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Abstract

How are sediments transported to, and deposited in, deepwater environments? This question has been the subject of discussion and debate since the 1870s and researched since the 1950s (see Shanmugam, 2000, for a comprehensive review). Collectively, the primary processes that transport sediment into deep water environments are called “sediment gravity flows” (Middleton and Hampton, 1973). Sediment gravity flows range from mass movements (rock falls) and cohesive debris flows at one end of the spectrum to fully turbulent flows at the other end of the spectrum (Fig. 4-1). Although the term “turbidity current” is often used to describe deep water transport processes, it is only one of several types of flows that transport sediment into deep water, and rework them once deposited.

Unfortunately, transport and depositional processes are only rarely observed or measured in deep marine and lacustrine environments. Thus, to understand sediment gravity flows, we must combine data sets from outcrops, subsurface well logs and cores, seismic reflection profiles, modern sea floor images and samples, laboratory flume experiments, and mathematical models. Fortunately, rapidly evolving imaging, measurement, and computing technologies— mainly driven by petroleum exploration and development—are providing new data and insights that are leading to improved understanding of the complexities of sediment gravity flows and their deposits.

This chapter summarizes our current state of knowledge by combining key historical concepts with more recent observations and analyses. The chapter is organized to discuss the initiation of sediment gravity flows, followed by the spectrum of processes

Definition of Sediment Gravity Flow

How are sediments transported to, and deposited in, deepwater environments? This question has been the subject of discussion and debate since the 1870s and researched since the 1950s (see Shanmugam, 2000, for a comprehensive review). Collectively, the primary processes that transport sediment into deep water environments are called “sediment gravity flows” (Middleton and Hampton, 1973). Sediment gravity flows range from mass movements (rock falls) and cohesive debris flows at one end of the spectrum to fully turbulent flows at the other end of the spectrum (Fig. 4-1). Although the term “turbidity current” is often used to describe deep water transport processes, it is only one of several types of flows that transport sediment into deep water, and rework them once deposited.

Figure 4-1.

Chart listing the different types of sediment gravity flows by their corresponding sediment support mechanism. The classification is mainly after Lowe (1982). The figure was provided by D. Pyles (2002).

Figure 4-1.

Chart listing the different types of sediment gravity flows by their corresponding sediment support mechanism. The classification is mainly after Lowe (1982). The figure was provided by D. Pyles (2002).

Unfortunately, transport and depositional processes are only rarely observed or measured in deep marine and lacustrine environments. Thus, to understand sediment gravity flows, we must combine data sets from outcrops, subsurface well logs and cores, seismic reflection profiles, modern sea floor images and samples, laboratory flume experiments, and mathematical models. Fortunately, rapidly evolving imaging, measurement, and computing technologies— mainly driven by petroleum exploration and development—are providing new data and insights that are leading to improved understanding of the complexities of sediment gravity flows and their deposits.

This chapter summarizes our current state of knowledge by combining key historical concepts with more recent observations and analyses. The chapter is organized to discuss the initiation of sediment gravity flows, followed by the spectrum of processes and resulting deposits, flow combinations and transformations, and temporal and spatial variations in processes. Processes of post-depositional reworking of sediment gravity flows on the sea floor are also discussed in a separate section. Finally, mention is made of allocyclic and autocycle processes, since vertical stacking patterns are important to interpreting larger scale evolution of sedimentary sequences and basin fill.

We want to stress that understanding of sediment gravity flow processes is continually evolving as results of new research are presented at technical meetings, research conferences and in publications. Because of this, the sections in this chapter summarize key points of deep-water processes and deposits and are not meant as exhaustive compilations of the existing literature.

Generation and Frequency of Occurrence of Sediment Gravity Flows

Upslope sediment gravity flows can be triggered by: (a) seismically-generated slides; (b) instability and slope failure resulting from rapid sedimentation, oversteepening and/or change in pore pressure; (c) hyperpycnal flow (underflow) produced when dense, high sediment concentration river effluent discharges into the sea, and (d) fine-grained underflows which trigger downslope sandy flows (Kneller and Buckee, 2000; Mulder et al., 2001a). Processes (a) and (b) are collectively termed “ignitive flows,” and (c) and (d) are termed “non-ignitive flows.” Sudden discharge of gas hydrates (clathrates) upward through slope sediment to the sea floor are also thought to be capable of initiating ignitive flows on submarine slopes.

The 1929 Grand Banks of Newfoundland, Canada earthquake generated an ignitive flow by slope failure (Piper et al., 1988; Cochonat and Piper, 1995; Mulder et al, 1997; Piper et al, 1999). Here, a single flow transported sediment for several hundreds of km into the deep Atlantic ocean floor. A second example of turbidity current generation by ignitive processes is the 1979 failure of part of the Nice, France airport (Piper and Savoye, 1993). The frequencies of occurrence of ignitive sediment gravity flows are not recorded, but in seismically-active areas, we may speculate that they could occur on the order of one flow every 10s to 100s of years.

At a smaller, and more recordable time scale are hyperpycnal flows, which are sustained flows that form at a river mouth during periods of high river discharge and move along the sea floor due to excess density relative to the ambient sea water (Mulder et al., 2003). Hyperpycnal flows can be generated at a frequency of years from rivers with extremely high suspended load (Mulder and Syvitski, 1995; Mulder et al., 2003). Eighty-four percent (84%) of the worlds’ rivers can produce hyperpycnal flows and can account for 53% of the world’s oceanic sediment load. During the time period between 1887 and 1937, 30 submarine cable breaks were recorded in the Congo Submarine Canyon (West Africa), apparently due to turbidity currents generated during times of high bed-load discharge from the Congo River (Heezen et al., 1964). On the Amazon Fan, age-dating of a 240 m thick, 20 ka year interval of thin-bedded and laminated, muddy sediment indicate an average occurrence of 1 flow event every 2 years (Pirmez et al., 2000). The Amazon Fan flows also are thought to have been triggered by flood discharges from the Amazon River system. In a core collected from the Var submarine canyon, located in the western Mediterranean Sea, 13–14 hyperpycnal flow deposits were recorded during the past 100 years and 9–10 were recorded during the past 50 years; this represents a frequency of occurrence of one hyperpycnal turbidity current every 5–7.5 years (Mulder et al., 2001a). An intense sediment gravity flow was recorded in the Zaire submarine valley in March 2001. This flow demonstrated that even during periods of relatively high sea level, sediment can be transported to deep water, in this case because the Zaire River is connected to the canyon and fan valley for a length of 760km (Khripounoff et al., 2003).

Sediment Support Mechanisms and Types of Sediment Gravity Flows

Once a sediment gravity flow is generated, a variety of processes may transport the sediment into the deep ocean, or into the deeper parts of lakes. Numerous studies suggest that the nature of a particular flow and its resultant deposit are a function of: (a) gravity, (b) velocity and fluid pressure within the flow in time and space (c) sediment support mechanism within the flow, (d) size frequency distribution, composition, and concentration of particles available for transport, (e) topography of the seabed over which the flow is transported and deposited, and (f) frictional forces between the seabed and the flow.

Lowe (1982) classified sedment gravity flow types according to flow behavior and sediment support mechanism. Sediment support mechanisms are illustrated in Figure 4-2. Creep and rockfall flow types, listed in Figure 4-1, were not included in Lowe’s (1982) original classification.

Figure 4-2.

Diagrams illustrating the sediment support mechanisms (after Lowe, 1982). With increasing concentration of sediment within a flow, the support mechanism changes from fluid turbulence to hindered settling, to dispersive pressure to matrix strength. The figure was provided by D. Pyles (2002).

Figure 4-2.

Diagrams illustrating the sediment support mechanisms (after Lowe, 1982). With increasing concentration of sediment within a flow, the support mechanism changes from fluid turbulence to hindered settling, to dispersive pressure to matrix strength. The figure was provided by D. Pyles (2002).

According to Lowe (1982), the support mechanism that prevails at any given instant during flow is principally a function of the flow velocity and concentration of sediment within the flow. Fluid turbulence (Newtonian flow) is the process of random motion of fluids within the flow, and dominates under conditions of relatively low sediment concentration. Turbidity currents are characteristic of fully-turbulent flow. Sediment concentration is low enough that upward-directed turbulence supports particles in suspension during transport.

With increasing sediment concentration, particles begin to settle from the flow, but upward flow of displaced water results in hindered settling of the particles (Fig. 4-2). With further increases in sediment concentration, dispersive pressure, caused by grain-to-grain collisions, becomes the predominant support mechanism (Fig. 4-2). At even higher sediment concentrations, plastic (Bingham) flows with a high matrix strength predominate (Fig. 4-2). The ratio of sediment to fluid is great enough to fully entrain the sediment as it flows in a plastic or laminar fashion.

Critical sediment concentrations, which distinguish plastic, fluidal or intermediate flows, are not presently clearly defined. Based on a compilation of published sediment concentration values of various flow types, Shanmugam (2000) suggests that the boundary between laminar and turbulent flows occurs at about 20–25% sediment volume within the flow. Recent experiments by Baas and Best (2001) have suggested that 3–4% clay content in a flow is the critical concentration distinguishing laminar from turbulent flows. Marr et al. (2001) place the critical clay concentration in experimental flows at 0.7–5 wt. % when the clay is bentonite and 7–25 wt. % when the clay is kaolinite. Numerical modeling has suggested a critical sediment concentration of 10% (Pratson et al., 2000).

Hyperpycnal flows differ from ignitive flows because the water within the flow is fresh rather than denser sea water. To overcome density contrasts and buoyancy effects, suspended sediment concentrations in excess of 36kg/m3 are required for the flow to sink to the sea floor and move downslope. Sedimentation rates on the order of 1–2 m/100yrs. can be generated by hyperpycnal flows (Mulder et al., 2003).

Sediment Gravity Flow Processes and Deposits

Historically, the most commonly cited classifications of sediment gravity flow deposits are those of Walker (1978) (Fig. 4-3), Lowe (1982) (Fig. 4-4), Mutti et al. (1999) (Fig. 4-5) and Kneller (1995) (Fig. 4-6). These classifications are discussed in subsequent sections of this chapter. Each author uses different terminology to describe the deposits of approximately equivalent flow types. An attempt at comparing the deposits of these flow types is shown in Figure 4-7, although comparisons are not possible for all categories by all authors. In the following sections, the deposits associated with the three main flow types—plastic flows, intermediate flows, and fluidal, turbulent flows—are discussed, along with more in-depth discussion of associated sediment gravity flow processes.

Figure 4-3.

Chart illustrating the sediment transport processes of sediment gravity flows and resulting deposits. Sediment support mechanisms (inset) are those illustrated in Figure 4-2; i.e., 1= fluid turbulence, 2 = hindered settling, 3 = dispersive pressure and 4 = matrix strength. The vertical axis depicts sediment concentration, and the horizontal axis represents time and/or space. According to this diagram, flows can follow a number of transport paths until sediments within them are deposited. (Modified from Walker, 1978). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-3.

Chart illustrating the sediment transport processes of sediment gravity flows and resulting deposits. Sediment support mechanisms (inset) are those illustrated in Figure 4-2; i.e., 1= fluid turbulence, 2 = hindered settling, 3 = dispersive pressure and 4 = matrix strength. The vertical axis depicts sediment concentration, and the horizontal axis represents time and/or space. According to this diagram, flows can follow a number of transport paths until sediments within them are deposited. (Modified from Walker, 1978). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-4.

Diagram illustrating the sediment gravity flow continuum (vertical column on left). The continuum figures reflecting various deposits of the continuum (1–13) move from bed 1 through bed 13. Lines without arrows (e.g., 1–2 and 1–3) connect members between which there probably exists a continuous spectrum of flow and deposit types, but which are not part of an evolutionary trend of single flows. Arrows connect members that may be parts of an evolutionary contnuum for individual flows. The transition from disorganized cohesive flows (1 and 3), to thick, inversely graded, density-modified grain flows and traction carpets (5) and to turbulent, gravelly, high-density turbidity currents (6) is speculative. (Modified from Lowe, 1982). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-4.

Diagram illustrating the sediment gravity flow continuum (vertical column on left). The continuum figures reflecting various deposits of the continuum (1–13) move from bed 1 through bed 13. Lines without arrows (e.g., 1–2 and 1–3) connect members between which there probably exists a continuous spectrum of flow and deposit types, but which are not part of an evolutionary trend of single flows. Arrows connect members that may be parts of an evolutionary contnuum for individual flows. The transition from disorganized cohesive flows (1 and 3), to thick, inversely graded, density-modified grain flows and traction carpets (5) and to turbulent, gravelly, high-density turbidity currents (6) is speculative. (Modified from Lowe, 1982). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-5.

Diagram illustrating the deposits of the sediment gravity flow continuum. According to this classification, flow processes change in the flow direction, so the type of sediment deposited also changes. Cohesive debris flows (CDF) are precursors to other flow types, such as gravelly high dencity turbidity currents (GHDTC), sandy high density turbidity currents (SHDTC) and low density turbidity currents (LDTC). Resulting deposits, such as debrites, massive sandstones (F7), and Bouma divisions are explained in the text. (Modified from Mutti et al., 1999). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-5.

Diagram illustrating the deposits of the sediment gravity flow continuum. According to this classification, flow processes change in the flow direction, so the type of sediment deposited also changes. Cohesive debris flows (CDF) are precursors to other flow types, such as gravelly high dencity turbidity currents (GHDTC), sandy high density turbidity currents (SHDTC) and low density turbidity currents (LDTC). Resulting deposits, such as debrites, massive sandstones (F7), and Bouma divisions are explained in the text. (Modified from Mutti et al., 1999). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-6.

