How Did Thin Submarine Debris Flows Carry Boulder-Sized Intraclasts for Remarkable Distances Across Low Gradients to the Far Reaches of the Mississippi Fan?

Abstract

Submarine density flows dominate sediment transport into many parts of the ocean and form submarine fans, which are some of the largest sediment accumulations on Earth. Previous studies often assumed that the distal fringes of submarine fans would be dominated by extensive sheet-like deposits from dilute and expanded turbidity currents, because only turbidity currents would transport large volumes of sediment for long distances across such low gradients. Deposits with a remarkable frondlike shape occur at the furthest fringe of the Mississippi submarine fan. Understanding the emplacement process of these deposits is important because their frondlike shape suggests deposition from debris flows rather than turbidity currents. Previous analyses concluded that these deposits comprise a complex arrangement of thin interbedded turbidity current and debris flow deposits. Here we propose a different internal geometry for the frondlike deposits. Intervals previously described as in place turbidites are interpreted to be clasts within a single debrite interval that is , 1 to 2 m thick. This debrite interval is underlain by clean sand. The debrite interval could have been emplaced by multiple debris flows, or one event comprising multiple pulses. Clasts in the debrite interval are much bigger than previously thought, including boulders with diameters that can exceed 50 cm. We show that thin (, 2 m) and extremely fluidal debris flows could have carried these boulders across gradients of only 0.06u to the fringes of one of the world's largest submarine fans, without hydroplaning. This long transport distance could occur if the density of the clasts was less than that of the surrounding debris flow. Emplacement of the basal clean sand layer appears closely linked to debris flow deposition because the clean sand layer pinches out in a location similar to that of the overlying debrite. We therefore suggest that the basal clean sand most likely settled out from the overlying debris flows, and that the clean sand is not a fore running turbidity current deposit. Shearing at the base of a mud-rich debris flow would most likely produce a muddy basal sand interval, rather than the clean sand that is observed. This basal sand interval is therefore less likely to be a record of a lubricating layer with high-pore-fluid-pressure on which the debris flows moved. This study shows how thin and highly mobile debris flows can redistribute large volumes of sediment including bouldersized clasts long distances into the deep ocean across remarkably low gradients.

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“…Such hybrid or linked turbidite-debrite beds were described previously (e.g., Wood and Smith, 1958;Middleton, 1979, 1980;RicciLucchi and Valmori, 1980;Zeng et al, 1991), but only recently has it become apparent that they are common in many locations worldwide (Haughton et al, 2003(Haughton et al, , 2009Sylvester and Lowe, 2004;Talling et al, 2004Talling et al, , 2007cTalling et al, , 2010Amy and Talling, 2006;Ito, 2008;Zeng et al, 1991;Amy et al, 2009;Hodgson, 2009;Jackson et al, 2009;Pyles and Jennette, 2009;Muzzi Magalhaes and Tinterri, 2010). Understanding these hybrid fl ows is important because they are a common feature of the most important processes for delivering sediment to the deep ocean.…”

“…6A, 6B) may have been supported because they are less dense than the surrounding matrix. A more detailed analysis of clast support in cohesive debris fl ows was provided in Talling et al (2010). This type of intermediate strength debris fl ow is broadly simi lar to intermediate strength debris fl ows responsible for thicker clast-rich debrites (e.g., Piper et al, 1999;Laberg and Vorren, 2000;Tripsanas et al, 2008), but it has a lower strength that permits a longer runout across low gradients.…”

“…It is therefore puzzling that basal clean sand layers formed in this way are seen only for the intermediate strength clast-rich debris fl ows, and not the clast-poor debris fl ows inferred to have low strength. However, it is a likely explanation for the structureless basal clean sands that pinch out in the same place as overlying cohesive clast-rich debrite (Talling et al, 2010), as such late-stage settling appears to be a rather general property of lower strength cohesive debris fl ow experiments.…”

Quantification of the Bed-scale Architecture of Submarine Depositional Environments and Application to Lobe Deposits of the Point Loma Formation, California

“…However, distal lobe deposits can also show abrupt thickness variations along-strike indicating a finger-like "dendritic" pattern (Twichell et al, 27 1992;Prélat et al, 2009;Talling et al, 2010) rather than a simple radial sheet commonly found in schematic models (Mutti and Normark, 1987). The low thinning rate values from distal locales ( Figure 7) suggests that neither the Point Loma nor the compiled distal lobe deposits contain these dendritic features, or that they are not sampled or recognized within the extent of the outcrops and within the database.…”

Aspects of marine geoscience: a review and thoughts on potential for observing active processes and progress through collaboration between the ocean sciences

“…However, detailed fieldwork on land and in the oceans by correlating beds across large distances has shown that the opposite can also happen, with densification of turbidity currents or embedded debris flow layers leading to depositing denser flows such as debris flows [268,269]. The high density of debris flows may also explain how some large boulders can be carried as far as the distal parts of fans [270]. These and many other issues have been addressed by UK groups with laboratory and numerical simulation of sedimentary flows [271][272][273][274][275][276].…”