ORIGIN AND TIMING OF THE MIOCENE±PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA

JULIE A. DICKINSON, MALCOLM W. WALLACE, GUY R. HOLDGATE, STEPHEN J. GALLAGHER, AND LINDSAY THOMAS School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia

ABSTRACT: An unconformity is present close to the Miocene±Pliocene a sea-level curve (e.g., Haq et al. 1988). We suggest that a major interval boundary in the onshore and nearshore portions of the Otway, Port of regional uplift and exhumation began in southeast Australia during the Phillip±Torquay, and Gippsland basins of southeast Australia. The un- late Miocene. Exhumation in localized areas was on the order of several conformity is angular (generally Ͻ 1±5؇ angularity), with the under- hundreds of meters to a kilometer or more (e.g., Otway Ranges). This and lying Miocene units having been deformed (gentle folding and reverse later Pliocene uplift is probably largely responsible for the present topo- faulting) and eroded prior to deposition of the Pliocene succession. The graphic relief of the southern Cretaceous Highlands (Otway and Strzelecki unconformity also marks a change from Oligocene±Miocene deposition ranges; Fig. 2) of Victoria. of cool-water carbonate sediments and brown coal-bearing successions The recognition of late Miocene uplift in southeast Australia has some to the accumulation of more siliciclastic-rich sediments in Pliocene important economic implications. These include the erosion of cover se- time. The Miocene±Pliocene boundary therefore represents an interval quences from the giant Oligocene±Miocene brown coal deposits of the of signi®cant regional uplift in the southeast Australian basins. The Latrobe Valley, making large volumes of coal economic for mining, and amount of section removed is greatest around the Otway and Strzelecki structures offshore produced in the late Miocene provide favorable targets ranges in Victoria, where up to a kilometer of section may have been for petroleum accumulation (e.g., the Nerita structure, Torquay sub-basin). removed. In most onshore sections of the Victorian basins hundreds of meters of section have been eroded. In distal offshore locations the REGIONAL GEOLOGY boundary becomes conformable. The timing of uplift and erosion is best constrained in the Otway and Port Phillip basins, where late Mio- The southeast Australian basins occupy an area along the southern mar- cene (N16 ϳ 10 Ma) sediments underlie the unconformity and earliest gin of the Australian continent. The basins developed during the rifting of Pliocene (ϳ 5 Ma) sediments overlie it. This timing coincides with a the Australian continent from Antarctica, Lord Howe Rise, and New Zea- change in the dynamics of the Australian plate, beginning at around land during the Cretaceous to early Tertiary (Etheridge et al. 1987), re- 12 Ma. Southeast Australia is currently under a NW±SE compressional sulting in a common structural and stratigraphic history. Consequently, all regime, and this has probably persisted since the late Miocene. In the the basins contain a largely nonmarine Lower Cretaceous rift±®ll succes- basins (as opposed to the basement-dominated highland areas), the late sion overlain by an Upper Cretaceous to Recent nonmarine and marine Miocene uplift event is more signi®cant than later Pliocene±Recent up- post-rift deposits. lift. The margins of the basins are partly de®ned by blocks of Lower Cre- taceous sediment (forming the southern Cretaceous Highlands), bounded by reverse faults that form structural highs and de®ne the present limit of INTRODUCTION Tertiary sediments. The Otway basin is separated from the Torquay Em- bayment and Port Phillip basin by the Otway Ranges, which is in turn Determining the relative importance of eustasy versus tectonic phenom- separated from the Gippsland basin by the Mornington Peninsula High and ena in controlling regression and transgression within sedimentary basins Strzelecki Ranges (Fig. 2). The northern margin of the basins is de®ned by has proved problematic. The Exxon sequence stratigraphic approach, de- the Western and Eastern Highlands, whilst the southern margin extends veloped during the 1970s and 1980s, provides a framework for deriving offshore (Fig. 1). relative sea level by analysis of the geometry of sedimentary packages. The Oligocene to Recent marine sediments of the Otway, Port Phillip Regional and global correlation of such data should distinguish between and Gippsland basins are dominated by cool-water and temperate-water tectonically and eustatically controlled sea-level change. However, wide- carbonates. The sedimentology and diagenesis of these nontropical carbon- spread, short-lived tectonic events can easily be confused with eustatic ates has been the focus of several recent studies (e.g., James et al. 1992; events, particularly when the additional problem of age resolution is con- Boreen et al. 1993). The Oligocene±Miocene carbonates that make up the sidered. The Miocene±Pliocene boundary is an example of an event (or bulk of the onshore marine sections are characterized by high-energy bry- events) where tectonism is apparently widespread across the globe (e.g., ozoal grainstones with little or no siliciclastic content. Nonmarine Oligo- Mercier et al. 1987; Dewey et al. 1989; Amano and Taira 1992; Walcott cene±Miocene onshore sediments are characterized by extensive accumu- 1998; Hill and Raza 1999), and potential exists for considerable confusion lation of brown coal, again with little or no siliciclastic content. Conversely, between eustatic and tectonic events. the Pliocene succession is typi®ed by calcarenites and unconsolidated sand There has been much discussion of the tectonic or eustatic signi®cance deposits which contain a signi®cant proportion of siliciclastic material, in- of the Miocene±Pliocene unconformity in the southeast Australian basins cluding quartz-rich pebble formations. Deposition of brown coal largely (Gippsland, Port Phillip, and Otway basins; Fig. 1; e.g., Mallett 1978; Cart- ended in the late Miocene, and there have been no signi®cant Pliocene er 1979). Unconformities of late Miocene±early Pliocene age (commonly coals discovered to date. Pliocene±Quaternary basalts, known as the Newer associated with phosphatic clasts) from several localities across Victoria Volcanics (Douglas and Ferguson 1988), generally overlie the marine Pli- have long been recognized as being stratigraphically signi®cant markers ocene. (Coulson 1932; Keble 1932; Singleton 1941; Gill 1957; Bowler 1963). Oligocene to Recent sediments in the onshore areas of the Otway and However, little research has been carried out on the regional signi®cance Gippsland basins are variously folded and faulted. Oligocene±Miocene sec- of this Miocene±Pliocene unconformity. tions commonly have gentle dips (generally Ͻ 10Њ) with monoclines and In this paper, we attempt to synthesize paleontologic, stratigraphic, and open folds dominating. Reverse faults are also common in some regions, structural data from the various Tertiary basins across southeast Australia particularly surrounding the Cretaceous Highlands. Tertiary sediments in in order to assess the timing and origin of these Miocene±Pliocene uncon- close proximity to reverse faults rarely have steep dips (up to vertical). formities. Through this we highlight the importance of independent inter- Pliocene and younger sections are generally less deformed, but some faults pretation of sequences rather than systematic correlation with templates for and monoclines do affect these sediments. In the offshore portions of these

