Abstract
The following abstract is from my dissertation, "Sedimentology of Antidune Flow: controls on sediment transport and stratification"
Antidunes are bed configurations that form in sediment under fast, shallow flows. These wave-like features are highly unstable and contrast with more common bedforms, like ripples and dunes, by migrating upstream and changing dramatically in morphology over very short periods of time. Antidunes often amplify rapidly, deforming the flow above them until the water surface becomes unstable and collapses, partially or completely destroying the bedform. Because antidunes display such a dynamic behavior, it is difficult to observe interactions between the flow and the bed and collect data from the flows. As a result, our understanding of how antidunes produce preservable sedimentary structures is limited. Without this understanding, the identification of antidune structures in ancient deposits is difficult and often suspect.
The present study aims to improve our understanding of antidunes and their sedimentary structures. It relies on field observations and descriptions of small streams that contain antidunes, the sedimentary structures produced by these streams, and similar structures in ancient deposits. Flow data from streams were collected by video taping antidunes and their associated flows. Experiments where stream channels were altered to form antidunes in rapidly aggrading settings augmented the observations and provided a direct link between antidunes and their internal structures.
Antidunes, when migrating in rapidly aggrading settings, produce an intricate pattern of stratification consisting of two distinct types of laminae. The most common type of laminae typically dip in a downstream direction at variable angles and are a type of translatent strata that forms as the antidune trough migrates on an aggrading bed. These thin laminae truncate underlying structures and form the bounding surfaces around inversely graded, lenticular packets of sediment. A second type of laminae mark the instantaneous position of an antidune's upstream face and occur within the lenticular packets. These laminae dip upstream, downlap onto the translatent strata and may occur sporadically. The appearance of antidune structures varies dramatically with aggradation rate and degree of stability displayed by the antidunes. As a result, these structures may be useful in interpreting paleoflow conditions.
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