Erosion and sedimentation associated with the last sea level rise offshore Hong Kong, South China Sea

Erosion and sedimentation associated with the last sea level rise offshore Hong Kong, South China Sea

Quaternary International 55 (1999) 93—100 Erosion and sedimentation associated with the last sea level rise offshore Hong Kong, South China Sea J.A. ...

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Quaternary International 55 (1999) 93—100

Erosion and sedimentation associated with the last sea level rise offshore Hong Kong, South China Sea J.A. Fyfe *, I.C. Selby , A.J. Plater, M.R. Wright Hong Kong Geological Survey, Civil Engineering Department, Hong Kong Government, Hong Kong, China CRA Exploration Pty. Ltd., PO Box 410, Karratha, Western Australia, Australia Department of Geography, University of Liverpool, UK

Abstract High-resolution seismic data reveals an extensive planar, seaward-dipping reflector offshore south of Hong Kong. This is interpreted as a ravinement surface created during the last transgression and is associated with ca. 2.5 m of shoreface sand, which is overlain by a 6—15 m thick sheet of prodelta-marine shelf sediments. On seismic profiles, the shoreface sand is characterised by a chaotic reflector pattern and the sheet by low-amplitude, parallel, flat or low-angle, continuous reflectors. Particle size data reveal a westward-coarsening and decrease in sorting, which reflects increasing proximity to the Pearl River. Foraminiferal and diatom assemblages demonstrate the same overall westward-shallowing trend, with increase in inner shelf foraminifera and brackish planktonic diatom species. A phase of early Holocene shoreface sand and intertidal silt deposition in eastern and central areas culminated in a brief period of subaerial exposure. This was succeeded by an extensive period of shelf and inner shelf deposition, brought about by variation in balance between the rate of sea-level rise and accommodation, and the discharge and sediment supply from the Pearl River.  1999 INQUA/Elsevier Science Ltd. All rights reserved.

1. Introduction The Quaternary sediments that surround the landmass of Hong Kong comprise a dominantly Middle to Upper Pleistocene and Holocene succession. Interpretation of seismic and borehole data has led to the development of a lithostratigraphy by the Hong Kong Geological Survey (Strange and Shaw, 1986; Langford et al., 1995). The oldest sediments are gravel, sand and mud of the Chek Lap Kok Formation, which overlies rock in various states of weathering. In western and southeastern waters, the Chek Lap Kok Formation is deeply incised and overlain by the Sham Wat Formation (Fyfe and James, 1995). The sediments of this formation comprise intertidal to marine mud with minor intercalated sand. An areally-restricted Waglan Formation has recently been identified in the southeast of the Territory and comprises marine sand and mud (Fyfe et al., 1998). This Pleistocene

*Corresponding author. Present address: British Geological Survey, Mutchison House, West Mains Rd., Edinburgh, UK. Present address: Coastal Geosciences, Salmon Rd., Great Yarmouth, Norfolk, UK.

succession was in part eroded during the last glacial maximum and overlain largely by the Holocene Hang Hau Formation, comprising early shoreface and channelfill sand and intertidal mud overlain by a blanket of marine shelf sediments. Although this lithostratigraphic scheme has been used for some time in Hong Kong, recent work in the field of sequence stratigraphy (Fyfe et al., 1998) has revealed a more complex pattern. A number of surfaces have been identified, including two major fluvial entrenchments and two ravinement surfaces. These are most clearly seen in the southern part of Hong Kong waters away from the influence of nearshore erosion and deposition. Three boreholes, HKGS-A, HKGS-B and HKGS-C have been drilled in this part of the Territory and have been correlated with the sequence stratigraphy. The boreholes have ielded foraminiferal assemblages (Twiddy, 1994) and diatoms have also been examined (Wright, 1994). Particle size analyses have been carried out on samples (Plater, 1996) and radiocarbon dating on in situ shells was undertaken by the Oxford AMS Laboratory. This paper describes the results of the borehole analysis and the correlation with the seismic interpretation. The microfaunal analyses have been integrated with the

1040-6182/99/$ — see front matter  1999 INQUA/Elsevier Science Ltd. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 8 ) 0 0 0 3 0 - 5

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stratigraphy and the Holocene succession placed into the context of a deltaic model of the Pearl River estuary.

