An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates

An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic... ABSTRACT The feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south‐west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (Rw), maximum projection sphericity (δp), oblate‐prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δp and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade‐shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and Rw is observed, hence the abundance of disc‐shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Sedimentology Wiley

An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates

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Publisher
Wiley
Copyright
Copyright © 1992 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0037-0746
eISSN
1365-3091
D.O.I.
10.1111/j.1365-3091.1992.tb02128.x
Publisher site
See Article on Publisher Site

Abstract

ABSTRACT The feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south‐west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (Rw), maximum projection sphericity (δp), oblate‐prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δp and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade‐shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and Rw is observed, hence the abundance of disc‐shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates.

Journal

SedimentologyWiley

Published: Jun 1, 1992

References

  • The shape of rock particles, a critical review
    Barrett, Barrett
  • The sedimentary history of some Triassic conglomerates in the Vale of Glamorgan, South Wales
    Bluck, Bluck
  • Boulder shapes and grain size distributions of debris as indicators of transport paths through a glacier and till genesis
    Boulton, Boulton
  • Pebble segregation and bed lenticularity in wave‐worked versus alluvial gravel
    Clifton, Clifton
  • Gravelly shoreface deposits: a comparison of modern and ancient facies sequences
    Hart, Hart; Plint, Plint

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