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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes
Vol. 70 (2000), No. 3. (May), Pages 461-469

Random Orientation of Wood on Two Tiny Alluvial Fans (Tavaputs Plateau, Ne Utah, U.S.A.)

Neil A. Wells, Shawn R. Rapp


Abundant bits of wood scattered over the surfaces of two nearly identical, adjacent, tiny, steep, grassy, and smooth alluvial fans near Duchesne, Utah, allowed orientation measurements of 390 elongate and straight pieces that varied in size from twigs to tree trunks. On each fan, the pieces of wood are spread outward from the mouth of a small fan-Previous HitheadTop channel, but they showed effectively random orientations relative to fan slope, regardless of size. Almost all the wood lies loose and unburied on the two fan surfaces, and is thus free of associated sand and mud. Slopes uphill are similarly free of dead wood, although both scattered and tangled wood covers adjacent slope toes.

One possible explanation is that a localized rain burst cleaned the upper slopes but failed to flush wood beyond slope toes wherever runoff was not concentrated. The lack of sediment can be explained if the ground was frozen and/or snow-covered at the time. If the wood was deposited by a sheetflood, then the results are significant to the small but ongoing controversy over whether running water should orient wood (1) transverse to flow (by rolling), (2) parallel to flow (by swiveling or sliding), (3) both, or (4) randomly. Alternatively, the wood may represent a snow or slush avalanche, in which case random orientations would not be surprising. In either case, the results provide a note of caution: because this study was prompted by seeing a variety of plausible preferred orientations in many small or local sets of waterlaid wood on other small fans and because randomness was (arguably) not evident on these two fans without extensive data collection, we have become mistrustful of inferring paleocurrents from small samples of fossil wood.

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