Different types of turbidity current deposits formed under a variety of spatial (accumulative, uniform and depletive) and temporal (waning, steady and waxing) flow conditions. Bouma divisions are shown by different colors. The horizontal thicknesses of individual deposits are indicators of thicknesses of the beds in nature. Arrows point in the downcurrent direction. The nine flow types and their five deposits are explained in the text (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-6.

Different types of turbidity current deposits formed under a variety of spatial (accumulative, uniform and depletive) and temporal (waning, steady and waxing) flow conditions. Bouma divisions are shown by different colors. The horizontal thicknesses of individual deposits are indicators of thicknesses of the beds in nature. Arrows point in the downcurrent direction. The nine flow types and their five deposits are explained in the text (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-7.

Chart comparing the common classifications of sediment gravity flow deposits shown in Figures 4-34-6. Figures in which symbols and names of deposits are explained are provided beneath author’s names. The various deposits are described in the text.

Figure 4-7.

Chart comparing the common classifications of sediment gravity flow deposits shown in Figures 4-34-6. Figures in which symbols and names of deposits are explained are provided beneath author’s names. The various deposits are described in the text.

Debris Flows and Debrites

Because of their high matrix strength, debris flows move in a plastic, laminar, cohesive state. Their flow properties have been likened to that of wet concrete (Pratson et al., 2000). Debris flows can be large or small, and can move for long distances down a slope and into a basin.

Debris flows are composed of a shear flow region and a plug flow region (Fig. 4-8). The shear flow region is located in the lower part of the flow. Shear stresses in the bottom of the flow, generated by its movement, exceed the matrix shear strength and cause the flow to shear. The shearing decreases upward through the flow. The plug flow region is located in the upper part of the flow at the height at which the shear stress becomes less than the matrix shear strength (Fig. 4-8). At this height, shearing stops and the flow moves as a plug with uniform velocity. The uppermost portion of the flow may exhibit a decrease in velocity due to frictional effects with overlying ambient sea water.

Figure 4-8.

Schematic cross section illustrating the internal flow structure of a debris flow. Velocity of the shear flow region (Us) increases upward from the base of the bed. Within the plug flow region, velocity (Up) remains constant. The symbols hs and hp refer to thickness of each zone. Z is the vertical dimension or thickness of the flow. Note that the front of the flow is above the base of the bed. S is the angle of the bed relative to the horizontal plane. A wedge of water lubricates this basal zone, giving rise to the process of “hydroplaning,” explained in the text. (Modified from Pratson et al., 2000). Reprinted with permission of Society of Sedimentary Geology (SEPM) and American Association of Petroleum Geologists.

Figure 4-8.

Schematic cross section illustrating the internal flow structure of a debris flow. Velocity of the shear flow region (Us) increases upward from the base of the bed. Within the plug flow region, velocity (Up) remains constant. The symbols hs and hp refer to thickness of each zone. Z is the vertical dimension or thickness of the flow. Note that the front of the flow is above the base of the bed. S is the angle of the bed relative to the horizontal plane. A wedge of water lubricates this basal zone, giving rise to the process of “hydroplaning,” explained in the text. (Modified from Pratson et al., 2000). Reprinted with permission of Society of Sedimentary Geology (SEPM) and American Association of Petroleum Geologists.

Laboratory experiments by Mohrig et al. (1998) and Marr et al. (2001) have documented the process of “hydroplaning” wherein the basal layer of a debris flow is lubricated by a wedge-shaped layer of water forced beneath the flow during downslope movement (Fig. 4-8). This water layer has the effect of deflecting upward the debris from the bed. The effect of basal lubrication is that the head of a debris flow moves downslope at a higher velocity than the body, attenuating and even detaching the head from the body. Other features associated with laboratory-generated deposits of debris flows include structureless and ungraded grain size distribution of the deposit, tension cracks, water-escape structures, compression ridges and imbricate slices (Marr et al., 2001).

Whether or not debris flows have large erosive capabilities is debateable. Posamentier (2003) have documented from shallow-subsurface 3D seismic data grooves with 40–500m of vertical relief that are associated with mass transport complexes 25–30 km long, indicating that erosion by mass movement can occur on the sea floor. On the other hand, Mohrig et al.’s (1998) laboratory experiments, as well as numerical modeling (Pratson et al., 2000), indicate that debris flows are not capable of eroding the substrate over which they move. In these experiments, the volume of sediment within the flow does not increase appreciably between when the flow starts and when it ends. The dense, mud matrix of the flow also tends to inhibit loss of sediment across its upper surface.

If the constancy of sediment/water ratio throughout the length of a flow does occur in the marine environment, as it does during laboratory experiments, then it provides one of the most significant differences between debris flows and turbidity currents. However, the height of the flow is inversely proportional to its velocity. If the flow velocity decreases, the flow will thicken, and internal sediment concentration will decrease. If this velocity increases, the flow will thin, and the internal sediment concentration will increase. The flow is driven forward (downslope) by the weight of the flow and is retarded by friction acting on the seabed. Internal fluid pressure causes the flow to spread radially as it travels. The flow will continue downs-lope to a lower gradient on the sea floor where the flow spreads radially and fluid pressure is reduced below the frictional threshold for movement, so forward movement ceases.

Deposits of debris flows, often termed “debrites,” may be composed of mud, mixtures of mud and sand, or mixtures of mud, sand, and gravel arranged in a disorganized or random manner (Fig. 4-9). The high matrix strength is sufficient to hold gravel-size clasts within the flow and resulting deposit. Internally, debris flows may exhibit random orientation of clasts or there may be some imbricate orientation of larger clasts. Because of the high matrix strength, sedimentary structures resulting from fluidal movement are lacking. Slide and deformation structures may be present.

Figure 4-9.

Outcrop photograph of a poorly sorted debrite bed. Note the large boulders are supported within a dense, fine-grained matrix. California, U.S.A.

Figure 4-9.

Outcrop photograph of a poorly sorted debrite bed. Note the large boulders are supported within a dense, fine-grained matrix. California, U.S.A.

Intermediate Flows and Their Deposits

Lowe (1982) defined three types of intermediate flows on the basis of sediment support mechanism: grain flows, liquified flows and fluidized flows (Fig. 4-1). Grain flows (Fig. 4-4) are dispersions of particles maintained within a current solely by dispersive pressure arising from grain-to-grain collisions (Fig. 4-2). This process implies a relatively high concentration of sediment within the flow. Lowe (1982) claims that grain flows can exist only on slopes approaching the angle of repose of subaqueous sand (18–28 degrees). In deep water, grain flows would form as thin beds of avalanche foresets on dune slipfaces (Fig. 4-4).

In liquified and fluidized flows (Figs. 4-1 and 4-4), pore fluids are forced upward during sediment transport, as particles settle toward the base of the flow. Upward-rising fluid causes the particles to remain in suspension. Deposits from liquified and fluidized flows may exhibit fluid escape structures (e.g., dishes and vertical pipes) (Fig. 4-10). Such structures form when vertically-escaping water creates a cavity within the flow, causing internal, localized collapse. Sandstones that do not exhibit fluid escape structures are more likely to reflect grain-to-grain collisions (dispersive pressure) during initial deposition, without forceful escape of fluids.

Figure 4-10.

Photograph of dish structures in a sandstone bed. Stratigraphic top is toward the top of the photo. Formation of dish structures is discussed in the text. The formation which contains the dish structures is unknown. California, U.S.A.

Figure 4-10.

Photograph of dish structures in a sandstone bed. Stratigraphic top is toward the top of the photo. Formation of dish structures is discussed in the text. The formation which contains the dish structures is unknown. California, U.S.A.

Pratson et al. (2000) and Marr et al. (2001) claim that with > 10% sediment concentration in a flow, grains will be constantly in mutual contact. Grain flows, liquified flows, and fluidized flows probably fall within this category (>10% concentration), but are not considered true debris flows in the sense of Lowe (1982).

Subsequent to Lowe’s (1982) classification of flow types, Lowe and Guy (2000) recognized an unusual deposit that they term a “slurry bed,” interpreted to form from a watery flow transitional between a turbidity current and a debris flow. Such beds contain 10–35% detrital mud matrix, are enriched in water-escape structures, and are grain-supported. Lowe and Guy (2000) interpret the slurry beds to have originated as low density turbidity currents containing an abundance of flocculated or ripped-up, sand-size, cohesive mud particles that behave in a hydrodynamically similar manner to more rigid silt- and sand-size quartz and feldspar grains. As the flow wanes, the mud particles settle toward the base of the flow. There, the grains abrade against more rigid quartz and/or feldspar grains and disaggregate into their component silt- and clay-size particles, giving rise to a mud-rich basal flow. The increased mud content near the base of the flow increases viscosity and cohesion and suppresses near-bed turbulence, thus creating a quasi-laminar basal flow, and the resulting slurry bed. Fluid escape structures are common in slurry beds, owing to the upward flow of water as the density and viscosity in the basal layer increase during particle breakup. Slurry beds have been recognized by Lowe and Guy (2000) in the Lower Cretaceous Britannia Formation of the North Sea, the Pennsylva-nian Jackfork Group of Arkansas, and the Cretaceous Great Valley Group of California.

Turbidity Currents and Turbidites

Processes

A turbidity current is a sediment gravity flow with fluidal (i.e., Newtonian) rheology in which sediment is held in suspension by fluid turbulence (Figs. 4-11, 4-12). A turbidity current contains a head, body, and tail. The head may erode the sea floor and entrain sediment back into the body (Kneller and Buckee, 2000). Owing to viscosity differences between the turbidity current and overlying ambient sea water, a series of billows, called “Kelvin-Helmholtz” instabilities (Allen, 1985), form at the frictional interface of the fluids (Figs. 4-11, 4-12).

Figure 4-11.

Schematic cross section of a high-density turbidity current. Arrows point to the flow directions within the current. Vertical flow velocity profiles are shown in shaded black. The flow is size-graded, with coarsest grains at the base of the bed. The head of this flow is thicker than the body. The billows at the top of the flow are termed “Kelvin-Helmolz Billows.” (Modified from Postma et al., 1988). Reprinted with permission of Elsevier Publishing Co.

Figure 4-11.

Schematic cross section of a high-density turbidity current. Arrows point to the flow directions within the current. Vertical flow velocity profiles are shown in shaded black. The flow is size-graded, with coarsest grains at the base of the bed. The head of this flow is thicker than the body. The billows at the top of the flow are termed “Kelvin-Helmolz Billows.” (Modified from Postma et al., 1988). Reprinted with permission of Elsevier Publishing Co.

Figure 4-12.

Photograph of a cross section of a turbidity current generated in a flume tank. The head and body of the flow are clearly shown. The sediment box from which the sediment was dropped through a removable bottom is in the upper right of the picture. Note that the head is thicker then the body, even after only a short distance of transport. Photograph is from the flume tank of the St. Anthony Falls Laboratory, University of Minnesota.

Figure 4-12.

Photograph of a cross section of a turbidity current generated in a flume tank. The head and body of the flow are clearly shown. The sediment box from which the sediment was dropped through a removable bottom is in the upper right of the picture. Note that the head is thicker then the body, even after only a short distance of transport. Photograph is from the flume tank of the St. Anthony Falls Laboratory, University of Minnesota.

Laboratory experiments have indicated that once a turbidity current is initiated, its internal sediment distribution and velocity structure are quite complex (Kneller and Buckee, 2000). A vertical velocity profile through an experimental turbidity current is shown in Figure 4-13, along with a variety of experimental sediment concentration profiles. The velocity of the flow is low near the base owing to frictional forces with the sea bed. Flow velocity reaches a maximum at some distance above the bed where the flow is fully turbulent, then decreases upward. Sediment concentration profiles vary according to the absolute sediment concentration and the near-bed processes of erosion and/or transport. Experiments measuring vertical grain size distributions show that fine-grained particles are distributed uniformly vertically through the flow, whereas coarser-sized grains diminish in abundance upward from the base of the flow (Garcia, 1994) (Fig. 4-13). Thus, within a flow containing a range of coarse- to fine-grained particles, the base of the flow may be more poorly sorted than the overlying parts. Conversely, well-developed slurry flows should exhibit greater mud concentrations near their base (Lowe and Guy, 2000).

Figure 4-13.

Four graphs illustrating velocity (solid lines) and associated sediment concentration (dashed lines) profiles for a variety of experimental turbidity currents: (a) two layer concentration model with a constant concentration lower interval and an upper region of sediment detrained from the head of the flow; (b) a smooth concentration profile, characteristic of low concentration, weakly depositional flows; (c) a stepped concentration profile observed in erosional flows; (d) a distribution observed in turbidity currents in which coarse material is concentrated towards the lower part of the flow and the fine-grained material is evenly distributed thoughout the flow (Modified from Kneller and Buckee, 2000). Reprinted with permission of International Association of Sedimentologists.

Figure 4-13.

Four graphs illustrating velocity (solid lines) and associated sediment concentration (dashed lines) profiles for a variety of experimental turbidity currents: (a) two layer concentration model with a constant concentration lower interval and an upper region of sediment detrained from the head of the flow; (b) a smooth concentration profile, characteristic of low concentration, weakly depositional flows; (c) a stepped concentration profile observed in erosional flows; (d) a distribution observed in turbidity currents in which coarse material is concentrated towards the lower part of the flow and the fine-grained material is evenly distributed thoughout the flow (Modified from Kneller and Buckee, 2000). Reprinted with permission of International Association of Sedimentologists.

