Late Quaternary geological history of the Sydney estuary, Australia

Abstract The present study provides valuable new information on the evolution of Sydney estuary by tracing the development of the complete marine–estuarine–fluvial system through a full glacial cycle (Last Interglacial, LIG, to the present Interglacial). Extensive seismic (361.3-line km) and sedimentological studies provided a sound foundation for production of a detailed litho- and seismic-stratigraphic record for the estuary. In the absence of reliable age data, a relative chronology was constructed based on Quaternary flooding surface elevations constrained by a recent local relative sea-level record supported by other global studies. A thick, ubiquitous estuarine unit deposited during the LIG period (MIS 5.5; 130–115 ka BP) was an important chronological marker horizon and played a critical role in controlling seismic interpretation and correlation throughout the estuary. Deposition during the MIS 5.1/5.3 interstadial period (100–80 ka BP) resulted in deposition of fine-grained, estuarine sediments in the lower estuary and time-equivalent, fluvial-sourced estuarine and channel sediments, and marsh sediments in the upper and central estuary, respectively. The MIS 3 interstadial event did not play a significant role in sedimentation in Sydney estuary. An eolian dune field formed adjacent to the southern shores of the estuary during the last glacial (31–24 ka BP) when most of the sediment in the lower estuary had been removed by fluvial erosion. Transgressive marine sand, which deposited in the lower paleovalley after the ocean re-entered the estuary, experienced repeated erosion and infilling by laterally migrating paleoriver channels. A marine flood-tide delta now occupies the estuary mouth, and the lower and upper/central estuary are mantled in a veneer (mean 7 m) of Holocene sand and mud, respectively. KEY POINTS A relative chronology was based on Quaternary flooding surface elevations constrained by relative sea-level. First geological history of the Sydney estuary with a complete marine–estuarine–fluvial system. A late Quaternary estuary evolution through a full glacial cycle. Geological history includes an interstadial (MIS 5.3/5.1) estuarine sequence.


Introduction
The Sydney estuary is one of the most beautiful waterways in the world, owing partly to the distinctive, small pocket sandy beaches separated by rocky headlands and deep tributaries (Lane Cove River and Middle Harbour), which occupy the northern shore. The south side of the estuary is characterised by four large, shallow bays (Homebush Bay, Hen and Chicken Bay, Iron Cove and Blackwattle/Rozelle bays) lined by mudflats and mangrove forests (Liu, 1989).
The ancient Parramatta River, which is now the Sydney estuary, was an old, mature river valley that meandered across a flat plain 80 million years ago (Herbert, 1983a(Herbert, , 1983b. The considerable depth of bedrock in the estuary is due to incision by the paleo-Parramatta River during times of uplift and repeated cycles of glacial lowstand over an extended period (Liu, 1989). During the Quaternary (last 2.58 million years, Ma), climate cycles of ca 40 000 years duration transitioned to ca 100 000 years at ca 1 Ma known as the 'mid-Pleistocene transition'. These cycles produced sea-level fluctuations from approximately present-day heights to 130 m below present sealevel (bpsl) during each oscillation (Rib o et al., 2020). After the last glacial maximum (LGM) at ca 21 ka BP (thousand years Before Present) sea-level rose rapidly from about 130 m bpsl to present-day levels. These global glacial and interglacial cycles resulted in alternating flooding and erosion of the estuary. It was the inter-relationship between the geology and marine eustacy that shaped the final character of the Sydney estuary, and this relationship is the subject of the current study. Substantial geological work, which has been undertaken in the upper and central estuary based on boreholes taken for bridge and tunnel construction (Irvine, 1980;Liu, 1989;Och et al., 2017), has not previously been integrated with un-interpreted seismic data available for the whole estuary (Lean, 1976(Lean, , 1978. A relative chronology based on Quaternary flooding surface elevations constrained by local relative sea-level is presented.

