Mass transport deposits in deep-water minibasins: Outcropping examples from the minibasins adjacent to the Bakio salt wall (Basque Country, Northern Spain)

Abstract

Recent subsurface studies show that mass-transport deposits (MTDs) in salt-controlled basins may correspond to local or regional bodies induced by either regional tectonics, or diapir growth. These MTDs are commonly considered as muddy bodies but they may alternatively incorporate a high amount of clasts and reworked beds with good reservoir properties and thus they are often challenging deposits in oil and gas exploration. The minibasins adjacent to the Bakio salt diapir, in northern Spain, provide a unique opportunity to study up to seven outcropping MTDs comparable in size to subsurface examples. Detailed structural analysis was used to reconstruct the transport direction for each MTD and to infer their source locations. In addition, facies analyses enabled the estimation of their percent of mud or matrix, allowing for a discussion on their potential reservoir and seal properties. At least six of the studied MTDs correspond to locally-derived MTDs sourced from the Bakio diapir or from the footwall of the adjacent sub-salt extensional faults. The primary trigger for these MTDs may be halokinesis, probably with contributions from other secondary processes, such as carbonate platform aggradation, high sedimentation rates and regional extension. Transport directions together with palaeoflow analysis suggests that regionally-derived turbidites flowed along the minibasin axis, while MTDs were transported laterally from the minibasin margins at high angle with the turbidity flows. We identified three types of MTDs: muddy siliciclastic-dominated MTDs, sandstone clast-rich siliciclastic-dominated MTDs and carbonate-dominated MTDs. Using this classification and subsurface analogs we propose a model of locally-derived MTDs according to the nature of the source area and the sedimentary facies reworked along the MTD downslope trajectories. This model suggests that reservoir and seal properties could be suggested for MTDs in subsurface studies by characterizing the nature of the diapir roof and the facies at the seafloor found along the MTDs trajectories.