ABSTRACT

The development of overpressured fluids during softsediment deformation and compaction can be illustratedusing an analogue experiment, which is a simple,inexpensive and reusable teaching tool. Sand, mud andwater are shaken in a plastic bottle. When the sedimentssettle and the water has cleared, the sand supports a layerof mud. Gentle squeezing of the bottle causes the sand topack more closely and liquefy. Water is expelled from thesand, and becomes trapped beneath, and thereforesupports, the mud layer. The pressure in the waterbeneath the mud changes from hydrostatic tohydrostatic plus the pressure exerted by the weight of themud layer, i.e. it becomes overpressured. Theoverpressured water causes blistering and eventualcracking of the mud layer, with a plume of muddy waterrising from the blister. The muddy water from the plumesinks because it is denser than the clear water. The mudlayer also eventually sinks to become supported by thesand, and hydrostatic pressure is restored to the watercolumn. Natural examples that are the result of thisbehavior include sub-horizontal calcite veins, clastic sills,and mud- and sand- volcanoes.

Keywords: Apparatus, Education – Laboratory,Structural Geology.

INTRODUCTION

This paper demonstrates a simple, inexpensiveexperiment for the development of overpressured fluidsduring compaction and subsequent soft-sedimentdeformation. Overpressured fluid exerts pressure thatexceeds that of the hydrostatic gradient at a specificdepth (e.g. Thomeer and Bottema, 1961; Osborne andSwarbrick, 1997). Overpressured fluids operate invarious geological processes. For example, the crucialrole of fluid pressure in controlling thrust faulting wasrecognized by Hubbert and Rubey (1959), who showedthat large thrust sheets can move on gently-dipping faultsurfaces only if the fluid pressure is close to, or exceeds,that of the overburden. The overpressurized fluideffectively supports the mass of the overlying thrustsheet. Evidence for overpressured fluids in thrust sheetsincludes sub-horizontal veins in the walls of thrusts,these having initiated as cracks caused by overpressuredfluid (e.g. Sibson, 1989; Teixell et al., 2000).Sub-horizontal veins are common in sedimentary rocks,and indicate that fluid pressure exceeded the hydrostaticpressure plus the pressure exerted by the weight of theoverlying rocks (e.g. Cosgrove, 2001). Furthermore,overpressured fluids are of great interest to thepetroleum industry because they influence hydrocarbonproduction. For example, gushers are wells that releaseoverpressured oil that fountains above ground level.

Because overpressured fluids are important in thefield of geology, simple experiments demonstrating thenature of their development are beneficial to students

and the classroom experience. Osborne and Swarbrick(1997) showed that overpressured fluids can beproduced by: (1) an increase in compressive stress, (2)fluid movement or buoyancy, and (3) changes in thevolume of the pore fluid or rock matrix. Theoverpressured fluids described in this paper areproduced by a change in volume of the rock matrixcaused by compaction of the sediment volume. Osborneand Swarbrick (1997) describe disequilibriumcompaction as being caused by rapid burial and theresultant rapid increase in overburden stress. Thisrequires rapid expulsion of fluids, with fluids beingoverpressured if the fluids cannot escape quicklyenough. Osborne and Swarbrick (1997) showed that theincrease of vertical loading during burial of sedimentcauses the rearrangement of the sand grains and somedissolution at grain contacts. This causes sandstones tocompact from about 39-49% original porosity at the timeof deposition to about 12-25% porosity after burial todepths of 2-3 km. Although the pore pressure generatedby disequilibrium compaction never exceeds thelithostatic pressure, it can exceed the tensile strength ofthe rock, and therefore it exceeds the fluid pressureneeded to open up extension fractures (Osborne andSwarbrick, 1997). Cracks that are opened up by anincrease in the internal fluid pressure are termedhydraulic fractures (e.g. Phillips, 1972; Cosgrove, 1997).