JOURNAL OF SEDIMENTARY RESEARCH,VOL. 72, NO.2,MARCH, 2002, P. 288±303 Copyright ᭧ 2002, SEPM (Society for Sedimentary Geology) 1527-1401/02/072-288/$03.00 MIOCENE±PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA 289

FIG. 1.ÐDigital elevation model for southeast Australia (from Australian Geological Survey Organisation) with the location of the major tectonic and geomorphological elements. basins, some reverse faults and folding are also present, but deformation gene) activity in southeast Australia. In the southern parts of Victoria, is generally less intense than in the onshore sections. In the offshore Gipps- northeast oriented reverse faults of Neogene age bound the Otway Ranges, land basin, Oligocene±Recent deformation has produced large-scale struc- the Mornington Peninsula, and the Strzelecki Ranges, which are similarly tural traps that play host to giant oil and gas ®elds. oriented (Fig. 2). Farther north, in the Murray basin, Neogene faults have There are numerous structures with evidence of relatively young (Neo- a NNE orientation. In South Australia, Neogene reverse faults are present

FIG. 2.ÐDigital elevation model for southern Victoria (from Australian Geological Survey Organisation) showing the Southern Highlands and major faults. 290 J.A. DICKINSON ET AL.

FIG. 3.ÐGeneralized chronostratigraphy of the southeastern basins during the Neogene. T.p. ϭ Tubuliforidites pleistocenicus, M.l. ϭ Meyeripollis lipsis, C.b. ϭ Cingulolatisporites bifurcatus, T.b. ϭ Triporollenites bellus, P.t. ϭ Proteacidites tuberculatus. Modi®ed from Abele et al. (1988). in the Flinders Ranges, Mount Lofty Ranges, and St. Vincent basin and data). Paleontological samples were disaggregated by hydrogen peroxide they trend N±NNE. This is consistent with the predominantly W±WNW and foraminifera picked, sorted, and identi®ed for qualitative biostrati- oriented compressional stress ®eld to which southeast Australia appears to graphic purposes. be currently subject (Coblentz et al. 1995). All 87Sr/86Sr measurements were made on calcitic and unaltered arago- The onshore parts of the southeast Australian basins contain a major nitic molluscs. Samples were cleaned with concentrated hydrochloric acid stratigraphic break within the late Miocene±early Pliocene at the change to remove any potential surface overgrowths and rinsed with distilled water. from carbonate-rich to more clastic-rich sediments (Fig. 3). Phosphatic Dissolution of powdered samples was carried out in 10% acetic acid. clasts and vertebrate remains are commonly present at this unconformity Chemical separations were performed using 1N HCl and 2.5N HCl as el- (Coulson 1932; Keble 1932). This regional unconformity has previously uents through AG 50W-X8 ion-exchange resin in 15 ml quartz columns been interpreted as being due to tectonic, eustatic, or climatic affects (e.g., for Sr separation. The Sr isotope composition of the separate was measured Bowler 1963; Carter 1978a; Mallett 1978). In this paper, we discuss the at the University of Adelaide with mass fractionation normalized to a 86Sr/ nature and signi®cance of this stratigraphic break at various localities in 88Sr ratio of 0.1194 and to a SRM 987 ϭ 0.710260. Analytical reproduc- the onshore and nearshore parts of the southeast Australian basins. ibility of samples is 12 ϫ 10Ϫ6 (2␴ of the mean). Calcite ages were cal- culated using Howarth and McArthur's (1997) Sr isotope Look-Up Table METHODS Version 3:10/99 (Table 1). Time±depth conversion of interpreted seismic sections from the Torquay basin was carried out using Seismic Plugin v. Stratigraphic data were obtained through detailed sections taken at lo- 3.0 (Jay Lieske, Jr.) for Adobe PhotoshopTM. Sonic velocities used were cations in the Otway, Port Phillip, and Gippsland basins. Borehole data 1500 m/s, 1800 m/s, and 2200 m/s for sea water, Pliocene±Quaternary were used from Sorrento (Mallett and Holdgate 1985) and from coal bores sediments, and Miocene Torquay Group sediments, respectively. in the Gippsland basin (Bolger 1991; unpublished Rural Water Commission MIOCENE±PLIOCENE BOUNDARY IN THE OTWAY BASIN Where the boundary between Miocene and Pliocene sediments is ex- TABLE 1.ÐMeasured 87Sr/86Sr ratios of calcitic carbonate material from strati- graphic sections used in this study. posed in the Otway basin, it is generally an unconformity (Fig. 3). The unconformity is marked by a planar erosional surface with no evidence of Stratigraphic oxidation and a signi®cant change in the lithology. The underlying Miocene Location height (m) Age (Ma) 87Sr/86Sra Std errorb strata exhibit a shallowing-upward (more calcareous upward) character. Hamilton 2.90 4.94 0.709039 Ϯ12 Gray fossiliferous marls (Gellibrand and Muddy Creek marls) and bryo- 2.90 5.03 0.709036 Ϯ12 zoan-rich ®ne-grained white limestones (Port Campbell and Bochara lime- 2.90 3.52 0.709061 Ϯ12 1.60 10.86 0.708865 Ϯ12 stones) with a nominal siliciclastic content typify the sequence. The over- 0.80 10.86 0.708865 Ϯ12 lying Pliocene succession represents a prograding quartz-carbonate barrier 0.80 11.13 0.708844 Ϯ12 system with the initial marine incursion marked at the base by quartz-rich Batesford 7.00 4.86 0.709041 Ϯ12 6.60 4.21 0.709052 Ϯ12 shallow marine silts and shelly sands (Whalers Bluff and Grange Burn 6.00 4.45 0.709049 Ϯ14 formations) and subsequent deposition of yellow ¯aggy calcarenites (Wer- Coghills 0.50 m below unconformity 10.26 0.708882 Ϯ12 rikoo Limestone). Beaumaris 4.30 4.52 0.709048 Ϯ12 2.80 5.24 0.709028 Ϯ12 2.50 5.67 0.709011 Ϯ14 Glenelg River Section a All results are normalized to the SRM-987 standard ϭ 0.710260 and to 86Sr/87Sr ϭ 0.1194. b Analytical uncertainty represents 2 standard errors of the mean of ϳ 100 individual measurements and In the Dartmoor region (Fig. 4) along the Glenelg River valley, Pliocene± refers to the last two digits of the ratios. Pleistocene quartzose calcarenites of the Werrikoo Limestone unconfor- MIOCENE±PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA 291