Numerical modeling by Pratson et al. (2000) suggests that turbidity currents move downslope by the combined influence of the weight of the current (controlled by sediment concentration) and internal fluid pressures that are counterbalanced by frictional forces between the seabed and the current. The body may have velocities that are greater than those of the head, so that with distance, the head may expand in thickness (Fig. 4-12) as the body attempts to outrun the head (Talling et al., 2001).

Turbidity currents are capable of eroding the substrate. Thus, sediment can be continually entrained into the head of the flow, even as sediment is deposited from within the body and tail. Also, Kelvin-Helmoltz billows may transport sediment from the head of the flow back into the body. Ambient water can also be entrained within the flow, acting to dilute the sediment concentration while, at the same time, increasing the overall thickness of the flow.

Thus, the actual sediment concentration can decrease and increase in a non-systematic fashion along the length of a flow, as well as vertically at any one position within the flow. If the amount of new sediment entrained by erosion is less than the amount lost through deposition, then the turbidity current eventually ceases. But, if the amount of new sediment entrained is greater than the amount lost through deposition, the flow gains momentum, and further erodes as it moves downslope. However, if sediment concentration becomes too high, turbulent flow can be suppressed and a different sediment support mechanism becomes operative (Fig. 4-2).

Even with this complex flow behavior, or perhaps because of it, a turbidity current can travel for long distances at high velocities. Single-event turbidity currents have been documented, which have transported sediment several hundred kilometers from their source. The best documented of these, the 1929 Grand Banks of Newfoundland turbidity current, is calculated to have traveled at the following velocities: approximately 20 m/sec at a distance of 300 km from the earthquake epicenter, 14 m/sec at a distance of 500 km from the epicenter, and 11 m/sec at a distance of 600 km from the epicenter. This single flow traveled over 600km in 13 hours (Uchupi and Austin, 1979) at a velocity sufficient enough to transport particles up to 3 cm in diameter in suspension. In a different flow, inferred flow velocities of the Nice airport turbidity current reached 30 m/sec., forming deep submarine scours and transport of cobbles and boulders as bed load, and coarse sand as suspended load (Piper and Savoye, 1993). On the Amazon Fan, individual flows are estimated over periods of several hours to days, at velocities of 1–3 m/sec (Pirmez et al., 2000). The Zaire submarine valley flow was documented at an average velocity (integrated over 1 hour) of 1.2m/sec (with higher instantaneous velocities) 150m above the sea floor, and coarse sand and plant debris were collected 40m above the floor (Khripounoff et al., 2003).

The areal extent and volume of single-event turbidity current deposits can also be quite large. For example, the Holocene Black Shell turbidite on the Hatteras abyssal plain covers an area of approximately 25,000 sq. km. and comprises a minimum volume of 100 cu. km (Elmore et al., 1979).

The Bouma Sequence

The Bouma Sequence (Bouma, 1962) has long been considered to be the fundamental sand/mud deposit from a turbidity current (Fig. 4-14). Bouma (1962) defined this sequence as the product of continuous deposition from a turbidity current. A complete Bouma sequence consists of a grain-size fining-upward succession of (a) massive or normally size-graded, sandy Bouma Ta division; (b) parallel laminated, sandy Bouma Tb division; (c) ripple/climbing-ripple laminated/convoluted, sandy Bouma Tc division; (d) parallel laminated to massive, silty Bouma Td division; and (e) silt-clay, often microfaunal-rich Bouma Te division (Fig. 4-14). Some outcrop examples of Bouma divisions are provided in Figures 4-15, 4-16 and 4-17.

Figure 4-14.

Sediment grain size, structures, divisions of a complete Bouma sequence, and sediment transport mechanisms. The different divisions— from Bouma Ta to Bouma Te—are explained in the text. (Modified from Jordan et al., 1993). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-14.

Sediment grain size, structures, divisions of a complete Bouma sequence, and sediment transport mechanisms. The different divisions— from Bouma Ta to Bouma Te—are explained in the text. (Modified from Jordan et al., 1993). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-15.

Photograph of beds illustrating Bouma Ta, Tb, and Tc divisons in outcrop. The upward gradational decrease in grain size is noted by a “smoothing” of the outcrop surface. Location of the outcrop is unknown.

Figure 4-15.

Photograph of beds illustrating Bouma Ta, Tb, and Tc divisons in outcrop. The upward gradational decrease in grain size is noted by a “smoothing” of the outcrop surface. Location of the outcrop is unknown.

Figure 4-16.

Photograph of beds illustrating Bouma Ta-Te divisions in outcrop. Note the normal size grading as evidenced by the decrease in size and abundance of coarse (white) particles upward through the Ta division. Name of the formation which contains this rock is unknown. Newfoundland, Canada.

Figure 4-16.

Photograph of beds illustrating Bouma Ta-Te divisions in outcrop. Note the normal size grading as evidenced by the decrease in size and abundance of coarse (white) particles upward through the Ta division. Name of the formation which contains this rock is unknown. Newfoundland, Canada.

Figure 4-17.

Photograph of Bouma Tb-Tc division in outcrop. The upward change from parallel laminated to ripple (including climbing-ripple) bedding is a result of decrease in traction current flow from upper to lower flow regime velocities. Upper Miocene Mt. Messenger Formation, New Zealand.

Figure 4-17.

Photograph of Bouma Tb-Tc division in outcrop. The upward change from parallel laminated to ripple (including climbing-ripple) bedding is a result of decrease in traction current flow from upper to lower flow regime velocities. Upper Miocene Mt. Messenger Formation, New Zealand.

The upward decrease in grain size and change in sedimentary structures are a result of gradually waning flow velocities, ultimately leading to deposition of progressively finer-grained sediment under progressively lower flow regime conditions. The Bouma Ta division is thought to be deposited rapidly, directly from suspension. The Bouma Tb and Tc divisions are the product of traction of grains along the sea bed under upper (Tb) and lower (Tc) flow regime conditions. The Bouma Td division is deposited by suspension from the tail of a turbidity current. The Bouma Te division probably is a mixture of fine-grained sediment from both the tail of the current and slow settling of pelagic grains. Mixtures of shallow- and deep-marine microfauna in the Te division are indicative of a turbidity current process, as is size-grading of silt- and clay-sized particles. Turbidite mudstones and shales also exhibit a characteristic suite of waning-flow sedimentary structures and textures (Fig. 4-18).

Figure 4-18.

Succession of sedimentary textures and structures in finegrained turbidites Grain size and sedimentary structures change systematically upward much as they do in sandy turbidity current deposits. In this diagram, the Bouma Tc and Td divisions are the same as described in Figure 4-14. However, the Bouma Td division is divided into T1 and T2 subdivisions, and the Bouma Te division is divided into six subdivisions (T3-T8) based upon variations in grain size and small-scale sedimentary structures. (Modified from Stow and Shanmugam, 1980). Reprinted with permission of Elsevier Publishing Co.

Figure 4-18.

Succession of sedimentary textures and structures in finegrained turbidites Grain size and sedimentary structures change systematically upward much as they do in sandy turbidity current deposits. In this diagram, the Bouma Tc and Td divisions are the same as described in Figure 4-14. However, the Bouma Td division is divided into T1 and T2 subdivisions, and the Bouma Te division is divided into six subdivisions (T3-T8) based upon variations in grain size and small-scale sedimentary structures. (Modified from Stow and Shanmugam, 1980). Reprinted with permission of Elsevier Publishing Co.

Laboratory experiments in which relatively dilute concentrations of sand were mixed with varying amounts of clay and water simulated development of some of the common features of Bouma divisions and provided insight into when these features form (Marr et al., 2001). For example, grain-size grading was found to occur only during flow deceleration, rather than during downslope movement of the flow (Marr et al., 2001). Water escape structures also formed soon after the flow came to rest, and continued for several minutes afterward. Hydroplaning was not observed to occur in the dilute flows.

Turbidity Current Classification Based Upon States of Flow

The implication of the Bouma Sequence is that turbidity currents will diminish in velocity over time and in the downcurrent direction, giving rise to deposition of progressively finer-grained Bouma divisions, both vertically and laterally. Although this must be true where Bouma divisions are present in their normal strataigraphic position (Fig. 4-14), Kneller and Branney (1995) have challenged the general assumption of progressively waning flow on the grounds that there is no real reason to believe that all turbidity currents in the deep ocean behave in this manner. Kneller (1995) proposed that flows may wax, wane, or remain constant both over time and distance (Fig. 4-19). Over time, at any one place on the sea floor, a flow can wax (increase in velocity), wane (decrease in velocity), or remain steady (constant velocity). Over distance along the sea floor, a flow can become accumulative (increase in velocity), depletive (decrease in velocity), or remain uniform. Based upon these temporal and spatial variations in flow velocity, Kneller (1995) classified flow types into nine possible combinations (Fig. 4-6). Sediment concentration does not directly enter into this classification scheme. Thus, different Bouma divisions can occur both vertically within a stratigraphic sequence or spatially along a deposi-tional profile depending upon the temporal and spatial variations in flow velocity.

Figure 4-19.

Graphs of time (t) vs. velocity (u) and distance (x) vs. velocity (u), showing different types of flow under a variety of conditions. In the upper diagram, flow velocity first waxes, then wanes, then becomes steady over time. In the lower diagram, flow velocity first increases (accumulative), then decreases (depletive), then becomes uniform with downcurrent distance. (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-19.

Graphs of time (t) vs. velocity (u) and distance (x) vs. velocity (u), showing different types of flow under a variety of conditions. In the upper diagram, flow velocity first waxes, then wanes, then becomes steady over time. In the lower diagram, flow velocity first increases (accumulative), then decreases (depletive), then becomes uniform with downcurrent distance. (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

The topographic relief and gradient of the seafloor are thought to play major roles in modifying the velocity of a flow as it travels down the depositional profile. Accumulative flows (Figs. 4-6, 4-19, 4-20) might form with a downcurrent increase in slope gradient or downcurrent convergence a of flow through a restriction on the sea floor. Uniform flows (Figs. 4- 6, 4-19, 4-20) might form over a floor with a progressively slight decrease in down-current gradient. Depletive flows (Figs. 4-6, 4-19, 4-20) might form with a downcurrent decrease in slope gradient or downcurrent divergence of flow as the flow moves beyond a constriction on the sea floor and becomes unconfined.

Figure 4-20.

Diagram illustrating some causes of spatially depletive (downcurrent decrease in flow velocity) and spatially accumulative (downcurrent increase in flow velocity) sediment gravity flows. The upper diagram, (a) is a plan view of depletive flow resulting from divergent flow on the sea floor as the flow becomes unconfined, and (b) is a cross section view showing downcurrent decrease in flow velocity due to reduction in slope gradient. In the lower diagram, (c) is a plan view illustrating accumulative flow resulting from convergence of flow on the sea floor , and (d) illustrates accumulative flow resulting from a downcurrent increase in the slope gradient of the sea floor. (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-20.

Diagram illustrating some causes of spatially depletive (downcurrent decrease in flow velocity) and spatially accumulative (downcurrent increase in flow velocity) sediment gravity flows. The upper diagram, (a) is a plan view of depletive flow resulting from divergent flow on the sea floor as the flow becomes unconfined, and (b) is a cross section view showing downcurrent decrease in flow velocity due to reduction in slope gradient. In the lower diagram, (c) is a plan view illustrating accumulative flow resulting from convergence of flow on the sea floor , and (d) illustrates accumulative flow resulting from a downcurrent increase in the slope gradient of the sea floor. (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Also, when a sediment gravity flow meets a seafloor obstacle, the flow can either partially or completely override the obstacle, be deflected around the side of the obstacle, be confined or ponded by the obstacle, or reverse flow direction and flow back down the obstacle (Fig. 4-21) (Kneller and Buckee, 2000). Which of these processes dominates is a function of the velocity and density of the current, the flow stratification within the current, and the dimensions of the obstacle.

Figure 4-21.

Schematic diagram of possible flow paths (arrows) for sediment gravity flows when they encounter a sea floor obstacle. Flows can partially or completely override the obstacle, can divert around the obstacle or flow partway up the obstacle then reverse flow. (Modified from Kneller and McCaffrey, 1999). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-21.

Schematic diagram of possible flow paths (arrows) for sediment gravity flows when they encounter a sea floor obstacle. Flows can partially or completely override the obstacle, can divert around the obstacle or flow partway up the obstacle then reverse flow. (Modified from Kneller and McCaffrey, 1999). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Low-and High-Density Turbidity Current Deposits

Lowe’s (1982) classification of “low-” and “high-” density turbidity currents, differentiated on the basis of concentration of sediments in the flow, is also widely cited (Fig. 4-4). He applies the designation S to sediment deposited from high-density turbidity currents (Fig. 4-22).

Figure 4-22.

Vertical profile of sediment grain size and sedimentary structures illustrating high-to low-density turbidity current deposits using the terminology of Lowe (1982). S and R designations, and processes, are explained in the text and in Figure 4-4. Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-22.

Vertical profile of sediment grain size and sedimentary structures illustrating high-to low-density turbidity current deposits using the terminology of Lowe (1982). S and R designations, and processes, are explained in the text and in Figure 4-4. Reprinted with permission of Society of Sedimentary Geology (SEPM).

