Delta_Roy.ppsx
- 1. Depositional Environments - Delta PROMIT ROY M. SC SEMESTER II
- 2. Outline Introduction Types of Deltas Shapes of Delta Delta Sub environment Structure of Delta Lithofacies In Deltaic Environment Characteristics Of Deltaic Deposits Ancient Deltas References 2
- 3. Introduction A delta can be defined as a ‘discrete shoreline protuberance formed at a point where a river enters the ocean or other body of water’. It is formed where sediment brought down by the river builds out as a body into the lake or sea. 3
- 4. Continued In contrast, an estuary is a river mouth where there is a mixture of fresh water and seawater with accumulation of sediment within the confines of the estuary, but without any build-out into the sea. ‘Ordinary’ river mouths are settings where there is no significant mixing of waters and any sediment introduced by the river is reworked and carried away by processes such as waves and tides. 4
- 5. River Dominated Delta Wave Dominated Delta Tide Dominated Delta Types of Delta 5
- 6. River Dominated Deltas Landform created by deposition of sediment that is carried by a river. Occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment. The size and shape is controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. E.g. The Mississippi River Delta 6
- 7. Process Controls A delta is regarded as river-dominated where the effects of tides and waves are minor. This requires a microtidal regime and a setting where wave energy is effectively dissipated before the waves reach the coastline. The unidirectional fluvial current at the mouth of the river continues into the sea or lake as a subaqueous flow. 7
- 8. Figure : River dominated Delta 8
- 9. Wave Dominated Delta Wave-driven sediment transport controls the shape of the delta. Much of the sediment emanating from the river mouth is deflected along the coast line. Waves can also be responsible for carrying sediments away from the river delta, causing the delta to retreat. E.g. The Rhône River Delta 9
- 10. Process Controls Waves driven by strong winds have the capacity to rework and redistribute any sediment deposited in shallow water, especially under storm conditions. Wave action is effective at sorting the bedload into different grain sizes and the mouth-bar deposits of a wave- influenced delta may be expected to be better sorted than those of a river-dominated delta. Wave-dominated delta deposits display well-developed mouth bar and beach sediments. 10
- 11. Figure : Wave Dominated Delta 11
- 12. Tide Dominated Delta They tend to produce a "dendritic" structure. New distributaries are formed during times when there is a lot of water around – such as floods or storm surges. The distributaries slowly silt up at a more or less constant rate until they fizzle out. E.g. The Ganga – Brahmaputra Delta 12
- 13. Process Controls First, the delta-top channel(s) are subject to tidal influence with reverses of flow and/or periods of stagnation as a flood tide balances the fluvial discharge. The tidal currents rework sediments at the river mouth into elongate bars (Modified mouth bars) that are perpendicular to the shoreline, and may show bi- directional cross – stratification. 13
- 14. Continued The deposits of a tidally influenced delta can be distinguished from other deltas by the presence of sedimentary structures and facies associations which indicate that tidal processes were active. The main distinguishing feature is that a delta is always a progradational feature, whereas an estuary commonly forms as part of a retrogradational, or transgressive, succession. 14
- 15. Figure : Tide Dominated Delta 15
- 16. Figure : Triangular Classification Of Deltas (Galloway, 1975) 16
- 17. Figure : Classification of deltas taking grain size, and hence sediment supply mechanisms, into account. 17
- 18. Figure :Controls on delta environments and facies. 18
- 19. Shapes Of Deltas Arcuate Deltas The term delta is derived from the upper-case Greek letter delta since a delta is expected to have a typical delta or triangular shape. E.g. Nile River Delta Cuspate Deltas A cuspate delta is formed in regions with stronger waves. It is quite similar to an arcuate delta in shape but is more pointed towards the sea. E.g. Mississippi River Delta 19
- 20. Bird-foot Deltas Such a delta is not exactly triangular-shaped but appears like a bird’s foot when viewed from above and the tributaries are more widely spaced. E.g. Mississippi River Delta 20
