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Kneller and Mccaffrey Rent THE INTERPRETATION OF VERTICAL SEQUENCES IN TURBIDITE BEDS: THE INFLUENCE OF LONGITUDINAL FLOW STRUCTURE BENJAMIN C. KNELLER1 AND WILLIAM D. MCCAFFREY2 1 Institute for Crustal Studies, University of California, Santa Barbara, California 93106, U.S.A. email: [email protected] 2 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, U.K. ABSTRACT: Because turbidite beds aggrade progressively beneath a Weisbrich et al. 1981) and is generally considered to occur in nature (Bou- moving current, the vertical grain-size pro®le of a bed is generally an ma 1964; Stow et al. 1996, and references therein, cf. Shanmugam 1997). indication of the longitudinal velocity structure of the ¯ow, and lon- Lastly, sediment may be deposited directly from suspension (``direct gitudinal gradients in suspended sediment concentration (``density''). suspension sedimentation'' of Lowe 1982) without any intervening traction, A current is more likely to show a simple waning ¯ow history farther even where the grain-size and shear stress are such that traction is poten- from its source; this is because faster-moving parts of the ¯ow overtake tially possible. This occurs when the ¯ux of material from suspension to slower moving parts, and the ¯ow organizes itself over time so that the the bed is so rapid as to preclude any tractional transport (Middleton 1967; fastest parts are at the front. Thus distal (e.g., basin plain) turbidites Lowe 1988; Arnott and Hand 1989; Vrolijk and Southard 1997) and results commonly show simple, normally graded pro®les, whereas more prox- in structureless deposits (Lowe 1988; Kneller and Branney 1995). imal turbidites often show complex vertical sequences within a bed, related to unsteadiness. A turbidity current may deposit a structure- Origin of Vertical Bed Pro®les less, poorly sorted bed where the capacity of the current is exceeded, i.e., where there is insuf®cient turbulent kinetic energy to maintain the Static settling can result in subtle normal grain-size grading in ®ne- entire suspended mass. Capacity-driven deposition may occur where grained turbidites or in the ®ne-grained tops of coarser turbidites, as a result the ¯ow decelerates. Where ¯ow nonuniformity is the cause of capacity- of differences in settling velocities of different grain-size fractions (e.g., driven deposition, a massive interval will form the lowest part of the Stow and Shanmugam 1980). Normal size grading in the sand fraction also bed, and will have a ¯at base. Where ¯ow unsteadiness is the cause, a has been ascribed to differences in the time taken for grains of different normally graded massive interval may overlie erosional features or sizes to settle through the ¯ow as a result of different settling velocities traction structures at the base of the bed. Based on the assumption of (Lowe 1982; Shanmugam 1997) once all turbulent support has decayed. longitudinal gradients in velocity, density, and grain-size distribution, This is more or less equivalent to static settling, and may be the case where the longitudinal density structure of a current may induce a switch, at the loss of grain support is virtually instantaneous, i.e., the ¯ow ``col- any given point, from capacity-driven deposition to either (1) bypass lapses.'' Flow collapse requires large spatial velocity gradients, either and resuspension, (2) bypass with traction, or (3) competence-driven where the ¯ow is completely blocked (i.e., has stopped more or less in- deposition, each resulting in a characteristic upward change in deposit stantaneously) or where there are rapid changes in slope or con®nement. character. The temporal evolution of the ¯ow at a point varies system- In this situation, all the grains, regardless of settling velocity, are falling atically in a streamwise sense. Taking account of these longitudinal towards the bed, and the resultant grading is simply a consequence of the variations permits predictions of complex vertical sequences within fact that the average time taken for ®ner grains to reach the bed is longer beds, and of their downstream relations. than that for coarser grains. Where the ¯ow has more or less come to rest, the dominant vertical particle ¯ux is likely to produce convective settling in which the downward movement of sediment occurs in downward-con- INTRODUCTION vecting cells (essentially vertical gravity currents) separated by regions of Turbidites are the deposits of submerged gravity-driven turbid suspen- upward-convecting lower concentration suspension (Kuenen 1968). The de- sions of ¯uid (usually water) and sediment. Deposition from turbidity cur- posit is likely to be poorly sorted at the base, with sorting improving up- rents occurs when the ¯uid and suspended sediment move down a gradient wards. This process can result in a normally graded bed only where the in shear velocity (generally equivalent to a velocity gradient). This can ¯ow is a single surge, because continuous ¯ow must eventually result in a arise when the velocity decreases spatially (for example due to decrease in uniform