Lowe (1982) further subdivided S sediments (Fig. 4-22). His S1 division is deposited from traction currents, thus exhibiting traction structures, mainly planar laminations and cross stratification. The S2 division contains stacked, thin, inversely-graded beds deposited from bed load. Grain collisions predominate during rapid sedimentation, thus suppressing turbulence. The S3 division is deposited during high sediment fallout rates. Deposits are massive to normally graded, and may exhibit fluid escape structures.

Low-density turbidity currents contain dilute concentrations of clay, silt, and fine- to medium-grained, sand-size particles (Lowe, 1982) (Fig. 4-4). Low-density turbidity currents have been defined as containing 1–23% sediment by volume and high-density turbidity currents have been defined as containing 6–44% sediment by volume (Shanmugam, 1996). The overlap in concentrations according to these definitions points to the present lack of clearly-defined criteria to define these types of flows on the basis of sediment concentration.

Massive (Structureless) Sandstones

Because the origin of massive (structureless) sandstones has generated considerable debate in the literature, they are discussed separately in this chapter. Massive sandstones are the most common type of sediment gravity flow sandstone observed in outcrops and cores (Fig. 4-23) (Kneller,1995). Both Walker (1978) and Lowe (1982) differentiated massive sandstones from graded Bouma Ta sandstones. Walker (1978) differentiated massive sandstone from the Bouma Ta division (Fig. 4-3) by (a) the common presence of fluid escape structures, (b) fewer associated shale interbeds, (c) an increase in erosionally-based and irregularly-bedded sandstone, (d) coarser grain size relative to associated sandstones, and (e) sandstone beds that are thicker than associated beds. Lowe (1982) referred to massive (S3) sandstones as fluidized or liquefied flow deposits, depending upon the presence or absence of fluid escape structures (Fig. 4-4).

Figure 4-23.

Core of unconsolidated sand (light color) and lithified shale (darker color) from Long Beach Unit, Wilmington oil field, California (Slatt et al., 1993). Individual sand beds are massive, structureless, and of uniform grain-size from base to top. Scale is in 0.1 ft. increments. Reprinted with permission of the Society of Sedimentary Geology (SEPM)

Figure 4-23.

Core of unconsolidated sand (light color) and lithified shale (darker color) from Long Beach Unit, Wilmington oil field, California (Slatt et al., 1993). Individual sand beds are massive, structureless, and of uniform grain-size from base to top. Scale is in 0.1 ft. increments. Reprinted with permission of the Society of Sedimentary Geology (SEPM)

Shanmugam (1996, 1997, 2000) has argued that massive sandstones are not turbidity current deposits, but are the product of deposition from plastic or laminar flows. He uses the term “sandy debrite” for a massive sandstone, and claims that a mud matrix as low as 1% is sufficient to provide cohesive strength to a flow. As mentioned previously, experimental work by Marr et al. (2001) support this interpretation if the clay matrix is composed of bentonite and the water content is 25–40%. More clay is required if the mineral is not bentonite. Marr et al (2001) stress that their results apply only to laboratory-scale, and not field-scale flows.

Shanmugam’s argument is at least partially based on semantics. He claims that a Bouma Ta bed must be size-graded to qualify as a turbidity current deposit, even though Bouma’s (1962) definition alludes to the fact that some Ta beds are size-graded and others are not. But, if there is not a range of particle sizes in the original flow—such as might occur from a pre-sorted sand— then size-graded beds cannot be deposited from the turbulent flow. Shanmugam (2000) further claims that the presence of shale clasts within a sandstone argues for cohesive, rather than turbulent flow, yet he claims that debris flows do not have the capability to erode the sea floor. Because many shale clasts found within deep water sequences appear similar to underlying shale beds, erosion of the muddy substrate by the flow must have occurred to generate the clasts.

Kneller’s (1995) classification shows that massive sand of uniform grain size will be deposited under “steady state” flow (over time) in combination with “depletive flow” (over distance) (Fig. 4-6). Even though grain size and bed thickness decrease in the downcurrent direction, the deposit remains massive and of uniform grain size at any one depositional site on the sea floor.

It is reasonable to expect relatively steady flow of a turbidity current over time on the sea floor. For example, steady flow has been suggested for a period of at least 2 hours for the Grand Banks turbidity current (Piper et al. 1988). This should be sufficient time to deposit a massive sand on the sea floor. The common occurrence of ungraded beds in the rock record suggests steady state flow is common.

Hyperpycnal Flows and Hyperpycnites

Hyperpycnal flows have been a topic of considerable discussion and interest at conferences during the past few years. According to Mulder et al., (2003), the importance of hyperpycnal floods as a sediment transport process in deep water has been underestimated for many years. The following discussion summarizes current knowledge about hyperpycnal flows and their deposits.

Mulder et al. (2003) differentiate turbidity currents that are generated by ignitive transformation of a submarine slide into a turbulent flow and those that are generated by non-ignitive conditions, such as from continuity of hyperpycnal discharge of a river during flood stage (Fig. 4-24) Because the fluid in such flows is fresh water, the density contrast between fresh water and sea water is such that a very high suspended sediment concentration is required for the flow to sink or plunge to the sea floor (Fig. 4-24). Mulder et al. (2003) place the critical sediment concentration at 36–43kg sediment/m3 fluid. Variations within this range are due to the variations in temperature and salinity of sea water at the river mouth.

Figure 4-24.

Schematic cross section of main types of flows debouching from a river mouth. Hyperpycnal flows are denser than ambient water, and flow along the sea bed. Hypopycnal flows are less dense than ambient water, so ride on the sea water surface. Interflows are at some intermediate density. and flow within the ambient water. Lofting occurs when the density of a hyperpycnal flow decreases due to loss of sediment within the flow by deposition, and the flow rises into the ambient sea water. (Modified from Mutti et al., 2003). Reprinted with permission of Elsevier Publishing Co.

Figure 4-24.

Schematic cross section of main types of flows debouching from a river mouth. Hyperpycnal flows are denser than ambient water, and flow along the sea bed. Hypopycnal flows are less dense than ambient water, so ride on the sea water surface. Interflows are at some intermediate density. and flow within the ambient water. Lofting occurs when the density of a hyperpycnal flow decreases due to loss of sediment within the flow by deposition, and the flow rises into the ambient sea water. (Modified from Mutti et al., 2003). Reprinted with permission of Elsevier Publishing Co.

Sediment concentrations measured from two sediment traps on the Zaire submarine fan valley (one in the channel and one on the levee 18km away), were 3.28kg sediment/m2 of trap area/day above the channel and a peak of 11kg sediment/m2/day at the levee site (Khripounoff et al., 2003).. Sediment in the channel trap consisted of silt and large plant remains with finer grained siliciclastic particles and 464 mg organic carbon/m2/day found in the levee sediment trap.

Hyperpycnal flows can be relatively long lived. During a major flood that lasted for three days in 1994, the Var River in France generated a hyperpycnal turbidity current that lasted for 18 hours (Mulder et al., 2003). The Var River transported 11–14 times the river’s normal particle load, which ultimately was deposited in the deep ocean. The total duration of the sediment gravity flow in the Zaire submare valley is estimated to have been 10 days, with a 3 day delay between the channel and the levee station 18km away. Flow thickness exceeded the 150m depth of the channel floor beneath its levee crest, allowing sediment to overflow (Khripounoff et al., 2003).

After a hyperpycnal flow debouches from the river mouth into marine water, it plunges to the sea floor and moves downslope. If sediment is lost from the flow by deposition, the density of the remaining flow can decrease to the extent that the flow detaches from the sea floor and rides within the ambient water column at an appropriate depth (Fig. 4-24). This fact, due to the initial fresh water nature of the internal fluid, provides a fundamental difference between normal marine turbidity currents and hyperpycnal turbidity currents. Mulder et al. (2003) state that hyperpycnal flows with relatively low sediment concentrations and density are the low-density turbidity currents of Lowe (1982). In contrast, turbidity currents generated by ignitive processes, in which the internal fluid is sea water, are the high-density turbidity currents of Lowe (1982).

The typical hyperpycnite deposit consists, from the base upward, of a coarsening-upward successsion overlain by a fining-upward succession of beds (Fig. 4-25). The lower succession is deposited during rising flood stage when the flow is waxing and river discharge is high at the river mouth (Figs. 4-6, 4-19, 4-25). After the period of peak discharge, the flow wanes (Figs. 4-6, 4-19, 4-25) and the upper succession is deposited. The stratigraphic continuity of a single hyperpycnite bed can be disrupted by an erosional or bypass surface if the waxing-stage flow reaches a threshold velocity capable of eroding the underlying rising-stage deposit (Fig. 4-25). Grain size of a typical hyperpycnite increases vertically from silt to fine sand, and then back to silt. Climbing ripples are a common sedimentary structure. Land-derived organic material also is present in hyperpycnites. A hyperpycnite bed is shown in Figure 4-26.

Figure 4-25.

Graph illustrating flows with different discharge through time, and the representative lithofacies. 1. Low magnitude flood generates a normally-graded bed. 2. Low magnitude flood which generates a complete inverse-to-normally graded hyperpycnite due to waxing, then waning flood stages. 3. Mid-magnitude flood generates a complete sequence which is coarser grained than the example in 2. 4. High-magnitude flood generates an inversely-graded bed from waxing flow, followed by an erosion or bypass surface due to peak flood flow velocity sufficient to erode the substrate, followed by the waning flow, normally-graded bed. (Modified from Mulder et al., 2001b). Reprinted with permission of Springer-Verlag Publishing Co.

Figure 4-25.

Graph illustrating flows with different discharge through time, and the representative lithofacies. 1. Low magnitude flood generates a normally-graded bed. 2. Low magnitude flood which generates a complete inverse-to-normally graded hyperpycnite due to waxing, then waning flood stages. 3. Mid-magnitude flood generates a complete sequence which is coarser grained than the example in 2. 4. High-magnitude flood generates an inversely-graded bed from waxing flow, followed by an erosion or bypass surface due to peak flood flow velocity sufficient to erode the substrate, followed by the waning flow, normally-graded bed. (Modified from Mulder et al., 2001b). Reprinted with permission of Springer-Verlag Publishing Co.

Figure 4-26.

Ouctrop photo of a hyperpycnite of the type shown in 4 of Figure 4-25. An erosional or bypass surface caps an inversely-graded bed, and is overlain by a normally-graded bed. Marnoso-arena-cea Formation, Apennine Mountains, Italy.

Figure 4-26.

Ouctrop photo of a hyperpycnite of the type shown in 4 of Figure 4-25. An erosional or bypass surface caps an inversely-graded bed, and is overlain by a normally-graded bed. Marnoso-arena-cea Formation, Apennine Mountains, Italy.

Whether the graded beds deposited by these flows should be termed hyperpycnites, hyper-pycnal turbidites, or turbidites is subject to debate (Mutti et al, 2002). Mutti et al (2003) suggest that the stratigraphic succession of beds deposited from hyperpycnal flows should be termed “mixed depositional systems” to emphasize their transitional character between truly basinal, ignitive turbidites and delta-fed, turbidite-like deposits derived from rivers in flood stage.

Modern hyperpycnites have been documented for a distance of 700km downcurrent from linked river- canyon-fan systems in the central Japan Sea in water depths to 3350m (Nakajima, 2006). Two 4m long cores contain alternating turbiditic silt and hemipelagic mud beds. Some of the silt beds exhibit a typical fining-upward grain size trend. However, other beds on the order of 5-10cm thick exhibit a distinct coarsening-upward trend with mean grain size in the range 25-40 microns, capped by an erosional surface, then overlain by 5-10cm of beds which decrease in grain size upward in the range 5-25microns. It is postulated that the duration of these flows was on the order of several days to 3-4 weeks in order to travel 750km, and that they could have maintained the density required for them to hug the sea floor by entraining sea water and eroding sediment into the flow. Estimated velocities were on the order of 0.3m/sec, and they have an estimated frequency of occurrence of 70 years.

Siltstones with similar characteristics have been documented for the Cretaceous Dad Sandstone member of the Lewis Shale leveed-channel deposits (Chapter 6 and Chapter 7) (Soyinka and Slatt, 2004; Soyinka, 2005). Laser grain size analyses of individual laminae and thin beds reveal systematic changes in mean grain size within the silt size range (Fig. 4-27A). In some intervals the change from coarsening- to fining-upward trend is separated by a subtle erosional surface. This same surface has been documented in the modern hyperpycnites from the Japan Sea, mentioned above (Fig. 4-27B). A major delta system is known to have fed the Dad Sandstone slope and basinal facies (Pyles and Slatt, 2007), so the likelihood of hyperpyc-nal processes and deposits is high.

Figure 4-27.

(A) Polished slab of siltstone and mudstone outcrop interval analyzed for grain size distribution by laser grain size analysis (Soyinka, 2005); coin for scale. The graph to the right of the slab is a plot of the modal grain size of individual samples analyzed over the length of the slab. Ha = basal unit; Hb = top unit; HL = sandy horizontal laminae; GC = gradational contact; SC = sharp contact; SBC = sharp basal contact. Coarsening-upward and fining-upward trends in modal size are noted. An erosional surface is shown in the thin-section photomicrograph. (B) Average grain size of samples over a 10+cm interval of siltstones and mudstones from the floor of the Japan Sea; these beds and laminae are interpreted as hyperpycnites (Nakajima, 2006). EC = erosional surface denoting the vertical change in average grain size from coarsening-upward to fining-upward. For comparative purposes, the vertical scales are the same in A and B.

Figure 4-27.

