Distribution and characterization of failed (mega)blocks along salt ridges, southeast Brazil: Implications for vertical fluid flow on continental margins

Three-dimensional seismic data are used to assess the control of halokinetic structures on the distribution of blocks in a mass transport deposit in the Espirito Santo Basin, southeast Brazil. In contrast to what is commonly observed over growing salt structures, the thickness of the MTD-A1 is larger on top of a northwest trending salt ridge. Emphasis was given to the statistical analysis of 172 remnant and rafted blocks identified within Eocene mass transport deposits (MTD-A1). Three styles of block deformation are identified and scale relationships between the geometry of blocks and their relative position on the salt ridge are presented. Average block height reaches 130 m. Average block area reaches 0.43 km(2), while 11.3% of the total area (A) investigated is covered by blocks (5% < A < 17%). On the basis of variations in block geometry (height, area, width/length ratio, orientation) and their relative distribution, we interpret that most failed strata have been remobilized by adjacent topography created during growth of the investigated salt ridge. We show that the origin of the blocks is linked to densely spaced sets of halokinetic-related faults that deformed the prefailure strata. The presence of underlying faults and blocks of remnant and rafted strata potentially induces sharp variations in the internal permeability of MTD-A1. Thus, the interpreted data shows that megablocks in MTDs can constitute viable fluid pathways on otherwise low-permeability units. This character can significantly decrease seal competence above and on the flanks of halokinetic structures.