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 ᭧ 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. Thus the ver- by lock-exchange surges in ¯ume tanks) in which the frontal part of the tical structure of a turbidite bed relates to the longitudinal velocity and current moves most rapidly and more hindward parts of the ¯ow follow density structure of the current. The vertical sequence of grain-sizes in a more slowly (Simpson 1997; Altinakar et al. 1996; Kneller et al. 1999; bed records the history of shear velocity of the current through time at a McCaffrey et al. in press; e.g., Fig. 1D). As a result, the time series of given point, and the sedimentary structures record the interplay between velocity at any point along the length of such currents is dominated by those grain-sizes and bed shear stress. Except in the case of static settling, waning ¯ow. Because the bed shear stress and shear velocity vary with which, as argued above, is likely to apply only to ®ne-silt-grade material ¯ow velocity, the vertical pro®le of grain-size and sedimentary structures or ®ner, the vertical sequence of grain-sizes in a bed bears no relation to records this history of waning ¯ow; i.e., normal grading, and a sequence the vertical structure of the current, and features of the deposit record only of sedimentary structures indicative of decreasing ¯ow strength (Fig. 2). ¯ow conditions very close to the bed. Little can be inferred directly from The Bouma sequence (Bouma 1962) is a special case of this. A conse- the deposit about higher parts of a ¯ow that continued down the transport quence of this longitudinal velocity structure is that the front of the current path and deposited their sediment load farther downstream. progressively pulls away from the more hindward parts and the current In the following analysis and discussion, we postulate that the principal becomes attenuated, as implied both by simple box models and more com- control on the development of vertical sequences of structures and textures plex numerical models of turbidity currents (Dade and Huppert 1995; Felix within turbidite beds is the longitudinal velocity and density structure of 2002) and by experimental data (McCaffrey et al. in press). As a result, the current, especially at its base, where it is interacting directly with the the current wanes more rapidly in proximal positions than in distal ones. bed to form the deposit. Combining observations of ancient deposits with Steady, Surging, and Waxing Flow theoretical considerations, we propose a model that relates various vertical sequences within deposits to properties of the parent ¯ows, and places these Flow that is steady at the point of initiation may remain so at all points in a predictive downstream sequence. downstream. This will also be re¯ected in the deposit, which will be un- 708 B.C. KNELLER AND W.D. MCCAFFREY

FIG. 4.ÐInversely graded turbidite beds produced by deposition from waxing ¯ow (Kneller 1995). A) Coarsening-upwards sequence consisting of a basal ripple cross- laminated interval (i) succeeded by parallel-laminated interval (ii), the two separated by a loaded surface; an interval of curving and slightly fanning lamination (iii) reminiscent of hummocky cross-strati®cation (Prave and Duke 1990) overlying a surface of erosion; a transition into massive sandstone of ``normal'' turbidite Ta FIG. 3.ÐUngraded bed of massive sandstone produced by deposition from steady interval (iv); Marnoso Arenacea, Miocene, northern Italy; this sequence has been ¯ow (Gres d'Annot, Oligocene, SE France). interpreted by Mutti (in press) as the deposit of a ¯ood-generated density current. B) Coarsening-upwards sequence from (v) parallel-laminated basal interval to (vi) massive upper interval, truncated by (vii) omission surface, possibly rippled (waxing ¯ow becomes suf®ciently energetic to be nondepositional), covered by (viii) mud graded (Kneller 1995) and may show little variation in sedimentary struc- drape. Lens cap in this and all succeeding photos is 6.5 cm in diameter (Gres ture through the bed (Fig. 3). Flow that is surging at the source or is d'Annot, Oligocene, SE France). affected by the generation or breakdown of internal waves such as internal roll waves (associated with very high bulk Froude numbers on slopes; Allen 1984), will include pulses of more rapidly moving ¯uid. Their arrival may developed the structure of a simple surge, with the velocity time series at be marked by waxing ¯ow. These pulses tend to advance through the ¯ow, a point