Delta details

Rivers carry huge volumes of sediment, but the amount of gravel, sand, silt and clay they can hold in suspension declines once they meet non-flowing water. These materials are accordingly deposited – but not all in the same place.

Delta details

As a river flows into the sea, its energy steadily falls and its sedimentary load gets deposited. The smallest particles separate out when the water finally comes to a standstill.

River-dominated deltas containing coarse deposits such as sand or gravel are known as Gilbert deltas, and are constructed with clearly defined top, inclined and bottom layers.

Taken in Sardinia, the photograph shows the top layers which defined the water surface during deposition. Inclined layers can be seen, which were deposited in front of the delta and point in the direction of flow. Their grains are rather smaller than in the top layer.

The finest sediments were deposited as bottom layers, further out in deeper water and not visible in the picture. The small crescent-shaped structures on the left are former river channels on the delta plain.

Knowing how sediments sift out is interesting because sand and gravel provide permeable deposits where oil and gas can migrate or accumulate. Finer silt and clay are virtually impermeable, and can prevent hydrocarbon flow.

The river which deposited the delta on Sardinia in the Miocene epoch has long since vanished. This Mediterranean island has subsequently been uplifted several hundred metres, and the former delta now provides local building materials.

  • Terje Solbakk (text) and Espen Simonstad (text and photo)