Depositional Processes in Latest Pleistocene and Holocene Ephemeral Streams of the Main Ethiopian Rift (Ethiopia)

  1. Michael D. Blum3,
  2. Susan B. Marriott4 and
  3. Suzanne F. Leclair5
  1. M. Benvenuti1,
  2. S. Carnicelli2,
  3. G. Ferrari2 and
  4. M. Sagri1

Published Online: 17 MAR 2009

DOI: 10.1002/9781444304350.ch16

Fluvial Sedimentology VII

Fluvial Sedimentology VII

How to Cite

Benvenuti, M., Carnicelli, S., Ferrari, G. and Sagri, M. (2009) Depositional Processes in Latest Pleistocene and Holocene Ephemeral Streams of the Main Ethiopian Rift (Ethiopia), in Fluvial Sedimentology VII (eds M. D. Blum, S. B. Marriott and S. F. Leclair), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304350.ch16

Editor Information

  1. 3

    Baton Rouge, Louisiana, USA

  2. 4

    School of Geography and Environmental Management, University of the West of England, Bristol BS16 1QY, UK

  3. 5

    Department of Earth and Environmental Sciences, Tulane University, Dimwiddie Hall, New Orleans, LA 70118, USA

Author Information

  1. 1

    Dipartimento di Scienze della Terra, University of Florence, Via G. La Pira 4, 50121 Firenze, Italy

  2. 2

    Dipartimento di Scienza del Suolo e Nutrizione della Pianta, University of Florence, P.le delle Cascine 18, 50144 Firenze, Italy

Publication History

  1. Published Online: 17 MAR 2009
  2. Published Print: 15 FEB 2005

Book Series:

  1. Special Publication Number 35 of the International Association of Sedimentologists

Book Series Editors:

  1. Ian Jarvis

Series Editor Information

  1. School of Earth Sciences and Geography, Centre for Earth and Environmental Science Research, Kingston University, Penrhyn Road, Kingston-upon-Thames KT1 2EE, UK

ISBN Information

Print ISBN: 9781405126519

Online ISBN: 9781444304350

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Keywords:

  • ephemeral streams of main Ethiopian rift;
  • depositional processes in latest Pleistocene and Holocene;
  • cyclic incision and aggradation of latest Pleistocene–Holocene ephemeral streams;
  • erosion and aggradation in dryland fluvial and slope systems - dynamic processes over spatial and temporal scales;
  • Silti-Debre Zeit Fault Zone (SDZFZ) to west and Wonji Fault Belt (WFB) to east;
  • modern fluvial systems and deposits;
  • upper quaternary deposits around Gademota ridge;
  • massflow facies

Summary

Discontinuous ephemeral streams were described originally from the south-west USA as consisting of alternating reaches of incised rectangular channels and aggradational channel-fan systems. Modern ephemeral streams on the flanks of the Gademota Ridge, in the Lake Region of the Main Ethiopian Rift, show a similar pattern, and deposits exposed in the walls of modern stream valleys record the development of these systems since the latest Pleistocene. Streamflow, sheetflow and massflow deposits are recognized to be the basic sedimentary facies in palaeovalley fills, and within palaeochannel–fan systems. These deposits are stacked in a cyclic pattern within multiple and single valley fills, with basal streamflow deposits overlain by hyperconcentrated sheetflow and eventually by debris-flow deposits. Debris-fall and debris-flow deposits related to bank failure occur interbedded with streamflow and sheetflow facies.

A model for the cyclic incision and aggradation of the latest Pleistocene–Holocene ephemeral streams in this area is proposed based on incision during transition to a more moist climate, followed by aggradation during two main genetic stages that contrast in discharge volumes. During the early stage, relatively high and continuous seasonal discharge transports coarse-grained material, and covers the incised channel base with gravelly and sandy dune-scale bedforms. Bank undercutting and piping produce instability of the channel margins, with accumulation of relatively thick aprons of fine-grained material. The channel fan system is supplied with pebbly–sandy material flushed from the valleys. During the late stage, a decreased volume of flashy discharge redistributes sediment stored in the bank aprons within the valleys and to the next downstream channel–fan system through hyperconcentrated sheetflows and debris flows. Spreading of debris flows and sheetflows on interfluves marks the overfilling of the valleys.