Inferred Mesozoic Origin of the Nautilus Basin, Arctic Ocean

Nina Lebedeva-Ivanova

The Nautilus Basin is a small (~124 000 km2) sub-basin forming northwestern CA Basin, and is surround by the Alpha-Mendeleev Ridge and Chukchi Borderland. The Nautilus Basin encompasses irregular seafloor topography at depths of 3000-3800 m. Several multichannel seismic lines (MCS) and sonobuoy seismic data were collected by US-CA expeditions in 2008, 2009, and 2011. These lines are in the central part of the Nautilus Basin and on two transects between the Chukchi Borderland and Alpha Ridge. Previous analyses have shown that High-Amplitude Reflective Packages (HARPs) underlie most of the northern CA Basin, including the Nautilus Basin. HARPs are often the most reflective events in the seismic section, sometimes exceeding the reflectivity of the seafloor. Where best developed, HARPs are ~100-300 ms TWTT, consisting of several high-amplitude wavelets, and have a narrow frequency band within the limits of ~10-30 Hz. It is consistent with patterns produced by constructive interference of thin beds of alternating higher and lower velocities. Two kinds of HARPs are observed in the Nautilus Basin; an upper reflective package, at 1-1.5 s below sea floor with highest amplitudes and a dominant frequency of ~25 Hz; and a deeper, less reflective HARP at ~0.2 s below the upper HARP, with a dominant frequency of ~20 Hz. Forward modeling of sonobuoy 2009-06 in the central Nautilus Basin, in combination with the multichannel data provides constraints on the velocity structure. The models suggest that the upper HARP consists of layers with velocities of ~3.5 km/s and ~2.5 km/s; the lower HARP is composed of layers with velocities of ~4.4 km/s and ~3.0 km/s. A possible sedimentary layer occurs between the two HARPs. Basement outcrops around the Nautilus Basin were dredged at the boundary with the Northern Chukchi Borderland. Dredge HLY0805-DR6 (Andronikov et al, 2008; Brumley et al, 2008) recovered basalts that have been dated at 80-88 Ma, consistent in age with clasts found within dredged volcanic sediments from the central Alpha Ridge (Van Wagoner et al, 1986) that were interpreted to have been deposited in shallow water. Dredge HLY0805-DR1 (Brumley et al, 2008, Brumley, 2009), located on southern Alpha Ridge along MCS line 2009-6, also recovered shallow-water volcaniclastic sediments, similar to those described by Van Wagoner et al. (1986). These described lithologies of inter-layered volcaniclasics and basalts are consitant with the alternating velocity parameters of the HARPs. This suggests that the deep rocks within Nautilus Basin may be of Late Cretaceous age. Normal faults offsetting the HARPs are younger which therefore constrains the timing of rifting within the basin. If this interpretation is correct, the layered structure of HARPs suggests that several phases of magmatism may have occurred within 80-88 Ma or possibly older.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013