strontium isotope dates of 11.13 Ϯ 0.5 and 10.86 Ϯ 0.5 Ma (Table 1; Fig. 5). The Miocene±Pliocene boundary is marked by a planar unconformity at the base of the Pliocene Grange Burn Formation, which overlies all older Miocene sediments (Fig. 5). The surface is characterized by the occurrence of a phosphatic nodule bed with abundant marine vertebrate remains. It also contains many fossils both reworked from the Muddy Creek Marl and autochthonous to the Grange Burn Formation, some attached to the phos- phatic clasts. The Grange Burn coquinas contain typical Kalimnan (Fig. 3) shelly fossils and are considered to be early Pliocene in age (Ludbrook 1973). Strontium isotopes taken of the mollusc fauna derive ages of 5.0± 4.0 Ma. The basalt has been dated by K±Ar at 4.35 Ma (Turnbull et al. 1965), placing deposition of the Grange Burn Formation during the very early Pliocene.

MIOCENE±PLIOCENE BOUNDARY IN THE CENTRAL COASTAL BASINS FIG. 4.ÐLocation map for the Otway basin showing the location of stratigraphic sections studied. The Port Phillip and Torquay basins lie between the Otway and Gipps- land basins and are bordered by uplifted areas of Lower Cretaceous sedi- ment and Paleozoic basement (Fig. 6). The Miocene±Pliocene boundary is amably overlie Eocene Knight Group and Miocene limestones of the Gle- well exposed in coastal and river sections and is characterized by a planar nelg Group (Fig. 5). The youngest age of strata preserved beneath the unconformity. The unconformity is typically represented by a sharp litho- unconformity is placed in the early Miocene (planktonic foraminiferal zone logical change from marl to quartz-rich carbonate sand (Figs. 7, 8A), with N5; Figs. 3, 5). The lower Werrikoo Limestone is here placed within the no evidence of karsti®cation or oxidation. There is commonly a discontin- Werrikooian (upper Pliocene) on the basis of the molluscan fauna found uous horizon of coarse quartz gravel containing distinctive phosphatic towards the base of the unit, typical of Singleton's (1941) Werrikooian clasts immediately overlying the unconformity. These phosphatic clasts stage, and the absence of planktonic foraminifera Globorotalia truncatu- commonly have borings (Fig. 8B). There has been much early work on the linoides. G. truncatulinoides does not become apparent until higher in the Miocene±Pliocene boundary in this region, largely because of the abun- section, placing the upper part in the Pleistocene (Fig. 5). In the vicinity dance of vertebrate remains (e.g., Coulson 1932; Keble 1932). of Jones Ridge basalt dated at 2.24 Ma by K±Ar (Singleton et al. 1976) The strata underlying the Miocene±Pliocene unconformity range in age overlies the lower Miocene sediments marking the unconformity, with de- from early (e.g., Shelford) through to mid- (e.g., Geelong) and late Miocene position of the Werrikoo Limestone above this. (e.g., Beaumaris and Point Nepean; Fig. 7), which is detailed below. It is A regional cross section produced by Kenley (1971) based on outcrop a succession of gray-buff calcareous clay-rich sediment with cemented fer- and well stratigraphy in the western part of the Otway basin (Fig. 5) in- ruginous horizons and layers of carbonate nodules and sporadic phosphate dicates that the Knight and Glenelg groups have undergone folding follow- concretions (Gellibrand Marl and Fyansford Formation). The overlying ing their deposition. This folding occurred prior to the deposition of the units are characterized by basal marine quartz-rich calcareous sandstones Werrikoo Limestone, which overlies the angular unconformity. The folding and molluscan-rich sandy calcarenites that grade upwards into a nonmarine is focused around the Merino High (as de®ned by Kenley 1971), an area sand (Moorabool Viaduct Formation and Brighton Group). The abundance of outcropping Otway Group (Lower Cretaceous) sediments (Fig. 4) that of quartz sand in these units contrasts with the quartz-poor clayey carbon- has been elevated and dissected. ates of the underlying Miocene.

