Effects of rapid sedimentation on developing the Nyegga pockmark field: Constraints from hydrological modeling and 3-D seismic data, offshore mid-Norway

Three-dimensional (3-D) seismic data in the Nyegga region expose hundreds of seafloor pockmarks and fluid escape chimneys in close proximity to the northern sidewall of the Storegga Slide, on the formerly glaciated Norwegian margin. As the up to 350 m wide individual craters of the Nyegga pockmark field postdate the Late Glacial Maximum (similar to 25 calendar (cal) ka B. P.) and because modern fluid flux is characterized as microseepage at most, it appears that the pockmarks developed nearly simultaneously with some specific event. External types of forcing such as earthquake loading, the Storegga Slide (8.1 cal ka B. P.), or rapid sediment loading represent potential trigger mechanisms for developing hundreds of pockmarks within a millennial time window. We integrate the 3-D seismic observations with a two-dimensional (2-D) sedimentation-fluid flow model to quantify the effects of sediment loading and erosion on overpressure generation and fluid expulsion. The models simulate the temporal evolution of compaction-driven overpressure and fluid expulsion based on sedimentation rates estimated from seismic data and hydrologic and sediment properties obtained from laboratory experiments and petrophysical data. The 2-D model predicts rapid overpressure generation in response to high sedimentation rates during the last (Weichselian) deglaciation period (25-18 cal ka B. P.). The high pressures significantly increase fluid expulsion, which peaks between 19 and 16 cal ka B. P. The modeled high-flux period and the following declining flux period exhibit an excellent correlation with existing age dates of methane-derived authigenic carbonate retrieved from Nyegga pockmarks. Accordingly, rapid sediment loading provides a mechanism to drive extensive cold seep formation. Our model suggests that the Nyegga pockmark field developed between 19 and 16 cal ka B. P. and therefore predates the giant Storegga Slide event.