Download The tenn karst defines a terrain with distinctive landfonns and

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
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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.
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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.
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