Revision of Systematics in Sedimentary Petrology with Special Reference on Sandstone Composition in Tectonic Provinces

Niichi Nishiwaki

Faculty of Social Research, Nara University,1500 Misasagicho, Nara City, Nara, 631-8502, Japan.

E-mail: niichi@daibutsu.nara-u.ac.jp

Abstract

The current systematics in sedimentary petrology is based on the texture, granulometry and modal composition

with reference to the sedimentological environment in the continental regions. In recent years, chemical and isotopic

composition data of sedimentary rocks are rapidly accumulated, and knowledge on sedimentological environment,

sedimentary facies and tectonic provinces is integrated, especially in the mobile zones. According to the drastic

change of quality and quantity of data and extended sedimentological information, it is urgently required to revise

the systematics in sedimentary petrology. The IUGS-CSP (Commission on Systematics in Petrology) is trying to

establish a working group of international sedimentological communities, in which the new systematics will be

proposed based on the detailed reviewing of the current classification and terminology. It is important for the new

systematics to clarify the data with mathematical and statistical analyses supported by various informational

techniques. The Japanese subgroup of the working group is constructing a database that contains various kinds of

sedimentological data and related information on sandstones in Japan and surrounded area, and analyzing the

relationship between the sandstone properties and geological and sedimentological settings by using several

statistical methods including factor and cluster analyses. Preliminary interpretation of the extracted factors

indicates that the chemical composition and other sedimentary properties are affected also by geological and

tectonic settings, not only sedimentary process, environment, diagenesis and weathering. Detailed statistical and

theoretical analysis on the relationships within different properties of sedimentary rocks is continuing, and it is

expected that many important suggestions will be obtained for the new classification and terminology of

sedimentary rocks.

Introduction

Geology was born from the study in continental areas, and many geological models and

theories were devoloped on the data from the continental regions. According to the development

of science and technology, new methods and techniques were applied, and geology was extended

its field to other kinds of areas, such as ocean floor and island arc. Geological models and

theories should be revised to adopt for the new areas and data.

Systematics in petrology is one of the classical but important achievement in geology, and it's

contribution to geological sciences and the human society is remarkable. It is, however,

necessary to be revised synthetically for the future application.

Classifications on Modal Composition

There are many researches on the systematics in sedimentary petrology, and the

classification of the sandstone was established mainly for the stable continental environments,

based on the modal and granulometric analysis, and using various ternary diagram (Pettijohn, 1954;

Okada, 1971). However the classification is difficult to adopted for the tectonic

environments, and there are many studies to extend the classification. For example, Dickinson et

al. (1983) tried to classify various provenances on the Q-F-L diagram (Fig. 1), Kumon et al. (1992)

proposed the discrimination of provenance on the Q-F-R diagram (Fig.2), and Takeuchi (1998)

tried to classify the sedimentary facies from the mode of rock fragments (Fig.3).

Fig. 1. Compositional fields indicative of sandstone derivation from different types of provenance

(Dickinson et al., 1983). Qm:monocrystalline quartz grain, F:total feldspar grains, Lt:total lithic

fragments.

Fig. 2. Tentative proposal for discriminating the provenance types of magmatic arc, based on the

traditional modal point-counting method. PVA: primitive volcanic arc provenance, EMA: evolved

and matured magmatic arc provenance, DMA: dissected magmatic arc provenance, DDMA: deeply

dissected magmatic arc provenance, RMA: renewed magmatic arc provenance. Solid and dashed

arrows show an evolving or roofing process, and open arrows indicate an unroofing or dissecting

process. Q: total quartz, F: total] feldspar, R: total rock fragments. (Kumon et al., 1992)

Fig. 3. Types of rock fragments in sandstones of the Shingai Formation. open square: normal

sediments, others: mélange facies. Lv:volcanic, Lm:metamorphic, Qp:polycrystaline quartz,

Ls:sedimentary, Lp:plutoniic. (after Takeuchi, 1998)

Other researchers focused on the specific grains in the sandstone to discriminate the

sedimentary environments. For example, Teraoka et al. (1997) tried to classify the clastic garnet

grains (Fig. 4), and Hisada et al. (1998) examine the chromian spinels in the sandstones of

metamorphic belt (Fig. 5).

Fig. 4. Classification of garnet type. (after Teraoka et al., 1997)

Fig. 5. Cr-Al-Fe3+ diagram of chromian spinels from the Hida marginal belts. front:Kuruma

Group, Back:Kotaki oriststrome. (after Hisada et al., 1998)

Classification on Chemical Composition

According to the development of analytical techniques, chemical composition is also used for

sandstone classification. Various kinds of diagrams were proposed to discriminate provenances,

especially in the magmatic arc regions. For example, Bhatia (1983) proposed four provenance

types on the Al2O3/Sio2 - Fe2O3+MgO diagram (Fig. 6), and Roser and Korsch (1986) proposed

three provenance types on the K2O/Na2O - SiO2 diagram (Fig. 7).

