| Popis: |
Enigmatic craters tens of kilometres in diameter have been mapped previously at the top of Oligo-Miocene ooze on the mid Norway margin. In this study seismic reflection data are used to map slides, craters and mounds in the M[unknown] Basin to better understand how the craters form. Many craters are filled by a slide named Slide W, which is by volume similar to the Storegga Slide, but which was translated only a few kilometres. Mounds mapped in the vicinity of craters filled by Slide W on the top surface of Slide W consist of sediment remobilised from the ooze in which the craters are incised. Mounds generally pinch out onto the top surface of Slide W and are emplaced on a palaeo-seabed, suggesting that failure of Slide W, incision of craters filled by Slide W and remobilisation of ooze occurred contemporaneously. A model is presented linking these processes to gas hydrate dissociation. Rapid deposition of less permeable sediment burying more permeable ooze leads to shoaling of the base of a gas hydrate stability zone towards the top of the ooze and the base of the rapidly deposited sediments. Dissociation of gas hydrate increases the pore pressure of the ooze. If pore pressure becomes equal to lithostatic pressure, fracturing of the overburden and venting of methane may occur, leading to slope failure. Alternatively, build up of pore pressure primes or triggers the slide. Ooze is liquefied and emplaced on the seabed through vents or faults developing in the slide, forming a crater in the subsurface into which the slide subsides. A new class of structure is defined (Subsurface Gas Expulsion Structures), and the significance of these structures is discussed. |
