Delta structure
Stratigraphic sequence.
The usual text-book picture of delta structure as a series of topset, foreset, and bottomset beds may be regarded as a rough but good generalization of the principal structural features of many coarse-grained deltas built out into deep water by rapidly shifting and oscillating braided-river systems. For fine-grained deltas and therefore also for the majority of large, modern deltas the terms "topset, foreset, and bottomset beds" often lack utility, and ought to be replaced by other terms for the different environments of deposition, such as channel deposits, levee deposits, inter-levee deposits, platform deposits, and frontal slope deposits.
In a single deltaic lobe the vertical distribution of sediments is roughly the same as their order of occurrence in a distal direction out into deep water. They are normally finest and most uniform in depositional fabric at the base, and become coarser grained and more complexly stratified upwards towards the top, where wave-agitated platform deposits are capped by a complicated series of channel, levee, and inter-levee deposits.
Large deltas, however, are usually built up in a step-by-step manner. Due to shifts in position and activity of the distributaries and to base-level variations, local delta construction is followed by partial destruction, abandonment, and initiation of a new cycle. Large deltas are therefore normally a composite of numerous lobes, with the younger lobes formed successively on top of, in front of, or in marginal depressions of the older lobes.
The Laitaure delta
Schematic cross-section through a part of the Laitaure delta. From Axelsson 1955 (Ymer 75).
As shown below the vertical and lateral extent of foreset slopes is small at the delta front in Lake Laitaure. This means that the extent of steeply inclined foreset beds also is small in the younger parts of the delta.
Vertical and lateral extent of foreset slopes (frontal slopes steeper than 10 degrees) at the delta front in Lake Laitaure (marked by striated areas). July 1958. Vertical exaggeration x 100. From Axelsson 1967 (Geogr. Ann. 49 A).
The depth to the upper and lower boundaries of the foreset slopes (as defined above) varies along the delta front and reflects the position of active and abandoned distributary mouths. The depth of distributary mouths bears a direct relation to distributary discharge and they tend to choke up when part of their flow is diverted. The depth to the upper boundary of the foreset slopes is therefore greater at the mouths of larger, active distributaries than at the mouths of the smaller, periodically inactive or abandoned distributaries. The depth to the lower boundary of the foreset slopes is more influenced by the original topography of the frontal lake basin but reflects also the position and the activity of the distributary mouths.
See also the radiograph of bottomset beds in a core from the Laitaure delta.
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