Lowe (1975) describes three processes by whichfluids escape from sediments. Liquefaction involves thesudden disruption of metastable, loosely packed grains.The grains become temporarily suspended in pore fluid,but settle rapidly with grain-support re-established (e.g.Terzaghi, 1950; DeGregorio and Ahmadi, 1990).Fluidization occurs when drag exerted by moving porefluids exceeds the effective weight of the grains (seeNichols et al., 1994). Grains are lifted and the grainframework is destroyed, with sediment strength beingclose to zero. Liquefaction and fluidization can produce arange of water escape structures, with grains havingbeen rearranged by the moving water. Seepage is therelatively slow movement of pore fluids through existingpores, without rearrangement of grains.

There have been various theoretical andexperimental approaches to studying liquefaction,fluidization, and the related changes in pore-fluidpressure. For example, DeGregorio and Ahmadi (1990)presented a continuum theory in which they comparedexperimentally measured values for shaken saturatedsand. Nichols et al. (1994) described an experiment inwhich fluidization is generated in layered sediments bypumping water from the base of a perspex container.Fluidization occurs when fluid pressure balances theweight of the overlying non-fluidized layer. Theimpermeable top layer lifts away from the fluidizinglayer as fluid pressure increases. Instabilities develop inthe top layer, with sand volcanoes developing at lowflow rates, and fragmentation occurring at high flowrates. The experiment presented in this paper is similarbut much simpler to build and demonstrate than theexperiments presented by Nichols et al. (1994).

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A Simple Experiment to Demonstrate Overpressured Fluids andSoft Sediment Deformation

D.C.P. Peacock Robertson Research International Ltd., Llandudno, LL30 1SA, U.K.,dcp@robresint.co.uk

SIMPLE ANALOGUE EXPERIMENT

The set-up of the experiment is shown in Figure 1A.Sand, mud and water are mixed together in a transparentplastic 2-liter bottle ( 121 mm wide, 82 mm thick and 200mm high). Salt is added to speed up the settling of themud, and bleach is added to stop organisms fromgrowing in the bottle. The relative quantities of thesediments do not appear to affect the results, but thereshould be enough sand for liquefaction to be significant,and the mud layer should not be < 1 mm or > 10 mmthick. The water above the settled sediments should betens of millimeters deep. The lid should be firmly

tightened, and the bottle shaken vigorously and allowedto settle for several hours on a stable surface. Care isneeded to make sure that sand has not stuck to the top ofthe bottle, because this can fall during the experimentand affect the results. The sand must be allowed to settleundisturbed; even slight disturbances during or aftersettling cause premature liquefaction.

Photographs of a typical sequence of events areshown in Figutr 1; this sequence takes seconds tominutes to occur. Several hours after being shaken, thewater has cleared and the mud has settled to form a layerseveral millimeters thick above the sand (Figure 1A). Thesand compacts and liquefies when the bottle is gentlysqueezed or knocked. Water is expelled from the sandand trapped under the mud, developing a layer of water(Figure 1B). Eventually, a blister develops in the uppersurface of the mud (Figure 1C), with cracks developingon and around the blister as the mud layer is stretched(Figure 1D). The blister eventually bursts, and a plume ofmuddy water extends up from the entrapped waterlayer, into the clear water above the mud layer (Figure1E). The plume water is muddy, and therefore denserthan the clear water, because material is suspended asthe plume rises through the mud layer. This dense plumewater sinks and the water starts to clear (Figure 1F). Themud layer eventually sinks back to the top of the sandlayer.

Blister size and the height of the plume appear to bepartly controlled by the thickness and composition of themud layer. Thin layers of mud (~ 1 mm thick) producemany small blisters, each of which may develop a smallplume. Thick layers of mud (~ 10 mm thick) producelarge blisters, but typically produce large blisters withsmall plumes or mud flows, apparently because therising plume of water entrains so much mud it becomesdense than the surrounding clear water. The highestplumes are created when the mud layer is ofintermediate thickness (~ 5 mm thick).

