Rock fragments from mud volcanic deposits of the Gulf of Cadiz: an insight into the Eocene–Pliocene sedimentary succession of the basin

Two hundred clasts from the mud breccia extruded by the Yuma mud volcano in the Gulf of Cadiz are used to establish an offshore stratigraphy. The clasts are a very complex mixture of material from the sedimentary successions through which the mud volcano erupted and provide important information about the composition and genesis of deep-seated strata. We deal with fragments of rocks that were deposited millions of years ago, subsequently buried and later erupted on to the seafloor. Rock fragments from the mud breccia, studied in thin section under a polarizing microscope, provide genetic indications suggestive of deep-sea conditions during the accumulation of strata. A reconstruction of the sedimentary succession through which the Yuma mud volcano erupted and the depositional evolution of the Gulf of Cadiz was made on the basis of lithology and age determinations of the rock. Age and depositional environment documented by the clasts indicate that a marine basin existed in the Gulf of Cadiz area at least from the Eocene, and that in the Early and Middle Eocene a deep-sea fan depositional environment prevailed in the region, resulting in the accumulation of thick turbidites. The Late Eocene and Oligocene are not represented among rock fragments from the mud breccia, suggesting regional uplift and non-deposition in the area. Sedimentation was re-established in the Miocene with the accumulation of a clayey Aquitanian–Burdigalian succession. Pelagic carbonate sedimentation became predominant in the Langhian time. The Serravallian–Early Tortonian is characterized by a high supply of terrigenous material, resulting in the deposition of turbiditic clays and sands. Shallow water sedimentation with accumulation of carbonate rocks prevailed in the basin during the Middle Tortonian.