downward grain ¯ux for all grain sizes. This is a fundamentally slope, ¯ow expansion, or reduced sediment load), or temporally (due to different process than the generation of normal grading by a waning cur- ¯uctuations in supply rate), or both (Kneller 1995; Kneller and McCaffrey rent. In the latter case, any grains that have not reached their suspension 1995). threshold may still be fully supported by the turbulence, and the generation Deposition of sediment from turbidity currents may occur in several of normal grading is entirely governed by the progressive decline in bed ways. Sediment may settle from a virtually static suspension once the cur- shear stress as the ¯ow wanes. With suf®ciently small rates of decline, the rent has come to rest or slowed to the point where turbulence is inadequate deposit may be well sorted throughout, showing distribution grading. to maintain the grains in suspension. This is the mechanism by which much However, because turbidity currents sensu stricto deposit progressively mud is deposited from turbidity currents, because the bed shear stress re- and not en masse (Hiscott et al. 1997; Kneller and Buckee 2000), it follows quired to keep such material in suspension is so small that only a virtual that the vertical sequence of grain-size and sedimentary structures in a bed absence of current will allow deposition to take place (although ¯occulated records the time history of ¯ow conditions and bedforms. This idea was clay may behave as coarser silt particles; Stow and Bowen 1980). explored by Myrow and Southard (1991, 1996) for storm deposits to show Alternatively, sediment may accrete to the bed in much the same way the potential variability of vertical sequences. In all cases (including that as it does beneath dilute shear ¯ows, falling from suspension beneath a of ¯ow ``collapse''), the succession of grain-sizes through the bed records moving current and experiencing a period of traction on the bed before the temporal evolution of the ¯ow passing a ®xed point. In the case of coming to rest. Such sedimentation, which produces sedimentary structures suspension fallout with traction, the progressive aggradation is recorded by such as parallel lamination and ripple cross-lamination in the resulting bed, the sequence of traction structures, whereas in the case of direct suspension has been demonstrated experimentally (e.g., Kuenen 1966; Luethi 1981; sedimentation it may be cryptic in the sense that no sedimentary structures JOURNAL OF SEDIMENTARY RESEARCH,VOL. 73, NO.5,SEPTEMBER, 2003, P. 706±713 Copyright q 2003, SEPM (Society for Sedimentary Geology) 1527-1404/03/073-706/$03.00 INFLUENCE OF LONGITUDINAL FLOW STRUCTURE ON TURBIDITES 707 FIG. 2.ÐNormally graded turbidite bed forming a Bouma sequence, produced by deposition from waning ¯ow (Peira Cava Sandstones, Oligocene, southern France). LONGITUDINAL VELOCITY STRUCTURE OF CURRENTS Flow unsteadiness consists of variations in ¯ow velocity with time as FIG. 1.ÐSchematic diagram of ¯ow velocity structure. A) De®nition sketch for seen at a ®xed point (Fig. 1A). All turbulent ¯ows are inherently unsteady steady and unsteady ¯ow. B) Longitudinal velocity structure at four different times for surging ¯ow; time 1 shows the ¯ow structure at an early time for an initially on short time scales because of the presence of large eddies and internal slow ¯ow followed by a surge of higher velocity than the initial current; times 2 waves (Kneller et al. 1997; Kneller et al. 1999). These can produce ¯uc- through 4 illustrate the eventual progression of the surge to the front of the current tuations in the style of deposition, or alternations between deposition and with time. C) Time series of current velocity for the surging current in a proximal erosion, during the passage of a single current. The result is diffuse band- position (waxing then waning ¯ow). D) Time series of current velocity for the ing, or internal scour surfaces (Lowe 1982; Hiscott 1994b). Over longer surging current in a distal position (entirely waning ¯ow). time scales, turbidity currents may remain quasi-steady for periods of hours to perhaps weeks (Lambert and Giovanoli 1988; Piper et al. 1988; Piper and Savoye 1993), or they may wane with time, particularly if triggered are preserved and there is consequently no direct record of bed aggradation, by catastrophic failure of the slope or a shelf-edge delta. Turbidity currents per se (Kneller and Branney 1995). related to ¯oods, or those that experience surging, may also have waxing phases. Relation to Flow Structure Waning Flow Not only do turbidites aggrade progressively, they generally do so be- The ¯ow unsteadiness at the point of initiation of a current translates neath a moving current. The lowest parts of beds are deposited from rel- into a longitudinal velocity structure. Currents generated by catastrophic atively frontal parts of ¯ows, and successively higher parts of the bed are failures are likely to take the form of ®nite volume releases (as modeled deposited from successively more hindward parts of ¯ows.
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