Abstract

Flume experiments were conducted to determine how a pervasive rain of sand might affect the development of bedforms, internal structures, and grain fabric in a bed aggrading from this fallout. During otherwise standard flume runs, sediment similar to that in the bed (M z = 2.13 phi ; sigma (sub phi ) = 0.63 phi ) rained from overhead bins extending the length of the 9-meter recirculating flume. Consequent rates of bed aggradation ranged to 4.2 cm min (super -1) . The experiments are viewed as having particular relevance to deposition from turbidity currents. Under upper-plane-bed conditions, so long as low rates of sediment feed prevailed, parallel lamination developed from rapidly migrating, millimeter-scale bed-load sheets. At high rates of feed, lamination was suppressed, suggesting that rapid aggradation can account for the "massive" structure characteristic of Bouma A divisions of turbidites. When flow conditions were maintained within the dune stability field for clear-water runs, established dunes persisted at all sediment fallout rates examined (bed aggradation <= 4.0 cm min (super -1) ), apparently contradicting a suspicion that the underrepresentation of medium-scale cross-bedding in turbidites might be attributable to suppression of dunes by sediment rain. It was not feasible to test whether sediment rain might inhibit development of nascent dunes, however. Ripples proved stable across the range of fallout rates examined. Depositional fabrics, deduced from anisotropy of magnetic susceptibility, confirmed that under upper-plane-bed conditions (representative of Bouma divisions A and B) grain orientation is current-parallel and equally well developed for all sedimentation rates. Up-current imbrication, consistently steeper than what is typically found in upper-plane-bed stream deposits, increased in steepness in direct proportion to sediment fallout and may provide a useful basis for estimating aggradation rates during deposition of divisions A and B of ancient turbidites.

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