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4b_G435.pps 1 Flow Regime and Sedimentary Structures An Introduction To Physical Processes of Sedimentation.

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Presentation on theme: "4b_G435.pps 1 Flow Regime and Sedimentary Structures An Introduction To Physical Processes of Sedimentation."— Presentation transcript:

1 4b_G435.pps 1 Flow Regime and Sedimentary Structures An Introduction To Physical Processes of Sedimentation

2 4b_G435.pps 2 Bed Response to Water (fluid) Flow Common bed forms (shape of the unconsolidated bed) due to fluid flow in –Unidirectional (one direction) flow Flow transverse, asymmetric bed forms –2D&3D ripples and dunes –Bi-directional (oscillatory) Straight crested symmetric ripples –Combined Flow Hummocks and swales

3 4b_G435.pps 3 Bed Response to Steady-state, Unidirectional, Water Flow Hydrodynamic variables –Grain Size| Most Important –Flow Depth |--> Variables in Natural Fluid Flow –Flow velocity | Systems –Fluid Viscosity –Fluid Density –Particle Density –g (gravity)

4 4b_G435.pps 4 Bed Response to Steady-state, Unidirectional, Water Flow FLOW REGIME CONCEPT –Consider variation in: Flow Velocity only Flume Experiments (med sand & 20 cm flow depth) –A particular flow velocity (after critical velocity of entrainment) produces –a particular bed configuration (Bed form) which in turn – produces a particular internal sedimentary structure.

5 4b_G435.pps 5 Bed Response to Steady-state, Unidirectional, Water Flow Consider Variation in Grain Size & Flow Velocity –for sand <~0.2mm:No Dunes –for sand ~0.2 to 0.8mmIdealized Flow Regime Sequence of Bed forms –for sand > 0.8:No ripples nor lower plane bed

6 4b_G435.pps 6 Bed Response to Steady-state, Unidirectional, Water Flow Lower Flow Regime –No Movement: flow velocity below critical entrainment velocity –Ripples: straight crested (2d) to sinuous and linguoid crested (3d) ripples (< ~1mλ) with increasing flow velocity –Dunes: (2d) sand waves with straight crests to (3d) dunes (>~1.5mλ) with sinuous crests and troughs

7 4b_G435.pps 7 Bed Response to Steady-state, Unidirectional, Water Flow Lower Flow Regime –No Movement: flow velocity below critical entrainment velocity –Ripples: straight crested (2d) to sinuous and linguoid crested (3d) ripples (< ~1m) with increasing flow velocity –Dunes: (2d) sand waves with straight crests to (3d) dunes (>~1.5mλ) with sinuous crests and troughs

8 4b_G435.pps 8 Dynamics of Flow Transverse Sedimentary Structures Flow separation and planar vs. tangential fore sets –Aggradation (lateral and vertical) and Erosion in space and time Due to flow velocity variation Capacity (how much sediment in transport) variation Competence (largest size particle in transport) variation –Angle of climb and the extent of bed form preservation (erosion vs. aggradation-dominated bedding surface)

9 4b_G435.pps 9 Climbing Ripples Angle of climb and decreasing flow capacity (downwards on figure)

10 4b_G435.pps 10 Bed Response to Steady-state, Unidirectional, Water Flow Lower Flow Regime –No Movement: flow velocity below critical entrainment velocity –Ripples: straight crested (2d) to sinuous and linguoid crested (3d) ripples (< ~1mλ) with increasing flow velocity –Dunes: (2d) sand waves with straight crests to (3d) dunes (>~1.5mλ) with sinuous crests and troughs

11 4b_G435.pps 11 Bed Response to Steady-state, Unidirectional, Water Flow Lower Flow Regime –No Movement: flow velocity below critical entrainment velocity –Ripples: straight crested (2d) to sinuous and linguoid crested (3d) ripples (< ~1mλ) with increasing flow velocity –Dunes: (2d) sand waves with straight crests to (3d) dunes (>~1.5mλ) with sinuous crests and troughs

12 4b_G435.pps 12 Bed Response to Steady-state, Unidirectional, Water Flow Upper Flow Regime –Flat Beds: particles move continuously with no relief on the bed surface –Antidunes: low relief bed forms with constant grain motion; bed form moves up- or down-current (laminations dip upstream)

13 4b_G435.pps 13 Flow regime Concept (summary)

14 4b_G435.pps 14 Application of Flow Regime Concept to Other Flow Types

15 4b_G435.pps 15 Application of Flow Regime Concept to Other Flow Types Deposits formed by turbulent sediment gravity flow mechanism –turbidites –Decreasing flow regime in concert with grain size decrease Indicates decreasing flow velocity through time during deposition

16 4b_G435.pps 16 Sediment Gravity Flow Mechanisms Sediment Gravity Flows: –20%-70% suspended sediment –High density/viscosity fluids suspended sediment charged fluid within a lower density, ambient fluid mass of suspended particles results in the potential energy for initiation of flow in a the lower density fluid (clear water or air) mgh = PE –M = mass –G = force of gravity –H = height –PE= Potential energy

17 4b_G435.pps 17 Distinction of Sediment Gravity Flow Mechanisms otbo Fluid Flow and Grain Support Mechanisms Newtonian Fluids (fluidal flows) –turbidity currents; grain support turbulence Plastics with a yield stress, or finite strength –High concentration sediment gravity flows: –debris flows; grain support fluid strength & buoyancy X X

18 4b_G435.pps 18 Sediment Gravity Flows Not distinct in nature Different properties within different portions of a flow Leading edge of a debris flow triggered by heavy rain crashes down the Jiangjia Gully in China. The flow front is about 5 m tall. Such debris flows are common here because there is plenty of easily erodible rock and sediment upstream and intense rainstorms are common during the summer monsoon season.

19 4b_G435.pps 19 Fluidal Flows Turbidity Currents –Re (Reynolds #) is large due to (relatively) low viscosity –turbulence is the grain support mechanism –initial scour due to turbulent entrainment of unconsolidated substrate at high current velocity Scour base is common

20 4b_G435.pps 20 Fluidal Flows Turbidity Currents –deposition from bedload & suspended load when F i >F m (F m = mobility forces; F i = grain inertia) –initial deposits are coarsest transported particles deposited (ideally) under upper (plane bed) flow regime

21 4b_G435.pps 21 Fluidal Flows Turbidity Currents –as flow velocity decreases (due to loss of minimum mgh) finer particles are deposited under lower flow regime conditions high sediment concentration commonly results in climbing ripples –final deposition occurs under suspension settling mode with hemipelagic layers

22 4b_G435.pps 22 Fluidal Flows The final (idealized) deposit: Turbidite –graded in particle size –with regular vertical transition in sedimentary structures Bouma Sequence and facies tract in a submarine fan depositional environment

23 4b_G435.pps 23 High Concentration Sediment Gravity Flows Grain Support –Matrix strength (yield stress) –Matrix density causing grain buoyancy in excess of clear water fluids Laminar flow mechanisms due to very high fluid viscosity (Re is low) Occur in both subaqueous (clear water is ambient fluid) and air Cessation of flow is by "freezing" (gravity stress < yield stress) XX

24 4b_G435.pps 24 High Concentration Sediment Gravity Flows Indicate generally unstable slopes (moderate to high relief) Internal sedimentary structures –little scour at base –very poor sorting, massive bedding –large particle sizes may be transported, matrix support –inverse to symmetric size grading –clast alignment parallel to flow surface X X

25 4b_G435.pps 25 Debrites Debris flow deposits –See Turbidites Turbidity current deposits


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