Anastomosed and Associated Coal-Bearing Fluvial Deposits: Upper Tongue River Member, Palaeocene Fort Union Formation, Northern Powder River Basin, Wyoming, U.S.A.

  1. R. A. Rahmani and
  2. R. M. Flores
  1. Romeo M. Flores and
  2. John H. Hanley

Published Online: 28 APR 2009

DOI: 10.1002/9781444303797.ch5

Sedimentology of Coal and Coal-Bearing Sequences

Sedimentology of Coal and Coal-Bearing Sequences

How to Cite

Flores, R. M. and Hanley, J. H. (1985) Anastomosed and Associated Coal-Bearing Fluvial Deposits: Upper Tongue River Member, Palaeocene Fort Union Formation, Northern Powder River Basin, Wyoming, U.S.A., in Sedimentology of Coal and Coal-Bearing Sequences (eds R. A. Rahmani and R. M. Flores), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303797.ch5

Author Information

  1. U.S. Geological Survey, Denver, Colorado 80225, USA

Publication History

  1. Published Online: 28 APR 2009
  2. Published Print: 26 MAR 1985

ISBN Information

Print ISBN: 9780632012862

Online ISBN: 9781444303797

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Keywords:

  • coal-bearing fluvial deposits - Upper Tongue River Member;
  • upper part of Tongue River Member, accumulating in ‘fluvial-lake-dominated environment’;
  • deposits of ‘fluvial-lake-dominated environment’, consisting freshwater, mollusc-bearing lacustrine limestone;
  • coarsening-upward deposits and limestone, representing common rock types;
  • crevasse splay and channel areal facies distribution

Summary

The Palaeocene Tongue River Member of the Fort Union Formation in the Powder River Basin is well known for its thick, extensive coals. In the northern Powder River Basin, economic and subeconomic coals are chiefly distributed in the lower 320 m and upper 140 m of the Tongue River Member, respectively. The economic coals of the lower Tongue River Member accumulated in vast floodbasin swamps marginal to meanderbelts of NE-flowing, meandering fluvial channels. The subeconomic coals of the upper Tongue River member formed in floodbasin swamps of anastomosed fluvial system and of a transition from a meandering fluvial system to an anastomosed fluvial system. Active detrital influxes into floodbasin swamps choked vegetal growth and diluted organic matter producing thin and impure coals. Widespread occurrence of small to large floodbasin lakes curtailed peat accumulation by drowning swamps. Lakes were inhabited by abundant freshwater gastropod and bivalves. Comparison with recent anastomosed fluvial deposits shows that facies and their areal distribution in the uppermost part of the Upper Tongue River Member are similar to those of the anastomosed reach of the lower Saskatchewan River in Saskatchewan, Canada and upper Columbia River in Alberta, Canada. Rapid basin subsidence accompanied by uplift in surrounding areas and damming downstream of the Powder River Basin fluvial system by the Cedar Creek Anticline in SE Montana during the late Palaeocene created an ideal setting for large-scale anastomosis in the intermontane Powder River Basin. This northerly-flowing anastomosed fluvial system served as a downflow continuum of basin margin, conglomeratic, braided and meandering tributary streams.