Glaciogenic Debris-Flow Deposits of Orphan Basin, Offshore Eastern Canada: Sedimentological and Rheological Properties, Origin, and Relationship to Meltwater Discharge

Abstract Glaciogenic debris-flow deposits (GDFs) have been recognized in the last decade seaward of many shelf-crossing ice streams. The rheology of GDFs remains poorly understood. Ultra-high-resolution sparker seismic profiles and 25 long piston cores were used to define the architecture, age, and sediment properties of the GDF deposits in Trinity trough-mouth fan (TMF), offshore northeast Newfoundland, and hence understand their origin and emplacement. The GDF deposits comprise poorly sorted gravelly mud. Individual GDF lenses are 5–30 m thick, 2–10 km wide, and up to 250 km long. Shear strength measurements and grain-size analysis indicate that GDFs have a different, more fluid rheology at their margins and tops, exhibiting a surging flow behavior. On the upper slope, the transition from hard over-consolidated till to thin proximal GDF deposits is exposed in a mid Holocene landslide scar. The transition between the two lithotypes appears gradual and no pre-Holocene failure scarps were detected. A process involving the continuous release of subglacial flowing material with high pore pressure and low shear strength is invoked for the production of GDFs. Five stacked GDF units (A–E) were deposited during the last glacial maximum (20.5–28 cal. ka), and can be correlated into a regional lithostratigraphy based on the presence of Heinrich beds and 8 local meltwater events (R1–8) represented by red plumite deposits south of the Trinity TMF. This correlation indicates that the timing of major GDF pulses corresponds to the early part of five meltwater discharge events (20.5–21, 23–23.5, 23.8–24.5, 25–27, and 27.5–28.5 cal. ka), so that GDF deposits represent only a small period of time during a major glacial advance. Three meltwater events (19.2–20, 23–23.5, and 25–27 cal ka) produced hyperpycnal flows that resulted in the formation of channel systems and distal sand turbidites. The presence of such erosional features in TMFs on the continental margins of the Norwegian–Greenland Sea suggests that this novel relationship between GDFs and hyperpycnal turbidites may be widespread and thus important for understanding the glaciological processes involved.

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