recording a sudden increase followed by a progressive decrease in overtaking slower-moving parts of the current until they reach a part whose velocity. This accounts for the relative simplicity of outer-fan and basin- mean velocity is equal to the velocity of the pulse, in the process devel- plain turbidites, in which simple normal grading is predominant, compared oping the structure of simple surges. If their mean ¯uid velocities are faster to the complexity of the depositional record of many turbidite sand beds than the rate of advance of the head of the current, they eventually catch in proximal positions. up with it, as long as the ¯ow does not dissipate ®rst. This establishes a Waxing ¯ow, for example at the onset of a ¯ood-generated turbidity simple longitudinal gradient in mean velocity (Fig. 1). The velocity time current, may produce complex signatures in the deposit. Where deposition series close to the source captures these surges, which may be expressed occurs during waxing ¯ow, it preserves a record of increasing shear stress in the deposit as alternating layers of inverse and normal grading (the with time in the form of an inversely graded bed, possibly with varying inverse graded layers becoming less well developed downcurrent as the bedforms (Fig. 4A; Kneller 1995). Waxing ¯ow may increase in strength velocity difference is reduced), or as internal erosion surfaces overlain by to the point that ¯ows that were initially depositional at a given point may normally graded layers (e.g., Lowe 1982). With increasing distance from become nondepositional, producing a surface of bypass or erosion at the the source, surges that propagate faster than the head will follow more top of the inversely graded layer (Fig. 4B), in the extreme case (the most closely upon the arrival of the head, and so the resulting inverse-to-nor- proximal situations) leaving no record of any original inversely graded mally graded layers tend to be con®ned to the lower parts of the bed. In layer. Waxing ¯ow is generally followed, in due course, by waning ¯ow, distal positions, such surges will have moved forward to a point where resulting ultimately in a normally graded layer. In proximal settings this their velocity is equal to the local ¯ow velocity, and the ¯ow will have overlies an erosion surface associated with the waxing phase, whereas in INFLUENCE OF LONGITUDINAL FLOW STRUCTURE ON TURBIDITES 709

FIG. 5.ÐDowndip pro®le of a deposit formed by a waxing-then-waning ¯ow (see also Mulder et al. in press). more distal settings it may directly overlie an inversely graded layer (Fig. absence of turbulence generation at the diffuse upper boundary of such a 5; Mulder et al. in press). Because the velocity peak ultimately works its layer, combined with internal turbulence suppression (Baas and Best 2002) way to the front of the current, any expression of the waxing phase likely may result in complete collapse of the turbulent suspension and growth of becomes less marked downstream, so that in the most downstream locations the ¯uid mud layer with thicknesses of centimeters to decimeters, and con- there is no inverse-graded layer at all (Figs. 1, 5). A proximal inversely centrations of clay perhaps reaching hundreds of grams per liter (Winter- graded deposit terminating in a surface of bypass or erosion (Fig. 4B) may werp 2001). thus be correlative of a normally graded deposit farther downstream that Turbulent kinetic energy in all ¯ows is lost by viscous dissipation, but records only waning ¯ow. in a turbid ¯ow it must also be converted to potential energy as sediment The relatively small thickness of inversely graded layers produced by is put into suspension. Because the amount of turbulent kinetic energy per waxing ¯ow (compared to the overlying steady or waning ¯ow deposit) unit volume of the ¯ow is ®nite, there must also be a limit to the amount and the more common development of traction structures during the waxing of sediment per unit volume that can be maintained in suspension in any stage, can be explained by the smaller particle deceleration and conse- given ¯ow; this limit represents the capacity of the ¯ow. Quantifying that quently smaller sediment fallout rate during the waxing phase (Kneller and limit is problematic because not only is the concentration of sediment de- McCaffrey 1995). pendent upon the turbulence, but the presence of sediment itself modi®es the turbulence (Gore and Crowe 1991; Sato et al. 1996). LONGITUDINAL DENSITY STRUCTURE OF CURRENTS Given these