(A) Polished slab of siltstone and mudstone outcrop interval analyzed for grain size distribution by laser grain size analysis (Soyinka, 2005); coin for scale. The graph to the right of the slab is a plot of the modal grain size of individual samples analyzed over the length of the slab. Ha = basal unit; Hb = top unit; HL = sandy horizontal laminae; GC = gradational contact; SC = sharp contact; SBC = sharp basal contact. Coarsening-upward and fining-upward trends in modal size are noted. An erosional surface is shown in the thin-section photomicrograph. (B) Average grain size of samples over a 10+cm interval of siltstones and mudstones from the floor of the Japan Sea; these beds and laminae are interpreted as hyperpycnites (Nakajima, 2006). EC = erosional surface denoting the vertical change in average grain size from coarsening-upward to fining-upward. For comparative purposes, the vertical scales are the same in A and B.

Gravel Deposits from Turbidity Currents

Gravelly deposits of turbidity currents are not as common as the sandier and muddier types described above. One reason is the general lack of pebbles and coarser grains within shallower water areas from which sediment gravity flows originate. Suitable source areas usually are confined to tectonically active basins with narrow shelves, and relatively high sedimentation rates (Reading and Richards, 1994). Although it might be surmised that such large grains would only be found in proximal deep water settings, gravel has been cored in the youngest channel of the Mississippi Fan a distance of approximately 220 km from the present shelf edge (Stelting et al., 1985).

Walker (1978) defined a series of gravelly, sediment gravity flow deposits on the basis of the abundance of pebbles and coarser grains and their sedimentary structures and stratification style (Figs. 4-3, 4-28). One type, pebbly sandstone (Fig. 4-3), contains dispersed or concentrated pebbles within a sandstone matrix. In outcrop, alternating pebble-rich and pebble-poor beds (Fig. 4-29) and trough or planar-tabular cross beds are the most characteristic internal sedimentary structures. Normal size grading (Fig. 4-30), large sole marks, lenticular-ity, and scoured bases of beds (Fig. 4-31) are also common features.

Figure 4-28.

Diagram illustrating the sedimentary features of the spectrum of clast-supported conglomerates with suggested downcurrent positions. (Modified from Walker, 1978). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-28.

Diagram illustrating the sedimentary features of the spectrum of clast-supported conglomerates with suggested downcurrent positions. (Modified from Walker, 1978). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-29.

Photograph of a stratified, pebbly sandstone to sandy conglomerate. Lower Pennsylvanian Jack-fork Group, Arkansas, U.S.A.

Figure 4-29.

Photograph of a stratified, pebbly sandstone to sandy conglomerate. Lower Pennsylvanian Jack-fork Group, Arkansas, U.S.A.

Figure 4-30.

Photograph of a graded, pebbly sandstone in outcrop. Top is toward the upper left corner. Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.

Figure 4-30.

Photograph of a graded, pebbly sandstone in outcrop. Top is toward the upper left corner. Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.

Figure 4-31.

Photograph of a series of amalgamated sandstone beds. The middle bed has been eroded, and the resulting scour (arrow) has been filled by a normally-graded, pebbly sandstone. Top is toward the upper right corner. Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.

Figure 4-31.

Photograph of a series of amalgamated sandstone beds. The middle bed has been eroded, and the resulting scour (arrow) has been filled by a normally-graded, pebbly sandstone. Top is toward the upper right corner. Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.

Walker (1978) classified other types of clast-supported conglomerates on the basis of type of grading (normal or inverse), presence or absence of stratification, and presence or absence of imbrication (Fig. 4-32). Based upon theoretical considerations, Walker defined gravelly deposits ranging from (upcurrent): (a) inverse to normally graded, imbricated conglomerates; to (b) normally graded, non-imbricated conglomerates; to (c) graded-stratified, imbricated conglomerates (downcurrent) (Fig. 4-28). Walker (1978) also recognized a fourth class, composed of disorganized, non-stratified, and non-imbricated conglomerates. All of these conglomerates tend to be lenticular, with scoured bases.

Figure 4-32.

Photograph showing imbrication in a shale-clast conglomerate stacked between two tan sandstone beds. Rock hammer is oriented approximately parallel to the orientation of the brown shale clasts. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-32.

Photograph showing imbrication in a shale-clast conglomerate stacked between two tan sandstone beds. Rock hammer is oriented approximately parallel to the orientation of the brown shale clasts. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

According to Lowe (1982), most coarse gravel is probably transported near the bed within a highly concentrated traction carpet, and in suspension in the lower part of a turbulent flow. Intergranular dispersive pressure maintains coarser grains in suspension, whereas finer grains filter to the sea bed, thus forming traction carpets near the bed. The coarser grains eventually fall from suspension to the bed, resulting in an inversely graded bed (Fig. 4-33). However, Legros (2002) has suggested that size segregation by the kinetic sieving process and/or by progressive aggradation of increasingly coarse-grained particles on the sea bed are more likely causes of inverse grading than is the maintainence of coarse grains within the flow by dispersive pressure. Lowe (1982) applies the designations R1 for coarse gravel with traction structures, R2 for inversely- graded gravel layers, and R3 for normally graded gravel layers (Fig. 4-22).

Figure 4-33.

Diagram illustrating the sedimentary processes of formation of inversely graded, traction carpet deposit. (A) Basal part of a high density flow shows development of a lower, inversely graded zone due to intergranular dispersive pressure. (B) Fallout of grains from suspension increases the clast concentration in the basal layer and results in formation of a traction carpet in which grains are supported by dispersive pressure. (C) Continued fallout from suspension increases the density of grains in the traction carpet and causes freezing in the upper part of the carpet. D) Final freezing of traction carpet results in formation of a new inversely graded basal layer above the deposit (Modified from Lowe, 1982). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-33.

Diagram illustrating the sedimentary processes of formation of inversely graded, traction carpet deposit. (A) Basal part of a high density flow shows development of a lower, inversely graded zone due to intergranular dispersive pressure. (B) Fallout of grains from suspension increases the clast concentration in the basal layer and results in formation of a traction carpet in which grains are supported by dispersive pressure. (C) Continued fallout from suspension increases the density of grains in the traction carpet and causes freezing in the upper part of the carpet. D) Final freezing of traction carpet results in formation of a new inversely graded basal layer above the deposit (Modified from Lowe, 1982). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Flow Combinations and Transformations

The various classification schemes discussed above all imply that flows can vary temporally and spatially in a predictable, downcurrent continuum, giving rise to different sediment gravity flow deposits along a single bed (Figs. 4-34-6). However, direct evidence of flow transformations within a single bed are relatively rare because most outcrops are of insufficient length and orientation with respect to bedding to trace a single bed laterally for a long distance. Some single bed flow transformations have been reported by Baruffini et al. (1994) and Drink-water and Pickering (2001). Al-Siyabi (1998) documented a transformation between a slurry bed and a massive sandstone in a single bed. Over a lateral distance of 5m (15ft.), this bed changes from one with a mud content of 9–20% and an abundance of fluid escape pipes, to a massive sandstone with a mud content of 4–7% and without fluid escape structures. Whether the slurry bed is upcurrent or downcurrent of the massive sandstone is not known, as no pale-ocurrent indicators are present on the bed.

Remacha and Fernandez (2003) state that individual beds within the Eocene Hecho Group (south-central Pyrenees, Spain) can be traced and correlated for ten’s of km in the downcurrent direction, where they grade from coarse-grained channel fill, to finer-grained channel-lobe transition deposits, to sheetlike lobes, and, finally, into basin plain deposits. Correlations of individual beds is possible because of the presence of numerous key marker beds that can be traced for these distances. Numbers of beds between markers, as well as their stacking patterns and facies characteristics, provide the means for correlation. In this manner, >50% of lobe beds (individual beds >9cm thick) have been traced downcurrent to form thin-bedded basin-plain facies (individual beds <9cm thick). There is only a decrease of 2.5 thin beds/km over these distances. The basin plain facies comprise classical Bouma turbidites (Fig. 4-5) (F8 and F9 beds of Mutti et al., 1999).

Laboratory experiments support the observations of downcurrent flow transformations. A flume experiment by Hampton (1972) provided a conceptual model for the evolution of a turbidity current from a debris flow. Marr et al. (2001) and Mohrig and Marr (2003) have generated turbidity currents from parent debris flows in laboratory flume experiments. Two processes that generate the turbidity currents from debris flows are: (1) grain-to-grain erosion of sediment from the surface of a debris flow and its subsequent ejection into the overlying water column, and (2) turbulent mixing of ambient water in front of a debris flow into its head, and subsequent dilution to form a turbidity current. In the first case, erosion of sediment from the head of the debris flow generates an overlying, less dense turbidity current that outruns the debris flow and continues advancing down slope. The resultant deposit is a thin turbidite bed on top of, and in front of the debrite. Whether the first or second process dominates in an individual flow depends upon whether dynamic stresses at the head of the flow are of sufficient magnitude to overcome the effective yield strength of the parent debris flow. If these stresses are sufficiently large, then ambient water in front of the flow can be entrained into the head of the flow, promoting transformation to a less dense turbidity current. If dynamic stresses are insufficient to overcome yield strength, then grain-to–grain erosion from the leading edge of the flow occurs, and eroded grains inject into the overlying water column to generate a turbidity current. The velocity at which the head is moving downslope appears to be a main factor in which of the two processes dominates.

Contrary to the examples presented above, Pratson et al. (2000) claim that it is not possible for debris flows to transform into turbidity currents. Their experimental work indicates that at a sediment concentration of > 10% , the flow is sufficiently cohesive to inhibit the exchange of water and sediment across the surface. Thus, the sediment concentration remains constant.

The presence of more than one gravity flow type within a single flow, documented experimentally by Marr et al. (2001) and Mohrig and Marr (2003), had been suggested ear- lier by others. Sanders (1965) interpreted the Bouma sequence as the deposit of two different kinds of flow: a basal “flowing grain layer” and an overlying turbulent flow to which he restricted the term “turbidity current” (Mutti et al., 1999). Allen (1985) and Postma et al. (1988) considered debris flows and low- and high-density turbidity currents as parts of a single sediment gravity flow (Fig. 4-11). The most recent model (Wagerle, 2001) places debris flows at the base of the flow where sediment concentrations are >50% (Fig. 4-34). The body and head of the flow contain 20–50% sediment concentration (high-density flow) in turbulent suspen-sion, whereas the low-density wake of the flow contains <20% sediment, also in turbulent suspension, and consists of the finest-grained sediment fraction. Sedimentary particles within each part of the flow are deposited at a different time and in a different position down the dep-ositionalaxis.

Figure 4-34.

Conceptual model (cross section) for a single sediment gravity flow composed of debris flow, high-density, and low-density turbidity current components. (Modified from Wagerle, 2001). Reprinted with permission of R. Wagerle.

Figure 4-34.

Conceptual model (cross section) for a single sediment gravity flow composed of debris flow, high-density, and low-density turbidity current components. (Modified from Wagerle, 2001). Reprinted with permission of R. Wagerle.

The process by which sediment gravity flows split apart from an original single flow is termed “flow stripping.” Flow stripping is the process of separation of the components of a sediment gravity flow as it travels within a sinuous channel (Piper and Normark, 1983), and is thought to be unique to sinuous submarine channels (Peakall et al., 2000; Posamentier, 2001). This process occurs along outside bends of sinuous channels, where turbidity current flows accelerate, similar to flow in fluvial channels. Sediment gravity flows of high velocity are unable to negotiate the bend, resulting in breaching of levees and deposition of sediment as splays on the outside of the levee, immediately downcurrent from the bend (Fig. 4-35). Flow velocity would normally be insufficient for all sediment to overtop the levee bend, so that the coarser-grained fraction of the flow remains within the channel while the finer-grained sediment is stripped out and transported to an extra-channel or levee location.

Figure 4-35.

Schematic illustration of a turbidity current in a channel and the process of flow stripping, where the upper part of the turbidity current flow overtops the outer levee at a bend in the channel. The lower, coarser-grained part of the flow remains in the channel. (Modified from Peakall et al., 2000). Reprinted with permission of Society of Sedimentary Geology (SEPM) and American Association of Petroleum Geologists.

Figure 4-35.

Schematic illustration of a turbidity current in a channel and the process of flow stripping, where the upper part of the turbidity current flow overtops the outer levee at a bend in the channel. The lower, coarser-grained part of the flow remains in the channel. (Modified from Peakall et al., 2000). Reprinted with permission of Society of Sedimentary Geology (SEPM) and American Association of Petroleum Geologists.

Mutti et al., (1999) refer to such flows as “bipartite gravity flows,” which include a basal, fast-moving, relatively coarse granular layer and an overlying turbulent suspension of finer-sized grains. Mutti et al. (1999) claims that the basal layer corresponds to the high-density turbidity current of Lowe (1982), in contrast to a low-density turbidity current which probably originated directly as a turbulent flow. One direcct line of evidence for bipartite gravity flows is the bidirectionality of paleocurrent indicators within an apparently single flow (Fig. 4-36). According to Mutti et al. (2002), these current indicators result from the lower, denser part of a bipartite flow moving in one direction within a channel, while the upper, less dense part of the flow splits along the channel margin, and then recombines with the denser part of the flow further downcurrent.

Figure 4-36.

Photograph showing two different orientations of paleocurrent directions in a single bed. The base of the sandstone exhibits flute casts which are oriented obliquely to the viewer. The upper part of the bed exhibits climbing ripples oriented at approximate right angles to the viewer. Marnoso-arenacea Formation, Apennine Mountains, Italy.

