402-pp5 - UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary...

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Unformatted text preview: UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary Petrology Lecture 5: Bedform Development (Flume Studies) Last Time 1. 2. 3. 4. 5. Six modes of sediment movement Real simple fluid dynamics (ideal conditions) Initiation of sediment movement Nasty mathematical relationships (Shield's Diagram) Useful empirical relationships (Hjulstrom's Diagram) Sediment Motion Rest (no movement) Roll Slide Saltation ("bouncing") Suspension Mass flow (viscous flow) Stoke's Law Vg = gd2(F-D) 18: g = gravitational constant (9.8 m/s2) d = particle size diameter (mm) F = grain density (g/cm3) D = fluid density : = dynamic fluid viscosity Vg = settling velocity Stoke's Law Vg . kd2 Vg is proportional to grain size or, alternatively, grain size is proportional to settling velocity Composite curves (Stoke's + Impact) 103 102 log Vg (cm/s) w Impact La 101 100 Sto k aw sL e' 10-1 10-2 10-4 10-3 10-2 10-1 100 101 102 103 log d (mm) Settling Curve (Graphic Representation) 103 102 log Vg (cm/s) 101 100 10-1 Settling Curve 10-2 (AKA Rubey's Curve) 10-4 10-3 10-2 10-1 100 101 102 103 log d (mm) Grain Size Analysis Sand and gravel are determined via sieving sand fractions: vc, c, m, f, vf, gravel fractions: not routinely done Analysis done using simple excel spread sheet (will be available on departmental computers and/or e-mailed to you) Sediment Entrainment (Hjulstom's Diagram) (from Blatt, H, Middleton, G. and Murray, R., 1980. Origin of Sedimentary Rocks. Prentice Hill, 782 p) Blatt, Rocks. Sediment Entrainment (Hjulstom-Sundborg Diagram) (from Collinson, J.D. and Thompson, D.B. 1982. Sedimentary Structures. George Allen and Unwin 194p) Collinson, Allen Today's Lecture 1. What's a flume? 2. Traction induced bed forms (sedimentary structures) 3. Flow regime Flume Studies A flume is an elongated plastic or glass tank through which a water current flows. Sediment is placed at the bottom of the flume and moves down current as bed load. Flume Studies A flume is an elongated plastic or glass tank through which a water current flows. Sediment is placed at the bottom of the flume and moves down current as bed load. Recall what this is Sediment loads Sediment moving along the base of a channel* that mostly stays in contact with the substrate is called... Current * A channel here is defined as a moving column of water that is confined to a narrow pathway Sediment loads Sediment moving along the base of a channel that mostly stays in contact with the substrate is called... Current Bed load Sediment loads Sediment moving along the base of a channel that mostly stays in contact with the substrate is called... Current Bed load saltation rolling sliding Types of bed load transport (traction) Flume Studies Flumes come in different sizes; from a few cm long to a few hundred metres long. Flume Studies small Flume Studies small current Flume Studies Big Flume Studies Big Flume Studies Flume Studies cu r ren t Sand (bed load) Sediment movement results in bed forms (sedimentary structures) Flume Studies Increasing current velocity under controlled situations (fixed water depth, constrained grain size) yields the following structures: Flume Studies Increasing current velocity under controlled situations (fixed water depth, constrained grain size) yields the following structures: Plan lamination (lower) Flume Studies Increasing current velocity under controlled situations (fixed water depth, constrained grain size) yields the following structures: Plan lamination (lower) Small current ripples Flume Studies Increasing current velocity under controlled situations (fixed water depth, constrained grain size) yields the following structures: Plan lamination (lower) Small current ripples Large current ripples (dunes) Flume Studies Increasing current velocity under controlled situations (fixed water depth, constrained grain size) yields the following structures: Plan lamination (lower) Small current ripples Large current ripples (dunes) Plan lamination (upper) Flume Studies Increasing current velocity under controlled situations (fixed water depth, constrained grain size) yields the following structures: Plan lamination (lower) Small current ripples Large current ripples (dunes) Plan lamination (upper) Antidunes Flume Studies Increasing current velocity under controlled situations (fixed water depth, constrained grain size) yields the following structures: Plan lamination (lower) Small current ripples Large current ripples (dunes) Plan lamination (upper) Antidunes Increasing water velocity Flume Studies A do it yourself recipe to make your own flume Flume Studies Take one large tank.... The best flumes are capable of passing a consistent current from one end to the next. Water is recycled through a recirculation pump Sediment/water movement is monitored through the glass sides of the flume Flume Studies Add water.... Flume Studies Add sediment.... Flume Studies current Add a current.... Flume Studies current Add a current.... .... Observe! Flume Studies: 1) lower plan lamination very slow currents (or none at all) U = very low Flume Studies: 1) lower plan lamination very slow currents (or none at all) U = very low Flat bed, containing finely laminated* parallel sedimentary structures Flume Studies: 1) lower plan lamination laminations versus bedding; it's all a matter of scale: Flume Studies: 1) lower plan lamination laminations versus bedding; it's all a matter of scale: laminations: parallel layers less than 1 cm apart: thick (1cm-5mm spacing); medium (5mm-1mm spacing); thin (<1mm spacing) Flume Studies: 1) lower plan lamination laminations versus bedding; it's all a matter of scale: bedding: parallel layers more than 1 cm apart: thick (> 50 cm spacing); medium (10 to 50 cm spacing); thin (1cm-10 cm spacing) Flume