- 22. Delta Plain Delta plains are extensive lowland areas which comprise active and abandoned distributaries' channels. Upper delta plains are essentially unaffected by basinal processes. Lower delta plains are also affected by fluvial processes but many are penetrated by saline water and by tidal processes. 22
- 23. Delta Front This is the area where sediment-laden fluvial currents enter the basin and interact with basinal processes. Where basinal processes are weak, fluvial processes predominate and deposit coarser sediment as distributary mouth bars, with shallow crests. Swash bars, beach ridges, beach spits and cheniers develop where wave processes predominate. Where tidal processes are important, tidal channels with tidal sandbars and ebb and flood tidal deltas develop. 23
- 24. Pro Delta The prodelta is that part of the delta which is unaffected by wave or tidal processes. It may not exist in shallow-water deltas. It comprises a relatively stable zone where mud and fine silt are deposited from suspension to form well-laminated sediment. Lamination is preserved where bottom waters are anoxic. Laminae are disturbed by bioturbation where bottom waters are aerobic. 24
- 25. Structure of a Delta Topset Deposits: Topset deposits of a delta are mainly made up of marsh deposits and delta front silts and sands. Foreset Deposits: Foreset deposits are made up of pro-delta silty days and rather coarse sand, silt, and clay deposits formed off the major deltaic distributaries. 25
- 26. Bottomset Deposits: Bottomset deposits are made up of offshore clays under the influence of active deltas. Marginal Deposits: These are transitional deposits between bottomset deposits and the deposits of the subsurface, which were deposited before the building up of the delta. 26
- 27. Figure : Cross Section Of A Delta (Reineck & Singh) 27
- 28. Lithofacies In Deltaic Environment Upper delta plain Migratory channel deposits: braided-channel deposits and meandering- channel deposits Lacustrine delta-fill deposits and flood-plain deposits Lower delta plain Bay-fill deposits (interdistributary bay, crevasse splay-natural levee, marsh) Abandoned distributary deposits. Subaqueous delta plain Distributary-mouth-bar deposit (prodelta distal bar, distributary-mouth bar) River-mouth tidal-ridge deposits Subaqueous slump deposit 28
- 29. 29 Figure : Facies Of A Delta System
- 30. Characteristics of deltaic deposits Lithologies – conglomerate, sandstone and mudstone Mineralogy – variable, delta-front facies may be compositionally mature Texture – moderately mature in delta-top sands and gravels, mature in wave-reworked delta-front deposits Bed geometry – lens-shaped delta channels, mouthbar lenses variably elongate, prodelta deposits thin bedded Sedimentary structures – cross-bedding and lamination in delta-top and mouth-bar facies 30
- 31. Continued Palaeocurrents – topset facies indicate direction of progradation, wave and tidal reworking variable on delta front Fossils – association of terrestrial plants and animals of the delta top with marine fauna of the delta front Colour – not diagnostic, delta-top deposits may be oxidised Facies associations – typically occur overlying shallow- marine facies and overlain by fluvial facies in an overall progradational pattern. 31
- 32. Ancient River Deltas Ancient river deltas are characterized by : a thick predominantly clastic succession that passes upward from offshore facies into continental, fluvial facies. a sediment body of restricted lateral extent since the delta formed a depocentre fixed around a river mouth. repetitive or cyclic successions due to repeated progradation and abandonment of the entire delta or lobes within the delta. 32
- 33. Recognition Of Ancient Deltas study of the vertical succession in boreholes and at outcrops where lateral data are sparse or even absent. the establishment of lateral facies changes by areal mapping and by correlation of adjacent sections. the identification of offlapping clinoforms on seismic sections and in very well-exposed outcrops with a low tectonic dip. 33
- 34. References Principles of Sedimentology and Stratigraphy SAM BOGGS Jr. Fifth Edition (2014) Sedimentology and Stratigraphy GARY NICHOLS Second Edition (2009) Ancient Sedimentary Environments And Their Sub- Surface Diagnosis Third Edition RICHARD C. SELLEY Sedimentary Environments: Processes, Facies and Stratigraphy Third Edition Edited by H.G. READING (1996) Depositional Sedimentary Environments With Reference to Terrigenous Clastics Second, Revised and Updated Edition H.E. REINECK, I. B. SINGH (1980) 34
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