Sydney estuary
The Sydney estuary (50 km 2 ) is a dendritic, drowned river valley 30 km long and up to 3 km wide in central New South Wales (Birch, 2017;Roy 1983); the catchment is small (500 km 2 ) and highly industrialised and urbanised (76%) (Birch et al., 2011(Birch et al., , 2015a(Birch et al., , 2015b. Low sedimentation rates recognised earlier (Laseron, 1947;Taylor et al., 2004) have been confirmed by exceptionally low sedimentation rates recently recorded in Middle Harbour  and are due to low discharge volumes (Birch & Rochford, 2010; and minor sediment loading (Beck & Birch, 2012a, 2012b. 'Sydney estuary' is used in the current paper to describe the entire estuarine system, instead of 'Sydney Harbour', or 'Port Jackson', which refers to restricted parts of the waterway (Birch & McCready, 2009;Birch et al., 2016aBirch et al., , 2016bMcLoughlin, 2000).

Objectives of the current study
The estuary was divided into upper/central  and lower (Birch & Lound, 2021) sectors, and described separately to facilitate publication. For a detailed discussion on methods, lithologies, age, geology and previous studies of the estuary, readers are referred to these manuscripts. The seismic units and lithology were correlated successfully using published geological cross-sections based on extensive borehole data ( Figure 1) and published for each sector separately.
The objectives of the present study were to combine the results of sedimentological and seismic investigations in the upper/central and lower Sydney estuary. The sedimentary/seismic units identified separately in these two sectors of the estuary are correlated into a single stratigraphy. A chronology was established for the combined litho-and seismic stratigraphy based on Quaternary flooding surfaces constrained by recent sea-level records allowing the geological history of the entire system to be constructed.

Sydney estuary environments
The three sectors of the Sydney estuary ( Figure 1) display contrasting geomorphological, sedimentological and structural features, which are depicted in 'typical' seismic sections and interpretations of the upper (Figure 2), central ( Figure 3) and lower (Figure 4) estuaries. The upper estuary (Ryde Bridge to Pulpit Point) is characterised by a narrow (50 m), sinusoidal, shallow (5-10 m) channel and irregular bedrock with predominantly muddy surficial sediments. The extensive, wide (up to 1 km) central estuary (Pulpit Point to Blues Point) comprises the Cockatoo Island basin (10-15 m deep), underlain by extensive Pleistocene sediments, and a depocentre at the Lane Cove River mouth with a considerable thickness of fluvially derived estuarine sediment. The large, lower estuary (Kirribilli Point to the Heads) is distinguished by a deeply, eroded paleochannel filled with mainly marine sediments and exhibits a strong marine character. The Sydney Harbour Bridge (SHB) is a transition zone (Blues Point to Kirribilli Point) separating a dominantly terrigenous-sourced upper/central estuary from a mainly marine lower estuary (Birch et al., 2015a(Birch et al., , 2015b. The present study considered all environments, except Lane Cove River and Middle Harbour, which are the subject of companion studies .

Available lithological and seismic data
Sedimentological information used for the upper and central estuary was obtained from 309 boreholes mainly associated with bridge construction (Irvine, 1980) and three shallow vibrocores (Liu, 1989). In the SHB area, abundant sediment data were available from the construction of two tunnels (Liu, 1989, three boreholes;Och et al., 2017, 32 boreholes;Pells & Wong, 1990, 11 boreholes), and in the lower estuary 13 boreholes were taken on seismic lines for naval works at Garden Island (Lean, 1978). Additional subsurface data were available from the outer estuary (Liu, 1989, four vibrocores; Sydney Storage Tunnel, four boreholes). The location of the boreholes is given in Figure 1, and the geological relationship between these investigations is presented in Table 1. A lack of subsurface data from the axis of the lower waterway limited lithological information for this area.
Structural information and seismic stratigraphy of the estuary were determined by interpretation of 361.3-line km of record obtained by Lean (1973Lean ( , 1976Lean ( , 1978 (Figure 1). Details of the seismic methods used in these studies are available in the Lean reports.

Stratigraphic control for interpretation of seismic units
Interpreted seismic transects from five representative locations in the estuary (Gladesville Bridge, Glebe Bridge, the SHB, Garden Island and Outer Harbour) provided by Birch and Lound (2021) and  correlated well with previously published sedimentary cross-sections based on multiple (376) boreholes (Irvine, 1980;Lean, 1978;Liu, 1989;Och et al., 2017;Pells & Wong, 1990). An additional 14 cross-sections from all three sectors of the estuary also demonstrated a close correlation between seismic and sedimentary units (Birch & Lound, 2021;.

