Exploring the Deep Sea and Beyond themed issue Hybrid submarine fl ows comprising turbidity current and cohesive debris fl ow: Deposits, theoretical and experimental analyses, and generalized models Peter J. Talling* National Oceanography Centre, European Way, Southampton, Hampshire SO14 3ZH, UK ABSTRACT transformations near the site of debrite depo- sured directly within such long run out fl ows. sition, and emplaced gently to avoid mixing This paucity of direct observations provides a Hybrid fl ows comprising both turbid- with surrounding seawater. The location and stark contrast to other major sediment transport ity current and submarine debris fl ow are geometry of cohesive debrites in hybrid beds processes such as rivers that have been closely a signifi cant departure from many previous are controlled strongly by seafl oor morphol- monitored in action. Understanding submarine infl uential models for submarine sediment ogy and small changes in gradient. Debrites fl ows remains a major challenge, as the only density fl ows. Hybrid beds containing cohe- occur as fringes around raised channel-levee record we have of most fl ows is the sediment sive debrite and turbidite are common in ridges, or in the central and lowest parts of deposit that they leave behind. distal depositional environments, as shown basin plains lacking such ridges. Small varia- Previous work has illustrated how long run by detailed observations from more than tions in mud fraction produce profound out distances can be achieved by turbulent sedi- 20 modern and ancient systems worldwide. changes in cohesive strength, fl ow viscosity, ment suspensions, called turbidity currents, that Hybrid fl ows, and cohesive debris fl ows more permeability, and the time taken for excess incrementally deposit layers of clean sand and generally, are best classifi ed in terms of a pore pressures to dissipate that span mul- mud. This work includes seminal contributions continuum of decreasing cohesive debris fl ow tiple orders of magnitude. Reduction in fl ow by Bill Normark and colleagues that elegantly strength. High-strength cohesive debris fl ows speed can also cause substantial increases in combined fi eld observations from the mod- tend to be clast rich and relatively thick, viscosity and yield strength in shear thinning ern seafl oor and ancient rock with quantita- and their deposit extends back to near the muddy fl uids. Small amounts of sediment can tive modeling (Bowen et al., 1984; Normark, site of original slope failure. They are typi- dampen or extinguish turbulence, especially 1989; Normark et al., 1993, 2002, 2006). Here I cally confi ned to higher gradient continental as fl ow decelerates, affecting how sediment address a second type of long run out fl ow that slopes, but may occasionally form megabeds is supported or deposited. This ensures that reaches the distal parts of submarine fans. These on basin plains, in both cases overlain by a cohesive debris fl ows and hybrid fl ows have a hybrid fl ows include both turbidity current and thin turbidite. Intermediate-strength cohe- rich variety of behaviors. mud-rich (cohesive) debris fl ow (Talling et al., sive debris fl ows typically contain clasts, 2012a), and their deposits comprise mud-rich but their deposits may be <1 or 2 m thick on INTRODUCTION debrite sand encased within turbidite clean sand low-gradient fan fringes, and are encased in and mud. This type of deposit was described turbidite sand and mud. Clasts may be far- Submarine fl ows of sediment driven by their fi rst by Wood and Smith (1958), and was noted traveled, and meter-sized clasts can be rafted excess density can run out for tens to hundreds subsequently by Van Vliet (1978), Hiscott and long distances across very low gradients if (and on occasions thousands) of kilometers, Middleton (1979, 1980), and Ricci-Lucchi and they are less dense than surrounding fl ow. sometimes across remarkably low seafl oor gra- Valmori (1980). However, only recently has Low-strength cohesive debris fl ows gener- dients of 0.1°–0.01° (Talling et al., 2012a). They it become apparent that this type of “linked” ally lack mud clasts, and as cohesive strength dominate sediment transport into many parts of debrite-turbidite bed is common in many loca- decreases further there is a transition into the deep ocean, and produce some of the most tions worldwide (Fig. 1; Haughton et al., 2003; fl uid mud layers that do not support sand. extensive and voluminous sediment accumula- Talling et al., 2004; Amy et al., 2009; Haughton Intermediate- and low-strength cohesive tions on Earth. Understanding these fl ows is et al., 2009). Hybrid fl ow deposits are the norm debrites are consistently absent