsu Formation. Based on the stratigraphic position of these biohorizons, the stratigraphic relationship between biohorizons of microfossils in the Boso Peninsula is revised. Comparison of the stratigraphic relations of diatom and calcareous nannofossil biohorizons between the Boso Peninsula and northeastern Japan areas suggests a diachronism for the last occurrence of calcareous nannofossil Cyclicargolithus floridanus.View full abstractDownload PDF (1873K) -
Keigo Hisadome, Makoto Ito2000 Volume 106 Issue 7 Pages 501-510
Published: July 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe upper Pleistocene shallow marine deposits defined as the Kioroshi Formation in the northern Shimosa Upland, Boso Peninsula of Japan, consist mainly of thick sands and locally intercalated gravel and mud beds. These sandstone-dominated shallow marine deposits are an important aquifer in this area and were investigated in terms of three-dimensional heterogeneity of lithofacies and permeability for characterizing hydrogeologic processes of the aquifer sediments.Three major lithofacies, i.e., hummocky cross-stratified sands (HCSS) (lower shoreface), trough cross-stratified sands (TCSS) (upper shoreface), and planar-stratified sands (PSS) (foreshorebackshore), were mainly analyzed in the present study. Heterogeneity of lithofacies was identified based upon variations in inclined directions of lamina planes (IDLP). HCSS are characterized by the most variable IDLP and are the most isotropic. IDLP of PSS are more consistent and indicate that PSS are the most anisotropic. TCSS are characterized by IDLP intermediate between those of HCSS and PSS. Heterogeneity of permeability of the three lithofacies was analyzed in terms of hydraulic conductivity (HC) of three different directions, i. e., vertical (VD), parallel to dip direction of lamina plane (DL), and parallel to strike direction of lamina plane (SL). In general, HC increase in response to the increase in both average grain size and sorting. TCSS and HCSS are characterized, respectively, by the highest and lowest HC and PSS are intermediate. Furthermore, HC of VD is the lowest and about half of other directions.On the other hand, there is not much difference between other two directions. Detailed investigation of three-dimensional heterogeneity of lithofacies and permeability of shallow marine sands, therefore, is interpreted to be very important for characterizing transmissivity anisotropy of aquifer sediments.View full abstractDownload PDF (1453K) -
Masayuki Ehiro, Kunihiro Ishizaki, Takayuki Sakai2000 Volume 106 Issue 7 Pages 511-519
Published: July 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSMiddle Permian fusulinaceans were discovered from the limestones, which have been believed to be a part of the Lower Permian Nishikori Formation, distributed in the northern part of the Maiya district, Southern Kitakami Massif. The stratigraphic unit Hatanosawa Formation is newly proposed for this Middle Permian limestone-dominated unit. The lower and upper parts of the Hatanosawa Formation are composed of massive limestone and alternating beds of limestone and calcareous shale, and the middle part consists of sandy shale and sandstone. The lower limestone contains fusulinaceans, such as Codonofusiella sp. and primitive neoschwagerinids, and the upper limestone yields Parafusulina cf. guatemalaensis, P. cf. lutugini, P. cf. motoyoshiensis, Colania cf. douvillei, and Lepidolina ? sp. Based on these fusulinaceans, the lower and upper parts of the formation are correlatable with the lower Kanokuran Series (Monodiexodina matsubaishi Zone) and upper part of the lower Kanokuran (Colania kotsuboensis Zone) to lower part of the upper Kanokuran (lower part of the Lepidolina multiseptata Zone), respectively.View full abstractDownload PDF (1449K) -
Isao Motoyama, Naotomo Kaneko, Hitoshi Tanaka, Toshiya Kudo2000 Volume 106 Issue 7 Pages XIII-XIV
Published: 2000
Released on J-STAGE: November 26, 2010
JOURNAL FREE ACCESS
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