Portland Shelford In the coastal cliffs north of Portland (Fig. 4), Pliocene Whalers Bluff West of Geelong, on the Leigh River near Shelford (Fig. 6), the under- Formation and a younger subaerial basalt unconformably overlie Miocene lying Miocene sequence is attributed to the Gellibrand Marl (Fig. 7; Ed- Port Campbell Limestone (Fig. 5). The top of the limestone is dated as late wards et al. 1996), and the upper part of the unit is dated as early Miocene Miocene by the presence of the planktonic foraminifera Globorotalia mio- (planktonic foraminiferal zone N7) on the basis of the presence of the tumida (planktonic foraminiferal zone N16; Fig. 3). The age of the initial planktonic foraminifera Globigerinoides trilobus. Quartz sands of the over- deposition of the Whalers Bluff Formation is determined to be early Pli- lying Pliocene Moorabool Viaduct Formation are poorly exposed. ocene (Zone N19) by the presence of Globorotalia punticulata. Boutakoff (1963) noted that the Pliocene sediments in®lled a karst topography de- Geelong veloped on Miocene limestones. The basalts that cap the top of the section are dated by K±Ar as 2.51 Ma (Singleton et al. 1976), giving an upper In the vicinity of Geelong, along the Moorabool River (old Batesford constraint on the age of the Whalers Bluff Formation. Quarry, Coghills section; Coulson 1932; Bowler 1963), the Miocene±Pli- ocene succession is represented by the Fyansford Formation, which is un- Hamilton conformably overlain by the Moorabool Viaduct Formation (Fig. 8C). The upper part of the Fyansford Formation, where exposed beneath the uncon- To the west of Hamilton in the Muddy Creek±Grange Burn area, the formity, varies in age from mid- to late Miocene (Fig. 3). A strontium Miocene±Pliocene sequence unconformably overlies mid-Paleozoic rhyo- isotope date from just below the unconformity at Coghills (Fig. 6) gives a lite and is capped by basalt. The Miocene Muddy Creek Marl contains a date of 10.26 Ϯ 0.5 Ma (Table 1). foraminiferal assemblage derived by Mallett (1977) that suggests that the Determination of the age of the overlying Moorabool Viaduct Formation unit spans the interval from foraminiferal zone N7 to N16. Directly beneath has been based largely on molluscan fauna. It is consequently considered the Miocene±Pliocene boundary a distinctive limonitic and glauconitic to be very late Miocene to early Pliocene (Abele et al. 1988), but there is green marl is present and contains abundant erosional lag beds yielding the suggestion that the unit is as young as late Pliocene (Bowler 1963). 292 J.A. DICKINSON ET AL.

FIG. 5.ÐMeasured stratigraphic sections across the Miocene±Pliocene unconformity in the Otway basin. Cross section is located on Figure 4 (modi®ed from Kenley 1971).

Strontium isotope analysis of marine molluscs at Batesford quarry places dated as mid-Miocene where it outcrops at Mornington farther to the south initial deposition at 4.5 Ϯ 0.5 Ma. Basalts that overlie the sequence are (Mallett and Holdgate 1985). The phosphatic nodule bed, which marks the dated at 2.0 Ma using K±Ar (Whitelaw 1989), constraining the upper age unconformity and consists of quartz pebbles, phosphatic clasts, and a rich of deposition. fauna of teeth, bones, and shell material (Fig. 7; Singleton 1941), is located 0.5 m beneath beach level. Beaumaris The exposure of the Pliocene (Black Rock Sandstone) at Beaumaris con- stitutes the type section of the Cheltenhamian Stage (Singleton 1941), plac- At Beaumaris, on the coast southeast of Melbourne, the age of the un- ing the unit in the upper Miocene (planktonic foraminiferal zones N17± derlying Fyansford Formation is poorly de®ned. However the upper part is 18). This is supported by the foraminiferal assemblage, which, although MIOCENE±PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA 293

FIG. 6.ÐLocation map of the central coastal basins showing the sections studied and the position of offshore seismic pro®les.

FIG. 7.ÐMeasured stratigraphic sections across the Miocene±Pliocene unconformity in the Port Phillip basin. 294 J.A. DICKINSON ET AL.

