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Interpretation of Ancient Fluvial Channel Deposits

Ethridge, Frank G.1
1 Geosciences, Colorado State University, Fort Collins, CO.

Holocene rivers have a high degree of morphological variability with end member channel and/or channel belt types recognized as straight, meandering, braided, anastomosing and wandering. Individual rivers also show a high degree of longitudinal and vertical (through time) variability as a result of changes in base level, climate, tectonics, tributary contribution, and/or valley slope. This high degree of variability in plan-view morphology of Holocene rivers is not reflected in interpretations of ancient fluvial deposits. Most fluvial deposits, in outcrop and in the subsurface, continue to be interpreted as meandering or braided channels and many interpretations continue to be suspect because of the myths about modern rivers that are ingrained in our thought process and because measured or described characteristics bear little relationship to plan view morphology (Bridge, 1985, 1993, 2006; Brierley & Hickin, 1991, Miall, 1995; Heller, et al., 2007).

Reconstructions of ancient channel plan forms are possible from exceptional 3D outcrop exposures of small- to medium-scale deposits and detailed evaluation of large-scale, inclined strata, produced by channel bar migration. A review of published literature on interpretations of the plan-view morphology of ancient fluvial deposits suggests that most lack any data on characteristics of channel bar deposits. Quantitative seismic geomorphology allows assessment of some channel morphologies and dimensions, and differentiation of channels and valleys in the subsurface. Successive time-slice images may even permit evaluation of lateral variability and evolution of ancient fluvial systems. In spite of the potential benefits of 3D seismic interpretations of channel patterns of ancient fluvial deposits from limited exposures and conventional subsurface data continue to be ambiguous. This situation raises some important questions. For example, do braided and low-sinuosity river systems represent the dominant fluvial style in the geologic past, even after the advent of land plants, as suggested by Gibling, 2006, or is the available database flawed? What are the implications of interpreting most coarse-grained, fluvial hydrocarbon reservoirs as braided river deposits?


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009