Typical seismic and sedimentary characteristics in the three sectors of the estuary
The seismic and sedimentary characteristics of the three sectors of Sydney estuary are substantially different owing to changes in structural, erosional and depositional controls.
An axial seismic transect at Rocky Point in the upper estuary showed basal sediments (SU 1) overlain by an 8 mthick Pleistocene deposit (SU 2/3) observed as multiple, bold reflectors on the seismic section. The SU 2/3 strata were overlain by 5 m of distorted beds of alternating lithologies (SU 4) and a (10 m) sequence of down-stream prograding beds, and a veneer of seismically opaque muds and sands (SU 8) mantled the surface of the channel (Figure 2). A typical section across the central estuary at Blues Point showed thin basal deposits (SU 1) and an eroded overlying sequence (SU 2) with a thick (25 m) estuarine section displaying planar and prograding reflectors (SU 4) and thin surficial sediments (SU 8) ( Figure 3). A typical seismic section across the lower estuary south of Robertson Point ( Figure 4) showed grey, mottle-textured seismic material (SU 6) overlying multiple, bold reflectors of the SU 2/3 sequence. In this sector, strong upward-concave, channel-like reflectors within the SU 6 indicated repeated periods of channel erosion and infilling (Emerson & Phipps, 1969;Harris, 2000). A thick sequence (>30 m) of strong, prograding reflectors (SU 7) overlying low-angle strata at mouth of Middle Harbour was typical of the outer estuary marine tidal delta ( Figure 5). This succession correlated with flood-tide delta sand overlying Pleistocene sand and mud intersected in four boreholes drilled for the nearby Sydney Northside Storage Tunnel (Birch & Lound, 2021, figure 6).

Lithology and distribution of seismic units
In this section, the five seismic units in the upper/central and five units in the lower sectors of the estuary identified in previous work (Birch & Lound, 2021; are correlated and combined into a single stratigraphy of  Seismic Units SU 1 and SU 2/3 Irregular, discontinuous basal reflectors (SU 1) corresponded to sands, gravels and boulders of residual lag deposits of SU 1, which Irvine (1980) observed throughout the estuary (Figures 2 and 3). The overlying unit (SU 2), comprising multiple, bold, near-horizontal reflectors up to 30 m thick, was confidently correlated with Unit 2 of Irvine (1980), which contained black, fossiliferous sand and silt, and with Unit F at Garden Island (Lean, 1978) comprising stiff, non-shelly mud. This strong, reliable seismic marker was observed extensively and proved to be important in the correlation of geophysical (seismic) and geological (borehole) data throughout the estuary. The overlying SU 3 intersected at Garden Island (Units E/D of Lean, 1978) was indistinguishable from SU 2 on seismic sections and was composed of similar sediments, i.e. stiff, shelly mud with peat and minor sand. The two estuarine lithologies (SU 2 and 3) at Garden Island were separated by an unconformity described as a 'major eroded surface produced by subaerial weathering, which removed shell and caused iron-staining and compaction' by Lean (1978, p. 9).
Strong multiple, SU 2/3 reflectors were most consistent and thickest (up to 15 m) adjacent to the southern shore of the lower estuary (at Garden Island, Woolloomooloo, Rushcutters, Double, Rose, Parsley and Vaucluse bays), while in the north-shore embayments (Neutral and Mosman bays and Shell and Little Sirius coves), the SU2/3 was discontinuous, thin and confined to shallow, small depressions. In the upper and central estuary, the SU 2 was mapped on all transects of the main channel and the unit also mantled most of the embayments, including the Cockatoo Island basin and was well developed (up to 25 m thick) between Manns Point and Blues Point.

Seismic Unit SU 4
In the upper estuary, the overlying SU 4 unit of discontinuous, cross-bedded reflectors and associated low-angle, planar prograding reflectors were commonly present as material filling depressions in the underlying SU 2 sediments and paleochannels ( Figure 2). These sediments were correlated to Units 3 and 4 of Irvine (1980) from the stratigraphic position directly overlying the estuarine seismic marker horizon (SU 2) and from internal sedimentary structure. Unit 3 of Irvine (1980) comprised fluvial, organicrich silts and clays and commonly included peat, whereas Unit 4 of Irvine was an upward-fining fluvial channel sand sequence. These two units merged laterally in some locations and could not always be differentiated seismically and have been combined in the present work into a single unit (SU 4). Some of these deposits in the upper estuary may also be mixed with backstepping materials of the latest transgression ( Figure 2). In the central estuary, the SU 4 was more extensive and thicker (>20 m) (Figure 3), indicating increased estuarine accommodation and substantial fluvial supply, possibly from the Lane Cove River and central channel.