2. The Pearl River delta Hong Kong lies on the northern coast of the South China Sea at the mouth of the Pearl River. This river has a catchment of around 425,000 km, an annual discharge of over 30,000 million m and an annual sediment load of 83 million tonnes (Long and Huo, 1990). The interface between the river and the continental shelf can be considered as a delta (Li and Wang, 1987; Li and Wei, 1987) and the influence of this delta on Hong Kong waters has been described by Shaw and Fyfe (1992). Galloway (1975) classified modern deltas on the basis of variations in transportation patterns. His three-fold subdivision into river-, wave- and tide-dominated types still forms the basis of the classification scheme used today. In this classification, the Pearl River delta falls into the river-dominated type, though it is also influenced by the estuary. Four zones have been identified (Li and Wang, 1987), namely delta plain, delta front, prodelta and shelf (Fig. 1). The present Pearl River delta developed during the Holocene (Huang, 1984). During the late Pleistocene, the shoreline lay a maximum of 130 km south of Hong Kong and the present offshore area was part of an alluvial plain (Feng and Shi, 1998). An increase in rainfall, coincident with deglaciation in boreal regions, provided an increase in river discharge and an initial pulse of sediment into the delta. As sea level rose, sea water inundated the area of the estuary, and by 6000 years BP the shore-

line was north of Guangzhou (Long and Huo, 1988). Since then, a minor fall in sea level and the advance of the Pearl River delta has led to the development of the current configuration of the estuary. The present-day hydrographical regime of the Pearl River estuary has been discussed by Watts (1973) and Morton and Wu (1975). In summer, the high rainfall and relatively quiet sea provide an environment for good estuarine stratification. In winter, the lesser discharge and turbulent sea means that mixing is greater, although a saline wedge intrudes further into the estuary. In the western part of Hong Kong waters, the estuarine influence of the Pearl River results in turbid and brackish waters. In the east, the influence of oceanic currents leads to clear and saline waters, with corals growing in Mirs Bay. Between these two limits is a zone of mixing.

3. Faunal assemblages The dominant foraminifera are Ammonia, including A. becarii, Bulimina marginata, Elphidium, Globigerina, Nonion and Quinqueloculina. These and other forms present in the assemblages, were divided into three ecological groups. Benthic genera that occur widely have been classified as shelf and those typical of shallower water as inner shelf. Of the inner shelf genera, Ammonia, Elphidium and Oolina are the dominant forms. Planktonic oceanic forms are represented in the assemblages by a single genus Globigerina. The dominant diatom ecological tolerance groups (according to the scheme of Vos and de Wolf, 1988) are the planktonic Cyclotella striata, Melosira sulcata, Cymatosira belgica groups, and the benthic Navicula digitoradiata group.

4. Offshore stratigraphic relationships 4.1. Southwestern waters

Fig. 1. Present-day sedimentological zones of the Pearl River delta (after Li and Wang, 1987) and location of boreholes HKGS-A, HKGSB and HKGS-C.

In the southwest (Fig. 2), a strong seismic reflector marks the base of the Holocene sequence, where the Hang Hau Formation overlies pre-Holocene marine sediments of the Sham Wat Formation. In borehole HKGS-C (Fig. 2) the Sham Wat Formation is topped by a 4 m thick palaeosol. This records a period of subaerial exposure and suggests that there was little erosion in the west before deposition of the Holocene sediments. Mapping of the base of the Holocene reveals a north—southtrending ridge of older sediments south of Lantau Island. This ridge was a positive feature in the early Holocene and was covered late during the transgression. The unit overlying the basal reflector is characterised on seismic profiles by a set of low-angle intraformational reflectors dipping to the east. This unit is correlated in the borehole with 10 m of sandy mud, the sand occurring as

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Fig. 2. Seismic profile SK1/10 and log of borehole HKGS-C.

