Interpreting ancient floodplain processes from hydromorphic paleosols
- Univ. of Colorado, Boulder, CO (United States). Dept. of Geological Sciences
Mudrock colors, pervasive mottling, and iron-oxide nodules indicate that alluvial paleosols in part of the Eocene Willwood Formation of Wyoming are hydromorphic. Hydromorphy in cumulative paleosols resulted principally from impeded drainage caused by clay-rich alluvium and was largely controlled by depositional processes rather than climatic or topographic gradients. Better-drained paleosols formed on coarser material deposited closer to channels: more strongly hydromorphic paleosols formed on finer grained, less permeable alluvium, more distal to channels. Smaller scale changes (tens to hundreds of meters) in hydromorphy are attributed to local incursions of somewhat coarser sediment and local topographic irregularities, both of which influenced soil drainage. Cumulative paleosols alternate with packages consisting of vertically stacked simple paleosols and crevasse-splay sandstones. The packages indicate very rapid deposition and are interpreted as ancient analogs to the avulsion-belt deposits described by Smith et al. (1989). Differences between the two types of paleosols indicate that their parent materials were deposited by somewhat different processes. The simple paleosols show that a significant portion of overbank deposits actually represent deposition from all stages of flow during avulsion. In contrast, the cumulative paleosols formed on true overbank deposits, which gradually accumulated after avulsion and establishment of a new channel. Willwood paleosols provide a simple means of identifying episodes of avulsion. The cyclic repetition of avulsion-belt deposits and cumulative paleosols, together with sediment accumulation rates, suggests that avulsive episodes occurred with a regular frequency of ca. 20k years and may have been induced by cyclic climatic processes.
- OSTI ID:
- 5690353
- Report Number(s):
- CONF-921058--
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Journal Name: Geological Society of America, Abstracts with Programs; (United States) Vol. 24:7; ISSN GAAPBC; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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