Progressive vs. Discrete Slumping: combining 3D seismics (Nile) with large-scale Outcrops (La Peña Canyon)

5. MILANA, J. P. AND , KNELLER, B. AND DYKSTRA, M.

Oslo

Congreso; International Geological Congress; 2008

IUGS

Mass transport deposits (MTDs) are not generated by unique mechanisms and also, the time elapsed for emplacement of a single MTD unit may be significantly different. Here we call discrete slumping to those MTD units suspected to be deposited relatively rapid, thus considered to be a “catastrophic” phenomena able to produce even physical large waves due to their movement. On the other extreme we have observed another variety of MTD that seems to occur slow enough in order to encompass some sedimentation on top of the deforming complex, adding mass to the MTD while it moves downslope. Two main forms of this progressive slumping are envisaged: the first can be compared to creep, and produces a very slow and constant movement of the mass, the second seems to occur by multiple episodes of discrete and time-limited events of movement. A good example of the later has been described for the Holocene of Spitsbergen. The internal composition and stratigraphy of the MTD would be quite different in all cases: discrete slumping would produce bodies with well defined boundaries, both laterally and downslope, but progressive slump deformed beds may fade away into the undeformed deposits that were accumulated during the time elapsed by the MTD emplacement. As a result, while the laterally equivalent non-MTD stratigraphy would partly correlate with episodes within the progressive MTD stratigraphy, it would not occur in the case of a discrete MTD where lateral equivalent deposits would not be time equivalent. This is because a discrete slump would determine a time-limited event, the progressive MTD would represent a long time period. The preliminary study of outcrop and 3D seismic datasat shows that progressive slumpling may often occur and that shows some determining characteristics recognizable both in seismics and in outcrop. As there is some accumulation during deformation, many progressive slumps share many characteristics with models of syntectonic sedimentation on piggyback small basins. The result of interplay between extension at the trailing edge and compression at the downslope front of a creeping slump, produces a complex play of onlaps, truncations and bed convergence (fanning) that can be well explained if a large amount of sedimentation of the slump has occurred while the MTD was moving. In these MTDs, the basal layer may act as a sliding layer and a thicker and progressively less deformed upper layer shows that deformation was progressive, in the way of block rotation, fragmentation and opening of small ponded mini-basins on top that shows sediment are progressively less disturbed upwards, making sometimes difficult to place an upper boundary to these MTDs. 3D seismic data shows this progressive slumping may occur in pulses and thus, a sort of depositional sequences are formed suggesting either that sediment input has been cyclical, or that slump movement has been episodic.  Understanding the way this process operates would increase our predictive capacity, mainly on the minibasins clearly formed on top of progressive slumping.