lenses, laminae and bands up to 4 mm thick. Radiocarbon age determination (OxA-5430) indicates that these sediments were deposited prior to 8000 years BP. The presence of the ridge of pre-Holocene sediments that was emergent during the early Holocene, however, implies that this unit of low-angled reflectors must have been deposited fairly rapidly. It is suggested that this deposition occurred during a period of advance of the delta front. The low-angled reflector unit is succeeded by one of flat-lying reflectors. This unit is represented in the borehole by clayey silt with very fine sand in thin lenses and laminae and a 4 m thick upward-coarsening bed of sand. The radiocarbon dates (OxA-5428, 5429) show an overall sedimentation rate in the last 8000 years of around 1 mm per year. The unit is interpreted as being estuarine to marine, deposited when the area was a part of the prodelta. The sand bed is believed to be the remains of a sand bank that existed off the nearby Soko Islands, later becoming moribund with changing hydraulic conditions in this area as sea level rose. The foraminiferal assemblage in HKGS-C is the richest in terms of the diversity of the identified genera. The assemblage is very stable in terms of ecology and is free of planktonic forms. The presence of large numbers

of Spiroculina, plus the low and constant levels of Miliolinella are indicative of an inner shelf environment. The upward increase in Elphidium, accompanied by a corresponding decrease in Spiroculina, reflects an overall trend towards shallower water in the upper part of the succession. In general, the assemblages suggest an inner shelf environment with significantly less oceanic influence than those from boreholes HKGS-A and HKGS-B. The diatom assemblage recovered from the upper 15 m of borehole HKGS-C is relatively stable in its ecology and is characterised by a predominant brackish planktonic influence, with subordinate marine and marinebrackish planktonic inputs. The increase in the marine influence, as illustrated by the diatom assemblage between approximately 9 and 8 m, is considered to correspond with a brief phase of tidal silt deposition. 4.2. Central southern waters To the east, in central waters (Fig. 3), the base of the Holocene sequence is marked by the contrast between the high-amplitude, sub-horizontal reflectors of the Chek Lap Kok Formation and undulating reflectors of the Hang Hau Formation. In borehole HKGS-B (Fig. 3), the underlying formation comprises mottled grey and brown

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Fig. 3. Seismic profile A14 and log of borehole HKGS-B.

firm silty clay and fine sand. A sharp erosional contact is seen between these and the overlying Holocene sediments. The Holocene sequence is clearly divided into two seismic units: a lower unit of undulating, continuous reflectors and an upper unit of parallel reflectors. In the lower unit, reflectors drape over the interfluves and into the channels. This draped reflector pattern may be produced by even, rapid sedimentation across a submerged topography or may be a secondary feature associated with compaction and dewatering. Borehole HKGS-B shows the sediments of the lower unit to be clayey sandy silt, with abundant plant fragments at the base. Upwards, the sediments become slightly shelly and the unit is topped by a thin sand layer. Radiocarbon dating (OxA5409, 5410) shows the unit to be around 9500—9000 years BP in age. The boundary between the lower and upper seismic units is interpreted as an unconformity (Fig. 3), demonstrated by the truncation of reflectors of the lower unit. The reflectors of the upper unit are sub-horizontal and of low amplitude, indicating widespread sedimentation. The existence of slight undulation of the basal reflectors in the upper unit suggests that compaction of the underlying units continued after the start of deposition of the upper

unit. Borehole HKGS-B shows the upper unit to be more mud-rich than the unit below. The radiocarbon date of 5425 years BP (OxA-5408) near the base of the unit confirms the inferred cessation of sedimentation and this and the radiocarbon dates from the sediments above (OxA-5406, 5407) indicate an overall sedimentation rate in the last 5000 years of around 1 mm per year, similar to that to the west. The shelly nature of the sediments and the presence of plant fragments in the basal part suggest that the lower unit is an intertidal deposit. It is therefore inferred that the topography was not fully submerged at that time and it is proposed that the draped reflectors are caused by compaction associated with a fall in sea level and subaerial exposure of the sediments. Huang et al. (1987) inferred an interruption in the post-glacial sea-level rise at around 10,000—8000 years BP, followed by a further rapid rise in sea level. There is additional evidence of change in global climate at this time from the work of Blunier et al. (1995) and Sowers and Bender (1995). The sedimentary succession from borehole HKGS-B is characterised by an upward increase in oceanic foraminifera, indicating a slight increase in water depth with time. However, some degree of shallowing or increase in depositional energy is evident between sample

