marginal-marine_c9_part_2

marginal-marine_c9_part_2 - Delta cycles During active...

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Delta cycles • During active constructional phase , deltaic deposits prograde basinward • Progradation generates coarsening-up succession – Delta-front buries prodelta • … delta-front and delta-plain also includes fine-grain sediment from marsh/bays • Regression halted by lobe abandonment – relative sea-level rise – upstream avulsion • Subsidence (or sea-level rise) produces accommodation over the abandoned lobe • Subsequent delta outbuilding fills accommodation and buries the abandoned lobe from previous regression • Cyclical pattern of delta deposits
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Vertical succession of single prograding delta
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Ancient fluival deltaic system • Carboniferous of Kentucky • DMB sandstones grade laterally into bay fill mud – 1.5 to 5 km wide sandstones – 15-25 m thick – Widest at base (similar to Mississippi bar fingers) – Gradational lower and upper contacts – Coarsening up and towards center of bars – Pebble-lag conglomerates at channel bases
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Model of carboniferous formations from eastern Kentucky
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9.3 beach and barrier island systems • Beach… – long, narrow sand accumulation – Shore parallel – Land attached – May form in association with deltas – Dynamic depositional environment – Dominated by marine processes – Microtidal settings – Minor eolian sand transport – Most are siliciclastic but some carbonate beaches – Studied extensively • Recreational use • Source of placer minerals • Erosion buffer • Reservoir rock
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Cape Sebastian, S. Oregon coast
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9.3 beach and barrier island systems • Barrier island beaches… – Similar to mainland beaches – Separated from land by shallow lagoon, estuary, or marsh – Commonly dissected by tidal channels – Microtidal settings
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Edingsville Beach barrier island. S.C. lagoon Tidal channel
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3 zones of beach morphology • 1. Backshore – Landward of the high-tide berm – Includes back-beach dune • 2. Foreshore – Intertidal zone • 3. Shoreface – From low-tide level to beach-shelf transition • I.e., to fair-weather wave base at 10-15 m water depth
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Generalized beach cross section
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Depositional processes • Beaches constructed by wave-related processes – Fair-weather and storm waves – Breaking waves – Translational waves – Wave swash – nearshore currents • Longshore • Rip currents • Wind also plays a role in sediment transport/deposition
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Depositional processes: wave processes • Water beneath waves moves in orbital paths – Orbits decrease with depth • When waves move into shallow water, orbital paths impeded by interaction with the bottom – paths flatten into to and fro water motions • waves entering shoaling zone eventually break – the wave slows, – the wavelength decreases – the wave height increases – Steepening waves break in the breaker zone
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Depositional processes: wave processes • Breaking waves generate turbulent surf zone – turbulence throws sediment into suspension • creates high-velocity translation wave
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This note was uploaded on 02/20/2010 for the course GEOL 3032 taught by Professor Bouma,a during the Fall '08 term at LSU.

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marginal-marine_c9_part_2 - Delta cycles During active...

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