SOUTH CASPIAN BASIN: A NATURAL LABORATORY FOR SEA LEVEL CHANGE AND GAS HYDRATE STABILITY

The presence of natural gas hydrates in the shallow, landlocked South Caspian Sea of Central Eurasia provides a potentially unique natural laboratory to study the relationship of sea level change and gas hydrate stability. Seismic reflection data from the offshore region of Azerbaijan document the occurrence of gas hydrates concealed beneath the seafloor (~300 m) in water depths ranging from ~400 to 650 m. Although identified in ~200 m thick layers on the seismic data, thermobaric modeling of gas compositions determined from shallow coring predicts minimum water depths of 150 m and maximum thickness of 1,300 m for hydrate stability. These are considerably shallower and thicker deposits than for other known hydrate provinces. Development of these gas hydrates near the base of the continental rise appears to control a large region (>200 sq. km) of shallow deformation. Shallow structural disruption, evident on detailed bathymetry of the seafloor, appears to be controlled by the base of the gas hydrate layer, while the zone of gas hydrate appears to be continuous across these shallow faults, implying rapid and dynamic re-equilibration of the gas hydrate stability field following very recent faulting. Rapid sedimentation rates in the basin during the Pleistocene averaged more than 2 m/1000 yrs. Historic sea level changes in the Caspian basin exceed 3 m over the last century, and are significantly amplified in comparison to global sea level changes during Pleistocene time. Detailed structural and stratigraphic analysis implies a correlation between sea level change, sedimentation rate, regional slope failure, and gas hydrate stability.