Deltaic Environments

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Deltas are depositional environments where a fluvial system’s mouth contacts an open or slow moving body of water, which typically is an ocean or sea. Deltas tend to form fan or triangular shaped deposits of sediment.[1] A delta’s morphology or shape is determined by sediment deposition and transport, as well as by several sources such as tides, wave-action, and fluvial input.[2] Deltas are environments of high ecological importance acting as barriers for storm events, areas of high amounts of biodiversity, and as breeding grounds for wildlife such as birds.[3]

Geomorphology of Deltas

A delta’s morphology fluctuates depending on the forces acting upon it, both in the fluvial system that meets the lower velocity body of water and the body of water itself. Deltas are commonly arranged in fan-like structures, also known as an alluvial fan.[4]

  • Alluvial Fans

Alluvial fans are generated by mass amounts of sediment deposition produced from the change in velocity seen when two contrasting fluid velocities make contact. These fans can be significant in their depth and size, with the Ganges Delta located in South Asia, being the widest in the world at 350km.[5] Alluvial fan morphology is not limited to triangular forms, but can experience shifts in morphology as sediment deposition rates fluctuate, and due to forces such as tidal influence and waves. The fluvial system which supplies sediment to the alluvial fan often forms distributary channels.

  • Distributary Channels and other structures

Distributary channels are smaller channels that originate from a delta’s fluvial source and often carve braided paths from the source, across the alluvial fan, and into the open body of water. Distributary channels fluctuate in size and sinuosity in turn with its fluvial source.[6] Other structures seen in relation to deltas are sand bars (otherwise known as tidal bars) and tidal flats. Sandbars are structures of loose, unconsolidated sediment which normally comprise of sand, gravel, and other debris.[7] Sandbars will often migrate as wave action transports sediment and the proceeding fluvial system deposits sediment. When tidal forces influence the morphology of a delta, estuaries tend to form.

  • Estuaries

Estuaries are similar to deltas in that they are contact points between a fluvial system and a larger body of water, but estuaries differ in that the tidal forces acting upon the fluid system cause the saline water to encroach into the fluvial system.[1] Within estuaries, tidal sand bars are common and behave as buffers for tidal intrusion into areas such as tidal flats or marshes. Tidal flats are regions of relatively flat sand and mud that experience rhythmic flooding events based on tidal cycles.[1]


Depositional processes and Associated Facies

Delta sedimentation occurs when sediment transported through a fluid system encounters a sudden change in velocity. This is typically seen when the fluid system’s “mouth” encounters an open body of water such as a lake, ocean, or sea. The sediment encounters a decrease in velocity due to the sudden increase in channel size. Sediment begins to accumulate at the precipice of the fluvial system and open body of water. As sediment deposits, it begins to extend laterally into the larger body of water in a fan-like shape. The sediment deposited is typically derived from sources upstream and can be a variety of particulates. Tidal fluctuations and storm events can often be recorded as facies within deltaic systems, as well as transgressive and progressive periods. Tidal fluctuations will be deposited as mud-dominated layers in Neap tides and sand dominated in Spring tides due to the changes in tidal activity.[8] Deltaic facies will vary depending on the position within the delta where deposition occurred.[8][9]

Delta facies associations can be segmented into a pro-delta (or pre-delta), a frontal splay, detached distal bars, distributary mouth bars, and terminal distributary channels.[9] Delta facies analysis can extend farther into a shallow marine environment or near shore environment to further evaluate deltaic influence on sedimentation within a marine environment.

Controls of the system’s evolution

The most common delta controls are fluvial changes, wave-action, and tidal action.[6] Deltaic morphology shifts based on the forces acting upon it, and when a primary force is identified as the system control, the delta is typically named based on this control.

  • Primary Deltaic Forms

Fluvial deltas are deltas where the primary force driving sediment transport and deposition is the fluvial system rather than the low velocity body of water. These are commonly referred to as river deltas but typically deltas do not have just a fluvial system controlling their morphology. The fluvial system which introduces sediment to the alluvial fan can experience changes to discharge rates, channel size, or sediment types which in-turn will affect the morphology of the alluvial fan. The Ganges Delta is a notorious example of an extensive fluvial delta.[5] Wave dominated deltas are ones that have a low velocity body of water impacting the morphology through wave-action. This is typically observed in marine environments, where wave action is consistently causing sediment erosion and transport. Wind waves discourage accumulation of fine-grained sediment at the delta mouth and tend to sculpt delta shorelines into a cuspate shape consisting of sandy shorelines composed of shoreline-parallel beach ridges.[10] Tide dominated deltas are similar to wave dominated since the main force influencing the delta’s morphology is a low velocity body of water. Rather than wave action driving morphological change, tidal rhythms drive sediment transport and deposition. Tidal rhythms can behave consistently in spring tides resulting in higher-than-average tidal levels and neap tides causing below-average tidal levels. However, storm events can cause major variations in both tidal forces and wave action which can greatly impact deltaic morphology. Storm-action can disturb sediment in higher amounts than normal and also transport sediment grain sizes which normally lay undisturbed. Tide dominated deltas tend to be highly variable and extensive in size.[11]

Facies Example Model

Deltas undergo changes throughout their lifespan depending on factors such as sediment transport rates or sea-level change.[12] However, examples of deltaic environments can often be preserved in the rock record. Beginning near the base or lower portion of a preserved delta and trending upwards, typical facies seen will range from finer-grained rocks such as siltstone and mudstones. These finer sediments can indicate a lower-energy environment such as a marine basin.[13] Moving upwards towards the shoreline will have heterogeneous sediments preserved such as siltstone and sandstone intermixed or interbedded. Once a flooding surface is reached, sandstone will be the primary lithological unit. An example of such a trend can be observed in Ahmed et al., 2014. When observing deltaic facies, tidal forces can cause cyclic deposits due to Spring and Neap tides, which will preserve sand-rich and mud-rich layers respectively. [8]

  • Commonly Seen Deltaic Structures

Typical lithological facies seen within a deltaic environment revolve around fine to coarse grained sediment types. Sandstone deposits within preserved deltas can be several meters thick, such as in the Sego Member, Colorado being 10m-20m thick.[14] Some commonly seen structures in delta-associated sandstones are cross-beds, planar laminations, wave-ripple laminations, and evidence of high energy rates of deposition through fluvial or shore-line forces.[9][15] Progressing towards a more marine environment presents facies that are associated with lower energy environments or forces such as wave and tidal action. In contrast to a higher energy setting, low energy localities have higher potential for the preservation of fossils.[16] Bioturbated siltstones and mudstones can be preserved in deltaic environments, with ichnofossils such as Skolithos, Palaeophycus, and Planolites indicating that marine life was once active in such an environment.[17] Along with ichnofossils or body fossils, massively bedded siltstone and mudstone can be common deltaic facies.[9] The Mcmurray Formation in Alberta, Canada demonstrates facies from an estuarine tidal bar paleoenvironment.[8] Starting from the fluvial source and transitioning towards a marine environment the formation includes: mud clast breccias, high-angle and low-angle cross stratified sands, consolidated sandy clay, sand wave bedded muds, very fine grained clays with sand interbeds, carbonaceous muds, interbedded sand and mud layers, laminated sands with cm-scale mud interbeds, cross-bedded sand, massive sandstone, bioturbated sand, and glauconitic muddy sand.[8]


References

[1] Water basics glossary. Water Resources Glossaries. (n.d.). Retrieved December 13, 2021, from https://water.usgs.gov/water-basics_glossary.html.</ref>

[2] Galloway, W. E. (1975), Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems, in Deltas: Models for Exploration, pp. 87–98, Houston Geol. Soc., Houston, Tex.

[3] Wildlife & Habitat - delta - U.S. fish and wildlife service. U.S. Fish & Wildlife Service. (n.d.). Retrieved December 13, 2021, from https://www.fws.gov/refuge/Delta/wildlife_and_habitat/.

[4] DeChant, L., Pease, P., & Tchakerian, V. P. (2021). Alluvial fan morphology: A self-similar free boundary problem description. Geomorphology, 375, 107532.

[5]. The Geological Society of London - Ganges Delta, Bangladesh and West Bengal. The Geological Society. (n.d.). Retrieved December 13, 2021, from https://www.geolsoc.org.uk/Policy-and-Media/Outreach/Plate-Tectonic-Stories/Stanage-Edge/Ganges-Delta.

[6] Kästner, K., Hoitink, A. J. F., Vermeulen, B., Geertsema, T. J., & Ningsih, N. S. (2017). Distributary channels in the fluvial to tidal transition zone. Journal of Geophysical Research: Earth Surface, 122(3), 696-710.

[7] Sandbar. SANDBAR | definition in the Cambridge English Dictionary. (n.d.). Retrieved December 13, 2021, from https://dictionary.cambridge.org/us/dictionary/english/sandbar.

[8] Tang, M., Zhang, K., Huang, J., & Lu, S. (2019). Facies and the architecture of estuarine tidal bar in the lower Cretaceous Mcmurray Formation, Central Athabasca Oil Sands, Alberta, Canada. Energies, 12(9), 1769.

[9] Ahmed, S., Bhattacharya, J. P., Garza, D. E., & Li, Y. (2014). Facies architecture and stratigraphic evolution of a river-dominated delta front, Turonian Ferron Sandstone, Utah, USA. Journal of Sedimentary Research, 84(2), 97-121.

[10] Curray, J. P., Emmel, F., & Crampton, P. (1969). Holocene history of a stran plain, lagoonal coast, Nayarit.

[11] Goodbred, S. L., & Saito, Y. (2012). Tide-dominated deltas. In Principles of tidal sedimentology (pp. 129-149). Springer, Dordrecht.

[12] Blum, M. D., & Törnqvist, T. E. (2000). Fluvial responses to climate and sea‐level change: a review and look forward. Sedimentology, 47, 2-48.

[13] Nelson, S. A. (2018, April 28). Sediment and Sedimentary Rocks. Sedimentary Rocks. Retrieved December 13, 2021, from https://www.tulane.edu/~sanelson/eens1110/sedrx.htm.

[14] Willis, B. J., & Gabel, S. (2001). Sharp‐based, tide‐dominated deltas of the Sego Sandstone, Book Cliffs, Utah, USA. Sedimentology, 48(3), 479-506.

[15] A. J. (1989). Controls on internal structure and architecture of sandstone bodies within Upper Carboniferous fluvial-dominated deltas, County Clare, western Ireland. Geological Society, London, Special Publications, 41(1), 179-203.

[16] Brett, C. E., & Baird, G. C. (1986). Comparative taphonomy: a key to paleoenvironmental interpretation based on fossil preservation. Palaios, 207-227.

[17] Kamola, D. L. (1984). Trace fossils from marginal-marine facies of the Spring Canyon Member, Blackhawk Formation (Upper Cretaceous), east-central Utah. Journal of Paleontology, 529-541.