Seismic Unit SU 5
The extensive, near-surface sediment accumulation (SU 5) observed on seismic sections off the southern shore of the lower estuary between Woolloomooloo and Rose bays terminated at the edge of the eroded main paleochannel south of Shark Island. These features were internally structureless, however at Rose Bay these sediments took the surface form of 20-m high dunes. This eolian unit was correlated with lower zone sediment in cores from Rose Bay, which contained well-sorted quartzose sand and no shells (Liu, 1989).

Seismic Unit SU 6
The central axis of the lower Sydney estuary was extensively eroded and filled with a grey, mottle-textured seismic material (SU 6), which in some locations overlay remnant, strong SU 2/3 reflectors (Figure 4). No boreholes have intersected the central axis of the lower estuary, however at Garden Island these sediments (Unit C of Lean, 1978) were described as well-sorted clean, very shelly sand (Lean, 1978) and were identified as a flood-tide delta deposit owing to abundant shell and repeated channelling on seismic sections (Lean, 1978). The SU 6 at the SHB was described as a marginal marine, well-sorted sand with shells by Och et al. (2017) and as a fine to coarse, soft, loose well-sorted sand by Pells and Wong (1990). SU 6 sediments were not observed on seismic sections, or in boreholes landwards of the SHB.

Seismic Unit SU 7
A flood-tide delta sand infilled the mouths of Middle Harbour and the Sydney estuary to thicknesses of 20 to 30 m. A seismic section through the landward edge of the marine tidal delta at the mouth of Middle Harbour ( Figure  5) showed a thick sequence (>20 m) of strong, prograding reflectors overlying low-angle strata, which intercalated distally with finely bedded layers of the deep mud basin. This section may depict two phases of sedimentation, i.e. a lower marine transgressive unit overlain by a strongly prograding flood-tide delta.

Seismic Unit SU 8 (surficial sediment)
In the upper and central estuary, the ubiquitous surficial estuarine mud deposit (SU 8) appeared opaque on seismic sections, whereas in the lower estuary, the SU 8 was observed as thin veneer and took on a mottled seismic expression.
In the lower estuary, the SU 8 comprised >90 vol% sand on the surface of the tidal delta and >70 vol% sand between the delta and the SHB (Irvine, 1980;Irvine & Birch, 1998). The sand was well sorted, fine to medium sand-sized and contained >30 vol% carbonate over large areas of the lower estuary (Liu, 1989). Surficial sediment in Rose Bay and Garden Island was approximately 1-2 m thick and up to 3.5 m thick, respectively (Lean, 1978;Liu, 1989). Small, isolated deposits of muddy sediment (>50 vol% mud) occupied bathymetric depressions and small embayments on the north and south shores. In Middle Harbor, seismically transparent muds up to 25 m thick occupied the central channel .
In the upper and central estuary, the composition of the SU 8 was a silty mud with variable shell content and between 7 and 15 m thick (Birch, 2017;Irvine, 1980). This work Lean (1978) Irvine (1980) Liu (1989) Pells & Wong ( Accumulations of 5-10 m were present in some deep holes in the paleochannel in the upper estuary and in the Cockatoo Island basin, as well as in some embayments and tributaries.

Depth to bedrock
The narrow (50 m  Depth to the top of the estuarine sequence (SU 2) The surface of the SU 2 deposit was commonly extensively eroded in the main paleochannel in the upper and central estuary leaving remnant wedges of sediment on the flanks and at the base of the ancient valleys ( Figure S1b). The surface of well-developed SU 2 sediments in embayments (Hen and Chicken Bay, Five Dock, Rozelle/Blackwattle Bay) and in some southern parts of the Pulpit Point-Kirribilli Point section of the estuary was close to present-day sealevel (2-15 m bpsl). However, the surface of this sequence was extensively eroded, especially in the central axis of the lower estuary where the sediment was commonly absent or present as remnant deposits. The SU 2 was well preserved in embayments on the southern lower shore where the surface was between 15 and 25 m bpsl.

