An improved Weichselian seismic stratigraphic framework of the Kongsfjorden-Isfjorden Interfan region off western Svalbard from high-frequency deep-towed seismic data and its implication on fluid migration and methane venting

Active methane venting in the upper continental slope off western Svalbard results from methane hydrate dissociation caused by bottom water warming. Lithological heterogeneity in the glaciomarine sediments influences fluid migration, accumulation, and methane venting. However, seismic imaging with an airgun source could not resolve glaciomarine stratal architecture in the top 50 m below the seafloor (mbsf). We address this limitation by collecting several deep-towed high-frequency (220–1050 Hz) SYSIF seismic data (vertical and horizontal resolutions 1 and 3 m, respectively). Based on seismic interpretation, we improve the seismic stratigraphic framework and infer the depositional processes controlling the distribution of glaciomarine sediments in the interfan region between the Kongsfjorden and Isfjorden Trough Mouth Fans. We identify six seismic reflectors that separate five seismic units, 1 (youngest)–5 (oldest), characterize the seismic facies, and map the facies distribution in different units. We assign ages to the reflectors based on the results from the drill core GS10-164-09PC collected at 846 m water depth. Basal unit 5 consists of low amplitude, chaotic reflections indicating poorly sorted glacial debris materials above a basal erosional surface. The upper part of this unit is characterized by isolated, parallel, high to moderate-amplitude reflections embedded within low-amplitude chaotic reflections. The overlying unit 4 shows parallel, well-stratified, continuous reflections pinching out upslope. Unit 3 consists of chaotic facies that occur as isolated lenses. Unit 2 consists of moderate amplitude, parallel, well-stratified, continuous reflections. The topmost unit 1 shows low amplitude, parallel, continuous reflection.

The erosional base of unit 5 is a result of incision by strong hyperpycnal flow during the onset of early mid-Weichselian glaciation. The chaotic facies within unit 5 (74–54 ka) is attributed to glacial debris flow (GDF). Glacial advancement to the shelf break led to the formation of a till delta and debris flow as the delta front steepened and failed. On the uppermost slope, eastward dipping toe thrusts within the GDFs formed due to frontal obstruction caused by pre-existing debris mound. The well-stratified layers embedded within the GDFs suggest sediment deposition from turbidity currents that emerge as a result of the mixing of debris flow with water. The well-stratified reflections within uniformly thick unit 4 covering the GDFs are primarily a result of the deposition of hemipelagic materials by contour currents between 54 and 38 ka. The isolated chaotic lens in unit 3 represents debris flow lobes detached from the parent debris unit. Their deposition occurred during the last glacial maxima (38–24 ka) when the ice sheet re-advanced to the shelf break. The well-stratified reflections in unit 2 represent plumite deposition since the last deglaciation (24–15 ka). The low-amplitude reflections in unit 1 indicate finer winnowed sediments (15 ka–Present). The well-stratified contourites and turbidites in units 4 and 5 are suitable reservoirs that can store and transmit fluid more efficiently than the GDFs. The clustering of methane seeps above these shallowest reservoirs indicates flow focusing in those sediments after the methane hydrates have completely melted.