Classification and Distribution the Sediment Classificationscheme Used on the 1:5 000 Map Series Is Primarily Facies- and Material- Related

Classification and Distribution The sediment classificationscheme used on the 1:5 000 map series is primarily facies- and material- related. The symbols on the maps comprisea prefix to indicatethe age of the sediments,followed by superscriptedlower-case letters to indicate environmentof depositionand upper-caseletters to indicate material. Ages of the sedimentsare Pleistocene(Qp), Holocene(Qh) and Quaternaryundivided (Q). The environmentsof depositionseen in the district (Figure 11) are slope (s), alluvial (a), beach(b), intertidal (i) and marine(m). The materialsare fill (F), debris (D), sand(8) andmud (M). The lithostratigraphicunits used in the 1:20000 mapseries are not used in the 1:5 000 series. The corre- lation betweenthese units andthe faciesmapped here is shownin Table2. Essentially,the Chek Lap Kok Formationin the district is representedby Pleistocenealluvial sandand mud,while the Hang Hau Forma- tion is representedby mud and sandof marineorigin. The preciserelationship between these formations and slope debris,onshore alluvium, and beachand intertidaldeposits is not currentlyestablished. Superficial sedimentsoccur mostly in the low-lying ground and offshore areas,although slope debris generallyoccurs on the higher ground,filling small valleys. Downslope,these depositsmay have been reworked by fluvial processes,and the distinctionbetween slope debris and alluvium is difficult to define. In the northeast,much of Tsing Yi New Town is underlainby shallowmarine, intertidal and alluvial sediments,representing deposits of Tsing Yi Bay and the inlet betweenTsing Yi and the island of Nga Ying Chau,mostly reclaimedduring the 1980s(Figure 12). Offshore, the seabedcomprises predominantly marine mud and sand of the Hang Hau Formation. Thin alluvial sedimentsof the Chek Lap Kok Formationoccur beneaththese marine sedimentsover much ofthe district and occur at seabedin areasof strongcurrents. In someparts of the RamblerChannel and inthe Ma Wan Channel,to the westofTsing Vi, currentshave scoured all sedimentsdown to bedrock. ChiS Qa QhbS QhiM OhmS HW Rock and weathered rock Figure J J -Schematic Section Showing the Relationship between Superficial Deposits and the Different Environments of Deposition ., Qp = Pleistocene; Qh = Holocene; Q = Quaternary (undivided); s = slope; a = alluvial; b = beach; i = intertidal; m = marine; F = fill; D = debris; S = sand; M = mud; HW = high-water level 34 9~~ Figure 12 -Pre-reclamation Geography of Tsing Yi Bay and Nga ring Chau Inlet ShowingSuperficial Sediments Slope Debris Depositsbelonging to this classification,which in Hong Kong are generallyreferred to as colluvium, are accumulationsof locally-derivedmaterial. In mostcases, these are debrisflows which havemoved down- slopeduring periods of high rainfall. In manycases on Tsing Vi, the debriscomprises a predominanceof boulders,particularly in steepshallow valleys alongthe southernand easternslopes of the centralrange of hills (Plate 14). Slope debris commonlyoccurs on hillslopes and in shallow valleys,where alluvial processescan sometimes be seento have reworkedthe debris. The debris flow frequentlygrades into alluvium at the foot of suchvalleys, and the boundarybetween the two faciesis difficult to determine.This is especiallythe case on the northwesternshore, where, in manycases, the inter-relationshipis maskedby coastaldevelopment. The matrix of the slope debris is generallya densefine to coarsesandy silt. Sedimentcolour rangesfrom greyish brown through yellowish brown to reddishbrown. Boulders compriseslightly- to completely- decomposedgranite and volcanic rocks, whichreflects the local bedrockgeology. 35 Plate 14 -Aerial Photograph ofEastern Tsing Yi ShowingTypical Boulder Fields, 1963 Alluvial Sediments Alluvial depositsmay be identified both onshoreand offshore.The onshorealluvium typically infills low- lying valleys, much now buried beneathdisused paddy fields and later coastaldevelopment. The offshore alluvium usuallyunderlies marine muds and sandsof the Hang Hau Formationand is designatedthe Chek Lap Kok Formation. Onshore,the alluvium generallycomprises medium dense yellowish brown silty sand with gravel, soft to firm yellowish brown clayey silt, and soft pale greyish yellow clay with fine sand and fine to coarse gravel. As much of this alluvium occursin low-lying land nearthe coast,most has now beencovered with reclamationfill. Some of this alluvium may be late Pleistocenein age but most is consideredto be Holocene. Offshore, the alluvium is entirely of late Pleistoceneage and belongsto the Chek Lap Kok Formation (Strange& Shaw,1986). Throughoutthe district, the formationis thin (up to 5 m) and appearsto fill hollows in the bedrock(Figure 13). It is poorly sorted,firm to