Repeated fluid expulsion through sub-seabed chimneys offshore Norway in response to glacial cycles

Abstract

Focused fluid flow through sub-seabed sediments is a common phenomenon on continental margins worldwide. However, the governing controls and timing of this fluid release have been difficult to understand, in particular, for fluid flow features buried beneath sub-surface sediments. A link between fluid flow activity, ensuing pockmark formation and the last glacial maximum has been hypothesized on the formerly glaciated Norwegian margin. New high-resolution P-Cable 3D seismic data from the Nyegga area on the mid-Norwegian margin reveal at least two more periods of fluid expulsion from sub-seabed sediments. The 3D seismic data show depositional patterns within chimney features, expressed as truncations of seismic horizons against the flanks of the chimneys. The truncations are interpreted, by analogy with present day observations, as evidence for buried carbonate mounds and/or sediment wash-out during formation of pockmarks in the past. The truncations are, hence, an indicator and a chronological marker for fluid expulsion in the past. The classification of chimneys results in three major groups: (1) chimneys that have been formed and consecutively reactivated one or two times during the last 200kyr and that have a fluid flow expression at the present seafloor; (2) chimneys that are approx. 125–160kaBP old without any associated fluid-flow expression at the present day seafloor; (3) chimneys with no stratigraphical evidence for reactivation formed after the last glacial maximum (18–25kaBP). The observations suggest that each activity period was likely related to the last stages of maximum glaciations in the region. The emplacement of thick sequences of glacigenic debris flow deposits during these maximum stages most likely caused rapid increase in overpressure and subsequently the formation of focused fluid flow features piercing through sediments of the Naust formation at Nyegga.