complexities, it is all but impossible with the present state of knowledge to evaluate ¯ow capacity quantitatively in situations of in- Many workers have recognized that turbidity currents are likely to have terest to sedimentologists, even given a detailed knowledge of the pro®les a discrete longitudinal structure (e.g., Kuenen and Menard 1952; Allen of turbulent kinetic energy. Nonetheless, it is clear that the potential limits 1991). Experimental and numerical modeling suggests that the highest con- to the mass concentration of suspended sediment are a function of the centrations of suspended sediment in surge-type turbidity currents occur turbulent kinetic energy, and may bear some relationship to the bulk Rey- towards the front (Altinakar et al. 1996; Felix 2002). Because suspended- nolds number of the ¯ow. load fallout rate is a function of ¯ow concentration (Lowe 1982, 1988), the highest deposition rates beneath turbidity currents are likely to occur towards the front, and therefore form the lowest part of the bed. This is Depositional Consequences where direct suspension sedimentation (and hence the deposition of struc- Where deposition from suspension occurs because of loss of competence tureless sands) is most likely to occur. of the current, the shear velocity of the current must fall below the sus- pension threshold of the coarsest grains present in suspension. However, The Capacity of Turbidity Currents where the sediment load exceeds the capacity of the current (most probably Hiscott (1994a) has argued that loss of capacity, not competence, is the because of deceleration and concomitant reduction in turbulence) no such fundamental process governing deposition from turbidity currents (see also condition need be met. In other words, regardless of shear velocity, a cur- Kuenen and Sengupta 1970). In the context of individual currents (includ- rent that is ``oversaturated'' with sediment may deposit grains whose set- ing turbidity currents) the concept of capacity implies a limit to the rate of tling velocity is less than their suspension threshold (i.e., grains that would suspended-sediment transport per unit cross-sectional area by a particular remain in suspension in a lower-density current). A current whose sus- ¯ow. Hiscott (1994b) argued convincingly that the massive intervals of pended load is initially within its capacity may come to exceed its capacity turbidite sandstone beds were deposited from ¯ows that had exceeded their (and deposit rapidly) in locations where it experiences a decrease in slope capacity to transport sediment in suspension. However, the controls upon or con®nement, regardless of its temporal velocity variation. It is likely capacity in turbidity currents and their depositional consequences remain that suspended-load fallout rates are high under these circumstances, and largely unexplored. that little sorting can occur; thus the deposit will be massive (re¯ecting Capacity represents the maximum sediment mass ¯ux per unit discharge. direct suspension sedimentation) and will essentially consist of a sample Where the bulk of the sediment load is in suspension, as is generally be- of the full range of grain-sizes present near the base of the current. We lieved to be the case in turbidity currents (Middleton 1993), it can be refer to this simply as the massive interval (Fig. 6). expressed as the fractional volume concentration of suspended particles. Where ¯ow nonuniformity is the cause of capacity-driven deposition, Suspension of non-cohesive particles is dependent largely on the upward sedimentation is likely to begin immediately upon arrival of the current. component of ¯uid turbulence and, in dense regions of the ¯ow near the The massive interval in this case forms the base of the bed, and is unlikely bed, on the transmission of energy by grain collisions. to be preceded by any erosion, and so the bed base is ¯at, without ¯utes For clay particles, whose settling velocities are very small, and which or scours (Fig. 6A). It is also possible that ¯ow unsteadiness could lead to experience signi®cant short-range electrostatic forces at high particle con- capacity-related deposition. A structureless but normally graded interval centrations, the concentration of suspended material can attain very high would be generated (Fig. 6B). In this case such deposition might not begin values (McCave and Jones 1988). Nonetheless, the amount of cohesive immediately upon arrival of the current, and capacity-driven deposition particles that can be sustained in turbulent suspension is also limited. Where may be preceded either by erosion (producing erosional structures on the rapid deposition of clay ¯ocs produces a ¯uid mud layer on the bed, the base of the bed) or by competence-driven deposition. The latter might be 710 B.C. KNELLER AND W.D. MCCAFFREY