Age and correlation of seismic units
In the absence of reliable absolute chronology for sediments of Sydney estuary, a relative chronology was determined based on Quaternary flooding surface elevations based on a recent relative sea-level record (Rib o et al., 2020), which coincided closely with other global studies (Grant et al., 2012;Miller et al., 2020;Waelbroeck et al., 2002; Figure 7a). This record defined highstand and modal sea-level ranges during the past glacial cycle, which allowed relative ages, spatial distributions ( Figure 7b) and correlations ( Figure 8) to be established for these units. Detailed discussion and evidence for assigning possible ages to these units have been provided in previous work (Birch & Lound, 2021;.

Seismic Unit SU 1
The irregular beds preserved in depressions in paleochannels (SU 1) probably accumulated as erosional-lag deposits during multiple lowstand events over an extended period during the Quaternary.

Seismic Units SU 2/3
The maximum elevation (within a few metres of the bpsl at the head of the estuary), spatial extent (entire estuary) and thickness (up to 30 m) of the SU 2 estuarine sequence indicated that this unit was deposited during a longstanding, , 2020). A Last Interglacial age was also indicated by a dinoflagellate cyst assemblage recovered from this unit (McMinn, 1989); moreover, an LIG age was identified for similar estuarine clay deposits in other NSW estuaries, e.g. Broken Bay (Roy, 1983), Botany Bay (Albani et al., 1978) and Hunter River (Roy et al., 1980). Any of the preceding major Interglacial events (MIS 7, 9 and 11) would also been capable of depositing the SU 2 estuarine sequence; however, erosion during the long intervening periods of lowstand would have been extensive, and an absence of evidence for multiple units separated by unconformities within the SU 2 in seismic sections does not support this origin. A last interstadial (Marine Isotope Stage, MIS 3) age was originally assigned to the SU 2 based on available dates (42.9 and 41.3 ka BP) at Garden Island (Irvine, 1980;Liu, 1989). However, a last interstadial (MIS 3) age for SU 2 is unlikely, as RSL was >70 m bpsl at this time based on recent relative sea-level records (Rib o et al., 2020). The position of the maximum sea-level during the MIS 3 has been controversial (Rodriguez et al., 2000), and relatively high shorelines have been reported for Texas (Rodriguez et al., 2000), the Red River delta in Vietnam (Hanebuth et al., 2006), and lagoonal-barrier systems in southern Brazil (Dillenburg et al., 2020). However, these features have been explained by a response to local differentiated sediment loading and compaction, or radiocarbon contamination by younger material. Radiocarbon dates (42.9, 41.3 and 39.2 ka BP) for the upper mud and peat section at Garden Island (Units D and E of Lean, 1978) and between 50 and 24 ka BP for bulk sediment at the SHB (Och et al., 2017) may have been affected by dating of mixed marine/terrestrial substances (Meadows et al., 1997;Southon et al., 1995, Stuiver & Braziunas, 1993 or contaminated material (Schiffer, 1986).
An unconformity based on strong evidence (subareal weathering, lack of shells and presence of iron-staining and compaction) (Lean, 1978) was placed within a 13 m-thick estuarine sequence at Garden Island. The top (19.5 m bpsl) of the upper (SU 3) of the two estuarine units directly overlying each other would need have to have been deposited during an RSL similar to the MIS 5.3 and MIS 5.1, which attained maximum heights of 20 m and 18 m bpsl, respectively (Figure 7a). Evidence supports deposition of the lower SU 2 estuarine unit during MIS 5.5, leaving the only highstands capable of depositing the overlying SU 3 unit to the later MIS 5.3 and 5.1 interstadial events. Deposition may have occurred during both these highstands, as the range and duration (40 ka) of this period were sufficient to provide extensive spatial and temporal deposition; however, the later MIS 5.1 event experienced less erosion and was the higher of the two events (Rib o et al., 2020). Extensive research in the Tuncurry-Forster region 300 km north of Sydney placed the first regressive barrier at 3-5 m bpsl dated by thermoluminescence at 96-87 ka BP (MIS 5.1/5.3) (Roy et al., 1997). However, an MIS 5.1 origin matches coastal barrier progradation and construction of onlapping eolian dune fields along the NSW coast, which occurred during MIS 5.5 and 5.1, but not during MIS 5.3 (Rib o et al., 2020).
Pleistocene estuarine sequences overlying bedrock, or basal residual deposits, have been reported for many east coast estuaries (Thom & Roy, 1985;Thom et al., 1992). Paleochannels eroded into bedrock in Broken Bay (20 km north of Sydney) were filled with Pleistocene estuarine sand and clayey sand deposited during the LIG (Roy, 1983;Roy & Thorn, 1981). Basal fluvial sand in Botany Bay (in south Sydney) graded into estuarine mud with increasing peat interbeds deposited during the LIG (Albani et al., 1978;Roy, 1983). In the Newcastle Bight (120 km north of Sydney) and at the Moruya River (250 km south of Sydney), a Pleistocene basal estuarine clay was overlain by fluvial/ deltaic deposits (Roy et al., 1980), and at Tuggerah Lake (65 km north of Sydney), bedrock channels were filled with at least two sequences of Pleistocene estuarine and fluvial/ deltaic sands (Roy & Peat, 1973). At Lake Macquarie and Lake Illawarra (100 km north and south of Sydney, respectively), at least two estuarine cycles were evident (Sloss, 2005). A basal Pleistocene sequence appears to be widespread in central NSW estuaries and, where dated, is related to the LIG event.