The experiment presented in Figure 1 producessimilar results to the fluidization experiments of Nicholset al. (1994), with an entrapped layer of water leading tothe development of water escape structures. There aredifferences, however, with the experiment illustrated inFigure 1, having the following characteristics: (1) it is aclosed system, in a sealed bottle, (2) liquefaction occursas the sand is disturbed, rather than fluidization as wateris pumped through the sand, (3) it is simpler and lessexpensive than the experiments of Nichols et al. (1994),and (4) it is reusable.

THE DEVELOPMENT OFOVERPRESSURED FLUIDS

Figure 2 shows a model for the sequence of eventsillustrated in Figure 1. Loosely packed sand supports thelayer of mud at the start of the experiment (Figure 2A),with hydrostatic pressure existing throughout the watercolumn. The sand liquefies when the bottle is gentlydisturbed (e.g. squeezed or knocked), with an entrappedlayer of water developing beneath mud (Figure 2B). Adensity contrast is developed in the material beneath themud, with denser sand below a layer of less dense water.The trapped water now supports the mud layer(assuming the saturated mud layer has little or nostrength or is not attached to the edge of the bottle), so thepressure of the water beneath the mud (PM) is given by:

PM = PH + PM (1)

Peacock - Experiment to Demonstrate Overpressured Fluids and Soft Sediment Deformation 411

Figure 1. Photographs of a typical sequence of eventsduring the experiment. (A) A 2 liter plastic bottle isfilled with sand, mud and water. When shaken andsettled, the sand is 102 mm thick, the mud is 5 mmthick, and the water above the mud is 93 mm deep. Saltand bleach are added to speed up settling of the mudand to prevent organisms from growing respectively.(B) The sand liquefies when the bottle is gentlysqueezed or knocked, and a layer of water is trappedbeneath the mud layer. (C) A blister develops in mudlayer. (D) Oblique view of a crack and plumedeveloping in a blister on the top of a mud layer in a 2liter, 105 mm diameter bottle. (E) A plume of muddywater from the overpressured water layer breaksthrough the cracked mud layer and rises into the clearwater. (F) The water from the plume sinks, and the mudlayer eventually sinks to the top of the sand as morewater escapes from the water layer.

where PH = hydrostatic pressure and PM = pressureexerted by the weight of the mud layer. The waterbeneath the mud is therefore at higher pressures than thewater above the mud, so is overpressured. The mudlayer is relatively thin, so the hydrostatic pressure aboveand below the mud is very similar, and PM becomes thedriving force for subsequent deformation. Water isnearly incompressible, so the volume of the water andsand beneath the mud layer is almost unchanged byliquefaction.

Instabilities then develop in the floating mud layer,possibly caused by variations in the thickness of the mudlayer. A blister typically develops (Figure 2C), althoughsometimes the overpressured water forces its wayupwards between the mud and the edge of the bottle.Expansion cracks develop within and around thegrowing blister as the mud layer stretches (Figure 1d).The blister eventually bursts, with a plume of waterrising from the water layer and exiting upwards throughthe mud layer and into the overlying water (Figure 2D).The plume is visible because mud is entrained as theoverpressured water rises through the mud layer. Theplume rises even though the entrained mud makes itdenser than the surrounding clear water, so theexpulsion of the plume illustrates that the water beneaththe mud layer is at higher pressures than is the waterabove the mud (Equation 1). The higher density of theplume water causes it to sink back towards the mud

layer. The mud layer eventually sinks to be supported bythe sand layer as the entrapped water escapes upwards(Figure 2E), and the originally entrapped pore water isrestored to hydrostatic pressures (Figure 2F).

