Depositional Sequences in Glaciolacustrine Deltas
Thomas C. Gustavson, Gail M. Ashley, Jon C. Boothroyd, 1975. "Depositional Sequences in Glaciolacustrine Deltas", Glaciofluvial and Glaciolacustrine Sedimentation, Alan V. Jopling, Barrie C. McDonald
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Sedimentary structures in distal outwash deposits, in glaciolacustrine deltas, and in lake sediments proximal to glaciolacustrine deltas are similar where the grain-size distributions of the sediment are similar, and where depositon occurs under smilar flow conditions.
Draped lamination, a common structure in distal outwash deposit, consists of parallel laminae of sand, silt, and clay deposited from suspension and draped over an underlying bedform. Thickness of the laminae remains essentially unchanged across the underlying bedform, and neither silt nor mica flakes are concentrated anywhere along draped laminae.
Overbank deposits in outwash decrease in grain size along the length of the stream, but contain similar sequences of sedimentary structures. Commonly, type A ripple-drift cross-lamination is overlain by low-angle type B, which in turn is overlain by draped lamination. This sequence of sedimentary structures suggests deposition under decreasing flow strength.
Ripple-drift sequences in glaciolacustrine deltas are separated in many cases by winter clay layers. The basic sequence begins as a thin unit of draped lamination deposited on a subjacent winter clay. A thin unit of type B ripple-drift cross-lamination follows and is in turn overlain by a relatively thick unit of type A. The type A grades upward into a second unit of type B. As the angle of climb of the type B ripple-drift cross-lamination increases to the vertical, type B grades upward into draped lamination. A superjacent winter clay layer completes the sequence. The basic sequence of sedimentary structures reflects a relatively rapid decrease in the ratio of rates of deposition from suspension to bed-load transport, followed by a gradual increase. Flow strength appears to increase quickly and then decrease. The thin graded beds and rare ripple- drift cross-lamination or isolated ripples observed within lacustrine varved clays are distal equivalents to the sequences observed within glaciolacustrine deltas.
Glaciolacustrine deltas are the products of rapid sedimentation into low-energy lake environments and are seldom seriously modified by wave action or currents. Deltas are built into lakes as overlapping lobes of sediment that were deposited from density currents issuing from meltwater distributaries. As distributaries migrate across the delta subaerial plain, lobes move laterally and overlap each other, forming an arcuate delta front.
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Glaciofluvial and Glaciolacustrine Sedimentation
This publication is the outgrowth of a symposium on Glacial Sedimentology that was held in Buffalo, New York, March 1972. The great interest generated in glacial phenomena during the nineteenth century had important implications and repercussions for the infant field of sedimentology. It provided its fair share of the background stimulus necessary to establish sedimentology as a separate branch of the earth sciences in the twentieth century. The time for reciprocity is now at hand; feedback from the expertise gained in the burgeoning field of sedimentology can greatly help the Quaternary specialist solve particular field problems. The last decade has witnessed a growing interest in the sedimentology of the Quaternary, and it seems appropriate now to summarize progress in the study of stratified drift, to present results of some recent studies, and to focus attention on avenues of research that should be explored in the near future.