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46170-AC8
Absheron Allochthon: Evidence for South Caspian Seafloor Deformation in Response to Climatically Driven Hydrate Dissociation

Camelia C. Knapp, University of South Carolina

This research aims to interpret and analyze a large (640 km², industry-quality, 3-D multichannel seismic reflection dataset (@ 3 s) and well-logs from the petroliferous South Caspian basin, offshore Azerbaijan, in order to clarify the origin, age, and areal extent of a large-scale (> 2,500 km²) late-Pleistocene zone of seafloor deformation and submarine slumping, the Absheron Allochthon. These data are uniquely set to evaluate proposed mechanisms for late-Pleistocene large-scale submarine slope failure, and favor a relationship with the dissociation of underlying gas hydrates.

Previous analysis of two regional seismic reflection profiles from the study area documented the occurrence of gas hydrates concealed beneath the seafloor (~300 m) in water depths of 400-650 m (Diaconescu et al., 2000; 2001; 2002). Historic sea level changes in the Caspian basin have exceeded 3 m over the last century, and were significantly amplified in comparison to global sea level changes during the Pleistocene. Since the inland Caspian Sea is affected by sea level fluctuations at much shorter time scales and much larger amplitudes than the world’s oceans, the premise is that climate-induced changes in sea level were responsible for large scale slope failure and seabed deformation through massive dissociation of buried gas hydrates during the sea level lowstands in the late-Pleistocene. This region may become a natural laboratory to study the relationships among the gas hydrate dissociation and seafloor deformation, and may be further used as a proxy for similar processes in the world’s oceans. The specific hypothesis of this project is that climate-induced changes in Caspian Sea level appear to have been responsible for large scale slope failure and seafloor deformation through massive dissociation of buried gas hydrates during the Late Pleistocene sea level fall.

The Absheron Allochton is the main feature of interest within the seismic dataset. It is composed of two main components, (1) a chaotic facies component in the western portion of the survey with discontinuous seismic reflections and relatively minor observable stratigraphic textures, and (2) an eastern component displaying much more continuous seismic reflections interrupted mainly by normal faulting and which is stratigraphically thicker than the chaotic facies component. The base of the Absheron Allochton generally displays a terraced geometry consisting of relatively low-relief areas bounded by steeply-dipping boundaries. The overall relief of the base of the Absheron Allochton is ~607 m, ranging from ~560 mbsl in the northwestern portion of the survey to ~1148 mbsl in the southeastern portion of the survey, and shallowing up to ~541 mbsl on the northern buttress of the mud volcano. The top of the Absheron Allochton displays more continuous relief, ranging from ~456 mbsl in the northern portion of the survey to ~753 mbsl in the southern portion of the survey. The isopach map of the Absheron Allochton shows the thinnest sections (< 20 m) in the westernmost portion of the survey consisting of the chaotic facies component, while the thickest section (> 500 m) resides in the northeastern portion of the survey consisting of the more continuous facies component.

Current work is focused on the dating of the emplacement of the Absheron Allochthon and correlation with the Pleistcene sea level changes in the Caspian Sea.

References

Diaconescu, C. C., and J. H. Knapp. 2000. Buried Gas Hydrates in the Deepwater of the South Caspian Sea, Azerbaijan: Implications for Geo-Hazards. Energy Exploration and Exploitation, 18 (4): 385-4000.

Diaconescu, C. C., R. M. Kieckhefer, J. H. Knapp. 2001. Geophysical evidence for gas hydrates in the deep water of the South Caspian Basin, Azerbaijan. Marine and Petroleum Geology, 18: 209-221.

Diaconescu, C. C., and J. H. Knapp. 2002. Gas Hydrates of the South Caspian Sea, Azerbaijan: Drilling Hazards and Sea Floor Destabilizers. Paper presented at the 2002 Offshore Technology Conference, May 6-9, in Houston, Texas.

Kennett, J. P., K. G. Cannariato, I. L. Hendy, R. J. Behl. 2003. Methane Hydrates in Quaternary Climate Change, Washington D. C.: American Geophysical Union.

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