--> Abstract: Structureless And Laminated Sands Of Dune-Sourced Alluvial Fans In The Nebraska Sand Hills, by M. R. Sweeney and D. B. Loope; #90928 (1999).

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SWEENEY, MARK R. and DAVID B. LOOPE
Department of Geosciences, University of Nebraska, Lincoln, NE, 68588-0340

Abstract: Structureless and Laminated Sands of Dune-Sourced Alluvial Fans in the Nebraska Sand Hills

Alluvial fans form in the Nebraska Sand Hills during intense summer rainstorms (at least 60 mm/hr of rain) when gullies are cut into the lee sides of large, grass-stabilized barchanoid-ridge dunes. The dunes are composed of fine- to medium-grained moderately well sorted, mature quartz sand. Clay and silt make up less than 3% by volume of the dune sediment; clays are present as coatings on sand grains. Two distinct facies, here interpreted as depositional couplets, are associated with these small-scale alluvial fans: 1) structureless debris flows, and 2) laminated sheetflood sands. Headward erosion of gullies and slumping of gully walls supply sand for the alluvial fans. In gullies with steep gradients (~25° slope), sand is initially transported to the fan surface as debris flows. Any further deposition on the fan is by sheetflood processes reworking debris flow sands or transporting any remaining sand from the gully. Debris flows have not been directly observed, but trenches in fresh deposits that originated from one rain event reveal massive sands underlying laminated sheetflood sands. Debris flow sands are often lobate in shape, with the thickest part of the flow in the center (50 cm or greater). Sheetfloods deposit sand with fairly uniform thickness, commonly 20 cm thick. Trenches in older deposits reveal couplets greater than one meter in thickness. Interpretation of structureless debris flow sands is complicated by rooting, trampling, and burrowing. Characterization of the massive debris flow and laminated sheetflood facies in this active environment will aid interpretation of structureless sandstones intercalated with eolian cross-strata.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas