Delta Sediments

As we know from previous post explained about sedimentary environments, at least we know we have 6 different types of sediment; alluvial fan sediments, fluvial deposits, delta deposits, lake deposits, barrier island deposits, and lagoon deposits. Now this article will explain about delta sediments.

 

Delta sediments definition

Delta known as the most important landform produced where a river enters a body of standing water. The term normally applies to a depositional plain formed by a river at its mouth. The implication that delta sediments accumulation at this position results in an irregular progradation of the shoreline. This surface feature was first recognized and named by the ancient Greek historian Herodotus, who noted that sediment accumulated at the mouth of the Nile River resembled the Greek letter Δ (delta). Even though a large number of modern deltas have this triangular form, many display a variety of sizes and shapes that depend on a number of environmental factors. Thus, the term now has little, if any, shape connotation. Deltas, in fact, exhibit tremendous variation in their morphological and sedimentologic characteristics and also in their mode of origin.

Delta sediments have been important to humankind since prehistoric times. Sands, silts, and clays deposited by floodwaters were extremely productive agriculturally; and major civilizations flourished in the deltaic plains of the Nile and Tigris-Euphrates rivers. Much of the world’s petroleum resources are found in ancient deltaic rocks, as what geologist discovered in recent years.

Delta sediments form where rivers carrying a large supply of sediments empty into a sea coast where the sediments cannot be transported away as fast as it is deposited (Peng 1996a). Thus, delta sediments lie in the transit region between the fluvial and marine environments. The Mississippi River Delta sediments is a good example.

 

Geomechanical behaviors of delta sediments deposits

The rocks in delta sediments deposits have the following geomechanical behaviors (Peng 1994):

  • Stratified with significant variations of lithology and thickness in the lateral direction.
  • Weak in weathering resistance, and the rock strength increases from the bottom to the top as the granularity increases.
delta sediments illustrations
illustration of delta and lake sediments (image from NASA)

 

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Factors influence to delta sediments

Delta sediments display much variation in size, structure, composition, and origin. These differences result from sediment deposition taking place in a wide range of settings. Numerous factors influence the character of a delta. The most important of which are: climatic conditions, geologic setting and sediment sources in the drainage basin, tectonic stability, river slope and flooding characteristics, intensities of depositional and erosional processes, and tidal range and offshore energy conditions. Combinations of these factors and time give rise to the wide variety of modern deltas. The presence of a delta sediments represents the continuing ability of rivers to deposit stream-borne sediments more rapidly than they can remove by waves and ocean currents.

 

Delta sediments components

Delta sediments typically consist of three components. The most landward section is called the upper delta plain, the middle one the lower delta plain, and the third the subaqueous delta, which lies seaward of the shoreline and forms below sea level.

That part of the river confined by valley walls, termed the alluvial plain, serves as a conduit through which sediment and water derived from the drainage basin are brought to the sea. At some point downstream the plain broadens out, and most river channels break up into more than one course. This is the apex of the delta and the beginning of the upper delta plain. All of this land lies at an elevation above the effective intrusion of tidal water and is form entirely by riverine processes. Areas between channels usually support broad freshwater marshes, swamps, or shallow lakes.

The lower delta plain is periodically experience the flood by tidal waters, and landforms result from the interaction of both riverine and marine processes. Areas between the channels show a variety of landforms, ranging from brackish water bays and mangrove swamps to saline tidal flats and beach ridges. The subaqueous delta forms entirely below the level of the sea. Thus, commonly constitutes the obvious bulge on the continental shelf seen seaward of many deltas. It serves as the submarine platform across which the exposed delta eventually builds. Variations in the proportions of each deltaic component give rise to the different sizes and slopes of the world’s deltas.

 

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