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Fluvial Reservoirs |
1 Department of Geological Sciences, University of Colorado, Boulde, CO 80309, USA
2 US Geological Survey, Denver, CO 80225, USA
Diverse palaeosols in the Eocene Willwood Formation illustrate how overbank deposits can help predict the location of fluvial sandbodies. Palaeosols of differing maturity formed in response to local fluvial processes and tectonically controlled sediment accumulation rates. The various palaeosols can be distinguished using criteria including colour, pedogenic nodules, and grain size trends, all of which should be observable in core.
Palaeosol maturity increases laterally with increasing distance from a coeval channel sandbody. Consequently, a specific type of Willwood palaeosol provides an estimate of the relative distance to a channel sandbody at a specific stratigraphic level. Vertical sequences of overbank deposits also show changes in palaeosol type as a result of channel avulsion. The vertical changes can thus help predict the position of channel sandbodies within a fluvial package. On a basin-wide scale, the rate of subsidence influenced both palaeosol maturity and the location of channel sandbodies. Areas of the basin that subsided rapidly (thus sediment accumulated rapidly) are commonly rich in channel sandbodies and also contain relatively immature palaeosols. In contrast, areas that subsided more slowly and in which channel sandbodies are not concentrated are characterized by relatively mature palaeosols.
