The Botany Basin; Its Bedrock Topography And Recent Geological History

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The Botany Basin; Its Bedrock Topography And Recent Geological History

A.D. Albani & P.C. Rickwood 1998
School of Geology, University of New South Wales, Sydney, NSW 2052.
The Botany Basin: its bedrock morphology and recent geological history, pp. 190-196.
In McNally, G.H and Jancowski, J, (Ed),
Collected Case Studies in Engineering Geology, Hydrogeology and Environmental Geology,
Geological Society of Australia.

Contents


Abstract

The bedrock topography of the Botany Basin has been determined from seismic sparker records obtained in Botany Bay, in the inner continental shelf and from a combination of seismic-refraction and gravity measurements on the Kurnell Peninsula; supplementary information has been obtained from boreholes.

The bedrock channel of the former Cooks River is about 30 m.b.s.l. near Kyeemagh, and that of the former Georges River is 75-80 m.b.s.l. at Taren Point. These rivers formerly constituted the main drainage of the Basin and after joining together at 90 m.b.s.l. beneath the present Kurnell Peninsula they, in turn, were joined by the Hacking River (at 100-105 m.b.s.l.) before proceeding to a depth of 120 m.b.s.l. at the former coastline, which was about 6 km east of the present position.

A lesser drainage system, that existed in the northeast of Botany Basin, generated the present entrance of Botany Bay beneath which the bedrock is now 110 m.b.s.l. This channel was separated from that of the 'Cooks/Georges Rivers' by a bedrock ridge which extended from beneath Sydney Airport to the northern extremity of the Kurnell Peninsula; over much of its length this divide is about 30 m.b.s.l. The formation of the Kurnell Peninsula tombolo led to the diversion of the 'Cooks/Georges River' through the mouth of Botany Bay at about 9000 years B.P. when the sea level became less than 30 m. below the present sea level.

A succession of environments from marine to terrestrial, have been recognised in cored sediment samples and a relative "minimum" chronology has been established which enabled correlation of erosional surfaces with sea-level fluctuations. Seismic surveys and borehole material from other sites indicate that the model presented here is applicable to other estuaries of N.S.W.

Introduction

The Botany Basin includes two large tidal bays; Botany Bay is located ~15 km south of central Sydney whereas Bate Bay is to the south of that and east of Cronulla. Kurnell Peninsula, the landing place of Captain Cook, separates these bays; it is a coastal sand barrier complex (Skinner, 1973) with large dunes reaching an elevation of 30-40 m that becomes an elevated sandstone plateau at the eastern end of the peninsula.

Botany Bay has a roughly circular shape of 6-7 km diameter and natural water depths that generally are less than 5 m but reach 10 m at the entrance and locally have been enhanced by dredging to 20 m.b.s.l.(metres below sea level). The entrance to Botany Bay is a narrow (1 km) gap between the rocky headlands of La Perouse and Inscription Point so Botany Bay is largely protected from sea storms.

Bate Bay is roughly oval with an entrance between Boat Harbour and Jibbon Point that seems to be very wide but three shallow rocky outcrops (Osborn Shoal, Merries Reef and Bombora off Jibbon) conceal an underwater ridge and the real entrance is at the southern end of Osborn Shoal. Necessarily, the Bay is exposed to the dominant southeasterly storm waves and swells. Although extensive dredging has been undertaken in the Port Hacking estuary, Bate Bay has not been so modified so water depths are never more than 20m. except at the rocky shallows over which waves break at high tide.

Two major rivers enter Botany Bay; Georges River (103 km in length - Warner, 1974) in the southwest and Cooks River (21 km long) in the northwest which has an artificial entrance (about 1.5 km to the south of the original) that was created in 1947 for the development of Sydney Airport (Griffin, 1963). The Hacking River (42 km in length - Albani et al., 1983) enters Bate Bay in its south-eastern corner and has a flow rate less than Georges River but greater than Cooks River.