in more challenging because they are remarkably dif- rather than the exception in the distal parts of proximal parts of submarine systems, where fi cult to monitor directly. The speeds of fl ows some submarine fans (Haughton et al., 2003, faster moving sediment-charged fl ows are that run out beyond the continental slope have 2009; Talling et al., 2004, 2012a, 2012b), and more likely to be turbulent. Intermediate- been measured accurately in only a few loca- they can involve very large amounts of sedi- strength debris fl ows can run out for long tions (Heezen and Ewing, 1952, 1955; Piper ment. One of the hybrid submarine fl ows that is distances on low gradients without hydro- et al., 1999; Piper and Savoye, 1993; Mulder described here transported 10 times the annual planing. Very low strength cohesive debris et al., 1997; Khripounoff et al., 2003, 2009; sediment fl ux of all of the world’s rivers com- fl ows most likely form through late-stage Vangriesheim et al., 2009; Hsu et al., 2008; bined (Talling et al., 2007a). Understanding Carter et al., 2012), and the vertical profi le of hybrid submarine fl ows is therefore important *Email: [email protected]. sediment concentration has never been mea- for determining how sediment is transported Geosphere; June 2013; v. 9; no. 3; p. 460–488; doi:10.1130/GES00793.1; 12 fi gures. Received 10 February 2012 ♦ Revision received 30 November 2012 ♦ Accepted 19 February 2013 ♦ Published online 17 April 2013 460 For permission to copy, contact [email protected] © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/3/460/3343593/460.pdf by guest on 28 September 2021 Hybrid submarine sediment fl ows A Agadir Basin - Bed 5 B Marnoso-arenacea Fm. (clast-poor debrites) 150 cm 150 cm m m µ µ 200 cm mud 1 mm 250 500 100 cm 100 cm dewatering pipes 150 cm 50 cm 50 cm 100 cm 0 cm 0 cm 50 cm m m m silt µ µ µ mud 1 mm 375 500 250 0 cm Sandstone C Mississippi Fan D Marnoso-arenacea Fm. (clast-rich debrites) Figure 1. Sedimentary logs show- 200 cm core top 100 cm Bed 3 ing different types of hybrid beds. Bed 2.5 (A) Bed 5 in the Agadir Basin off- 100 cm shore NW Africa (Talling et al., 100 cm clasts length to 320 cm 2007a). (B) Clast-poor debrites 100 cm 50 cm 30 cm in the Marnoso-arenacea For- clasts 50 cm 50 cm mation in the Italian Apennines 50 cm mud 375 500 (Talling et al., 2012b). (C) Distal 187 250 1000 lobe of the Mississippi fan in the 0 cm 0 cm 0 cm Gulf of Mexico (Talling et al., 0 cm ripples deep grooves 2010). (D) Clast-rich debrites in the Marnoso-arenacea Forma- E Jurassic & Paleocene, North Sea subsurface reservoirs F Karoo Group, South Africa 10 tion. (E) Jurassic and Paleocene 2 15 subsurface reservoir units in 0.5 2 the North Sea (Haughton et al., 8 0 1 60–100 cm 2009) (vf—very fi ne; f—fi ne; 60–100 cm m—medium; c—coarse; vc— 10 6 1 very coarse). (F) Permian Karoo 0 Group in South Africa (Hodg- 4 0.5 son, 2009). (G) Dysodilic Shale 0 G Oligocene, Carpathians, Romania 5 0 100 cm in the Carpathians in Romania 1 G 0 (Syl vester and Lowe, 2004). 2 1 J 0.5 (H) Banded slurry beds in the 0 50 cm 50 cm 0 Britannia Formation, North Sea 0 0 vf f m c vc silt (Lowe and Guy, 2000; Lowe et al., clay sand 2003). (I) Megabed in the Hecho clean sandstone parallel lamination mud clasts 0 cm muddy sandstone 0 cm Group, Spanish Pyrenees (Payros dewatering pipes sandy mudstone consolidation laminae vf m vc vf m vc sand KEY et al., 1999). (J) Debrites with mud sand mudstone dish structures convolutions mud low mud content from the Boso Penin sula, Japan (Ito, 2008). I MEGA-BEDS M3 Hecho Group, Pyrenees J Boso Peninsula, Japan M3 clean sandstone debrite matrix 30 M2b sand matrix ~10% < 20 µm 20 M2a M2a mud clasts meters Britannia Formation, N. Sea Britannia Formation, M2b H M4 10 M2a 1 m to several tens of m tens several 1 m to M2b 0 20 cm M2a carbonate or marl clasts Geosphere, June 2013 461 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/3/460/3343593/460.pdf by guest on 28 September 2021 Talling globally. Hybrid deposits often contain large turbidite mud VARIATIONS amounts of organic carbon, and may be a sig- A upper clean sand from H5 H4 missing nifi cant process for burying and sequestering dilute turbidity current H4 that organic carbon in deep water (Galy et al., few or no clasts 2007; Saller et al., 2008). Hybrid debrite-turbi- H3 cohesive (mud rich) debrite in debrite dite deposits are also important because they are sharp contact sharp or transitional boundary H2 a signifi cant departure from widely cited mod- banding corrugations deformed els for submarine fl ow deposits, such as those dune x-bedding of Bouma (1962), Lowe (1982), Mutti (1992), basal clean sand from dewatering and Mulder and Alexander (2001).