Figure 4-36.

Photograph showing two different orientations of paleocurrent directions in a single bed. The base of the sandstone exhibits flute casts which are oriented obliquely to the viewer. The upper part of the bed exhibits climbing ripples oriented at approximate right angles to the viewer. Marnoso-arenacea Formation, Apennine Mountains, Italy.

According to Kneller’s (1995) classification, temporal variations in flow can result in a variety of flow transformations and resulting deposits in one position on the sea floor (Fig. 4-6). One example of a set of alternating Bouma Tb-Tc beds, interpreted to represent the deposit of a single fluctuating flow, is shown in Figure 4-37. An unusual deposit has been observed in some outcrops of deep water strata. The deposit consists of a lower, massive or dewatered sandstone capped by a sandstone bed with shale and sandstone clasts, and with complex soft sediment-deformed and/or undulatory structures (Fig. 4-38). Haughton et al. (2001) refer to such beds as a “linked” sediment couplet; possible origins for the upper part of this couplet include: (a) vestige of a debris flow that only partially transformed to a turbidite, and (b) bulking of a lagged tail of a sandy turbidity current and overriding of the flow. Another possibility is flow stripping, whereby the lower and upper parts of the flow split in different directions, then recombined further in the downcurrent direction (Mutti et al., 2002).

Figure 4-37.

Photograph of alternating planar laminated (P) Bouma Tb and ripple bedded (R) Bouma Tc beds in outcrop, suggesting surge flow (alternating higher and lower velocity flow) during deposition. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-37.

Photograph of alternating planar laminated (P) Bouma Tb and ripple bedded (R) Bouma Tc beds in outcrop, suggesting surge flow (alternating higher and lower velocity flow) during deposition. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-38.

Outcrop photograph of a “linked couplet” (Haughton et al., 2001). The lower part of this bed is evenly bedded, but the upper part has been contorted and the beds have been detached. The scale is in 0.1 ft. increments. Lower Pennsylvanian Jack-fork Group sandstone.

Figure 4-38.

Outcrop photograph of a “linked couplet” (Haughton et al., 2001). The lower part of this bed is evenly bedded, but the upper part has been contorted and the beds have been detached. The scale is in 0.1 ft. increments. Lower Pennsylvanian Jack-fork Group sandstone.

Post-Depositional Reworking of Sediment Gravity Flows

Reworking of deep marine sands on the ocean floor by a variety of currents is well documented (Heezen et al., 1966; Bouma and Hollister, 1973; Stow and Lovell, 1979; Stow and Holbrook, 1984, Shanmugam 1997). In this section we discuss the two major types of currents, large-scale contour currents and smaller-scale, local currents. Contour currents, which are major, unidirectional oceanic currents that parallel slope contours (geostrophic currents), have been invoked for large-scale reworking of turbidite deposits into “contourite drifts” (Mutti, 1992). The internal stratigraphy of contourites has been discussed extensively by Stow and Lovell (1979) and Stow and Holbrook (1984), who point to the difficulty in differentiating contourites from fine-grained turbidites in the rock record owing to similar characteristics. Reworking of sands by more localized, unidirectional bottom currents can winnow mud and improve sorting (Fig. 4-39), thus improving potential reservoir quality when buried to depth. As shown in Figure 4-39 if reworking is too prolonged or intense, the resulting sand beds may become very thin and isolated, thus reducing their reservoir potential. An example of reworked Bouma Tb-Tc beds is shown in Figure 4-40; in this example, reworking is noted by a sharp top to an asymmetric ripple that is steep-sided in the opposite direction to primary cross-stratification.

Figure 4-39.

Schematic illustrations showing post-depositional processes by which a complete Bouma sequence (Fig. 4-14) might be progressively winnowed and reworked by ocean bottom currents (after Stanley, 1993). Reprinted with permission of Elsevier Publishing Co.

Figure 4-39.

Schematic illustrations showing post-depositional processes by which a complete Bouma sequence (Fig. 4-14) might be progressively winnowed and reworked by ocean bottom currents (after Stanley, 1993). Reprinted with permission of Elsevier Publishing Co.

Figure 4-40.

Outcrop photograph of a bottom-current reworked Bouma Tb-Tc bed. Note the sharp top and reverse orientation of the steep side of the ripple top relative to the direction of internal cross-stratification. Precambrian turbidite, Newfoundland, Canada.

Figure 4-40.

Outcrop photograph of a bottom-current reworked Bouma Tb-Tc bed. Note the sharp top and reverse orientation of the steep side of the ripple top relative to the direction of internal cross-stratification. Precambrian turbidite, Newfoundland, Canada.

In addition to unidirectional currents, internal waves and tidal currents within confined areas such as submarine canyons (Shepard et al., 1969; Bouma and Hollister, 1973) can rework sediments and provide bi-directional ripples in sandstones. Complex ripples have been noted on the modern deep sea floor (Fig. 4-41) as well as in deepwater sandstones (Fig. 4-42).

Figure 4-41.

Photograph of lunate- and lingoid-shaped ripples from the Scotian Sea floor at 4,010 m water depth, southeast of Terra del Fuego, southern Atlantic Ocean. (Reineck and Singh, 1980, Fig. 6-64). Reprinted with permission of Elsevier Publishing Co.

Figure 4-41.

Photograph of lunate- and lingoid-shaped ripples from the Scotian Sea floor at 4,010 m water depth, southeast of Terra del Fuego, southern Atlantic Ocean. (Reineck and Singh, 1980, Fig. 6-64). Reprinted with permission of Elsevier Publishing Co.

Figure 4-42.

Outcrop photograph of ripples on a sandstone bedding plane surface. The ripples are similar in appearance to those shown in Figure 4-41. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-42.

Outcrop photograph of ripples on a sandstone bedding plane surface. The ripples are similar in appearance to those shown in Figure 4-41. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Allocyclic vs. Autocyclic Processes and Vertical Stacking Patterns

The issue of transport and deposition of sediment gravity flows is significant to the larger considerations of interpreting allocyclic (extrabasinal) and autocyclic (intrabasinal) processes from vertical stacking patterns observed in outcrop or core. Vertical successions represent the product of localized deposition at one position on the sea floor.

At the single bed scale, a progressive upward decrease in sediment grain size (waning flow deposits of Fig. 4-6) implies decrease in depositional energy over time at one location. Conversely, a progressive upward increase in grain size (waxing-flow deposits of Fig. 4-6) implies an increase in depositional energy over time.

At the larger scale, a succession of sharp-based, amalgamated, massive or pebbly sandstones in sharp contact with underlying finer-grained Bouma Tb-d beds might be interepreted as recording a rapid drop in relative sea level and resulting seaward shift in the axis of deposition (allocyclic process) (Mutti et al., 1994).

However, without other criteria, an equally plausible explanation for this vertical succession might be a lateral shift in the depositional axis, with higher energy flows being deposited atop lower energy flows without any change in relative sea level (autocyclic process). In this case, successive sedimentation events in one area would provide bathymetric relief over time. When the relief becomes too high for sediment gravity flows to override, the flows are diverted to the bathymetrically lower, adjacent sea bed (Fig. 4-21). This style of deposition, which does not require a change in relative sea level, is termed “compensation bedding” (Mutti and Sonnino,1981). Compensation bedding occurs at all scales, from major sediment buildups (Walker, 1978, his Fig. 18), to thinner successions of beds (Jordan et al., 1993, their Fig. 69). This principle that the same vertical association of strata can be achieved either by allocyclic or autocyclic processes is shown in Figure 4-43. This principle dictates that caution should be used when making three-dimensional interpretation of processes from one-or two dimensional data (logs, cores, outcrops, and 2D seismic).

Figure 4-43.

Schematic cross sections of depositional intervals with differing grain size and how their vertical stacking arrangement (area in box) would be identical through either (A) landward shift (allocyclic) or (B) lateral shift (autocyclic) in depositional axis between deposition of the lower and upper intervals. (Modified from Al-Siyabi, 1998).

Figure 4-43.

Schematic cross sections of depositional intervals with differing grain size and how their vertical stacking arrangement (area in box) would be identical through either (A) landward shift (allocyclic) or (B) lateral shift (autocyclic) in depositional axis between deposition of the lower and upper intervals. (Modified from Al-Siyabi, 1998).

Summary: Lessons learned

Our understanding of sediment gravity flow processes continues to evolve at a rapid rate owing to a resurgence of research into their hydrocarbon-bearing deposits. Key points presented in this chapter are summarized below.

  1. 1.

    Sediment gravity flows can be initiated in a number of ways, from seismically-induced slides to flood stage discharge of rivers into the ocean. Such flows can occur with a frequency of years to 100s of years between event, depending upon the type of flow.

  2. 2.

    Critical factors that contol the type of flow and deposit include gravity, sediment concentration and flow velocity over time and space, These factors are related to sediment source area and type, climate, tectonics, and topography of the sea floor.

  3. 3.

    End member gravity flows, which are most common in the rock record, are cohesive debris flows and fluidal turbidity current flows. Intermediate flow types between true debris flows and turbidity currents also exist. A combination of observations of rocks and rock sequences, measurements in modern sedimentary environments, laboratory flume experiments, and computer modeling have illustrated the complexity of sediment gravity flows. Contrary to historical perspective, flows can wax, wane, or remain steady in both time and space, giving rise to complex sets of deposits, both laterally and vertically.

  4. 4.

    For descriptive, as well as some genetic classification, the Bouma sequence remains the fundamental deposit from a turbidity current. Different terms sometimes have been applied to divisions of the Bouma sequence by different authors. Other types of sediment gravity flows that do not conform to this classification include muddy sandstones, slurry beds, and massive sandstones (though the latter is sometimes classed as a Bouma Ta bed). Hyperpycnites (deposits from hyperpycnal flows) can be described by Bouma divisions even though their vertical stratigraphy may differ from that of a classic Bouma Sequence.

  5. 5.

    Turbidity currents generated from hyperpycnal flows are probably of greater significance than previously recognized, particularly off of major deltas. A distinctive coarsening-upward, followed by fining upward vertical sequence distinguishes hyperpycnites from other types of sediment gravity flows.

  6. 6.

    Downcurrent flow transformations of one type of sediment gravity flow to another type of flow undoubtedly occur in nature. Deposits that document flow transformations are difficult to find in outcrop, though rare outcrops have been identified. Flow transformations have been generated in flume tanks and by experimental modeling.

  7. 7.

    Once deposited on the sea floor, sediment gravity flows can be subjected to reworking by long-lived bottom currents as well as by internal waves and tidal currents (in the shallower portions of submarine canyons). Reworking can winnow the finest-grained fraction from sands, and sort them, to give rise to excellent potential reservoir rock upon burial.

  8. 8.

    Vertical stacking patterns of sediment gravity flows are important for interpreting the larger-scale filling history of a basin. A similar vertical sequence of sediment gravity flows can be produced by processes related to fluctuating base level (allocyclic processes) or fluctuations in depositional processes and sites without a change in base level (autocyclic processes). These differences should be considered when interpreting the depositional history of a sequence of sediment gravity flow deposits.

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Figures & Tables

Figure 4-1.

Chart listing the different types of sediment gravity flows by their corresponding sediment support mechanism. The classification is mainly after Lowe (1982). The figure was provided by D. Pyles (2002).

Figure 4-1.

Chart listing the different types of sediment gravity flows by their corresponding sediment support mechanism. The classification is mainly after Lowe (1982). The figure was provided by D. Pyles (2002).

Figure 4-2.

Diagrams illustrating the sediment support mechanisms (after Lowe, 1982). With increasing concentration of sediment within a flow, the support mechanism changes from fluid turbulence to hindered settling, to dispersive pressure to matrix strength. The figure was provided by D. Pyles (2002).

Figure 4-2.

Diagrams illustrating the sediment support mechanisms (after Lowe, 1982). With increasing concentration of sediment within a flow, the support mechanism changes from fluid turbulence to hindered settling, to dispersive pressure to matrix strength. The figure was provided by D. Pyles (2002).

Figure 4-3.

Chart illustrating the sediment transport processes of sediment gravity flows and resulting deposits. Sediment support mechanisms (inset) are those illustrated in Figure 4-2; i.e., 1= fluid turbulence, 2 = hindered settling, 3 = dispersive pressure and 4 = matrix strength. The vertical axis depicts sediment concentration, and the horizontal axis represents time and/or space. According to this diagram, flows can follow a number of transport paths until sediments within them are deposited. (Modified from Walker, 1978). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-3.

Chart illustrating the sediment transport processes of sediment gravity flows and resulting deposits. Sediment support mechanisms (inset) are those illustrated in Figure 4-2; i.e., 1= fluid turbulence, 2 = hindered settling, 3 = dispersive pressure and 4 = matrix strength. The vertical axis depicts sediment concentration, and the horizontal axis represents time and/or space. According to this diagram, flows can follow a number of transport paths until sediments within them are deposited. (Modified from Walker, 1978). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-4.

Diagram illustrating the sediment gravity flow continuum (vertical column on left). The continuum figures reflecting various deposits of the continuum (1–13) move from bed 1 through bed 13. Lines without arrows (e.g., 1–2 and 1–3) connect members between which there probably exists a continuous spectrum of flow and deposit types, but which are not part of an evolutionary trend of single flows. Arrows connect members that may be parts of an evolutionary contnuum for individual flows. The transition from disorganized cohesive flows (1 and 3), to thick, inversely graded, density-modified grain flows and traction carpets (5) and to turbulent, gravelly, high-density turbidity currents (6) is speculative. (Modified from Lowe, 1982). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-4.