FIG. 6.ÐDown-dip pro®les of the proximal region of deposits formed by currents with marked longitudinal concentration pro®les, showing schematic relationships between massive, reworked, and Bouma-like intervals. A) Case where deposition of massive interval is triggered by ¯ow non-uniformity (i.e., spatial gradient in ¯ow velocity). B) Case where deposition of massive interval is triggered by ¯ow unsteadiness (temporal change in ¯ow velocity). The latter case may include Lowe's (1982) S1 interval. Note that the lower part of the massive interval downstream correlates to early bypass (S1) upstream, whereas the upper part correlates to the reworked interval upstream, potentially creating a break in deposition in the downstream section. Note also that smaller-scale surges may superimpose internal erosion and grain-size ¯uctuations as illustrated by Lowe (1982) (see also Kneller et al. 1999). These sketches represent end-member cases; the case shown in Figure 5 may evolve downstream into the case shown in Figure 6B. Horizontal scales may vary. accompanied by the development of bedforms. Thus the presence of basal suspension the entire grain-size range both of its existing suspended load traction structures (such as the S1 interval of the high-density turbidity and the previously formed massive deposit. Deposition ceases as soon as current model proposed by Lowe 1982) and/or basal erosion should be the suspended-sediment concentration of the current falls below its capac- accompanied by normal grading in any overlying massive interval (Fig. ity, and the bypassing current progressively erodes the massive interval 6B; Lowe 1982). previously deposited by the same current. The massive interval is bounded at the top by an erosion surface (Fig. 7), or it may be removed altogether. Competence and Bypass Where there is a negative downstream velocity gradient (depletive ¯ow sensu Kneller and Branney 1995) this may pass downstream into Case B Following a period of capacity-induced deposition, a current may return (below). to a state where the suspended load is at or below capacity at a given Case B (Fig. 6) is where the current may be competent to maintain its location. This occurs if the concentration of suspended sediment within the suspended load but not to resuspend the coarsest component of the previ- ¯ow arriving at that point decreases as a consequence of a longitudinal ously formed massive interval. In this case, the current winnows the top gradient of concentration (decreasing hindwards) and/or increasing turbu- of the massive interval, resuspending the ®ner fraction and moving the lent kinetic energy. Under these circumstances three alternative hypothet- coarser grains in traction. Proximally, the basal massive interval may be ical situations may arise, here described in order of decreasing ¯ow removed completely (Fig. 8) or be bounded at the top by an erosion or strength. In practice these cases are thus likely to occur in a streamwise reactivation surface (Figs. 9A, 9B). This is overlain by an interval of trac- sequence (Fig. 6). Case A (Fig. 6) occurs where the current is competent to maintain in tionally reworked sediment of the massive interval, related to the bypass of the ¯ow. We term this the reworked interval. The boundary between the two intervals is abrupt, with a conspicuous lithofacies change from massive poorly sorted (capacity-related) to clean, well sorted, parallel-, wavy- or cross-strati®ed sand. The grain-size of the reworked interval (traction de- posit) is as coarse as the coarsest grains in the underlying massive poorly sorted interval, or it may be slightly coarser because of transport from a coarser-grained part of the massive interval upstream. With increasing dis- tance downstream, the reworked interval (generated by reworking of the basal deposit upstream) is likely to become thicker. The sea ¯oor at this stage would consist of a scoured surface succeeded downstream by a ®eld of bedforms. The reworking continues as long as the ¯ow remains more or less steady, or does not ¯uctuate beyond a lower threshold that would allow deposition and the upper threshold that would lead to erosion. Both the upstream erosional limit of the early-formed massive deposit and the bedform ®eld migrate downstream, and assuming a downstream decrease in bed shear stress, the downstream part of the reworked interval thickens with time. Case C (Fig. 6) is where the current is not competent to keep the coarsest sediment in suspension (i.e., its shear velocity is less than the settling ve- FIG. 7.