Seismic Unit SU 4
Sediments of SU 4 were interpreted to have been deposited during a late stage of the second interstadial (MIS 3) event based on radiocarbon dates (42.9-39.2 ka BP) of equivalent strata from Garden Island (Irvine, 1980). The 39.2 ka BP date came from a sample at 19.5 m bpsl, and the SU 4 at SHB and Blues Point was at 15 m bpsl (Liu, 1989), which placed deposition of this sequence well beyond the maximum height of the ocean during this period (65 m bpsl), and the maximum height of the MIS 3 sea was only 45-50 m bpsl (Rib o et al., 2020). Based on recent sea-level records (Rib o et al., 2020) and mapping of the SU 2/3 in the current work, the maximum ocean flooding of the estuary was seawards of Bradleys Head during the MIS 3 interstadial ( Figure  7b). Sediments of the SU 4 could not have been laid down during the MIS 3 event, and no significant sedimentation could have taken place in the majority of Sydney estuary during this period. Only the MIS 5.3/5.1 event could have provided sufficient accommodation in these parts of the estuary to facilitate deposition of the SU 4.
The SU 4 organic-rich muds, containing peat, landward of the SHB were considered to be deposited under swamp conditions in shallow water with the overlying, upward-finning fluvial channel sediments to be deposited soon thereafter (Irvine, 1980). Thick, low-angle planar and prograding SU 4 sediments observed in seismic sections between the mouth of Lane Cove River and the SHB suggested deposition in shallow water. Deposition of the SU 4 in this area may have taken place during the MIS 5.3/5.1 period, while sea-level was between 20 and 40 m bpsl for an extended interval (Rib o et al., 2020) and when the ocean would have fluctuated between these two locations (Figure 7b). Flooding of this area may have resulted in a low-energy, estuarine environment and favoured deposition of fine, organic-rich sediment and peat as described by Irvine (1980) and as was observed in seismic sections in this area. The thickness (20 m) of the SU 4 between the mouth of Lane Cove River and Blues Point suggests moderate accommodation and depth of estuarine water. This environment of declining energy would also explain the upward-finning nature of the upper fluvial channel sediments of SU 4.
Dinoflagellate cysts and pollen assemblages in these sediments suggested a shallow-water, marsh environment of deposition (McMinn, 1989). The restricted nature of this deposit is possibly related to the relatively short period the sea occupied this location, as well as reduced tidal flows and relatively low fluvial supply.
The SU 4 would then correlate with the marginal marine, well-sorted, medium-grained sands with shells observed in Sydney Harbour Metro boreholes (Och et al., 2017;Skilbeck, 2015) and with the upper estuarine sequence (Units D and E of Lean, 1978) at Garden Island, all deposited during the MIS5.3/5.1 interstadial events.

Seismic Unit SU 5
Sand dunes in Rose Bay and along the southern shores of the lower estuary were correlated with onland eolian sediments transported northwards from Botany Bay (Roy, 1983). The onland deposit was a shell-free, mediumgrained, well-sorted quartzose sand up to 30 m thick (Roy, 1983) and was deposited between 31 and 24 ka BP, immediately prior to the LGM (Thom & Oliver, 2019).