Studies: 2) small current ripples slow currents: U = low Something wonderful happens.... Flume Studies: 2) small current ripples Laminations start to fade... Flume Studies: 2) small current ripples Flat bed, passes gradually into a "rippled" bed form Flume Studies: 2) small current ripples Asymmetrical Flume Studies: 2) small current ripples ide oss s St Asymmetrical Flume Studies: 2) small current ripples e de iid es es Le Le Asymmetrical Flume Studies: 2) small current ripples Lee side slopes range from 20-34o Flume Studies: 2) small current ripples ripple height: 3 to 5 cm Flume Studies: 2) small current ripples ripple wavelength: 4 to 40 cm Flume Studies: 2) small current ripples ripple wavelength: 4 to 40 cm Ripple index (height to wavelength ratio): 10 to 40 Flume Studies: 2) summary ripple morphology From Collinson, J.D. and Thompson, D.B. 1982. Sedimentary Structures. George Allen & Unwin, 194p. Collinson, Allen Unwin, Flume Studies: 2) small current ripples Ripples migrate down current Flume Studies: 2) small current ripples Ripples migrate down current Flume Studies: 2) small current ripples Ripples migrate down current Flume Studies: 2) small current ripples current Net result is that an inclined lamination (marking the former lee side of the ripple) develops Flume Studies: 2) small current ripples current Net result is that an inclined lamination (marking the former lee side of the ripple) develops = cross stratification Flume Studies: 2) small current ripples Flume Studies: 2) small current ripples ? Flume Studies: 2) small current ripples Flume Studies: 2) small current ripples Dune fraction Bypass fraction Flume Studies: 2) small current ripples Which grains become part of the dune fraction; which ones are part of the bypass fraction? Big quartz Small quartz Heavy minerals Flume Studies: 2) small current ripples Heavy and larger grains usually become part of the dune fraction; smaller and lighter grains become part of the bypass fraction Big quartz Small quartz Heavy minerals Flume Studies: 2) small current ripples Migration direction Flume Studies: 2) small current ripples Migration direction Ripple co-set Flume Studies: 2) small current ripples 38 cm 59 cm Video of ripple migration in a flume (Bird' eye view) Click image to start The movie was compiled from 1161 video images collected at the rate of 1 per minute for a duration of approximately 19 hours. Source: USGS Coastal & Marine Geology Web page (http://walrus.wr.usgs.gov/seds/). (http://walrus.wr.usgs.gov/seds/). Flume Studies: 2) small current ripples Computer animation of ripple migration Click image to start Source: USGS Coastal & Marine Geology Web page (http://walrus.wr.usgs.gov/seds/). (http://walrus.wr.usgs.gov/seds/). Flume Studies: current ripple crest morphology increasing velocity (or decreasing water depth) straight sinuous linguiodal From Collinson, J.D. and Thompson, D.B. 1982. Sedimentary Structures. George Allen & Unwin, 194p. Collinson, Allen Unwin, Flume Studies: 3) large current ripples Moderate currents U = moderate Flume Studies: 3) large current ripples Moderate currents U = moderate Small current ripples gradually pass into larger ones Flume Studies: 3) large current ripples Lee side angle: 10 to 34o Ripple height: >5cm (commonly exceeds 10's of m) Wavelength: 60 cm to 100's of m Note: also known as megaripples and dunes Flume Studies: 4) upper plan lamination High currents; U = fast Flume Studies: 4) upper plan lamination High currents; U = fast Something wonderful happens again! Large current ripples start to fade and.... Flume Studies: 4) upper plan lamination .... plan lamination forms again Flume Studies: 4) upper plan lamination This form of plan lamination forms only during very fast currents; sediment is literally streaming along the substrate. Flume Studies: 5) antidunes Very high currents; U = very fast Flume Studies: 5) antidunes Very high currents; U = very fast Upper plan lamination fades and is replaced by.... Flume Studies: 5) antidunes Very high currents; U = very fast Upper plan lamination fades and is replaced by.... .... antidunes Flume Studies summary of structures Collinson, Allen Unwin, From Collinson , J.D. and Thompson, D.B. 1982. Sedimentary Structures. George A llen & Unwin , 194p. Flume Studies Collinson, Allen Unwin, From Collinson , J.D. and Thompson, D.B. 1982. Sedimentary Structures. George A llen & Unwin , 194p. Plan lamination (lower) Small current ripples Large current ripples (dunes) Plan lamination (upper) Antidunes Chutes & pools (erosion) Increasing velocity Flume Studies and Flow Regime From Collinson, J.D. and Thompson, D.B. 1982. Sedimentary Structures. George Allen & Unwin, 194p. Collinson, Allen Unwin, Flume Studies and Flow Regime Lower From Collinson, J.D. and Thompson, D.B. 1982. Sedimentary Structures. George Allen & Unwin, 194p. Collinson, Allen Unwin, Flume Studies and Flow Regime Upper From Collinson, J.D. and Thompson, D.B. 1982. Sedimentary Structures. George Allen & Unwin, 194p. Collinson, Allen Unwin, Current velocity (cm/s) Modified Hjustrom's diagram Grain size (mm) Collinson, From Collinson , J.D. and Thompson, D.B. 1982. Sedimentary Structures. Unwin, George Allen & Unwin , 194p. Water depth as a variable Water depth (cm) Collinson, From Collinson , J.D. and Thompson, D.B. 1982. Sedimentary Structures. George Allen & Unwin , 194p. Unwin, Upcoming Stuff Homework 1) Data plotting exercise (due Thursday: no redo) 2) Writing Assignment 2-redo (Grain Size hypothesis/methods: Due Thursday 3) Grain Size Analysis Today's Lab Grain Size Analysis Next Lecture: More Sedimentary Structures Upcoming Activity (week 5): Observations in our flume ...
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This note was uploaded on 02/05/2012 for the course GLY 402 taught by Professor Staff during the Fall '11 term at S. Alabama.

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