Areas of exposed rock are limited within Botany Basin and consist of Triassic Hawkesbury Sandstone but the overlying Wianamatta Group shale crops out on adjacent high ground to the west and southwest (Griffin, 1963). These rocks have been shown to dip towards a meridional axis passing through the centre of the study area. Unconsolidated sediment dominates in the Botany Basin so indirect methods have to be employed to establish the positions of the bedrock drainage channels. Using similar methods in investigations of the continental shelf near Sydney we have shown that these channels extend for varying, but considerable, distances onto the shelf (Albani et al., 1985; 1988; White, 1990). Moreover, our studies of bays and river mouths indicated that bedrock drainage basins formed when the sea level was more than 110 m lower than at present (Albani et al., 1973; Albani & Johnson, 1974; Johnson & Albani 1975; Albani et al., 1976, 1978, 1983, 1991; Johnson et al., 1977).

The geological history of Botany Bay, and of similar estuaries along the east coast of New South Wales (Albani et al. 1973, 1974), was strongly influenced by the climatic events of the Quaternary Period. During conditions of low sea level, the numerous coastal streams were rejuvenated and river valleys were entrenched and accentuated: a large volume of eroded material was deposited on the inner shelf. During high sea-level stands, some of the newly deposited sediment was remobilised by marine processes and transported back into the drowned river valleys causing partial siltation. The oscillating cycles of sea level of the Pleistocene are well recorded in the sediment cover of Botany Bay.

This paper discusses our results and conclusions in Botany Basin but necessarily draws on our earlier work. Throughout it has been necessary to refer to existing rivers as well as bedrock channels of ancient rivers, some of which we have named. For clarity, the names of the ancient rivers and topographic features are always shown in the text between quotation marks.

Previous Work

Griffin (1963) identified several bedrock valleys running southwards into Botany Bay by analysing data from 334 boreholes in the north west of Botany Basin and subsequently, Smart (1974) obtained data from 135 boreholes drilled in a small area (4.5 km2) of industrial development at Banksmeadow and produced a more detailed map of the underlying bedrock topography. Very few of the boreholes put down for Sydney Airport reached bedrock (Goodwin 1970, 1976) and no significant geological information was gained from them.

A sparker survey conducted by the Geological Survey of N.S.W. in 1969 (Ringis, 1974) covered the entire area of Botany Bay but Ringis wrote, ' results of sufficiently good quality for meaningful interpretation were obtained only in the eastern part of the bay' where the presence of a deeply buried (100 m.b.s.l.) bedrock channel between the rocky headland at the entrance to Botany Bay was first detected. Little information was also yielded by a seismic refraction survey of a small part of Kurnell Peninsula, between Quibray Bay and Boat Harbour. Thus, Emerson (1966, p.204) wrote that 'the bedrock is considered to be essentially plane and horizontal at an R.L. of about 20 feet below sea level under the survey lines' but Healey & Ringis (1966) noted a more undulating bedrock surface in a nearby area. Bedrock was intercepted in holes drilled by The NSW Department of Mines on, or close to, 'Kurnell Island' and both Goodwin (1970) and Ringis (1974) reported it to be shallower than 45 m.b.s.l.

Thus the knowledge of the bedrock topography was scant before we undertook our investigation using a sparker system that has been described by Johnson et al. (1977). This was supplemented by a seismic refraction survey on Kurnell Peninsula at a site in a mangrove area chosen because it is well away from the surf noise which plagued Emerson (1966). The resulting data was readily interpreted by the reciprocal method (Hawkins, 1961) and gave depths accurate to within 2-3 m. Supplementary information was gained from a gravity survey conducted with a Worden Master gravimeter, at an interval of approximately 1 km, along the road from Woolooware to Kurnell and along Towra Point Peninsula; it could not be carried out on the Bate Bay side of Kurnell Peninsula because of the vibrations caused by breaking waves. The data were reduced by standard means and negative values clearly indicate the presence of bedrock channels and hence the gravity survey was successful in facilitating interpolation between the sparker surveys in Bate Bay and Botany Bay.

Bedrock Topography

A map of bedrock contours (Figure 1) combines the interpretations of the sparker records, seismic-reflection survey and gravity survey; it has been linked to Griffin's (1963) contour map of depth to bedrock for the most northern part of the study area and enhanced by additional bore data made available by the Maritime Services Board.

