Calcretes of Olduvai Gorge and the Ndolanya Beds of Northern Tanzania

  1. V. Paul Wright2 and
  2. Maurice E. Tucker3
  1. R. L. Hay and
  2. R. J. Reeder

Published Online: 8 APR 2009

DOI: 10.1002/9781444304497.ch1

Calcretes

Calcretes

How to Cite

Hay, R. L. and Reeder, R. J. (1991) Calcretes of Olduvai Gorge and the Ndolanya Beds of Northern Tanzania, in Calcretes (eds V. P. Wright and M. E. Tucker), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304497.ch1

Editor Information

  1. 2

    Postgraduate Research Institute of Sedimentology (PRIS), University of Reading, UK

  2. 3

    Department of Geological Sciences, University of Durham, UK

Author Information

  1. Department of Geology and Geophysics, University of California, Berkeley, California, USA

Publication History

  1. Published Online: 8 APR 2009
  2. Published Print: 13 JUN 1991

ISBN Information

Print ISBN: 9780632031870

Online ISBN: 9781444304497

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Keywords:

  • Quaternary calcretes;
  • Calcretes of Olduvai Gorge and the Ndolanya Beds - northern Tanzania;
  • replacement, major process in formation of massive calcretes;
  • calcite, referring to low-Mg calcite;
  • pellet, referring to rounded aggregate of clay or micrite;
  • calcretes of the Olduvai Beds, supporting vegetation;
  • non-calcite fraction of calcretes, mixture of inherited and authigenic materials

Summary

Pedogenic calcretes are closely associated with Pliocene to Holocene wind-worked deposits of volcanic ash in the Olduvai and Ndolanya Beds of northern Tanzania. The typical profile with calcrete consists of an unconsolidated sediment layer, an underlying laminar calcrete, and a lowermost massive calcrete. The laminar calcrete is a relatively pure limestone, whereas massive calcrete is aeolian tuff cemented and replaced by calcite. An Olduvai calcrete profile can develop to a mature stage in only a few thousand years. Carbonatite ash was the dominant source for most of the calcite in the calcretes.

Replacement was a major process in formation of the massive calcretes, and oolitic textures have resulted from micrite replacing pelletoid clay coatings around sand grains. Phillipsite and possible other zeolites were extensively replaced in the massive calcretes. Replacement of clay by micrite in the Olduvai calcretes is accompanied by dissolution or leaching of phengitic illite and the formation of clay approaching the composition of halloysite or kaolinite. In the upper calcrete of the Ndolanya Beds, montmorillonite was altered to a kaolinite-type mineral and to dioctahedral chlorite. Authigenic dolomite, zeolite, and dawsonite in the Olduvai calcretes probably received at least some of their components from replaced materials.