Erosive sub-circular depressions on the Guadalquivir Bank (Gulf of Cadiz): Interaction between bottom current, mass-wasting and tectonic processes

Morphological features (escarpments, depressions and valleys) identified on the Guadalquivir Bank, within the middle slope of the Gulf of Cadiz, have been investigated based on high- and mid-resolution geophysical datasets. The morphological and seismic stratigraphic analyses allowed their interpretation as the result of the interaction between oceanographic, mass-wasting and tectonic processes. A phase of enhanced tectonic activity occurred during the Mid-Pleistocene related to the activity of diapiric structures, the front of the allochthonous units, and/or basement uplift or adjustments. This event seems crucial for the origin of widespread mass-wasting events along middle slope sheeted drift plateaus that have been further reworked by the Mediterranean Outflow Water. This resulted in the erosion of a marginal valley to the north of the Guadalquivir Bank, while fluid escape processes also have a minor role in the creation of crescent-shaped depressions. The Diego Cao channel is the result of a complex evolution, from a contourite moat associated to a separated drift to a multi-crested drift and moat system during the Pliocene and Early Quaternary. During the Mid-Pleistocene, the mass-wasting phase produced a prominent erosive surface that opened a deep gateway into the Guadalquivir Bank uplifted basement, allowing the onset of the Mediterranean Intermediate Branch to flow towards the N-NW. The system evolved into a contourite system composed of the present-day channel and an associated deposit on its western side (Bartolomeu Dias sheeted drift plateau) that forms a particularly complex separated drift. This drift displays a series of circular depressions that are similar in morphology to collapse features or pockmarks, but are in contrast, the result of the interaction between the bottom current and the irregular mass-wasting-related scars.