NATURAL EXAMPLES

The natural examples in this section illustrate visualsimilarities with the range of behaviors shown in Figure1. Sub-horizontal fibrous calcite veins with cone-in-conestructure (prismatic- or beef-veins) are common in theJurassic shales and marls of Dorset, England (Figure 3A).The prismatic veins in Devon are up to about 20 mmthick, and individual veins may be meters long. Isotopedata suggest that prismatic veins formed diageneticallyat depths of tens to hundreds of meters (Marshall, 1982).Jenkyns and Senior (1991) discuss Jurassic-age faulting inthe Wessex Basin, which includes Dorset. Evidenceincludes fault-related variations in Jurassic facies andJurassic rocks in neptunian dykes in Carboniferousbasement. Extension in the Wessex Basin probablyoccurred throughout the Mesozoic (e.g. Stoneley, 1982;Karner et al., 1987). It is likely, therefore, that normalfaulting and earthquakes occurred during depositionand diagenesis of the sedimentary rocks illustrated inFigure 3A. This tectonic activity may have triggeredliquefaction and dewatering of the sediments, leading tothe development of the prismatic veins (cf. Figure 1B).

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Figure 2. Model for the sequence of events shown in Figure 1. (A) Poorly-sorted sand supports the mud layer,with hydrostatic pressures throughout the water column. (B) The sand liquefies when the bottle is gentlysqueezed. Water is expelled from the sand but is trapped by the mud, causing a layer of water to develop beneaththe mud layer. The water layer supports the mud layer, so the pressure of the water beneath the mud layer is thehydrostatic pressure plus the pressure exerted by the weight of the overlying mud. (C) A blister develops in themud layer, caused by higher water pressures beneath the mud layer than above. (D) Cracks develop in the blisteras the mud layer is stretched, allowing a plume of water to rise up through the mud layer. This plume water isdenser than the clear water because of material entrained as flow occurs through the mud layer. The plumeillustrates that the water beneath the mud layer was at higher pressures than above the mud, with the plumerising in spite of being denser. (E) The muddy plume water sinks, illustrating it is denser than the surroundingwater. The mud layer sinks as more water escapes from the layer of water, with hydrostatic pressures restored tothe water column. (F) Graph of the variation in fluid pressure with depth during the sequence shown in Figure2A-E. Hydrostatic pressures occur throughout the water column in Figures 2A and 2E.

Clastic sills (Figure 3B) are injection structures thatare commonly seismically induced, and that can be usedto determine the timing and magnitude of prehistoricearthquakes (Obermeier, 1996, 1998). Clastic intrusionsdevelop when water and sediment, usually sand, flowupward into cracks along the base of finer-grainedoverlying units. The resultant geometry can be similar tothe blistering of the mud layer illustrated in Figure 1C.The example shown in Figure 3B is similar to the idealform of an igneous laccolith, as illustrated by Corry(1988; based on Gilbert, 1877). Laccoliths are formedwhen pressurized magma causes a blister in theoverlying layered sedimentary rocks.

Mud volcanoes are volcano-shaped cones of mudand clay, usually < 1-2 m tall. The mud volcanoillustrated in Figure 3C was built-up by a mixture ofhydrothermal water and mud that poured gently from avent similar to a fluid lava flow. It has a similarappearance to the plume of muddy water illustrated inFigures 1D and 1E. Graue (2000) describes mudvolcanoes from offshore Nigeria that are 1-2 km indiameter and tens to hundreds of meters high. Thisregion exhibits rapid deltaic sedimentation of sand-richsediments, with active normal faulting. The mudvolcanoes are inferred by Graue (2000) to havedeveloped above faults, especially in rollovers abovelistric normal faults. Graue suggests the mud volcanoesare related to overpressuring caused by gas expansionand by escaping compaction water and hydrocarbons,

with periodic expulsion of water, gasses and solidmaterial.