FIG. 8.ÐA) The unconformity (arrow) between Miocene and overlying Pliocene at the old Batesford Quarry. B) Phosphatic clasts from above the unconformity showing marine borings on their surface. C) Miocene±Pliocene succession at the old Batesford Quarry with the unconformity indicated by the arrow. The Moorabool Viaduct Formation overlies the unconformity, while the Fyansford Formation underlies it. Newer Volcanics are visible at the top of the cliff. Height of section at arrow is 11.3 m. D) Tilting (white arrows) in coal measures as exposed at Loy Yang coal ®elds beneath the unconformity (black arrows) and the Haunted Hill Formation. poor, places deposition of the unit within the same time (Mallett 1977). bonate-rich marls of the Torquay Group are offshore equivalents of the However, strontium isotope analysis of molluscs within the unit (Fig. 7) Oligocene±Miocene sequence. The sub-basin has been the subject of pe- suggests that the deposit formed during the early Pliocene (5.5±4.5 Ma). troleum exploration, and although there is little sample material for study, The Black Rock Sandstone grades upward into terrestrial sands and clays much seismic and well log data are available (Trupp et al. 1994). Several that are regarded as Pliocene (Abele et al. 1988). shallow seismic lines have also been run across the sub-basin. Using pa- leontological data from Nerita #1 (Fig. 6) and the nearby Nepean bores, it Sorrento appears that the youngest Torquay Group (Trupp et al. 1994) ranges in age The Nepean Peninsula extends northwest into the center of the Port Phil- from early to late Miocene. Well-log data from Wild Dog #1 (Fig. 6) also lip basin from the southern end of the Mornington Peninsula. A number indicate that the succession becomes less clay-rich upwards, as does the of bores are situated at Sorrento, where sediments of the Port Phillip basin section onshore. are thickest. The Fyansford Formation is here present at its youngest, rang- On industry seismic sections, the Torquay Group and underlying suc- ing up to upper Miocene. Elsewhere in the basin, the strata above the cessions are folded and faulted (Fig. 9). Reverse faults in the underlying unconformity are Pliocene. At Sorrento, however, the overlying Brighton Otway Group and Paleozoic basement generally grade into monoclinal Group is considered to be upper Miocene (Mallett 1977). In contrast, the structures in the Torquay Group sediments. Folding and faulting affects the Wannaeue Formation (Fig. 7), as proposed by Mallett and Holdgate (1985), entire section up to the upper Miocene. Furthermore, much erosion of sec- contains a rich faunal component that indicates an early to mid Pliocene tion is evident at the sea ¯oor. On the Nerita #1 structure 400 m of Miocene age. Both the age and the lithology are comparable to the Whalers Bluff section has been eroded and outcropping sediments at the sea ¯oor are Formation at Portland (Mallett and Holdgate 1985). However, the unit is lower Miocene (Fig. 9; Shell 1967). only apparent in the subsurface and has not been recorded in any other part Near the Otway coastline and the Mornington Peninsula, the entire Tor- of the Port Phillip area, reaching a thickness of 88 m in the center of the quay Group and underlying successions (Angahook Formation and Boonah Sorrento Graben and becoming thinner towards the margins. Formation) have been removed by erosion, indicating more than 600 m of erosion (Fig. 10). No thinning of the Torquay Group towards the Otway TORQUAY SUB-BASIN Ranges or the Mornington Peninsula is visible on seismic, and this suggests The Torquay sub-basin, offshore from the Port Phillip basin (Fig. 6), that large thicknesses of Torquay Group have been stripped from these contains an upper Tertiary succession similar to that exposed onshore. Car- highs. Deformation and erosion must postdate the upper Miocene Torquay MIOCENE±PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA 295

FIG. 9.ÐSeismic pro®le (OS88A±12) across the Torquay sub-basin (Fig. 6) showing folding of Miocene and older sediments against the structural highs to the east and west. Note the doming of the Nerita-1 structure. The Miocene±Pliocene unconformity is just visible at the top of the section.

