Sediment properties, flow characteristics, and depositional environment of submarine mudflows, Bear Island Fan

摘要

Recent gravity flows on the Bear Island Fan consist of fine-grained sediments that presently exhibit high densities (1.8-2.0 g/cm~3), low water contents (30-40 percent dry weight), and high (estimated) yield strengths (1100-1350 Pa). Rheological analyses, however, reveal exceptionally low yield strengths (3.5 Pa and lower) for laboratory slurries (<1.5 g/cm~3) made with the mudflow sediments. The low yield strengths are inconsistent with previous interpretations that the mudflows were spawned from glacial sediments that were rapidly deposited as till deltas on the upper slope of the fan, or that they were emplaced as low-density, low-cohesion, fluidized mud. Abundant glacial flour in the mudflow samples is indicative of subglacial meltwater discharge. This, in turn, suggests that the sediment originated from turbid plumes that settled into a low-density, high-water-content mud deposit in an open-marine, tidewater glacier environment. Such an environment may have existed during the mid/late Weichselian when an embayment in the Barents shelf ice-sheet left much of the inner shelf ice-free. Subsequent advance of the ice front closed the embayment, compressing the mud. Loss of water during compression increased the bulk density and yield strength of the mud. The modified embayment mud was gradually pushed off the shelf, generating mudflows on the fan. Morphologic evidence suggests that some mudflows may have hydroplaned, indicating that flow speeds varied above and below a critical speed (ca. 5 m/s) marking the onset of hydroplaning.