Diagram illustrating the sediment gravity flow continuum (vertical column on left). The continuum figures reflecting various deposits of the continuum (1–13) move from bed 1 through bed 13. Lines without arrows (e.g., 1–2 and 1–3) connect members between which there probably exists a continuous spectrum of flow and deposit types, but which are not part of an evolutionary trend of single flows. Arrows connect members that may be parts of an evolutionary contnuum for individual flows. The transition from disorganized cohesive flows (1 and 3), to thick, inversely graded, density-modified grain flows and traction carpets (5) and to turbulent, gravelly, high-density turbidity currents (6) is speculative. (Modified from Lowe, 1982). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-5.

Diagram illustrating the deposits of the sediment gravity flow continuum. According to this classification, flow processes change in the flow direction, so the type of sediment deposited also changes. Cohesive debris flows (CDF) are precursors to other flow types, such as gravelly high dencity turbidity currents (GHDTC), sandy high density turbidity currents (SHDTC) and low density turbidity currents (LDTC). Resulting deposits, such as debrites, massive sandstones (F7), and Bouma divisions are explained in the text. (Modified from Mutti et al., 1999). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-5.

Diagram illustrating the deposits of the sediment gravity flow continuum. According to this classification, flow processes change in the flow direction, so the type of sediment deposited also changes. Cohesive debris flows (CDF) are precursors to other flow types, such as gravelly high dencity turbidity currents (GHDTC), sandy high density turbidity currents (SHDTC) and low density turbidity currents (LDTC). Resulting deposits, such as debrites, massive sandstones (F7), and Bouma divisions are explained in the text. (Modified from Mutti et al., 1999). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-6.

Different types of turbidity current deposits formed under a variety of spatial (accumulative, uniform and depletive) and temporal (waning, steady and waxing) flow conditions. Bouma divisions are shown by different colors. The horizontal thicknesses of individual deposits are indicators of thicknesses of the beds in nature. Arrows point in the downcurrent direction. The nine flow types and their five deposits are explained in the text (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-6.

Different types of turbidity current deposits formed under a variety of spatial (accumulative, uniform and depletive) and temporal (waning, steady and waxing) flow conditions. Bouma divisions are shown by different colors. The horizontal thicknesses of individual deposits are indicators of thicknesses of the beds in nature. Arrows point in the downcurrent direction. The nine flow types and their five deposits are explained in the text (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-7.

Chart comparing the common classifications of sediment gravity flow deposits shown in Figures 4-34-6. Figures in which symbols and names of deposits are explained are provided beneath author’s names. The various deposits are described in the text.

Figure 4-7.

Chart comparing the common classifications of sediment gravity flow deposits shown in Figures 4-34-6. Figures in which symbols and names of deposits are explained are provided beneath author’s names. The various deposits are described in the text.

Figure 4-8.

Schematic cross section illustrating the internal flow structure of a debris flow. Velocity of the shear flow region (Us) increases upward from the base of the bed. Within the plug flow region, velocity (Up) remains constant. The symbols hs and hp refer to thickness of each zone. Z is the vertical dimension or thickness of the flow. Note that the front of the flow is above the base of the bed. S is the angle of the bed relative to the horizontal plane. A wedge of water lubricates this basal zone, giving rise to the process of “hydroplaning,” explained in the text. (Modified from Pratson et al., 2000). Reprinted with permission of Society of Sedimentary Geology (SEPM) and American Association of Petroleum Geologists.

Figure 4-8.

Schematic cross section illustrating the internal flow structure of a debris flow. Velocity of the shear flow region (Us) increases upward from the base of the bed. Within the plug flow region, velocity (Up) remains constant. The symbols hs and hp refer to thickness of each zone. Z is the vertical dimension or thickness of the flow. Note that the front of the flow is above the base of the bed. S is the angle of the bed relative to the horizontal plane. A wedge of water lubricates this basal zone, giving rise to the process of “hydroplaning,” explained in the text. (Modified from Pratson et al., 2000). Reprinted with permission of Society of Sedimentary Geology (SEPM) and American Association of Petroleum Geologists.

Figure 4-9.

Outcrop photograph of a poorly sorted debrite bed. Note the large boulders are supported within a dense, fine-grained matrix. California, U.S.A.

Figure 4-9.

Outcrop photograph of a poorly sorted debrite bed. Note the large boulders are supported within a dense, fine-grained matrix. California, U.S.A.

Figure 4-10.

Photograph of dish structures in a sandstone bed. Stratigraphic top is toward the top of the photo. Formation of dish structures is discussed in the text. The formation which contains the dish structures is unknown. California, U.S.A.

Figure 4-10.

Photograph of dish structures in a sandstone bed. Stratigraphic top is toward the top of the photo. Formation of dish structures is discussed in the text. The formation which contains the dish structures is unknown. California, U.S.A.

Figure 4-11.

Schematic cross section of a high-density turbidity current. Arrows point to the flow directions within the current. Vertical flow velocity profiles are shown in shaded black. The flow is size-graded, with coarsest grains at the base of the bed. The head of this flow is thicker than the body. The billows at the top of the flow are termed “Kelvin-Helmolz Billows.” (Modified from Postma et al., 1988). Reprinted with permission of Elsevier Publishing Co.

Figure 4-11.

Schematic cross section of a high-density turbidity current. Arrows point to the flow directions within the current. Vertical flow velocity profiles are shown in shaded black. The flow is size-graded, with coarsest grains at the base of the bed. The head of this flow is thicker than the body. The billows at the top of the flow are termed “Kelvin-Helmolz Billows.” (Modified from Postma et al., 1988). Reprinted with permission of Elsevier Publishing Co.

Figure 4-12.

Photograph of a cross section of a turbidity current generated in a flume tank. The head and body of the flow are clearly shown. The sediment box from which the sediment was dropped through a removable bottom is in the upper right of the picture. Note that the head is thicker then the body, even after only a short distance of transport. Photograph is from the flume tank of the St. Anthony Falls Laboratory, University of Minnesota.

Figure 4-12.

Photograph of a cross section of a turbidity current generated in a flume tank. The head and body of the flow are clearly shown. The sediment box from which the sediment was dropped through a removable bottom is in the upper right of the picture. Note that the head is thicker then the body, even after only a short distance of transport. Photograph is from the flume tank of the St. Anthony Falls Laboratory, University of Minnesota.

Figure 4-13.

Four graphs illustrating velocity (solid lines) and associated sediment concentration (dashed lines) profiles for a variety of experimental turbidity currents: (a) two layer concentration model with a constant concentration lower interval and an upper region of sediment detrained from the head of the flow; (b) a smooth concentration profile, characteristic of low concentration, weakly depositional flows; (c) a stepped concentration profile observed in erosional flows; (d) a distribution observed in turbidity currents in which coarse material is concentrated towards the lower part of the flow and the fine-grained material is evenly distributed thoughout the flow (Modified from Kneller and Buckee, 2000). Reprinted with permission of International Association of Sedimentologists.

Figure 4-13.

Four graphs illustrating velocity (solid lines) and associated sediment concentration (dashed lines) profiles for a variety of experimental turbidity currents: (a) two layer concentration model with a constant concentration lower interval and an upper region of sediment detrained from the head of the flow; (b) a smooth concentration profile, characteristic of low concentration, weakly depositional flows; (c) a stepped concentration profile observed in erosional flows; (d) a distribution observed in turbidity currents in which coarse material is concentrated towards the lower part of the flow and the fine-grained material is evenly distributed thoughout the flow (Modified from Kneller and Buckee, 2000). Reprinted with permission of International Association of Sedimentologists.

Figure 4-14.

Sediment grain size, structures, divisions of a complete Bouma sequence, and sediment transport mechanisms. The different divisions— from Bouma Ta to Bouma Te—are explained in the text. (Modified from Jordan et al., 1993). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-14.

Sediment grain size, structures, divisions of a complete Bouma sequence, and sediment transport mechanisms. The different divisions— from Bouma Ta to Bouma Te—are explained in the text. (Modified from Jordan et al., 1993). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-15.

Photograph of beds illustrating Bouma Ta, Tb, and Tc divisons in outcrop. The upward gradational decrease in grain size is noted by a “smoothing” of the outcrop surface. Location of the outcrop is unknown.

Figure 4-15.

Photograph of beds illustrating Bouma Ta, Tb, and Tc divisons in outcrop. The upward gradational decrease in grain size is noted by a “smoothing” of the outcrop surface. Location of the outcrop is unknown.

Figure 4-16.

Photograph of beds illustrating Bouma Ta-Te divisions in outcrop. Note the normal size grading as evidenced by the decrease in size and abundance of coarse (white) particles upward through the Ta division. Name of the formation which contains this rock is unknown. Newfoundland, Canada.

Figure 4-16.

Photograph of beds illustrating Bouma Ta-Te divisions in outcrop. Note the normal size grading as evidenced by the decrease in size and abundance of coarse (white) particles upward through the Ta division. Name of the formation which contains this rock is unknown. Newfoundland, Canada.

Figure 4-17.

Photograph of Bouma Tb-Tc division in outcrop. The upward change from parallel laminated to ripple (including climbing-ripple) bedding is a result of decrease in traction current flow from upper to lower flow regime velocities. Upper Miocene Mt. Messenger Formation, New Zealand.

Figure 4-17.

Photograph of Bouma Tb-Tc division in outcrop. The upward change from parallel laminated to ripple (including climbing-ripple) bedding is a result of decrease in traction current flow from upper to lower flow regime velocities. Upper Miocene Mt. Messenger Formation, New Zealand.

Figure 4-18.

Succession of sedimentary textures and structures in finegrained turbidites Grain size and sedimentary structures change systematically upward much as they do in sandy turbidity current deposits. In this diagram, the Bouma Tc and Td divisions are the same as described in Figure 4-14. However, the Bouma Td division is divided into T1 and T2 subdivisions, and the Bouma Te division is divided into six subdivisions (T3-T8) based upon variations in grain size and small-scale sedimentary structures. (Modified from Stow and Shanmugam, 1980). Reprinted with permission of Elsevier Publishing Co.

Figure 4-18.

Succession of sedimentary textures and structures in finegrained turbidites Grain size and sedimentary structures change systematically upward much as they do in sandy turbidity current deposits. In this diagram, the Bouma Tc and Td divisions are the same as described in Figure 4-14. However, the Bouma Td division is divided into T1 and T2 subdivisions, and the Bouma Te division is divided into six subdivisions (T3-T8) based upon variations in grain size and small-scale sedimentary structures. (Modified from Stow and Shanmugam, 1980). Reprinted with permission of Elsevier Publishing Co.

Figure 4-19.

Graphs of time (t) vs. velocity (u) and distance (x) vs. velocity (u), showing different types of flow under a variety of conditions. In the upper diagram, flow velocity first waxes, then wanes, then becomes steady over time. In the lower diagram, flow velocity first increases (accumulative), then decreases (depletive), then becomes uniform with downcurrent distance. (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-19.

Graphs of time (t) vs. velocity (u) and distance (x) vs. velocity (u), showing different types of flow under a variety of conditions. In the upper diagram, flow velocity first waxes, then wanes, then becomes steady over time. In the lower diagram, flow velocity first increases (accumulative), then decreases (depletive), then becomes uniform with downcurrent distance. (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-20.

Diagram illustrating some causes of spatially depletive (downcurrent decrease in flow velocity) and spatially accumulative (downcurrent increase in flow velocity) sediment gravity flows. The upper diagram, (a) is a plan view of depletive flow resulting from divergent flow on the sea floor as the flow becomes unconfined, and (b) is a cross section view showing downcurrent decrease in flow velocity due to reduction in slope gradient. In the lower diagram, (c) is a plan view illustrating accumulative flow resulting from convergence of flow on the sea floor , and (d) illustrates accumulative flow resulting from a downcurrent increase in the slope gradient of the sea floor. (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-20.

Diagram illustrating some causes of spatially depletive (downcurrent decrease in flow velocity) and spatially accumulative (downcurrent increase in flow velocity) sediment gravity flows. The upper diagram, (a) is a plan view of depletive flow resulting from divergent flow on the sea floor as the flow becomes unconfined, and (b) is a cross section view showing downcurrent decrease in flow velocity due to reduction in slope gradient. In the lower diagram, (c) is a plan view illustrating accumulative flow resulting from convergence of flow on the sea floor , and (d) illustrates accumulative flow resulting from a downcurrent increase in the slope gradient of the sea floor. (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-21.

Schematic diagram of possible flow paths (arrows) for sediment gravity flows when they encounter a sea floor obstacle. Flows can partially or completely override the obstacle, can divert around the obstacle or flow partway up the obstacle then reverse flow. (Modified from Kneller and McCaffrey, 1999). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-21.

Schematic diagram of possible flow paths (arrows) for sediment gravity flows when they encounter a sea floor obstacle. Flows can partially or completely override the obstacle, can divert around the obstacle or flow partway up the obstacle then reverse flow. (Modified from Kneller and McCaffrey, 1999). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-22.

Vertical profile of sediment grain size and sedimentary structures illustrating high-to low-density turbidity current deposits using the terminology of Lowe (1982). S and R designations, and processes, are explained in the text and in Figure 4-4. Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-22.

Vertical profile of sediment grain size and sedimentary structures illustrating high-to low-density turbidity current deposits using the terminology of Lowe (1982). S and R designations, and processes, are explained in the text and in Figure 4-4. Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-23.