Group sediments, because the entire Torquay Group appears conformable of-coal surface. The unconformity has been well documented by Bolger and is uniformly affected by deformation. (1984, 1991). On shallow seismic sections (Fig. 11), a spectacular angular unconfor- The major structures of the onshore Gippsland basin are illustrated in mity is visible (also just visible on industry seismic, Fig. 9). From corre- Figure 12. Faulting associated with the folding tends to be NE-trending lation with the Nepean bores and Nerita #1, the unconformity postdates the and is both extensional and compressional, resulting in horsts or tilted horst upper Miocene Torquay Group sediments and appears to be the lateral blocks (Smith 1982; Hocking et al. 1988). Many of the blocks and mono- equivalent of the Fyansford Formation±Brighton Group unconformity in clines are now bounded by and overlie reverse faults at depth, and appear the Nepean bores (i.e., the Miocene±Pliocene unconformity). This would to be inverted structures. Tertiary drape over these reverse faults de®nes indicate that the overlying undeformed sequence is the equivalent of the monocline warping (Barton 1981), which formed largely after Miocene Brighton Group and Wannaeue Formation described by Mallett and Hold- coal-measure deposition. gate (1985) from the Nepean bores. The Miocene±Pliocene unconformity on seismic (at Nerita #1 and on Latrobe Valley Depression shallow seismic) indicates that most deformation and erosion of the un- derlying Torquay Group took place during the late Miocene. The Nerita The Tertiary coal measures of the Latrobe Valley Depression occupy an #1 structure, for example, appears to be almost entirely due to late Miocene elongated asymmetric east-pitching syncline known as the Latrobe and deformation. On shallow seismic near the Otway Ranges, however, some Traralgon synclines (Fig. 12). Within these synclines over 700 m of Oli- folding is evident within the overlying Pliocene sediments, indicating con- gocene±Miocene coal measures are present. Peripheral to the central syn- tinued deformation into the Pliocene. clines are a series of en echelon structures that separate the Latrobe Valley into a number of blocks where the dips on Tertiary coal measures can reach angles exceeding 45Њ. The coal seams exposed in open cuts on the Mor- GIPPSLAND BASIN well±Yallourn and Loy Yang blocks (Fig. 12) are also strongly jointed The onshore Gippsland basin is characterized by an Eocene to Miocene because of this folding. Most joints trend NNW to NW, swinging more coal-dominated sequence that is laterally equivalent to the Oligocene±Mio- northerly at Morwell because of regional stress accompanied by shear on cene cool-water carbonates of the Seaspray Group in marine sections of the Morwell Monocline/Fault (Fig. 13; Barton 1979). The structures post- the basin. A number of coal seamsÐTraralgon, Morwell, and YallournÐ date coal-seam deposition, inasmuch as they are unconformably overlain make up the Tertiary brown coal measures, which constitutes the bulk of by ¯at-lying Haunted Hill Formation sediments, which are also seen to ®ll the Latrobe Valley Group. This is unconformably overlain by a siliciclastic the joints. sand- and gravel-dominated unit (Pliocene Haunted Hills Formation) and Post-coal measure folding, uplift, and subaerial erosion of the Latrobe laterally equivalent marine sandy calcarenites and coquinas (Jemmys Point Valley Group and equivalents dominates the near-surface geology of the Formation). In the Latrobe Valley, the Miocene±Pliocene unconformity is Latrobe Valley (Thomas and Baragwanath 1949, 1951; Gloe 1960, 1976; expressed as an angular difference accompanied by erosion and weathering Bolger 1984, 1991). Evidence for this erosion can be seen in all the anti- (oxidation) of coal seams, together with pronounced burn holes in the top- clinal structures adjacent to the Balook Block, and along the northern and 296 J.A. DICKINSON ET AL.

FIG. 10.ÐSection of the seismic pro®le OS88A±1 projected up onto the Otway Ranges (Fig. 6) with reconstruction of the eroded pre-Pliocene succession.

FIG. 11.ÐShallow seismic pro®le 82-K-1, Kimbla Cruise, February 1982, across the Torquay sub-basin (Fig. 6) illustrating the angular unconformity at the Miocene± Pliocene boundary. MIOCENE±PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA 297

FIG. 12.ÐLocation map of the Gippsland basin showing the main structural features and positions of cross sections. southern basin margins (Figs. 14, 15). The Miocene±Pliocene boundary is clines is evident on cross sections (Fig. 14), but in the center of the syn- well exposed in all the brown coal open-cut exposures, where the Pliocene clines more continuous sedimentation appears to have occurred between Haunted Hill Formation rests unconformably on coal seams (Fig. 8D). Ar- coal measures, post-Yallourn Seam clays, and Haunted Hill Formation eas of maximum uplift have largely been stripped of their Tertiary cover, (Bolger 1991). and this stripping may exceed 200 m in places (e.g., the Baragwanath The timing of structural uplift for the Tertiary coal measures in Gipps- Anticline and Loy Yang Dome; Fig. 12; Dickinson et al. 2001). In the land is closely related by their peripheral occurrence to the Balook Block adjacent synclinal areas, evidence for the unconformity is provided by sub- and Eastern Highlands uplift: surface data. Post-Yallourn Seam truncation on the major bounding mono- (1) The isopachs of the Oligocene to lower Miocene Morwell 2 coal

FIG. 13.ÐCorrelation of stratigraphy from borehole data across the Gippsland basin showing palynology and microfossil ages for the main formations. Biostratigraphy sourced from Partridge (1971), Mallett (1977), and Dawson (1983). 298 J.A. DICKINSON ET AL.