Seismic Units SU 6/7 post-glacial transgressive marine sedimentary units
The sea reached the mouth of Sydney estuary at approximately 14.5 ka BP and may have attained a maximum height slightly greater than present (þ1 to þ2 m) (Bryant et al., 1992;Flood & Frankel, 1989;Lewis et al., 2013) at ca 6.8 ka BP (Dougherty et al., 2019;Sloss, 2005). The transgressive marine sand (SU 6) penetrated rapidly 7 km into the paleovalley as barriers and tidal inlet deposits.
Holocene transgressive marine sand deposits have been reported for several nearby estuaries. The surface of a Pleistocene sequence in the mouth of the Hawkesbury River in north Sydney was eroded during last glacial (LG) lowstand and mantled by an extensive deposit of Holocene fluvial sand 7 km long; a similar succession was observed in Pittwater at the mouth of Broken Bay (Roy, 1983). A sedimentary cross-section at the rail bridge over the Hawkesbury River upstream of Broken Bay showed Pleistocene clay eroded and filled with fluvial sand and muds in a 'near duplicate' of the transect across the Sydney estuary at the SHB (Roy, 1983, figure 12c). Transgressive sand sheets extend for long distances landward in other estuaries, e.g. Hunter River for at least 15 km (Roy et al., 1995), Port Hacking (7 km long), Lake Macquarie (6 km long) and Shoalhaven River (5 km long) (Roy, 1994). The Holocene transgressive marine deposit occupying 7 km of the lower Sydney estuary is thus consistent with that of other NSW estuaries.

Seismic Unit SU 8
Dated sediment indicated marine flooding first took place at Rose Bay in the lower estuary before 6.9 ka BP after eolian sedimentation (Liu, 1989). A eucalypt tree stump in a growth position at 15 m bpsl at Garden Island was dated at 8.36 ka BP (Gill, 1970, reported by Roy, 1983. This tree was growing on a pre-flooded land surface, 3.3 km landwards of Rose Bay, suggesting over 1000 years of fluctuating, shallow-water inundation and deposition of ironstained, shelly, muddy sand with peat (Liu, 1989) in the lower estuary at this time.

Classification of estuaries
Southeastern Australia is characterised by a high-energy, embayed coast with various degrees of infilling by fluvial and marine sediments (Roy et al., 1980). Three main types of estuaries have been recognised, i.e. tide-and wavedominated and intermittently open-closed estuaries based on geology and entrance condition (Roy et al., 2001). The Sydney estuary is classified as a tide-dominated, drowned river valley characterised by a deeply (>50 m) incised bedrock paleovalley (Roy, 1994) and oriented normal to the coastline. Drowned river valleys with open mouths and full tidal ranges exhibit large transgressive marine sand bodies that form rapidly during rising sea-level and extend for long distances landward. Four geomorphic zones are also recognised in all southeast Australian estuaries (Roy et al., 2001), i.e. a marine flood-tide delta; a deep, central mud basin; fluvial delta; and riverine channels and alluvial plains (Figure 9).
The Sydney estuary accords well with the classification of coastal waterbodies; however, the zonation of the estuary appears not to fit all criteria. The marine flood-tide delta, fluvial delta and riverine channels are clearly recognisable, but the presence of a deep, central mud basin is less obvious (Figure 9). The lower estuary is occupied by a thick, sandy (>90 vol% sand) flood-tide delta (2.5 km long) and transgressive marine shelly sand (>70 vol% sand) to the SHB (9 km from the mouth) (Supplemental data, Figure  S2a, b). The central estuary (15 km from the mouth) is 5-14 m deep and is mantled with muddy sand (50-70 vol% sand) with sandy mud (30-50 vol% sand) in local depressions. Off-channel embayments (Blackwattle/Rozelle, Iron Cove, Five Dock, Hen and Chicken and Homebush bays) and the Lane Cove tributary contain muddy sediment (0-10 vol% sand) but are shallow (5-7 m deep). These environments could not be classed as 'deep, central mud basins' as compared with the deep basins of the Hawkesbury River (Cowan Creek, Pittwater) (20-30 m deep) (Roy, 1994), Port Hacking (20-25 m deep) (Roy, 1994) and Middle Harbour (30 m deep) , which demonstrate classic examples of deep mud basins. The central estuary, off-channel embayments and Lane Cove River of the Sydney estuary are occupied by considerable (up to 25 m) late Pleistocene deposits, which were not substantially eroded during the LG. The sandy nature of sediment mantling the shallow, central estuary suggests tidal velocities are sufficiently high to prevent deposition of fine material under present conditions, a conclusion reached in earlier studies . Fine sediment mantling the off-channel embayments and Lane Cove River indicates lower tidal velocities in areas isolated from the main channel. The off-channel embayments are surrounded by small catchments (1-18 km 2 ) resulting in minimal erosion and shallow bedrock. These environments were supplied with fine sediment, mostly from the main channel during multiple highstand events  and preserved during glacial times owing to minor fluvial erosion. These processes have resulted in a moderately sandy, shallow central estuary, which is classified as a mature Stage D drowned river valley type estuary (Roy et al., 2001); however, infilling took place mainly during the Pleistocene and not Holocene. Middle Harbour, a tributary at the entrance of the Sydney estuary is a Stage A drowned river valley estuary the same as Cowan Creek, a tributary of the Hawkesbury River, which is at Stage C in its evolutionary process. The Hawkesbury River and Sydney estuary display a similar mix of environments at different evolutionary stages.

