Transition from debris flow to hyperconcentrated flow in a submarine channel (the Cretaceous Cerro Toro Formation, southern Chile)

Debris flow is an important sedimenttransport mechanism in subaerial and subaqueous environments. Its properties change almost continuously as sediment and water are added to or subtracted from it (Smith and Lowe, 1991; Vallance, 2000). In subaerial environments, debris flows are commonly diluted into hyperconcentrated flows when they encounter a streamflow (Pierson and Costa, 1987; Costa, 1988). A transition facies is produced during these events, comprising both debris-flow and hyperconcentratedflow deposits in one sedimentation unit (Pierson and Scott, 1985; Scott, 1988; Scott et al., 1995; Sohn et al., 1999) (Fig. 1A,B). In subaqueous environments, debris flows are much more vulnerable to flow dilution because of larger flow resistance (Norem et al., 1990), lack of surface tension effects by interstitial water (Mulder and Alexander, 2001) and prompt disintegration of debris in the case of hydroplaning debris flows (Mohrig et al., 1998). Sohn (2000b) has suggested that submarine debris flows can be efficiently diluted when they hydroplane, and that two different associations of debris-flow deposits and diluted-flow deposits (turbidites) can be produced depending on whether a debris flow hydroplanes or not (Fig. 1C,D). This study reports another example of flow transitions in a submarine environment, which involved progressive dilution of debris flows into hyperconcentrated flows and turbidity currents. This study suggests that the transition from debris flows to hyperconcentrated flows can occur in the submarine environment in a manner similar to the subaerial counterparts but under different conditions.