FIG. 14.ÐCross section across the Yallourn Monocline at Yallourn North coal mine illustrating the angularity of the Miocene±Pliocene unconformity. seam show thickening towards the Yallourn Monocline and later uplift to at the end of the Miocene (an interval of 8±4 Ma). Farther east towards form the Yallourn North and Extension coal ®elds (Holdgate 1985). The the Bairnsdale±Lakes entrance area, the same relationship and ages are original extent of the Morwell 2 Seam is conjectural, but the present extent observed between the Tambo River Formation and the Jemmys Point For- is clearly seen to be controlled by later uplift and erosion. The Morwell 2 mation (Carter 1964, 1978a, 1978b; Mallett 1978), where horizons of phos- (Latrobe) Seam is also cut and offset by reverse faulting along the Yallourn phatic clasts are widespread along the unconformity. Monocline at Yallourn Power Station (Fig. 15) and is present on the up- thrown Haunted Hill Block. DISCUSSION (2) The earliest appearance in the Traralgon Syncline of the lower Cre- taceous Strzelecki Group detritus (taken to indicate erosion from the Balook The stratigraphic relationship summarized above points to a major and Narracan blocks) is observed in upper Miocene clays above the Yal- change having taken place in the sedimentological regimes of the southeast lourn Seam (Bolger 1984). Australian basins during the late Miocene to early Pliocene: In the Traralgon and Latrobe synclines, clayey sediments of the Hazel- 1. An unconformity of approximately late Miocene±early Pliocene age is wood Formation, reaching over 100 m thickness, conformably overlie the present in every onshore succession from the Otway, Port Phillip, and Yallourn coal seam (Fig. 13). Both are folded and unconformably overlain Gippsland basins. Indeed, very few sections onshore from any of these by sands and gravels of the Haunted Hills Formation. Constraints from basins have uppermost Miocene sediments preserved, and in many cases palynology dates suggest that the Hazelwood Formation extends through subsequent deposition did not occur until much later in the Pliocene. the Upper T. bellus and lower part of the C. bifurcatus zones (Fig. 3), 2. Where the Miocene±Pliocene boundary is preserved in marine succes- whereas the basal units of the Haunted Hill Formation include the upper sions, the unconformity is typically sharp and planar, with no evidence part of the M. lipsis Zone (Fig. 13; Dawson 1983). This effectively indi- of oxidation or karsti®cation of the underlying strata (the only exception cates a hiatus during which folding took place spanning the time interval being Portland). Furthermore, the earliest sediments overlying the un- recorded between the upper C. bifurcatus and lower M. lipsis zones, i.e., conformity are of marine origin. This indicates that the unconformity between 7 and 4 Ma (Fig. 3). was produced by marine erosion, or that any evidence of subaerial ero- The correlation of stratigraphy across the onshore Gippsland basin (Fig. sion has been removed by subsequent marine erosion. 13) is consistent with the other southeast Australian basins. Here the best 3. The Miocene±Pliocene break is generally an angular unconformity, with constraint, by palynology dates, microfossil biostratigraphy, and structural differences in dip ranging from 0.5Њ to nearly 90Њ (most commonly in control, con®nes the timing of unconformity development to the latest Mio- the range 0.5Њ to 5Њ). The underlying Miocene and older successions cene. are folded, whereas the overlying Pliocene succession is generally not. 4. The Oligocene±Miocene carbonates of southeast Australia are charac- Alberton Depression teristically clastic-poor successions. In contrast, the majority of onshore The structural pattern of the pre-middle Miocene Alberton Coal Mea- Pliocene to Recent marine sequences are characterized by mixed clastic- sures in the Alberton Depression of South Gippsland is broadly similar to carbonate facies. that of the Latrobe Valley Depression. The coal measures are folded and 5. Accumulation of clastic-poor Oligocene±Miocene brown coal seams unconformably overlain by ¯at-bedded Haunted Hill Formation clastic sed- ceased during late Miocene±early Pliocene time. Nonmarine Pliocene to iments and Pliocene coquinas of the Jemmys Point Formation (Hocking Recent sediments overlying the coals are characterized by high-energy 1976; Greer and Smith 1982; Holdgate 1982; Thompson and Walker 1982). siliciclastic ¯uvial successions. The timing of the unconformity, which marks this change in sedimen- tation, is con®ned by the deposition of sediments above and below the Lake Wellington Depression unconformity. In many cases the youngest deposits beneath the boundary Down-basin towards the Sale area, the timing of folding is further con- are lower to middle Miocene. Precise dating of the overlying units is made strained. Here the latest age for the underlying folded Miocene (Tambo dif®cult owing to a lack of diagnostic fauna. The timing is best constrained River and Boisdale formations; Fig. 13) is dated as planktonic foraminifera by the hiatus that is apparent in the stratigraphy at Portland and Hamilton. zone N17 (Fig. 3), whereas the oldest part of the overlying Jemmys Point At both locations upper Miocene sediments are preserved beneath the un- Formation is con®ned to planktonic foraminifera zone N20. Therefore the conformity (ϳ 11 Ma from 87Sr/86Sr marine carbonate dating at Hamilton). age of folding, given the constraints on the dating, must have taken place Sequences above the unconformity are earliest Pliocene at Portland by MIOCENE±PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA 299