ÐCase A. Sharp top to massive, poorly sorted coarse sandstone of massive locity of the largest grains). In this case sedimentation continues after the interval, separated by sharp grain-size break from succeeding parallel to ripple cross- laminated ®ne sandstone at the base of the normally distribution-graded Bouma-like suspended load has fallen within the capacity of the current, though perhaps interval; bypass with possible erosion followed deposition of massive interval (Gres with lower suspended-load fallout rates, and with a higher probability of d'Annot, Oligocene, SE France). traction and sorting. This deposit is produced by deposition (from suspen- INFLUENCE OF LONGITUDINAL FLOW STRUCTURE ON TURBIDITES 711

FIG. 8.ÐCase B. The massive interval is absent, presumably having been entirely reworked or never deposited; the reworked interval in this case consists of three 12 to 25 cm stacked sets of cross-strati®ed coarse to very coarse sandstone; this is overlain by normally graded Bouma-like interval of parallel-laminated ®ne sandstone to mudstone with a sharp grain- size break between the reworked and Bouma-like intervals. Notebook is 12 cm wide (Gres de Peira Cava, Oligocene, SE France). sion) of material carried in suspension within the ¯ow where it was induc- most likely moderately well sorted and with traction structures (for example ing traction upstream. The boundary between the two deposits (capacity- Bouma Tb through Te intervals). We refer to this as the Bouma-like inter- related massive below and competence-related reworked above) may be val. In the most proximal areas of Case A, the normally graded Bouma- diffuse (Fig. 10). As long as the current continues steadily, the correspond- like interval overlies a sharp boundary (an erosion surface) above deposits ing part of the reworked interval is ungraded. of earlier ¯ows. Farther downstream, where part of the early-formed (ca- pacity-related) massive interval survives, this surface is marked by signif- Final Stages of Flow icant textural changes (massive and poorly sorted below, structured and better sorted Bouma-like above), and possibly a marked grain size change In all cases the current must eventually wane, leading to progressive loss also (Fig. 7). In Case B, the Bouma-like interval overlies the reworked of competence, producing a normally distribution-graded top to the bed, interval with a sharp grain size break (Figs. 8, 9). In Case C, this normally

FIG. 9.ÐCase B. A) Turbidite sand bed showing massive, reworked and Bouma-like intervals; the massive interval is massive, poorly sorted coarse sand; the reworked interval is well sorted coarse sand with a single set of cross- strati®cation, the sharp base of which is interpreted as an erosional contact; the Bouma- like interval is a normally distribution-graded ®ne sandstone to mudstone with ripple cross- lamination in the lower part. B) Very coarse structureless, poorly sorted sandstone of massive interval capped by erosion surface; massive interval was largely reworked during bypass that created reworked interval, represented here by dunes of about 10 m wavelength (dune migration direction in reworked interval consistent with ¯utes on base of massive interval). (Gres de Peira Cava, Oligocene, SE France). 712 B.C. KNELLER AND W.D. MCCAFFREY

Occurrences in the Ancient Record We recognize, both from our own ®eldwork and from the literature, candidates for all the variants described here. While the nomenclature used above is genetic, and does not strictly describe lithofacies, the massive interval is generally structureless, ungraded, poorly sorted sandstone or pebbly sandstone. The reworked interval is generally clean, well-sorted, cross-strati®ed or less commonly parallel- or wavy-laminated sandstone. The Bouma-like interval is a moderately well-sorted normally graded unit with traction structures, often of Bouma type, commonly beginning with