Two drainage systems are separated by a barrier which is located beneath the main N-S runway of Sydney Airport and extends to Inscription Point at a general depth of 30 m.b.s.l.; we have called this the 'Runway Ridge'. No headwater channels cross Runway Ridge, so establishing that it was once a significant divide.

The northern system was partly identified by Griffin (1963) who called it the Lakes Valley, and also by Ringis (1974) who recognised a small portion. The ancient river, which we call 'Botany River', descended from an altitude of 40-60 m. on the southern flank of the Paddington ridge to a depth of 110 m.b.s.l. beneath to the present entrance to Botany Bay via a course 15 km in length. This river system is characterised by short, steep, tributaries.

The major drainage basin lies south and west of the Runway Ridge and includes the precursors of the present day Cooks and Georges Rivers. 'Cooks River' flowed generally southwards from its present entrance into Botany Bay, at Kyeemagh, to the vicinity of Towra Point where it was joined by three tributaries which we have called the 'Towra River', 'Bonna River', and 'Kurnell River'. 'Georges River' flowed eastwards from its present location between Taren Point and Sans Souci to join the ancient Cooks River beneath the Kurnell sand barrier and the unified system continued southwards into Bate Bay.

Under the middle of Bate Bay the combined 'Cooks/ Georges River' was joined by another tributary originating just west of Boat Harbour and Osborn Shoal which we have named the 'Osborn River'. Between Osborn Shoal and Port Hacking Point (the southern headland of the entrance to Port Hacking) the predecessor to the present Hacking River also joined the ancient 'Cooks/Georges River' at a depth in excess of 110 m.b.s.l. Both the 'Cooks/Georges River' and the 'Botany River' reached the old coastline about 6 km east of the present coast and at a depth of about 120 m.b.s.l.

The drainage system observed in the bedrock topography is characterised by a change in gradient in the vicinity of 30-40 m.b.s.l. Above this depth, the bedrock gradients are quite low and appear to form a plateau. Below this depth the gradients are much greater and the valleys are narrower and steeper.

Stratigraphy of Unconsolidated Sediments

Because of the close correlation between borehole data and seismic records, this part of the investigation was concentrated in the area or the Port Botany (Figure 1) and the chronology was established for the 'Botany River' valley. However, the seismic data available for the remainder of Botany Bay, and additional boreholes, indicate that the sequence is applicable to the whole Bay, and probably to Bate Bay as well.
 
Fig. 1 -  The bedrock morphology of the Botany Basin.
Fig. 1 - The bedrock morphology of the Botany Basin. The source of the "Botany River" lies on the Paddington ridge, to the north of this map (Griffin, 1963) (enlargement available)

 
Fig. 2 - Sea Level, Environmental Change, Stratigraphy.
Fig. 2 - Sea Level, Environmental Change, Stratigraphy. (enlargement available)

Several seismic reflectors have been recognised within the unconsolidated sediments; they represent surfaces of erosion with well-defined drainage patterns that were cut by runoff water and therefore relate to former subaerial conditions during periods of lower sea level. A cross section diagram through the 'Botany River' valley was drawn (Figure 2C ) to illustrate the various events envisaged to have occurred during the Pleistocene and Holocene periods within the Botany Basin. The vertical sequence of sediment consists mainly of sand and clay with minor peat layers; occasional horizons of shell-rich material also are found. Borehole No. 420 was selected for a detailed analysis of the benthic foraminiferal fauna in each horizon (Albani, 1980). By combining the seismic, paleontological and lithological data, the unconsolidated sediments can be grouped into four units each characterised by a distinct environment of formation; they are discussed in reverse chronological order.

Unit 1. This unit comprises the most recent marine deposit; it is primarily sand but with silt and clay becoming more predominant in areas with less tidal flow. Occasional detrital peat accumulated in more sheltered conditions. A marine fauna is present and its composition is indicative of the various hydrological environments characteristic of the estuaries around Sydney (Albani, 1968, 1978). Unit 1 varies in thickness from a few meters to 20-30 m. at the entrance of Botany Bay.