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depths of 12 and 11 m and at ca. 9 m, corresponding with the presence of sandy horizons in the lower unit. A dominant brackish planktonic influence in the diatom assemblage from the very fine silt between depths of 13.3 and 10.1 m in borehole HKGS-B suggests a shallow water lagoon or estuary of moderate salinity with both freshwater and marine influences. The occurrence of Pinnularia major group at ca. 11 m is indicative of an inter- to lower supratidal mudflat or lagoon in the early Holocene. Indeed, brackish conditions predominate at the base, fresh to brackish conditions between approximately 12 and 11 m, and brackish to marine conditions between approximately 11 and 10 m. This indicates an overall increase in the marine influence. Furthermore, the environments of deposition in the silt between 10.1 and 7.0 m are characterised by a continued increase in the marine influence. The reversal of the upward-coarsening trend is accompanied by the establishing of an open marine environment with a strong brackish water influence as observed in the upper sediments from borehole HKGS-A. 4.3. Southeastern waters In the southeast (Fig. 4), the base of the Holocene succession is interpreted as a ravinement surface and is

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marked by a change in character from low-amplitude subparallel reflectors of the underlying Waglan Formation to the chaotic reflector pattern of the basal Hang Hau Formation. In borehole HKGS-A, the Waglan Formation comprises soft, grey clayey silt with pale yellowish brown mottles and colour bands. The boundary between this and the overlying Holocene sediments is marked by the incoming of a fine to coarse sand horizon. The basal part of the Holocene succession is represented on seismic profiles by a unit of chaotic reflectors. Borehole HKGS-A shows this to be a 2.5 m thick bed of fine to coarse shelly sand. The unit is believed to represent the remains of retreating shoreface sand that were deposited nearshore, on the shelf during the rise in sea level associated with the Holocene transgression. The ravinement surface dips southwards at an angle of 0.2°. This unit is succeeded by one of low-amplitude, subparallel, continuous reflectors, which is correlated in borehole HKGS-A with 10 m of very soft to soft mud that is bioturbated and contains shells and shell fragments. Radiocarbon ages (OxA-5400-5402) show that the sediment was deposited after 5000 years BP, with an overall sedimentation rate of almost 2 mm per year. The greater sedimentation rate in this part of the Territory,

Fig. 4. Seismic profile C29 and log of borehole HKGS-A.

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away from the primary sediment source, is attributed to an increase in accommodation eastwards, which, together with weaker currents, allowed a more or less continuous succession of Holocene sediments to accumulate. The foraminifera at ca. 13 m comprise the shallowestwater assemblage in the borehole. A high-energy environment is also suggested, possibly as a result of wave-induced sediment movement accompanying tidal processing. With time, there is an increase in water depth and a decrease in energy until conditions similar to the present occur at ca. 6 m. Diatoms are present only in the uppermost 11.5 m of borehole HKGS-A. The predominance of marine planktonic forms in the diatom assemblages, with subordinate marine epiphytic/benthic species, suggests an open marine environment. However, the brackish and marinebrackish planktonic components indicate a significant estuarine influence on sedimentation, which increases slightly upwards in the borehole.

5. Environmental interpretation The relationship between borehole stratigraphy and seismic sequences in southern Hong Kong waters is

shown in Fig. 5 and illustrates the development of a delta during the Holocene. The overall pattern of sedimentation is reflected by the grain size distribution (Plater, 1996). The most significant trend is a westward coarsening and decrease in sorting of sediments. The proximity to the Pearl River, which provided the most significant source of sediment to feed this delta, is the main contributory factor and suggests a westward-shallowing environment. The palaeoenvironmental evidence obtained from the foraminiferal and diatom analysis reveals the same overall trends, although the detail may appear somewhat contradictory in places. The reason for any disparity is the fact that the foraminiferal assemblages are dominated by benthic forms, whilst the diatoms are primarily planktonic. Hence, the foraminifera are more reliable indicators of autochthonous palaeoenvironments, and the diatoms provide evidence on allochthonous inputs. This, in itself, is a useful characteristic of the combined palaeoecological methodology. The main trend in the foraminifera is a westward increase in numbers of inner shelf forms which may reflect increasing proximity of the influence of the Pearl River estuary. The high proportion of porcelaineous foraminifera (e.g. Quinqueloculina, Spiroculina and Miliolinella) rather than calcareous benthic forms, and rare

Fig. 5. Relationship between sequence stratigraphic units and a model of the development of the Pearl River delta.

