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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 56 (1972)

Issue: 3. (March)

First Page: 654

Last Page: 655

Title: Sequence of Bedform and Stratification in Silts, Based on Flume Experiments: ABSTRACT

Author(s): J. B. Southard, J. C. Harms

Article Type: Meeting abstract


Flume experiments with 2 silt sediments indicate development of a sequence of bed forms and stratification which is systematically related to flow intensity. At a fixed flow depth, each silt is transported as ripples over a wide range of mean velocities above the threshold for movement. With still higher velocities, ripples disappear abruptly and a flat-bed mode of transport occurs. Dunes are not present at velocities intermediate between rippled and flat beds, as they are for sand.

At lower velocities, ripples develop forms very similar to sand ripples: planar lee slopes accrete by slumping

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and meet the crest and trough at sharp angles; stoss slopes are wholly erosional. At higher velocities, ripples become progressively lower and more rounded, and the resulting laminae are complex and sigmoidal; stoss-slope laminae become more common, resulting in temporary development of ripple-drift configurations, even in the absence of net bed aggradation. Low ripples are succeeded by a flat bed with relatively crude lamination and textural lineation parallel with flow.

At the lowest velocities capable of maintaining ripple migration, sediment is transported entirely as bed load. Suspended-sediment concentration is high at velocities near and above the transition from ripples to flat bed, but the sediment-water mix behaves as a turbulent fluid rather than as a slurry.

The 2 silts, derived from Illinois Pleistocene loess, have median diameters of 30 and 40 ยต, are well sorted, and consist mainly of angular quartz chips. The flume used is a recirculating open channel 36 ft long and 3 ft wide.

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