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From Turbid to Lucid: a Straightforward Approach to Sediment Gravity Flows and Their Deposits

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The Sedimentary Record CLASSIFICATION OF SEDIMENT GRAVITY FLOWS: BACKGROUND From Turbid to Lucid: AND REVIEW The initial definitions of turbidity currents A Straightforward Approach to Sediment and debris flows were purely descriptive, with- out being specific about the physical proper- Gravity Flows and Their Deposits ties of the current. Therefore, from the begin- M. Royhan Gani ning, questions revolved around what should Department of Geosciences be the main basis of classification of sediment University of Texas at Dallas gravity flows. Different authors emphasized P.O. Box, 830688, Richardson, TX, 75083-0688, USA different parameters in their classification [email protected] schemes (sediment concentration: Bagnold, 1962; rheology: Dott, 1963; fluid turbulence: Sanders, 1965; sediment-support mechanisms: ABSTRACT Middleton and Hampton, 1973; combination Deepwater sediment gravity flows are categorized on the basis of a combination of four of rheology and sediment-support mechanism: parameters – sediment concentration, sediment-support mechanism, flow state (laminar Lowe, 1982; combination of physical flow or turbulent), and rheology. Because there is no agreement among sedimentologists properties and sediment-support mechanism: about which of these parameters should be the decisive one, one school’s turbidites Mulder and Alexander, 2001). Among the become another school’s debrites, and vice-versa. Except for rheology, all of these four most important parameters (sediment parameters change gradationally from one end member to another.Therefore, rheologi- concentration, sediment-support mechanism, cal classification of sediment gravity flows should be the most straightforward and the flow state, and rheology) of sediment gravity least controversial.These flows can be either Newtonian (i.e., turbidity currents), or flows, sediment concentration (by volume) non-Newtonian (i.e., debris flows). However, identification of flow rheology by examining directly affects other three parameters. the deposits may not be easy.Although we may confidently identify some rocks as tur- Therefore, sediment concentration appears to bidites and others as debrites, there are some transitional deposits, here called densites, be the most pragmatic parameter for defining that share both the characteristics of turbidites and debrites. Densites are the deposits the various types. Unfortunately, we can not of dense flows, which are rheologically stratified flows having a composite rheology of establish specific threshold values for various Newtonian fluids and non-Newtonian fluids. Moreover, the absence of a general term for types of sediment gravity flows (Shanmugam, all types of sediment gravity flow deposits has resulted in overuse and misuse of the 1996) because grain size and concentration of term turbidite.The term ‘gravite’ is proposed here for deposits of any kind of sediment clay minerals offset these threshold values. gravity flow, irrespective of their depositional environment. Sediment-support mechanisms include matrix strength, dispersive grain pressure, escaping pore fluid, and fluid turbulence. INTRODUCTION fluid is moved by gravity dragging the sedi- These mechanisms may change gradationally The term ‘turbidity current’ was introduced ment along, whereas in former gravity moves with increasing fluid content, and more than by Johnson (1938) and applied to a current the sediment, which drags the fluid along. A one support mechanism may operate simulta- generated due to turbid or muddy water. turbidity current is only one type of sediment neously for a specific type of sediment gravity Later, Kuenen (1957) introduced the term gravity flow. The center point of the turbidite flow. Similarly, the flow state may change gra- ‘turbidite’ for the deposit of a turbidity cur- controversy lies in the classification scheme of dationally and back-and-forth between a lami- rent, and Bouma (1962) introduced a classic sediment gravity flows, which is so far poorly nar state and a turbulent state with the change five-fold vertical facies model for turbidites. constrained and a bit ambiguous. This contro- of sediment concentrations or basin slopes. Soon, the terms ‘turbidites,’ ‘Bouma versy can lead to erroneous numerical model- On the other hand, the rheology of sediment sequences,’ and ‘deepwater deposits’ became ing of sediment gravity flows frequently used gravity flows is expressible in a straightforward almost synonymous in many published in submarine construction and hydrocarbon and simplified mathematical way in a 2-D accounts. Although the overuse and misuse of exploration because specific mathematical for- graph (Fig. 1). Most importantly, the rheologi- the terms ‘turbidity current’ and ‘turbidite’ mulae govern specific types of sediment gravi- cal types do not vary gradationally among was first indicated by Sanders (1965), the tur- ty flows. Therefore, it is felt that this classifica- each other. Therefore, rheology may be the bidite controversy has recently caught wide tion scheme needs to be reviewed to clarify the one parameter that can be used least ambigu- attention (e.g., Shanmugam, 2000; Lowe and controversy. In this paper, I take a simple and ously to define various types of sediment grav- Guy, 2000; Kneller and Buckee, 2000; Mulder straightforward approach to classifying sedi- ity flows. and Alexander, 2001). ment gravity flows. I also suggest some key According to Figure 1, there are only two Sediment gravity flows play a major role in depositional features on which flow types can basic types of rheology in sediment gravity transporting and depositing sediments in be interpreted least equivocally. Although the flows – Newtonian and non-Newtonian. If a deepwater environments, and can be defined suggestions made here are applicable irrespec- sediment gravity flow deforms instantly with as a complex mixture of sediment and fluid tive of depositional environments, this paper applied stress and develops a linear relation- that flows down slope due to the action of mainly deals with deepwater sediment gravity ship between shear stress and strain rate, it is gravity. Sediment gravity flows are different flows and their deposits. called a Newtonian fluid. Any deviation from from fluid gravity flows, because in the latter, this characteristic results in non-Newtonian 4 | September 2004 The Sedimentary Record the flow rheology of a deposit by examining its depositional features may be challenging. There are sediment gravity flow deposits that share both the characters of turbidites and debrites (deposits of debris flows). If we follow the turbidite controversy for the last ten years, it becomes obvious that these hybrid deposits (Non-Newtonian Bingham Plastic) are the main issue of debate. A plethora of ter- Cohesive debris flow minology (e.g., high density turbidity cur- rents, sandy debris flows, slurry flows, concen- trated density flows) has been applied to these Shear stress rocks. Most of these deposits originate from (Newtonian fluid) rheologically stratified (or bipartite) sediment gravity flows (e.g., Sanders, 1965; Tinterri et al., 2003) with commonly a lower zone of Turbidity current non-Newtonian dilatant fluid (non-cohesive debris flow) overlain by a Newtonian fluid (turbidity current). Because these types of Yield strength Non-cohesive debris(Non-Newtonian flow dilatant fluid) flows can frequently change the intra-flow rhe- ological boundaries, and can generate a single Strain rate (du/dy) event bed, a separate name is needed for these flows and their deposits. In this study, these Figure 1. Basic types of rheology in sediment gravity flows. According to this diagram, turbidity currents are Newtonian fluids, whereas debris flows are not. Debris flows can be either non-Newtonian Bingham flows are called ‘dense flows’ (after Allen, plastics (cohesive debris flows; e.g., mud flows) with a certain yield strength, or non-Newtonian dilatant 1997), as they show an intermediate density fluids (non-cohesive debris flows; e.g., grain flow) without any yield strength. (due to intermediate sediment concentration) between turbidity currents and debris flows rheology. Sediment gravity flows can show two issue of turbidite controversy. (Fig. 2A), and their deposits are named ‘den- types of non-Newtonian rheology (Fig. 1). In It may be easy to determine the rheology of sites.’ However, it is emphasized that accord- a non-Newtonian Bingham plastic, a critical flows in the laboratory. However, interpreting ing to rheology there are only two basic types value of shear stress (called yield stress) has to be crossed before there is any deformation, Non-Newtonian dilatant fluid after which the deformation is linear (i.e., a A ? Newtonian fluid Bingham plastic Bingham plastic is a combination of an ideal ? ? Figure 2. plastic and a Newtonian fluid). In a non- 0203010 40 50 60 70 80 90 Distribution of dif- ferent types of sedi- Newtonian dilatant fluid there is no yield Gravel ment gravity flows strength, but the deformation is nonlinear in 2-D space of sedi- with applied stress in such a way that it Coarse sand ment concentration becomes progressively harder to deform the vs. grain size (A), fluid (Fig. 1). Applying the above concepts, I Fine sand Dense flow Debris flow and Reynolds num- recommend that sediment gravity flows with (Hybrid of ber vs. Froude num- Silt turbidity current Grain size ber (B). Note that Newtonian rheology should be called ‘turbidi- and non-cohesive debris flow) dense flows (rheolog- ty currents,’ and those with non-Newtonian
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