Mud volcanoes from the Alboran Sea: materials and vulcano-sedimentary and diagenetic processes involved

Abstract

In the westernmost Mediterranean, a broad field of mud volcanoes (MVs) and pockmarks occurs in the West Alboran Basin (WAB). These typical sea-floor structures can be found in the inner part of the Gibraltar Arc and occur above a major sedimentary depocenter encompassing early Miocene to Holocene sedimentary sequences, up to 7 km thick. In this region the development of mud volcanism is related to the presence of a large shale-diapiric province in the WAB. The shale-diapiric province underlying the MV fields is derived from deep overpressured units (undercompacted Miocene shales and olistostromes), which in turn overlie a metamorphic basement. MV occurrences are related to tectonic features (faults): they rise under Holocene tectonics, and grow atop diverse diapiric structures. Various studies have focused on the tectonic context and morphology of these mud volcanic edifices. Still, however, little is known about the nature and components of the extruded materials (solid and fluids phases). This PhD thesis involves mineralogical, geochemical and biogeochemical investigations of mud-volcanic products and related sediments from some MVs and pockmarks of the Alboran Sea, named Perejil, Kalinin, Schneider´s Heart, Carmen, Maya Dhaka, Marrakesh, Granada and Mulhacen, and also Crow´s foot pockmark. The research is performed on gravity cores that comprised extruded materials’ from the MVs (mud breccias) and associated hemipelagic sediment samples. A entailed multi-proxy approach including organic (lipid biomarkers) and inorganic (mineralogy, sedimentology and pore water geochemistry) determinations have been performed both on fluids and solid phases. This study provides important insights about the composition and provenance of the volcanic extruded material (mud breccia) and hemipelagic sediments involved also including biogeochemical results closely related with the discharge of hydrocarbon-rich fluids to the seafloor through MVs. It also pursues a better understanding of the origin of the extruded muds as well as of the mud volcanism in the WAB. Hemipelagic facies draping mud volcanic structures exemplify mud volcanic activity. The existence of several interbedded intervals of hemipelagic sediments between mud breccias indicates the recurrence of volcanic pulses (episodes of mud extrusions), which result in discrete and successive emission of volcanic fluxes in the form of mud breccia intervals. Presence or absence of hemipelagic facies drapping mud edifices inform about the age of the mud volcanic events. Active edifices present a lack of hemipelagic drapes covering the last (uppermost) mud breccia interval. Only craters of Carmen and Perejil MVs lack hemipelagic drapes which indicate current or very recent mud flow and fluid eruptions as well as their current activity. This is supported by further evidence of most recent activity at these two MVs, such as gas bubbling or living chemosynthetic fauna. Thus, the occurrence of hemipelagic intervals intercalated with mud breccias points to successive episodes of volcanic activity resulting in mud flows that were draped by normal marine sediments. Inactive MVs present well developed hemipelagic drapes that lay covering the MV deposits. The absence of detectable methane discharge and nonexistence of living chemosynthetic fauna at these MVs support their inactivity. These features are found at Kalinin, Maya, Dhaka, Marrakech, Granada and Mulhacen which are considered inactive. The relative homogeneity in mineralogy and geochemistry of these sediment drapes, covering or interbedded with mud breccia intervals, indicates that they are generally not affected by the seepage venting. Radiocarbon dating of Globigerina Bulloides in hemipelagic intervals reveals the age of the older eruption occurred 10.8 Ka BP at Carmen whereas the most recent eruption happened 0.7 ka BP at Marrakech.