Unit 2. Sediments of Unit 2 almost always consist of quartz sand, with very low carbonate content, and with some lenses of peaty material. It is characterised by a complete absence of foraminifera and other marine benthic life which strongly indicates a subaerial environment of deposition such as a sand dune. A peat sample from this unit (4.70 m.b.s.l. in borehole No. 410; P1 in Figure 2) has a radiocarbon age greater than 35,000 years B.P. (Radiocarbon Laboratory, University of Sydney). In other estuaries where dune formation did not occur, this unit is generally replaced by a deposit similar to either Unit 3 or 4.

Unit 3. Clay and clayey sand with some peat horizons predominate. Occasional concentrations of shell fragments, and the lack of abundant benthic foraminifera, suggest an environment characterised by near-brackish water conditions. These are found in modern lagoons separated from the open sea by wide sand barriers and with adequate fresh water to prevent the establishment of semi-permanent marine conditions in the bottom waters (Albani & Brown, 1976, Roy et al., 1980).

Unit 4. Lithologically, this unit is very similar to Unit 3 but it differs in possessing a very rich foraminiferal fauna such as occurs in the deeper water areas of' present day estuaries. The abundance and variety of marine benthic and pelagic foraminifera indicates an open tidal environment (Albani, 1980).

The chronology given in Figure 2B is based on a simplified sea-level curve (Bloom et al., 1974) (Fig. 2A); no vertical movement of land masses has been envisaged. For simplicity, the dates shown on Figure 2B for the culmination of depositional or erosional settings are those of the maxima or minima of the sea-level curve; we recognise that these processes extended until the area was either covered by water (end of an erosion phase) or exposed to subaerial conditions (end of a deposition phase) and it is unlikely that there was exact correspondence to the peaks of the sea-level curve.

These series of events must have been repeated several times during the Pleistocene Epoch and it is likely that some remnant of the earlier interglacial deposits may be present in the stratigraphic sequence but has not been recognised. Total removal of such deposits, by surface erosion in the periods of low sea level that correspond to previous glaciations, has been inferred so that the chronology given here is the youngest possible.

Conclusions

We have been able to supply the evidence to substantiate Griffin's (1963, p.16) prediction that, 'It is also thought that Bate Bay was once the main entrance to Botany Bay, Kurnell being an island'. The bedrock topography of Botany Basin is characterised by steep valleys incised into rock but the valleys have shallow gradients at levels above 30-40 m.b.s.l.

The deepest erosion of the bedrock occurred when the sea was more than 100 m below its present level. Since that time the gradual rise of sea level has led to progressive infilling of the bedrock channels with a tombolo forming between Kurnell and Cronulla. The present form of Botany Bay could not have existed until the water level had risen to about 30 m below present day sea level to cover the lowest part of Runway Ridge and inhabitants of Kurnell Island could have walked to Kyeemagh along this ridge until about 9000 years B.P.

In most estuaries along the east coast of Australia, seismic investigations have revealed the presence of a number of reflectors within the sediment infilling. The borehole information across the 'Botany River' valley showed that these reflectors do not correspond to lithological changes but are erosional features. Recognition of this has enabled a chronology of the environmental evolution of the Bay to be established.

Acknowledgements

This project was financially supported by the Sutherland Shire Council and we are most grateful for the continual encouragement given by the successive Shire Presidents--Councillors K.M. Skinner, P.C. Lewis and M. Tynan and particularly Mr A.G. Hill, the Shire Clerk at the time that the field work was carried out.

We are particularly keen to acknowledge the assistance of Mr Ken Gibbons, Dr B.D. Johnson and Mr J.W. Tayton (all formerly of Macquarie University). The field work could not have been completed without the help of many people; Mr Eric Potts supplied the vessel from which we worked; The University of New South Wales Regiment kindly maintained a radio communication network for us. Mrs C.A. Johnson, Ms P. Sayers, Mrs W. Ambler, the late Mr J.C. Arden and Messrs. J. Gough, M. O'Toole, and I. Pauncz kindly manned theodolite stations at various times. We also acknowledge the cooperation of the Australian Oil Company, the Maritime Services Board, and the New South Wales Golf Club, who permitted us to place theodolite stations on their property. The authors are grateful to the Maritime Services Board of New South Wales for making the borehole material available for study

References

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