Geological history
The basal relict, coarse-grained deposit of pebbles, boulders and bedrock (SU 1) observed throughout the Sydney estuary probably accumulated during multiple Pleistocene and older sea-level lowstand events. The age and stratigraphic position of the overlying estuarine sequences (SU 2 and 3) were inferred mainly through constraints provided by recent sea-level records (Rib o et al., 2020). The maximum elevation of the SU 2 unit strongly favoured deposition during the Last Interglacial event (MIS 5.5). The formation of the overlying estuarine sediment (SU 3) was most likely during the MIS 5.3/5.1 highstand event when sea-level reached a maximum height of 20 m bpsl (Rib o et al., 2020). The SU 4 sediments comprising fill material in paleochannel depressions in the upper estuary and exhibiting thick, low-angle, planar and prograding strata over large areas in the central estuary were considered landward equivalents of the estuarine sandy muds (SU 3) of the lower estuary owing to its position overlying SU 2 strata.
A sustained period of glacial conditions and sea-level fall was interrupted by minor sea-level recovery associated with the MIS 3 interstadial event. MIS 3 interstadial seas flooded only in the lowermost Sydney estuary and played no significant part in sedimentation of the landward part of the waterway. The prolonged period of lowstand (MIS 2) resulted in extensive erosion and removal of much of the sediment occupying the lower Sydney paleovalley. Eolian sands (SU 5) were deposited adjacent to the southern shores of Sydney estuary between 31 and 24 ka BP before the LG. Deglaciation and melting of the ice sheets occurred rapidly, and sea-level rose quickly (15 m/ka, Thom & Roy, 1985) before the ocean re-entered the estuary at ca 14.5 ka BP and reached a maximum of slightly higher than present (þ1 to þ2 m) (Bryant et al., 1992;Flood & Frankel, 1989). Transgressive marine sand (SU 6) penetrated rapidly into the paleovalley as thick (up to 40 m) and extensive barriers, and tidal inlet deposits transported by tidal currents and storm surges formed a marine flood-tide delta (SU 7) at the mouth of the estuary (Roy et al., 1995(Roy et al., , 1997. Lateral migration of the paleoriver in the lower estuary during deposition resulted in repeated channel erosion and infilling in the central axis of the estuary. Surficial sands and muds (SU 8) were deposited throughout the lower and upper/ central estuary, respectively, since the ocean reached present-day levels.

Conclusions
Abundant seismic coverage combined with a reliable local sea-level record has allowed seismic units to be allocated to marine flooding events, which provided relative ages to these units. The proposed stratigraphy and evolution of the estuary were similar to other central NSW drowned river estuaries; however, the role of interstadial events in the geological history of this coastal feature is rarely observed in the adjacent coastal zone.
A similar stratigraphy of adjacent estuaries with extensive seismic data and strong subsurface control gave confidence to the current interpretation in the lower Sydney estuary; however, the sparse distribution of borehole data needs to be addressed, and deposition of the MIS 5.3/5.1 interstadial estuarine-fluvio-deltaic units (SU 3 and 4) requires additional verification. It is hoped that the present study will stimulate further fieldwork and research in this, the most iconic waterway in Australia.