the Tb or Tc interval. Massive beds with sharp upper boundaries have been described by many authors, and have been interpreted by some as the deposits of sandy debris ¯ows (e.g., Shanmugam et al. 1995). They are commonly overlain by a discrete normally graded (Bouma-type) unit with a sharp textural and grain- size break at the boundary, which we interpret as Case A. The cross-strati®ed reworked interval corresponds to Mutti's facies F6 (Mutti 1992), and with the ``reworked top variant'' of thick-bedded sands (Pickering et al. 1989), and is also described by Walker (1978) and Amy et al. (2000), amongst others. It generally overlies a structureless ungraded unit (commonly across a sharp boundary), and is generally sharply overlain by a normally graded Bouma-type upper unit. While it is possible in in- dividual cases to argue that the individual units are the deposits of unrelated events, the common occurrence of this vertical sequence strongly suggests FIG. 10.ÐCase C. Normally graded turbidite sand bed showing diffuse boundary between massive and reworked intervals (Gres de Peira Cava, Oligocene, SE a genetic link. France). DISCUSSION AND CONCLUSIONS graded top (Bouma-like) interval may overlie the earlier-formed deposits Consideration of the longitudinal velocity and density structure of tur- of the same ¯ow in a smooth upwards transition, and essentially form the bidity currents suggests that a wide variety of vertical sequences should be upper part of a classic sandy turbidite bed (Fig. 10). expected within turbidites. Temporal variations in velocity translate into In some cases more complex vertical sequences are observed within the spatial variations that are expressed in the vertical sequence. Deposits in Bouma-like interval. In particular, abrupt reductions in grain-size across a proximal locations are more likely to capture temporal ¯uctuations in ¯ow discrete surface are observed within an overall normally graded Bouma- velocity than more distal locations, where the ¯uctuations have evened out, like interval. Because this interval is deposited under a competence-con- and where the resulting bed structure is commonly simpler. This is true trolled regime, such grain-size breaks cannot be related to longitudinal gra- both for irregularly surging ¯ows and for ¯ood-generated ¯ows with an dients of concentration within the ¯ow. One possibility is that the discrete initial waxing phase. breaks in grain-size may represent the expression on the bed of discrete Gladstone and Sparks (2002) have recently interpreted certain grain-size breaks in the longitudinal gradient of grain-size within the ¯ow. That is to breaks in turbidites as consequences of the vertical density structure of the say, grains of the grain-size class that is ``missing'' in the deposit may parent turbidity currents. In our view, this can be the case only where simply have been absent within the ¯ow. Perhaps more likely is that such deposition of one or more layers of the deposit takes place en masse (com- grain-size breaks may represent the depositional effect of irregular longi- pare Postma et al. 1988) because during progressive deposition from tur- tudinal gradients in critical shear stress along the ¯ow. That is, the missing bidity currents of low or moderate density only the lowermost parts of the grain-size class in the deposit may have been present in the ¯ow, but have current directly affect the deposit. Thus the upper, low density, ®nes-rich bypassed the depositional site because the shear stress in the ¯ow may have part of the ¯ow can make no direct contribution to the deposit, since it is remained above the suspension threshold for grains of that size, perhaps separated from the bed by the higher-density suspension that constitutes because of a surge or phase of steady ¯ow. the lower part of the ¯ow. However, because the density structure of the The uppermost part of a turbidite (where not eroded) represents depo- current probably involves upcurrent-dipping surfaces of equal density and sition from the tail of the current and/or a residual cloud of ®ne suspended maximum suspended grain-size, the tail region of the current may be com- material, and commonly consists of current laminated silt and hydraulically positionally similar to the upper part of the current nearer to the head. We equivalent clay ¯ocs (Td of Bouma 1962; Stow and Bowen 1980) overlain thus concur with Gladstone and Sparks (2002) that the density structure of by homogeneous mud (Te). In some turbidites, however, these divisions the current is implicated in these grain-size breaks, but we differ in inter- are represented by a ``slurried'' interval consisting of streaky mud and silt, preting the vertical structure of the bed as an expression of the longitudinal commonly with disruption of the