Sedimentary analysis of mud patch fields in the subtidal zone of the Cádiz Bay (SW Spain)

Abstract

In the subtidal zone close to the northern edge from the Cádiz Bay (SW Spain), between the Rota harbour and the La Galera shoal, several mud patch fields have been identified through side scan sonar records. These appear in the sonographic records as patches of whitish colour, which are surrounded by darkgrey sands. In order to establish the factors that control the presence of these bedform fields, a sedimentological study has been carried out. The results indicate that the normal hydrodynamic agents, such as tidal currents and waves, did not have capacity to cause these morphologies, therefore, other factors are necessary to explain their formation. Depositional processes, such as, hydrated gas scape, major storms, earthquakes and tsunamis, could cause the reactivation and reworking of these bedforms, but they did not cause their formation. The data indicate that the underwater landslides are related to the presence of submarine outcrops of Miocene diatomite marls, which constitute the continuity seaward of diatomite marls that outcrop in the coastal cliff from the nearby Almirante beach. Both emerged and submerged outcrops show numerous lobed morphologies caused by surface erosion and gravitational slides, which must have been formed in terrestrial environments, before the sea reached its current level. Later, the submerged outcrops were modified by the tides and waves, as well as by the action of other processes and events that partially modified the deposits.