Fig. 6. Al203/Si02-Fe203+MgO diagram. O.ARC: oceanic island arc, C.ARC: continental island

arc, AM: active continental margin, PM: passive margins (Bhatia 1983).

Fig. 7. K2O/Na2O-SiO2 diagram. The environmental divisions are after Roser & Korsch, 1986.

ARC: oceanic island arc margin, ACM: active continental margin, D: passive margin.

New discriminating indeces were also proposed for sandstone classification. For example,

Kiminami et al. (1992) proposed the Basicity Index and the B.I. diagram to discriminate the four

provenance types of magmatic arc. (Fig. 8). Fedo et. al. (1995) proposed the Chemical Index of

Alteration, and the A-CN-K diagram to discriminate two provenance trends (Fig. 9).

Fig. 8. Trend of magmatic arc sandstones (Segments A to C) on the Al2O3/SiO2 -

(FeO+MgO)/(SiO2+K2O+Na2O) diagram.

Fig. 9. A-CN-K diagram, modified after Fedo et al. (1995). CIA:Chemical index of alternation.

A:Al, CN:2Ca+Na, K:K.

Diagrams are revised and new diagrams were proposed for the discrimination of

provenances and classification of sandstone. For example, Kiminami et al. (1998) has revised

the B. I. diagram, and Musashino (1998) has discussed its resemble diagrams (Fig. 10). Roser

(1998) proposed Th/Sc - Zr/Sc diagram for five volcanic arc trends using trace elements

composition (Fig. 11).

Fig. 10. K*/A* - (Fe*+Mg*)/Al* daigram on sandstones from western Buller terane, New Zealand.

(after Musashino, 1998)

Fig. 11. Example Th/Sc-Zr/Sc diagrams for Triassic Murihiku sediments from Southland. St HAB

(high-Al basalt), LSA (low-Si andesite). AND (andesite), DAC (dacite), RH (rhyolite) are average

volcanic rock compositions, which define a primary source evolution line for first cycle

sediments.(after Roser, 1998a)

Statistical and Mathematical Treatment

According to the development of mathematical theories and accumulation of

sedimentological data, databases were constructed and used for the statistical study of systematics

in petrology. For example, Nishiwaki (1998) described the sedimentary database of the Japanese

sandstone, and it was used to for statistical analyses. Roser and Korsch (1988) proposed the

major element provenance diagram in which the discriminating functions are used for its axes, and

the trend of six volcanic arcs were clarified in the divided quadrant (Fig. 12).

Fig. 12. Major element provenance diagrams for Shimanto sandstones, Kii Peninsula (discriminants

of Roser & Korsch 1988). Stars BA(basalt) AN(andesite) DA(dacite) RD (rhyodacite). RH

(rhyolite) are average volcanic rock compositions, which define a primary source line for first cycle

sediments. Arrow indicates direction of maturation. (after, Roser, 1998b)

Statistical and mathematical researches on the sedimentological data are important to certify

the diagrams and classification by using them. As many new diagrams were empirically designed

in the first step, they should be examined with different kinds of data set before confirming the final

form of them. They should be examined by theoretical points of view to clarifying the meaning

of the diagram, including axes, scale, threshold value and discriminated groups. For example,

Nishiwaki (1999) tried to examine the discrimination with the B. I. diagram (Kiminami, 1998) by

discriminant analysis (Fig. 13), and it is indicated from the structure matrix (Table 1) that the

chemical composition and other sedimentary properties are affected also by geological and tectonic

settings, not only sedimentary process, environment, diagenesis and weathering.

Fig. 12. Scatter plot of tectonic provinces in the discriminant functions 2 and 2 space on the

Japanese sandstone data.

Table 1. Pooled within-groups correlations between discriminating variables and canonical

discriminant functions. Variables are ordered by size of correlation within function. Function 1 Function 2 Function 3

Ti02 .65167* .16618 -.29385

TFe2O3 .64222* .31070 -.25245

FeO -.54843* .18983 -.28367

MgO -.46854* .44068 -.05662

BaO -.05346* .04865 -.02164

SiO2 -.37128 .49780* .38615

MnO .15338 -.28177* .04811

CaO .13081 -.27926* .05382

P2O5 .14601 -.26798* .23239

Na2O .14694 -.05905 -.60718*

Al2O3 .25534 -.28347 -.51806*

K2O -.11609 .16824 -.21190*

* denotes largest absolute correlation between each variable and any discriminant function.

Revision of Systematics

The IUGS Commission on Systematics in Petrology is continuing the discussion on the

revision of the systemtics. The Subcommissions on Igneous Petrology and Metamorphic

Petrology have been working on the revision, and many important proposals and achievements

were given. On the other hand, the Subcommission on Sedimentary Petrology has been inactive

for many years. The Commission has decided to examine the feasibility of organizing a working

group for the revision of the systematics in sedimentary petrology.

Many participating members are deeply appreciated for the working group from various

sedimentological communities.

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