Chaotic Deposits: Examples from the Brazilian Offshore and from Outcrop Studies in the Spanish Pyrenees and Northern Appennines, Italy
Chaotic deposits are common in ancient and modern basins in both divergent- and convergent-margin settings and vary in scale from mappable units up to 100's of m thick and traceable over considerable distances to cm-thick units observable within individual sandstone beds. Similarly variable is the origin of these units that, at a large scale, most commonly derive from instability processes along basin margins associated with relative sea level variations, rapid sedimentation, tectonic oversteepening, and earthquakes or, to a small scale, from density gradients established between packets of newly deposited sand laminae and underlying water-impregnated mud. In all cases, these sediments experienced more or less severe post depositional deformation, which commonly obliterates the original stratification and geometry of the deposit, giving way to a wide range of chaotic and/or structureless facies. The correct understanding of the behavior of these sediments as reservoir or even as seal plays a very important role in the economic assessment for exploratory activities mainly in deep-water frontiers. This paper focus on large-scale chaotic units recognized through extensive seismic mapping in the Brazilian offshore and from outcrop studies in the south-central Pyrenees, Spain, and northern Apennines, Italy. The examples discussed are interpreted as mass transport deposits and include slide blocks, internally folded and thrusted slump units, debris flow deposits (commonly with large floating blocks), cohesionless debris flow deposits, mostly represented by breccias and conglomerates, and possibly by thick-bedded and essentially structureless sandstone facies. Excess pore pressure seems to have played a major role in most of the examples considered.