siltier laminae including small-scale iso- structure of the current rather than its vertical structure. clinal folds; such intervals frequently include a high proportion of ®nely Spatial variations in density can produce complex vertical sequences in disseminated ¯aky carbonaceous material. We suggest that these slurried which the Bouma-like, massive and reworked intervals may appear suc- tops derive from a ¯uid mud layer associated with collapse of the muddy cessively downstream, and may be separated by sharp interfaces. Their suspension in the tail of the current. The streaky textures probably result most downdip expression, however, may show continuous deposition and from shear within the mud layer during formation. The presence of organic approximate the ``classic'' turbidite sequence. Where deposition is trig- material is related to its buoyancy and hydraulic equivalence to silt and gered by ¯ow unsteadiness, the massive interval is normally graded and ¯occulated clay. Thus the upper interval, generated under laminar ¯ow, can may overlie an erosion surface. There may also be an intervening unit with be interpreted as a type of hydraulic segregation-related ``linked'' debrite, traction structures (Lowe's S1 interval; Lowe 1982; see also Amy et al. sensu Haughton et al. (2003). 2000) related to early bypass before waning ¯ow triggered capacity-driven INFLUENCE OF LONGITUDINAL FLOW STRUCTURE ON TURBIDITES 713 deposition. Erosion during bypass can truncate early-formed massive de- KNELLER, B.C., BENNETT, S.J., AND MCCAFFREY, W.D., 1999, Velocity structure, turbulence and ¯uid stresses in experimental gravity currents: Journal of Geophysical Research, v. 104, p. posits, and create dunes (reworked) sometimes in stacked sets up to a meter 5281±5291. or more in thickness, which locally may dominate the bed. KUENEN, P.H., 1966, Experimental turbidite lamination in a circular ¯ume: Journal of Geology, In summary, a consideration of the longitudinal variation within turbidity v. 74, p. 523±545. KUENEN, P.H., AND MENARD, H.W., JR., 1952, Turbidity currents, graded and non-graded de- currents, and its evolution through time, allows the prediction of a variety posits: Journal of Sedimentary Petrology, v. 22, p. 83±96. of vertical sequences of structures within the resulting beds. These vertical KUENEN, P.H., AND SENGUPTA, S., 1970, Experimental marine suspension currents, competency sequences occur in predictable downstream sequences, as shown in Figure and capacity: Geologie en Mijnbouw, v. 49, p. 89±118. LAMBERT, A., AND GIOVANOLI, F., 1988, Records of riverborne turbidity currents and indications 6, and in general become simpler downstream, revealing less of the com- of slope failures in the Rhone Delta of Lake Geneva: Limnology and Oceanography, v. 33, plexity of the parent current. These sequences suggest that sediment may p. 458±468. experience multiphase histories of transport and sedimentation by individ- LOWE, D.R., 1982, Sediment gravity ¯ows; II, Depositional models with special reference to ual currents. the deposits of high-density turbidity currents: Journal of Sedimentary Petrology, v. 52, p. 279±297. LOWE, D.R., 1988, Suspended-load fallout rate as an independent variable in the analysis of ACKNOWLEDGMENTS current structures: Sedimentology, v. 35, p. 765±776. LUETHI, S., 1981, Experiments on non-channelized turbidity currents and their deposits: Marine The ideas presented here were developed during a study funded by a consortium Geology, v. 40, p. M59±M68. of oil companies, including Amerada Hess, Amoco, Arco, British Gas, BHP, BP, MCCAFFREY, W.D., CHOUX, C.M., BAAS, J.H., AND HAUGHTON, P., in press, Spatio-temporal evolution of velocity structure, concentration and grain-size strati®cation within experimental Chevron, Conoco, Elf, Enterprise, Fina, Mobil, Shell, and Texaco. We thank Emi- particulate gravity currents: Marine and Petroleum Geology. liano Mutti for sharing his experience of rocks in the Hecho Group, and Juan Pablo MCCAVE, I.N., AND JONES, K.P.N., 1988, Deposition of ungraded muds from high-density non- Milana for astute and seminal observations in the ®eld. Constructive and thoughtful turbulent turbidity currents: Nature, v. 333, p. 250±252. reviews by Jeff Parsons, Tom Hickson, and Associate Editor Paul Myrow are grate- MIDDLETON, G.V., 1967, Experiments on density and turbidity currents; 3, Deposition of sed- fully acknowledged. iment: Canadian Journal of Earth Sciences, v. 4, p. 475±505. 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