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Lecture-14_deltas - Lecture 14 Deltas and Delta Deposits...

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1 Lecture 14: Deltas and Delta Deposits Class reading from Boggs, Principles of Sedimentology and Stratigraphy p.289 - 305. History of DELTAS Herodotus (c. 400 BC) - thought the alluvial plain at mouth of Nile looked like a D Barrell (1912) - described the ancient (Devonian) Catskill Delta and defined a delta as "a deposit partly subaerial built by a river into or against a body of permanent water. The outer and lower parts are necessarily constructed below water level, but its upper and inner surface must be land maintained . . . by a river . . . therefore consists of a combination of terrestrial and marine, or at least lacustrine strata.“
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RIVER input is key to delta formation. 2
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Sedimentation at a river mouth: Characteristic pattern of sedimentation associated with the loss of confinement and a spatial deceleration of current where flow runs into standing body of water. Spatial structure of the flow emanating from a river mouth is often characterized as a jet . Deposition at end of channel = mouth bar . Local areas of flow expansion lead to bar deposition and flow splitting. 3
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(C) The abandoned delta lobe will subside over time without an influx on new sediment. (D) Delta lobe switching happens on the time scales of decades to many millennia. Modern and ancient channels of Brazos River delta 4 (A) Channel avulsions distribute sediment across the delta surface. (B) A new delta lobe form and prograde where a newly avulsed channel hits the shoreline. Modern Brazos River delta
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5 Lobes of Mississippi River delta John W. Day, Jr., et al. Science 315, 1679
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Mississippi 6 Delta classification scheme based on processes controlling coastline morphology (Galloway, 1975)
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River-dominated deltas: (A) Relatively unaltered channel- forms protrude into the standing body of water.
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