Gas-charged sediments and large pockmark-like features on the Gulf of Cadiz slope (SW Spain)

Gassy sediments and large seafloor pockmarks are identified from seismic profiles in the Gulf of Cadiz slope. Gas-charged sediments are represented by zones of acoustic turbidity and other acoustic anomalies on seismic profiles. Most of the gas is believed to be biogenic in origin, resulting from the decay of organic matter contained within rapidly developed regressive shelf-margin deltas. A group of large asymmetric seafloor pockmarks also occurs downslope of the gassy area, both on the seafloor surface and deeply buried in the sediment column. Sediment sorting by the Mediterranean Undercurrent seems to act as a controlling factor for the development of each type of gas-originated feature. The area where the strong Mediterranean Undercurrent outflow impinges the seafloor is covered by sandy sediments of high porosity which allow the interstitial gas to escape through the sediment pores, whereas the areas affected by weaker currents are covered by clays, and the gas tends to accumulate in the sediment beneath impermeable horizons.