Core of unconsolidated sand (light color) and lithified shale (darker color) from Long Beach Unit, Wilmington oil field, California (Slatt et al., 1993). Individual sand beds are massive, structureless, and of uniform grain-size from base to top. Scale is in 0.1 ft. increments. Reprinted with permission of the Society of Sedimentary Geology (SEPM)

Figure 4-23.

Core of unconsolidated sand (light color) and lithified shale (darker color) from Long Beach Unit, Wilmington oil field, California (Slatt et al., 1993). Individual sand beds are massive, structureless, and of uniform grain-size from base to top. Scale is in 0.1 ft. increments. Reprinted with permission of the Society of Sedimentary Geology (SEPM)

Figure 4-24.

Schematic cross section of main types of flows debouching from a river mouth. Hyperpycnal flows are denser than ambient water, and flow along the sea bed. Hypopycnal flows are less dense than ambient water, so ride on the sea water surface. Interflows are at some intermediate density. and flow within the ambient water. Lofting occurs when the density of a hyperpycnal flow decreases due to loss of sediment within the flow by deposition, and the flow rises into the ambient sea water. (Modified from Mutti et al., 2003). Reprinted with permission of Elsevier Publishing Co.

Figure 4-24.

Schematic cross section of main types of flows debouching from a river mouth. Hyperpycnal flows are denser than ambient water, and flow along the sea bed. Hypopycnal flows are less dense than ambient water, so ride on the sea water surface. Interflows are at some intermediate density. and flow within the ambient water. Lofting occurs when the density of a hyperpycnal flow decreases due to loss of sediment within the flow by deposition, and the flow rises into the ambient sea water. (Modified from Mutti et al., 2003). Reprinted with permission of Elsevier Publishing Co.

Figure 4-25.

Graph illustrating flows with different discharge through time, and the representative lithofacies. 1. Low magnitude flood generates a normally-graded bed. 2. Low magnitude flood which generates a complete inverse-to-normally graded hyperpycnite due to waxing, then waning flood stages. 3. Mid-magnitude flood generates a complete sequence which is coarser grained than the example in 2. 4. High-magnitude flood generates an inversely-graded bed from waxing flow, followed by an erosion or bypass surface due to peak flood flow velocity sufficient to erode the substrate, followed by the waning flow, normally-graded bed. (Modified from Mulder et al., 2001b). Reprinted with permission of Springer-Verlag Publishing Co.

Figure 4-25.

Graph illustrating flows with different discharge through time, and the representative lithofacies. 1. Low magnitude flood generates a normally-graded bed. 2. Low magnitude flood which generates a complete inverse-to-normally graded hyperpycnite due to waxing, then waning flood stages. 3. Mid-magnitude flood generates a complete sequence which is coarser grained than the example in 2. 4. High-magnitude flood generates an inversely-graded bed from waxing flow, followed by an erosion or bypass surface due to peak flood flow velocity sufficient to erode the substrate, followed by the waning flow, normally-graded bed. (Modified from Mulder et al., 2001b). Reprinted with permission of Springer-Verlag Publishing Co.

Figure 4-26.

Ouctrop photo of a hyperpycnite of the type shown in 4 of Figure 4-25. An erosional or bypass surface caps an inversely-graded bed, and is overlain by a normally-graded bed. Marnoso-arena-cea Formation, Apennine Mountains, Italy.

Figure 4-26.

Ouctrop photo of a hyperpycnite of the type shown in 4 of Figure 4-25. An erosional or bypass surface caps an inversely-graded bed, and is overlain by a normally-graded bed. Marnoso-arena-cea Formation, Apennine Mountains, Italy.

Figure 4-27.

(A) Polished slab of siltstone and mudstone outcrop interval analyzed for grain size distribution by laser grain size analysis (Soyinka, 2005); coin for scale. The graph to the right of the slab is a plot of the modal grain size of individual samples analyzed over the length of the slab. Ha = basal unit; Hb = top unit; HL = sandy horizontal laminae; GC = gradational contact; SC = sharp contact; SBC = sharp basal contact. Coarsening-upward and fining-upward trends in modal size are noted. An erosional surface is shown in the thin-section photomicrograph. (B) Average grain size of samples over a 10+cm interval of siltstones and mudstones from the floor of the Japan Sea; these beds and laminae are interpreted as hyperpycnites (Nakajima, 2006). EC = erosional surface denoting the vertical change in average grain size from coarsening-upward to fining-upward. For comparative purposes, the vertical scales are the same in A and B.

Figure 4-27.

(A) Polished slab of siltstone and mudstone outcrop interval analyzed for grain size distribution by laser grain size analysis (Soyinka, 2005); coin for scale. The graph to the right of the slab is a plot of the modal grain size of individual samples analyzed over the length of the slab. Ha = basal unit; Hb = top unit; HL = sandy horizontal laminae; GC = gradational contact; SC = sharp contact; SBC = sharp basal contact. Coarsening-upward and fining-upward trends in modal size are noted. An erosional surface is shown in the thin-section photomicrograph. (B) Average grain size of samples over a 10+cm interval of siltstones and mudstones from the floor of the Japan Sea; these beds and laminae are interpreted as hyperpycnites (Nakajima, 2006). EC = erosional surface denoting the vertical change in average grain size from coarsening-upward to fining-upward. For comparative purposes, the vertical scales are the same in A and B.

Figure 4-28.

Diagram illustrating the sedimentary features of the spectrum of clast-supported conglomerates with suggested downcurrent positions. (Modified from Walker, 1978). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-28.

Diagram illustrating the sedimentary features of the spectrum of clast-supported conglomerates with suggested downcurrent positions. (Modified from Walker, 1978). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-29.

Photograph of a stratified, pebbly sandstone to sandy conglomerate. Lower Pennsylvanian Jack-fork Group, Arkansas, U.S.A.

Figure 4-29.

Photograph of a stratified, pebbly sandstone to sandy conglomerate. Lower Pennsylvanian Jack-fork Group, Arkansas, U.S.A.

Figure 4-30.

Photograph of a graded, pebbly sandstone in outcrop. Top is toward the upper left corner. Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.

Figure 4-30.

Photograph of a graded, pebbly sandstone in outcrop. Top is toward the upper left corner. Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.

Figure 4-31.

Photograph of a series of amalgamated sandstone beds. The middle bed has been eroded, and the resulting scour (arrow) has been filled by a normally-graded, pebbly sandstone. Top is toward the upper right corner. Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.

Figure 4-31.

Photograph of a series of amalgamated sandstone beds. The middle bed has been eroded, and the resulting scour (arrow) has been filled by a normally-graded, pebbly sandstone. Top is toward the upper right corner. Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.

Figure 4-32.

Photograph showing imbrication in a shale-clast conglomerate stacked between two tan sandstone beds. Rock hammer is oriented approximately parallel to the orientation of the brown shale clasts. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-32.

Photograph showing imbrication in a shale-clast conglomerate stacked between two tan sandstone beds. Rock hammer is oriented approximately parallel to the orientation of the brown shale clasts. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-33.

Diagram illustrating the sedimentary processes of formation of inversely graded, traction carpet deposit. (A) Basal part of a high density flow shows development of a lower, inversely graded zone due to intergranular dispersive pressure. (B) Fallout of grains from suspension increases the clast concentration in the basal layer and results in formation of a traction carpet in which grains are supported by dispersive pressure. (C) Continued fallout from suspension increases the density of grains in the traction carpet and causes freezing in the upper part of the carpet. D) Final freezing of traction carpet results in formation of a new inversely graded basal layer above the deposit (Modified from Lowe, 1982). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-33.

Diagram illustrating the sedimentary processes of formation of inversely graded, traction carpet deposit. (A) Basal part of a high density flow shows development of a lower, inversely graded zone due to intergranular dispersive pressure. (B) Fallout of grains from suspension increases the clast concentration in the basal layer and results in formation of a traction carpet in which grains are supported by dispersive pressure. (C) Continued fallout from suspension increases the density of grains in the traction carpet and causes freezing in the upper part of the carpet. D) Final freezing of traction carpet results in formation of a new inversely graded basal layer above the deposit (Modified from Lowe, 1982). Reprinted with permission of Society of Sedimentary Geology (SEPM).

Figure 4-34.

Conceptual model (cross section) for a single sediment gravity flow composed of debris flow, high-density, and low-density turbidity current components. (Modified from Wagerle, 2001). Reprinted with permission of R. Wagerle.

Figure 4-34.

Conceptual model (cross section) for a single sediment gravity flow composed of debris flow, high-density, and low-density turbidity current components. (Modified from Wagerle, 2001). Reprinted with permission of R. Wagerle.

Figure 4-35.

Schematic illustration of a turbidity current in a channel and the process of flow stripping, where the upper part of the turbidity current flow overtops the outer levee at a bend in the channel. The lower, coarser-grained part of the flow remains in the channel. (Modified from Peakall et al., 2000). Reprinted with permission of Society of Sedimentary Geology (SEPM) and American Association of Petroleum Geologists.

Figure 4-35.

Schematic illustration of a turbidity current in a channel and the process of flow stripping, where the upper part of the turbidity current flow overtops the outer levee at a bend in the channel. The lower, coarser-grained part of the flow remains in the channel. (Modified from Peakall et al., 2000). Reprinted with permission of Society of Sedimentary Geology (SEPM) and American Association of Petroleum Geologists.

Figure 4-36.

Photograph showing two different orientations of paleocurrent directions in a single bed. The base of the sandstone exhibits flute casts which are oriented obliquely to the viewer. The upper part of the bed exhibits climbing ripples oriented at approximate right angles to the viewer. Marnoso-arenacea Formation, Apennine Mountains, Italy.

Figure 4-36.

Photograph showing two different orientations of paleocurrent directions in a single bed. The base of the sandstone exhibits flute casts which are oriented obliquely to the viewer. The upper part of the bed exhibits climbing ripples oriented at approximate right angles to the viewer. Marnoso-arenacea Formation, Apennine Mountains, Italy.

Figure 4-37.

Photograph of alternating planar laminated (P) Bouma Tb and ripple bedded (R) Bouma Tc beds in outcrop, suggesting surge flow (alternating higher and lower velocity flow) during deposition. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-37.

Photograph of alternating planar laminated (P) Bouma Tb and ripple bedded (R) Bouma Tc beds in outcrop, suggesting surge flow (alternating higher and lower velocity flow) during deposition. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-38.

Outcrop photograph of a “linked couplet” (Haughton et al., 2001). The lower part of this bed is evenly bedded, but the upper part has been contorted and the beds have been detached. The scale is in 0.1 ft. increments. Lower Pennsylvanian Jack-fork Group sandstone.

Figure 4-38.

Outcrop photograph of a “linked couplet” (Haughton et al., 2001). The lower part of this bed is evenly bedded, but the upper part has been contorted and the beds have been detached. The scale is in 0.1 ft. increments. Lower Pennsylvanian Jack-fork Group sandstone.

Figure 4-39.

Schematic illustrations showing post-depositional processes by which a complete Bouma sequence (Fig. 4-14) might be progressively winnowed and reworked by ocean bottom currents (after Stanley, 1993). Reprinted with permission of Elsevier Publishing Co.

Figure 4-39.

Schematic illustrations showing post-depositional processes by which a complete Bouma sequence (Fig. 4-14) might be progressively winnowed and reworked by ocean bottom currents (after Stanley, 1993). Reprinted with permission of Elsevier Publishing Co.

Figure 4-40.

Outcrop photograph of a bottom-current reworked Bouma Tb-Tc bed. Note the sharp top and reverse orientation of the steep side of the ripple top relative to the direction of internal cross-stratification. Precambrian turbidite, Newfoundland, Canada.

Figure 4-40.

Outcrop photograph of a bottom-current reworked Bouma Tb-Tc bed. Note the sharp top and reverse orientation of the steep side of the ripple top relative to the direction of internal cross-stratification. Precambrian turbidite, Newfoundland, Canada.

Figure 4-41.

Photograph of lunate- and lingoid-shaped ripples from the Scotian Sea floor at 4,010 m water depth, southeast of Terra del Fuego, southern Atlantic Ocean. (Reineck and Singh, 1980, Fig. 6-64). Reprinted with permission of Elsevier Publishing Co.

Figure 4-41.

Photograph of lunate- and lingoid-shaped ripples from the Scotian Sea floor at 4,010 m water depth, southeast of Terra del Fuego, southern Atlantic Ocean. (Reineck and Singh, 1980, Fig. 6-64). Reprinted with permission of Elsevier Publishing Co.

Figure 4-42.

Outcrop photograph of ripples on a sandstone bedding plane surface. The ripples are similar in appearance to those shown in Figure 4-41. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-42.

Outcrop photograph of ripples on a sandstone bedding plane surface. The ripples are similar in appearance to those shown in Figure 4-41. Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.

Figure 4-43.

Schematic cross sections of depositional intervals with differing grain size and how their vertical stacking arrangement (area in box) would be identical through either (A) landward shift (allocyclic) or (B) lateral shift (autocyclic) in depositional axis between deposition of the lower and upper intervals. (Modified from Al-Siyabi, 1998).

Figure 4-43.

Schematic cross sections of depositional intervals with differing grain size and how their vertical stacking arrangement (area in box) would be identical through either (A) landward shift (allocyclic) or (B) lateral shift (autocyclic) in depositional axis between deposition of the lower and upper intervals. (Modified from Al-Siyabi, 1998).

Contents

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