Sand- and Mud-Dominated Alluvial-Fan Deposits of the Miocene Seto Porcelain Clay Formation, Japan

  1. N. D. Smith2 and
  2. J. Rogers3
  1. K. Nakayama

Published Online: 17 MAR 2009

DOI: 10.1002/9781444304213.ch28

Fluvial Sedimentology VI

Fluvial Sedimentology VI

How to Cite

Nakayama, K. (2009) Sand- and Mud-Dominated Alluvial-Fan Deposits of the Miocene Seto Porcelain Clay Formation, Japan, in Fluvial Sedimentology VI (eds N. D. Smith and J. Rogers), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304213.ch28

Editor Information

  1. 2

    Department of Geosciences, 214 Bessey Hall, University of Nebraska, Lincoln, NE 68588-0340, USA

  2. 3

    Cape Town, South Africa

Author Information

  1. Department of Geoscience, Shimane University, Matsue 690-8504, Japan

Publication History

  1. Published Online: 17 MAR 2009
  2. Published Print: 7 OCT 1999

ISBN Information

Print ISBN: 9780632053544

Online ISBN: 9781444304213

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Keywords:

  • sand- and mud-dominated alluvial-fan deposits of Miocene Seto Porcelain Clay Formation, Japan;
  • Late Miocene Seto Porcelain Clay Formation;
  • alluvial fans - defined by geomorphological character;
  • sand- and mud-dominated alluvial-fan deposits;
  • facies associations;
  • facies association 3;
  • spatial and stratigraphical variation of facies associations;
  • palaeocurrent analysis;
  • grain composition and size;
  • estimation of fan morphology

Summary

The Late Miocene Seto Porcelain Clay Formation, best exposed in the northern part of Seto City in Japan, was deposited over a period of 1 Myr around 9 Ma. It is distributed sporadically in small collapse basins less than 30 km2 in area. The formation is underlain mainly by Cretaceous granite, as well as Palaeozoic-Mesozoic pelagic formations and Early to Middle Miocene shallow-marine formations (22–15 Ma). After deposition of the marine formations, the granite was strongly weathered and decomposed in situ, forming sand- and mud-size material. Many plant fossils occur within the Seto Porcelain Clay Formation, which suggest a warm climate with a low annual temperature range.

Three facies associations (FA1–FA3) are recognized. Facies association 1 consists mainly of poorly sorted gravel beds and cross-stratified sandy gravel beds, with associated massive mud beds. Gravel clasts were derived from nearby uplifted basement blocks. Sand beds commonly occur in channels, and mud beds contain rare plant roots. Facies association 2 is dominated by cross-stratified coarse- to very coarse-grained sand beds, with mud beds that commonly contain plant roots, and is subdivided into FA2a and FA2b on the basis of sand-bed architecture. Sand beds in FA2a commonly occur in channels, whereas those in FA2b have a sheet-like geometry. Plant roots are more abundant in the mud beds of FA2b than in FA2a. Facies association 3 comprises mainly plant-root- and plant-debris-dominated mud and lignite beds, with subordinate massive muddy sand beds.

Palaeocurrents occur in two radiating domains. In each domain, facies associations are ordered FA1, FA2a, FA2b and FA3 away from the palaeopiedmont line. Palaeocurrents indicate that two alluvial fan systems existed, and the arrangement of facies associations implies lateral facies change. The fans have radial lengths of 1.5km and 2.0 km. Slope gradient in FA2 is estimated to be about 0.1° to 0.2°, based on channel depth and grain size in sand beds. The combination of voluminous sand supply from decomposed granite, together with collapse of small basement blocks and mud entrapment by vegetation, enabled the formation of the sand- and mud-dominated alluvial-fan systems.