Sand liquefaction is common during earthquakes(e.g. Seed, 1968) and can cause sand blows, sand boilsand sand volcanoes to form as pressurized water andentrained sediment reach the surface (Figure 3D). Theycan form in sub-marine (e.g. Papatheodorou andFerentinos, 1997) and sub-aerial (e.g. Sieh, 1978)environments, and are commonly triggered byearthquakes. The sand blow illustrated in Figure 3D isanalogous to the settled plume material illustrated inFigure 1F.

Conditions Favoring Liquefaction and Over-pressuring - Osborne and Swarbrick (1997) showed thatdisequilibrium compaction is particularly likely to occurin low permeability sediments (e.g. thick clays andmuds) and in adjacent high permeability sediments (e.g.sands) undergoing rapid burial. Liquefaction andfluidization can be triggered by external disturbances,including earthquakes and rapid deposition ofsediments (e.g. Lowe, 1975; Sieh, 1978). The developmentof water escape structures resulting from liquefactionand fluidization is favored in fine- to medium-grainedsands with high instantaneous and mean sedimentationrates, especially where permeability decreases upwardsin the sediment column. Lowe (1975) showed that theseconditions are common in normally graded turbiditesand deep-sea flysch pro-deltas. Liquefaction and

Peacock - Experiment to Demonstrate Overpressured Fluids and Soft Sediment Deformation 413

Figure 3. Natural examples of the geometries illustrated in Figures 1 and 2. (A) Sub-horizontal veins (prismaticveins) in the Jurassic shales from the cliff face of Black Ven, Charmouth, Dorset. This vein probably formed fromcalcite crystallization from a fluid-filled water layer (c.f. Figure 1B). (B) Laccolith-shaped clastic sill (white sand)between blue clay beds (c.f. Figure 1C). Photograph from Obermeier (1998), source of imagery courtesy of theUSGS. (C) ~ 400 mm tall mud volcano in the Norris Geyser Basin, Yellowstone National Park, Wyoming (c.f.Figures 1D and 1E). Source of imagery courtesy of the USGS. (D) Sand blow, formed as water and entrained sandwere extruded through a mud layer (c.f. Figure 1F).

overpressuring is therefore most likely to occur intectonically active regions in which rapid sedimentationand burial occurs. Nichols et al. (1994) showed thatfactors controlling the vigor of rupture of the top layer,and therefore the style of fluid escape structures, include:(1) difference in grain size between top and bottomlayers, (2) grain shape and density, (3) thickness oflayers, (4) layer cohesion, and (5) water pressure.

CONCLUSIONS

A simple, inexpensive experiment illustrates thedevelopment of overpressured fluids and subsequentsediment disruption during the compaction andliquefaction of soft sediments. Sand, mud and watershaken together in a plastic bottle and allowed to settleresult in the sand supporting a layer of mud. The bottle,when gently squeezed or knocked, causes liquefaction ofthe sand. Water is expelled from the liquefied sand and istrapped as a layer of water beneath the mud layer. Thetrapped water then supports the weight of the mud andso becomes overpressured. Next, the mud layer blistersand cracks, with a plume of muddy water being expelledthrough the mud layer and into the clear water above.The denser plume water subsequently sinks towards themud layer, and the mud layer sinks to be supported bythe compacted sand, with hydrostatic pressures restoredto the water column. The experiment illustrates variousgeological processes, including the development ofsub-horizontal cracks in soft sediments, intrusion ofclastic sills, the processes similar to the development ofigneous laccoliths, and mud- and sand- volcanoes(Figure 3).

ACKNOWLEDGEMENTS

Elisabeth Parfitt and John Peacock are thanked for theirsuggestions about overpressuring and related processes.Graeme Caselton, Steven Brantley and Martitia Tuttleare thanked for providing Figures 3A, 3C and 3Drespectively. The United States Geological Survey isthanked for permission to use Figures 3B and 3C. I amvery grateful to Tim Demko, Russell Dubiel and StevenGood for providing careful reviews, and to MartinTraugott and an anonymous reviewer for providinghelpful comments on an earlier version of this paper.

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