Naturaleza de la brecha fangosa de volcanes de fango del Golfo de Cádiz: sistema diapírico del Guadalquivir y zona Tasyo

Abstract

This study presents stratigraphic and compositional data from gravity cores of the mud volcanoes located in the middle slope of Gulf of Cádiz. The collective data were obtained during the cruise Anastasya 01/09 in 2001on board of the R/V Cornide de Saavedra. Five gravity cores up to 3 m long collected from mud volcanoes structures, have been studied by means of sedimentological and mineralogical techniques. The aim was to define the units and the main features of the matrix of the mud volcano breccia samples and compare the obtained data of the two different studied areas and also with the interbedded hemipelagic sediment. The main characteristics of the matrix of the mud breccia are: green to dark colours, a bimodal clastometric distribution and a mineralogical composition made of detrital minerals as clay minerals (illite, chlorite, kaolinite and smectite), quartz and carbonates. Dolomite and pyrite are also present and they are of a probable diagenetic origin. Mud breccia of the two different areas is quite similar and is always sealed by sandy sediments (TASYO area) or sandy and hemipelagic sediments (SDG area). The different described units for both areas imply differences on the thickness of expelled sediments, and in the sequence, having the SDG area more interlayered hemipelagic sediments than the TASYO area. At present, no activity has been detected in any of the studied sites but alternating dormant and expulsion periods of mud have been detected at both sites. Finally, the results of this study show that the source units of the mud breccia have to be deep Miocene clayey-rich units, similar to those source units described for some of the Mediterranean Mud volcanoes.