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The tenn karst defines a terrain with distinctive landfonns and drainage arising from greater rock solubility in natural waters thanelsewhere(Jennings,1987). Karst hasalso beendefinedas a diagenetic facies; an overprint in subaerially-exposedcarbonatebodiesproducedand controlled by dissolutionand migration of calcium carbonatein meteoric water, occurring in a wide variety of climatic and tectonic settings and generatinga recognisablelandscape.The realisation that karst processeswere active in carbonaterocks throughoutgeologicaltime addeda further dimensionto researchin this field and led to the introductionof the tenn palaeokarstmeaningsimply fossilisedcarbonatefeatures. In the Yuen Long district, carbonaterock exists in the fonn of marble which by metamorphicand tectonic action is in part well-jointed, faulted and folded,resulting in the juxtapositionof many different lithologies. Such situations are ideal for the development of karst features given the necessary hydrological conditions (plates 15 & 16). Karst is found in all latitudes ranging from the poles to the equator,but the higher the temperaturethe fasterthe dissolutionof carbonatesby acid groundwater.The rate of karst developmentdependsalso on the hydraulic conditions, which together with the strengthof the calcareousrock controls the erosion rate. The Yuen Long marble is a very strong rock when fresh, with uniaxial compressivestrength ranging from 65 MPa to 140 MPa (GCO, 1990), ensuring a slower rate of erosion than for softer carbonatessuch as chalk. However, the Ma Tin Member of the Yuen Long Fonnation is chemically almost pure calcium carbonate,and although one of the strongestrocks of the district, is more susceptibleto dissolution than the marble from the weaker more impure Long Ping Member. This reversal results from a high insoluble residue in the Long Ping Member which slows down the dissolutionprocessand at times may fonn a protective barrierto further reaction. Plate 15 -Karst Featuresin BoreholeCores: Solution FeatureShowingFerruginous Staining Plate 16 -Karst Featuresin BoreholeCores: Solution WidenedJoint ShowingFretted Sulfate 48 Notwithstanding the above variables,no rock surfaceis ever unifonn, evenif horizontal and of recent age. It will have suffered variable pressuresresulting from the diageneticprocesses,experiencedlocal compactioneffects, and may have beensubjectedto regional tectonic and metamorphicevents.Bodies of rock respond to stressesoccurring during geological history by fonning joints and fractures. It is along suchhydraulic channelsthat the initial attackby acidic water is concentrated,leaving other zones virtually unchanged.Selective chemical weatheringand erosion is therefore started,which with time results in the many types of karst geomorphology now well documented throughout the world (Jennings, 1987). Examples of some of the types detected in the palaeokarstof Yuen Long are describedbelow. A site adjacentto Wang Lok Street[212 357] on the Yuen Long Industrial Estatewas investigatedby 24 drillholes, and the configurationof the marble rockheadis shown in Figure 18. It is dominatedby a north-southtrending ridge with a steepeasternface. The marble shelvesboth to the eastand west to depths greaterthan -70 mPD, with the top of the ridge closeto -20 mPD proving a relief of some 50 m. A nearby site [213 356] proved by thirteen boreholesshows a depressionwith a diameter of some 100 m and side slopes inclined at angles between20. and 40. (Figure 19). The bottom of the depressionis at about -75 mPD, confinning a range in relief of about 50 m. The bedrock here comprisesboth marble and granodiorite,and the faulted/intrusiveboundary is obviously an important factor in the fonnation of this fossil sinkholestructure. The tenns'sinkhole, swallow hole or swallet refer to closed depressionsin karst. These English terns have, however, been used very loosely and the word doline is now acceptedfor closed depressions varying from circular to oval in plan and dish-shaped,conical or cylindrical in cross-section.Such variety of fonns results from the solution, erosionand collapse processesoperating (Jennings,1987; James & Chokuette, 1987). A doline, therefore,is an indication at the rock surface of a process of removal undergroundof a considerablevolume of material.Numerouscloseddepressionsexist within the marble rockheadof the district, and clay-filled joints and cavities occur commonly at depth. It can be logically concluded that the dolines and cavities are connected.In Yuen Long, doline fonnation was halted at least 50 000 years ago, based on the age of the overlying sediments,with the connecting channelsbetweendoline and the epikarstbeing sealed. The geological model derived from nearly one hundred drillholes at a site on Hop Choi Street [2115 3375] in the southeast of Yuen Long illustrates the juxtaposition and alignment of the irregularities in a palaeokarstterrain (Figure20). It also shows not only the surfacephenomenaof karst, but also more fundamentalstructural trends in the north of the site where the marble surface dips steeply beneathoverlying fonnations. Examination of the direction of jointing for site YL1L 429 [2065 3355] in the south of Yuen Long adjacentto Ma Tin Road (Houghton, 1988) showeda wide spreadof values but with a dominantstrike along a northwest-southeast axis. Most joints dip at angles greaterthan 50.. The fITstfew metresof the karst top are usually poorly recoveredin drillholes, and the core consistsof numerousbroken fragmentsof stainedmarble. Although this surfaceprofile of the karst is irregular, possibly with sporadic detachedcorestones,the drilling processhas howeverexaggerated this disruption of the surface. Inspectionof more than 1 000 drillholes through marble in the district shows that in generalthe top 5 m of the karstic rockheadis most affected by dissolution (plates 15 & 16). The next 25 m also shows karstification which decreasesin intensity with depth until the only indications of dissolution are stained joint planes. This total zone of karstic erosion is tenned the epikarst. One of the main concernsfor developmentin an area of karst terrain is the presenceof cavities within the carbonatefonnation. These may be shallow structures closely linked by widened joints to the surface or deep-seatedvoids, partially or totally fllled by recentsediments.Such sedimentsare usually poorly cementedsilts and sandsor soft clays, commonlywith an organiccontent. Cavities within the district are of threetypes: 1 Cavities in the epikarstusually in the top 30 m of the marble bedrock 2 Cavities beneaththe epikarst,usually30 to 60 m below the marblerockheadbut probably linked to the surfaceby major joints or discontinuities 3 Cavities associatedwith other lithologies or faults Recoveryof cavity-fill material from drillholes is usually poor, with the soft sedimentsbeing washed away by the drill flush. However in BGS 2 a maziersampleprovided an excellent section from 76.28 to 77 m of part of a 3.45 m wide cavity (Figure 21) which indicatesa repetitive cyclical sedimentation. The basal 100 mm of the recoveredsectionconsistsof cross-beddedsilty fine sand with interlaminated clay layers, proving an initial surgeof sedimentsin a relatively high energyenvironment.These sands are overlain by structurelessgrey to black organic sandyclays, suggestingslow depositionpossibly in brackishwater. The clays contain angular fragmentsof chert up to 20 mm long which may have fallen from a vein in the roof or side of the cavity. The clays passupwardsby interlaminationinto grey silts. 49 cavities are near vertical or steeplydipping joint-controlled featureswhich have been widened,and in places filled or partly filled with sandsand clays. Thesejoints are the dominant overprint in the marble of the regional metamorphismof the district, and therefore form the commonesttype of cavity. Their steeply dipping attitude explains why one borehole will show well developedkarstification, whilst an almost adjacentborehole proves sound rock. In Florida it has been calculated that some 6% of the limestone has beendissolved in one area affected by karst processes.Similar calculations in the Yuen Long area show a range of 6% to 50% dissolutionof the marble in the karst zone. The high figures are derived, however, from many boreholeswhich have beendrilled vertically down karstic joints. If most of the cavities are formed along near vertical joints then an inclined borehole should pass through multiple sets of cavities, joints or discontinuities within the rock formation. This was confirmed by Houghton(1988) for a site in southernYuenLong. Whilst the majority of cavities occur in the epikarst zone within some 30 m of the rockhead,some boreholes have encounteredclay-filled cavities at depths down to -80 mPD. Such voids must have developedat a stage of the karstification processwhen the sea was at its lowest level relative to the land. The hydraulic link betweenthese deep cavities and the rockheadsurface must have been very tenuous, particularly with the discontinuousnature of the marble outcrop; such cavities should be infrequentand small. In parts of the district a large numberof major discontinuitiessuchas faults and lithological boundariesexist which have beena focus for karstification,and in many caseshave given rise to cavities with a greaterlateral extent thanthosedevelopedfrom joints. Figure 22 shows the position of a thrust plane separatingmarble from siltstone and sandstonenear On Hong Road [20503365] towardsthe southernmargin of YuenLong. This plane was borderedby a zone several metres wide containing numerous fractures associated with tectonic movements. Water movementappearsto have been channelledinto this zone, with dissolution and erosion creating an inclined void some 50 m deep by 10 m wide. This void, which tapers with depth, probably does not extendbeyond -100 mPD. It is one of the deepestcavitiesidentified in the district and is filled with soft silts and sands. Reactivation of Karst Features In certain parts of the world where shallow buried karst exists, catastrophicground collapses occur where palaeokarstsinkholesare reactivated.In most casesthesecollapsesare causedby fluctuations in gr()undwaterlevel. Old sinkholesform a vertical hydraulic link betweendiffering lithologies and in the Yuen Long area are now plugged by sedimentssuch as recent silts sandsand clays. Erosion of this sedimentplug over a period of time may underminethe adjacentstrataand transmit the disruption to the surface with resulting ground movement and damage to structures built upon it (Culshaw & Waltham, 1987). There are paleokarstsinkholesbeneaththe district which, if reactivated,are potential areasof collapse. However, the fluctuations of the groundwaterare slight (Gale & Cook, 1989) so the opportunities for disturbanceof sinkhole plugs are limited. Under certain abnormal conditions, such as surface water inundation by a broken water main or flood, sufficient erosive force from surface water may weaken sedimentplugs if the water table has beenlowered locally by excessiveextraction of groundwaterby pumping. Chan (1988) consideredthe factorsaffecting sinkhole formation, and concluded that their widespread recurrencein Yuen Long was unlikely, provided that the water table remainedabove rockhead.There has been no evidenceof sinkhole formation at the surfacesince the sealingof the karst surface by the overlying superficial deposits, despite hydrogeologicalconditions which must have been far more conducive to sinkhole developmentthan those of the presentday. However, civil engineeringworks may disturb the existing hydrogeologicalregime, for exampleby causingponding of surfacewater,and drilling operationsmay also induce local groundcollapse. 54