stiff, yellow to reddishbrown sandysilty clay with gravel. Recognitionof this formationon seismicprofiles is very difficult in the district becauseof its thinnessand impersistence. Chek Lap Kok Formationis much thinner than in many other parts of the Territory. This suggests that pre-Holoceneerosion cut deeplyinto rockheadand alluvial sedimentshere before depositionof the comparativelythick sequenceof marinesediments of the Hang HauFormation. Beach Sand Over 80% of the coast of Tsing Yi has been developed and consists of reclaimed land. Of the remaining natural coastline, all in the northwestern part of the island, much of the shore is rocky with occasionalbeaches. This shore is now being rapidly reclaimed, and it is anticipated that by late 1995, the entire Tsing Yi coastline will be man-made. Beach sand also exists beneath the extensive reclamation in the northeast and in the south of the island. The sand is dominantly loose yellowish brown fine- to medium-grained with shell fragments and gravel. In some boreholes on the east coast, it is described as grey to black silty clayey medium sand. The colour change may relate to chemical reduction of the organic debris in the sand. 36The Intertidal Sediments Intertidal mud and sand are presentbeneath the reclamationin the northeastof the island (Figure 12). The extent of thesesediments has beeninterpreted from pre-reclamation(1963) aerialphotographs. The sedimentshave beendivided into intertidal mud (QhiM),intertidal sand (QhiS)and undifferentiatedsedi- ments (Qhi)and have beensampled in a numberofboreholes. The intertidal mud is generallya soft dark grey to yellowish brown sandyclay with shell and coral fragments and lithic gravel. The muds are usually confmedto the inner parts of the esfuarywhere current energyis low. Intertidal sandtends to be argillaceousand is interbeddedwith soft greyishbrown clayey silt and sandwith somegravel. Sandsoccur in more openparts of this area and interdigitatewith the marine sands. The boundarybe- tweenthese two facies is difficult to defme. Intertidal sandis also identified near to the coast,where it occursalongside beach sand and alluvium. In someinstances, areas may be identified from aerialphoto- graphsas intertidal, but lack of boreholedata prevents differentation into sandor mud. 10 00 20 -0 C 0 U OJ .~ = 30 .5 OJ E ~ ""Qj 40 ~ ...'- >- CO S: I 0 50 S: f- 0 0 )(0) )«(\1 .-a) ,-=0) "-It) It) 00 ~Q) 5 ..I::..- 0. Q) "tJ Q) "+ro E 'x ...0 0. «0. Figure 13 -Seismic Profile Showing the Character of the Chek Lap Kok (QCK) and Hang Hau Formations in the Tsing Yi Channel(for location,see Figure 4) 37 60 Marine Sediments Sedimentsbelonging to this classificationhave beenlaid down in a sub-littoral shallow marine environ- ment. They comprisemuds and sandsof the Hang Hau Formation(Strange & Shaw, 1986) and are entirely of Holocene age. They occur in the offshore areasand beneathcoastal reclamation fill. Marine sandalso occurs beneathfill in the inlet betweenNga Ying Chauand Tsing Yi in the northeasternpart of the island (Figure 12). Interpretationof seismicrecords demonstrates a large variation in thicknessof the Hang Hau Formation aroundthe district. In the Ramblerand Ma Wan channels,currents have scoured sediments down to bedrock, and in theseparts the formationis absent. It thickeIis markedlyon channelmargins (Figure 13), in small bays near the coastand in hollows in the irregularly-erodedbedrock north of the island, where it reachesa thicknessof over 25 m. To the southof the island,the formation generallythickens away from the coastlinebut, as in the north, local thickening in rockheadhollows is seennear the shore. Marine mud is presentat seabedin the southernpart of the RamblerChannel and the westernpart of the Tsing Yi Channel. It also occurs in small bays on the westerncoast of Tsing Yi and at TsuenWan. It comprises'o'ery soft 10 soft, very dark grey clayey silt with shell fragments. Off the southwesterlytip of the island,marine mud is overlain by marine sand. In the westernpart of the Tsing Yi Channel,marine mud overlies thin marinesand. The marine sandis dominantlysoft grey clayeymedium-grained and containsshell fragments.These shell fragments,sometimes with lithic gravel, are often concentratedin bands,which may be indicative of storm events. The sandusually overlies stiff clay and densesand of the Chek Lap Kok Formation,but to the southwestof Tsing Yi, overliesmarine mud. In the easternpart of the Tsing Yi Channeland the north RamblerChannel, the marinesand is very thin. Modification of the SeaBed The seabedin the district has beenconsiderably modified by man. Southwestof Tsing Vi, sandbas been dredgedas fill for reclamation;the dredgedarea is shownon the map sheet. The sand,which is alluvial and part of the Chek Lap Kok Formation,occurs in a positive bathymetricfeature

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