The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 112 , Issue 7
Showing 1-8 articles out of 8 articles from the selected issue
  • Hiroyoshi Arai, Tohru Ohta
    2006 Volume 112 Issue 7 Pages 439-451
    Published: 2006
    Released: November 08, 2006
    In the field of geology, compositional data, such as petrochemical compositions, faunal compositions and modal compositions of sandstones, are common. This type of data contains an awkward mathematical problem known as constant-sum constraint. To resolve this problem, logratio and simplicial analyses have been developed in the last two decades. However, zero and missing values are common in practical compositional data, which are troublesome for logratio or simplicial analysis because neither logarithm nor geometric mean can take zeros. In this context, many authors have suggested nonparametric replacement methods of zero and missing values to overcome this problem. We review these nonparametric methods, additive replacement and multiplicative replacement, with their merits and limitations, after showing types and nature of zeros: rounded zeros stemmed from a detection limit of apparatus and essential (or true) zeros designating nothing. Zero replacement, however, may create outliers of data vectors and would lead us to erroneous conclusions. For this reason, we also review how to assess the outlier: by atypicality indices of data vectors and by confidence regions of a population. To disseminate statistically rigorous replacement and outlier detection, computer programs for open source statistical environment `R', which replace zeros in a given data set and calculate atypicality indices, were developed.
    Download PDF (1604K)
  • Ken-ichi Hirauchi, Ken-ichiro Hisada, Yasuhiro Iba
    2006 Volume 112 Issue 7 Pages 452-458
    Published: 2006
    Released: November 08, 2006
    Serpentinite bodies are distributed along fault boundaries between the Sanchu Cretaceous and the rocks of the Southern Chichibu Belt in the northwestern Kanto Mountains, central Japan. It has been thought that the relationship of the Sanchu Cretaceous and the serpentinite is a fault contact, but we found an outcrop where the Shiroi Formation (brackish sediments) of the Cretaceous covers the serpentinite unconformably. The contact surface is undulatory in geometry and is oblique to the bedding plane of the basal Shiroi Formation. Serpentinite clasts are included in the basal part of the Shiroi Formation. The microscopic texture of serpentinite clasts consists mostly of antigorites and differs from the textures of underlying serpentinite.
    It is clear that the serpentinite occurs in two different manners, that is, a constituent of the fault zone and a basement rock of the Sanchu Cretaceous. Almost serpentinite clasts were derived from antigorite-rich serpentinite bodies nearby. We can conclude that the serpentinite body along the Otchizawa River was protruded on the brackish area immediately before the initial sedimentation of the Shiroi Formation (Hauterivian).
    Download PDF (1652K)
  • Eiji Sasao, Hideki Iwano, Tohru Danhara
    2006 Volume 112 Issue 7 Pages 459-468
    Published: 2006
    Released: November 08, 2006
    The depositional age of the Toki Lignite-bearing Formation of the lowermost part of the Miocene Mizunami Group, which is distributed in the southeastern part of Gifu Prefecture, central Japan, has been investigated by fission track (FT) dating and petrographic description of four tuffaceous sandstones.
    An investigation of the refractive indices of plagioclase suggests that the tuffaceous sandstones are a mixture of andesitic tuff and granitic detritus. Two age populations, 17-21 Ma (early Miocene) and 56-67 Ma (late Cretaceous to early Tertiary) were obtained by a statistical treatment of the distribution of zircon grain ages. The younger age cluster is regarded as the age of andesitic tuff and the older one is probably the age of detrital zircon derived from the Toki Granite (59-61 Ma). The younger age implies the depositional age of the middle part of Toki Lignite-bearing Formation. Based on the obtained and previously reported FT ages, the depositional age of the Toki Lignite-bearing Formation except for the basal conglomerate is inferred to range from 18 to 20 Ma.
    Download PDF (1428K)
  • Hidetaka Horikawa, Kohki Yoshida
    2006 Volume 112 Issue 7 Pages 469-477
    Published: 2006
    Released: November 08, 2006
    The sedimentary environment of reddish clastic rocks in the lower most horizon of the Lower Triassic Hiraiso Formation, which is in lower member of the Inai Group, South Kitakami Terrane, is reviewed. The sequence at the Kawabukuro quarry, in Toyoma City, Miyagi Prefecture, consists of several fining-upward successions of varicolored conglomerate, sandstone, and reddish sandy mudstone with desiccation cracks and carbonate nodules. The sedimentary feature of this section is interpreted as a deposition in the fluvial environment at the lower part and in the distributary channel affected by a tidal current at the upper part. A part of the reddish sandy mudstone shows an altered vitro-clastic texture under microscope. The immediate covering of the reddish sandy mudstone in the form of a wave ripple is suggestive of the ferric alternation of the fine volcanic material in the hinterland not by weathering after deposition. Meanwhile, the existence of rip-up clasts of the reddish mudstone in the massive sandstone indicates redeposition following the erosion of the mudstone, which was once deposited as mudstone strata, under a high flow regime condition.
    XRD analysis of the reddish sandy mudstone reveals the formation of hematite as the material responsible for the reddish color of the mudstone. The whole rock geochemistry of the reddish sandy mudstone is characterized by high-Fe2O3/Al2O3 and equivalent (K2O+Na2O)/Al2O3 ratios in contrast to the mudstone of the Upper Permian Toyoma Formation. The existence of the carbonate nodules scattered in the reddish sandy mudstone with small-scale bioturbation indicates weak soil formation in semi-arid environments.
    Download PDF (1953K)
Prompt Report