Sedimentary Structure:
These are large-scale features or patterns of sediments that are produced as a result of the migration of bed forms.
Classification of primary Sedimentary Structure:
The most simple classification of sedimentary structure is:
Primary structure:
These types of sedimentary structures are generated during or shortly after the deposition of sediments by physical processes.
Secondary structure:
These types of sedimentary structures form sometimes after deposition by chemical processes.
Relationship between bed forms and Sedimentary structure:
With the initiation of grain movement under a lower flow regime, ripples, sand waves, and dunes form with increasing flow strength over a plane bed of sand. The water waves are out of phase with the bed forms.
With further increases in flow velocity, dunes are destroyed, giving way to an upper flow regime condition of flow and form planner beds with in-phase water waves. Finally, at higher velocities of flow, beds are migrated into antidunes.
Sequences of development sedimentary structure in terms of flow regime concept:
Froud number of 1 separates two flow regime. lower flow regime (when Fr<1) and upper
regime (when Fr>1)
Fr = u/√9D
(U→ mean velocity of flow, D→ Depth of flow, g> Acceleration due to gravity)
- With initiation of grain movement under (ower flow regime, ripples, sandwaves and dunes form with increasing flow strength over a plane bed of sand the water waves are out-of-phase with the bed forms.
- These bed formòmigrated downstream sediment is eroded from the upstream of the bedforms and carried upto the crest, where it avalanches down to the Lee slope.
- Avalanching leads to the formation of cross lamination that deep downstreams at angle upto 30°.
- Further increases of flow velocity, dune are destroyed giving way to an upper flow regime condition of flow with in-phase water waves.
- Rapid sheet like flow causes, intense sediment transport as plane bed and give rise to horizontal lamination.
- At, still higher velocity of flow, plane bed give way to antidunes that migrate upstream and produce low angle (lower than 10°) cross bedding.
Finally, when flow velocity increases above the antídune stage, chute and pool structure develops. chutes form where flow is shallow and rapid and pool form where flow is tranquit.
The succession of bed form produce duning, lower flos) regime and upper flow regime depends on flow velocity, grain size and water depth.