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planktonic genera would also support this interpretation. In addition, the succession in the west exhibits the least variable and probably ‘shallowest’ environment of deposition throughout the Holocene, whilst that in the east exhibits the greatest variation and a period of high oceanicity. Although the diatoms are predominantly planktonic, there does appear to be a consistent westward increase in the brackish water influence on deposition. Those from the western section exhibit the least variable and most estuarine environment, whilst those in the east would appear to be the most oceanic of the three sites. Intermediate between these two end-members, in central southern waters, the diatoms exhibit a clear increase in marine influence with time. However, this trend is interrupted by an inter- to lower supratidal mudflat or lagoonal environment in the early Holocene. In the central area, the palaeoecological indicators suggest a predominant marine littoral environment or tidal inlet, with occasional shallowing events resulting in the establishment of tidal channels or a shallow subtidal environment in an estuarine system. A number of similar channels are identified from seismic records to the north. This area is one of significant acoustic turbidity caused by gas in the sediments and this is attributed to bacterial decomposition of organic debris in the tidal channel mud. In the early Holocene, this region was part of an intertidal zone that filled the area between the contemporaneous ridge south of Lantau island and the islands of Lamma and Hong Kong.

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affect Hong Kong waters until after 5500 years BP. The subsequent prodelta and shelf sedimentation led to the deposition of a marine mud sheet at an average sedimentation rate of ca. 1 mm per year. Both the foraminifera and the diatoms exhibit a westward increase in the influence of the Pearl River on the environments of deposition. A marine shelf environment predominates, with a westward ‘shallowing’ trend. The succession in the east shows the greatest range of depositional environments, while that in the west is the most consistent in terms of preserved environments. The ‘upward-deepening’ trend in the east is accompanied by an increase in the allochthonous brackish water influence on shelf deposition. The succession in the central area exhibits very little change in the environment of deposition in terms of its foraminiferal assemblages, but the diatoms provide clear evidence of an upward increase in oceanicity. Again, this culminates in an increase in the brackish water influence. The strong brackish water influence on the successive environments of deposition, as illustrated by the allochthonous components of the diatom assemblages, is attributed to an increasing input from the Pearl River as the tidal pathways through the Hong Kong region were breached with rising sea-level (Fyfe and Shaw, 1995). There is no net prodelta-shelf sedimentation at the present day as the natural sea bed is more or less in hydraulic equilibrium.

Acknowledgements 6. Conclusions The rise in sea level during the Holocene led to the deposition of a blanket of muddy sediments of variable thickness offshore Hong Kong. In the east, the transgression resulted in shoreface erosion and the development of a distinct seaward-dipping ravinement surface. Sedimentation following the shoreface erosion created a sandy horizon up to 2.5 m thick. In central and western areas, and near the shore, the initial sedimentation was an intertidal facies, filling fluvially-entrenched channels. Interbedded silt and sand deposited around 9000 years BP in an inter- to lower supratidal mudflat or lagoon were subaerially exposed by a brief fall in sea level, before being submerged by the renewed sea-level rise. An initial pulse of sediment a short time before 8000 years BP resulted in large-scale low-angle foreset bedding in the west. The rise in sea level from 8000 years BP to the global maximum at 5000 years BP led to increase in accommodation on the outer shelf. The availability of this accommodation is not synchronous with the sedimentation offshore Hong Kong, however, indicating that the main sediment supply from the Pearl River did not

This paper was presented at the 14th INQUA Congress in Berlin, 1995 and was a contribution to the Symposium of the Commission on Quaternary Shorelines. The authors would like to acknowledge the work of Dr. Edward Twiddy at the Environmental Research Centre, University of Durham on foraminiferal analyses and Dr. Housley of the Oxford AMS Laboratory, now at the University of Strathclyde, for helpful comments on the radiometric dating. The paper is published with the permission of the Director of Civil Engineering, Hong Kong Government.

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