FIG. 15.ÐReverse offset on the Yallourn Monocline/fault as exposed in the Yallourn coal mine indicating a pre-Pliocene (pre-Haunted Hill Formation) age for deformation and erosion. planktonic foraminifera (N19) and 5.5±4.0 Ma (early Pliocene) by 87Sr/86Sr moved by erosion at the sea ¯oor as the Otway Ranges are approached dating at Hamilton. This places the commencement of the interval of un- (Fig. 10). conformity development during the late Miocene (11±5 Ma), with cessation Very similar relationships exist around the Strzelecki Ranges in the La- of the event by the Pliocene. Similar age relationships exist at Beaumaris trobe Valley. Near the margins of the ranges, Oligocene±Miocene brown (Mallett and Holdgate 1985), and in the Gippsland basin, a range of 8±4 coals are completely removed by erosion, with little or no evidence of Ma is determined for the unconformity. stratigraphic thinning towards the ranges (Fig. 14). The signi®cant emer- gence of the Balook Block above depositional base level and into an active Uplift and Exhumation of the Southern Highlands denudation zone did not occur until post-Yallourn Seam times in the late Miocene. The stratigraphic and structural relationships around the Creta- Blocks of Lower Cretaceous sediments are present across the southeast ceous highlands indicate several hundred meters or more of erosion from Australian basins as structural highs. These blocks consist of sediments that the Cretaceous blocks during the late Miocene±Pliocene. The offshore seis- are principally feldspathic sandstone, mudstone, and shale, with conglom- mic pro®les and regional cross sections in the vicinity of the Early Creta- erate in places prominent at the base. The relationship of Oligocene±Mio- ceous highs show that folding and faulting is most strongly developed in cene sediments to these Cretaceous highs is important in determining their regions adjacent to these uplifted areas (Fig. 11; Skeats 1935). timing of uplift and exhumation and consequently the origin of the Mio- The history and type of tectonism is illustrated best on shallow seismic cene±Pliocene unconformity. sections from the Torquay sub-basin. Here, the bulk of folding, faulting, Where Oligocene±Miocene sediments ¯ank the shoulders of these Me- and erosion occurred during the late Miocene. This has largely taken the sozoic highs (e.g., Otway Ranges±Torquay basin, Strzelecki Ranges±La- form of doming (e.g., line OS88A±1, Fig. 9) and reverse faulting in the trobe Valley) the stratigraphy is dominated by the deposition of carbonate- Otway Group (producing monoclines in the Torquay Group) along the mar- rich sediments (e.g., Port Campbell Limestone, Gellibrand Marl, Fyansford gins of the Otway Ranges and Mornington Peninsula. The Miocene±Plio- Formation) or nonmarine brown coal seams (e.g., Latrobe Valley Group). cene unconformity overlies much of the eroded and folded Miocene sec- The siliciclastic component is everywhere low or nonexistent in these ¯ank- tion, indicating a pre-Pliocene origin for most deformation and erosion. ing Oligocene±Miocene sediments. This suggests that the Lower Creta- However, the Miocene±Pliocene unconformity in the Torquay sub-basin ceous blocks had not been exhumed and exposed at this time, inasmuch as has a gentle synformal morphology with dips of less than 1Њ on the limbs the erosion of the feldspathic beds would cause a signi®cant clastic input (Fig. 11). The initial surface is likely to have been a relatively planar feature to the surrounding depositional regimes and be evident in the resulting (particularly because it is almost certainly of marine origin, discussed strata. Such a clastic component within the Tertiary succession is not ap- above) and the synformal geometry must be a result of Pliocene and youn- parent until the Pliocene, as re¯ected in the Brighton Group, the Haunted ger deformation. Pliocene and younger sediments appear to have ®lled the Hill Formation and the Jemmys Point Formation. syncline as it was formed. Most deformation and erosion therefore took Further evidence for the Early Cretaceous highs not being as signi®cant place during the late Miocene, with signi®cant, but less intense uplift oc- during the Oligocene±Miocene comes from seismic pro®les in the Torquay curring during the Pliocene±Quaternary. Similarly, in the Gippsland basin, sub-basin (Figs. 9±11). The Oligocene±Miocene Torquay Group overlying tilting and faulting of the Haunted Hill Formation across the more active the Otway Group sediments in the subsurface shows no indication of thin- monoclines indicates that compressional uplift continues to the present day. ning or onlap onto what would have been a topographic high had the Otway Ranges been present. Instead, the uniform thickness of strata indicates that Late Tertiary Uplift in Southeast Australia deposition occurred on a relatively low-relief basin ¯oor that has subse- The evidence for a late Miocene tectonic uplift event is not con®ned to quently been uplifted. Furthermore, the Torquay Group sediments are re- the southeastern margin. In the St. Vincent basin, which ¯anks the western 300 J.A. DICKINSON ET AL.

FIG. 16.ÐTectonic setting of the Indo± Australian plate and stress orientation for southeast Australia. H ϭ Himalaya, S ϭ Sumatra Trench, J ϭ Java Trench, B ϭ Banda Arc, PNG ϭ Papua New Guinea, SM ϭ Soloman Trench, NH ϭ New Hebrides, TK ϭ Tonga±Kermadec trench, NZ ϭ New Zealand, MOR ϭ Mid Ocean Ridge. The plate boundaries are characterized as cb (collisional boundary), sz (subduction zone), and ia (island arc). Modi®ed from Coblentz et al. (1995). margin of the Mount Lofty Ranges (Fig. 1), the sedimentary succession tion. It is clear that the late Miocene uplift event we have documented is shows siliciclastic Pliocene±Recent deposits lying upon an angular uncon- part of the same major episode of tectonism as the late Pliocene±Pleistocene formity over a mid-Eocene to mid-Miocene dominantly cool-water carbon- Kosciusko Uplift. However, the Kosciusko Uplift has historically always ate sequence (Lindsay and Allen 1995; Tokarev et al. 1998). Evidence from been considered to be late Pliocene±Pleistocene in age, and to extend this morphological features and drainage systems in the Mount Lofty Ranges terminology to a much older event may cause considerable confusion in in conjunction with the neighboring stratigraphy suggests that two phases the literature. of tectonism took place (Tokarev et al. 1998), the ®rst in the mid-Eocene Correlation of the Miocene±Pliocene unconformity across the southeast and the second, of more relevance here, around 5 Ma. This second phase margin indicates that the uplift event occurred over a regional area. Several is associated with reverse faulting, as demonstrated by Bourman and Lind- datasets (in situ stress measurements, seismicity, and young faults) indicate say (1989). that a compressional regime now characterizes the Australian continent Similarly, on the western side of the Flinders Ranges, there is evidence (Coblentz et al. 1995), resulting in the reactivation of older structures as of young uplift. At Wilkatana North Creek, Pleistocene fanglomerates are features of compression. In southeast Australia the regional stress ®eld is affected by reverse faulting (Williams 1973). A series of uplifted and dis- orientated E±W to WNW±ESE (Fig. 16; Tokarev et al. 1998), which is sected ``pediments'' are also present on the western side of the Flinders consistent with the ENE±WSW orientation of the Early Cretaceous highs Ranges, and these appear to be due to late Tertiary reverse faulting (Twi- and the reverse faults that bound them. The stress ®eld has been attributed dale and Bourne 1996). to changes in the relative motion and forces at the boundary between the In the Murray basin, a comparable stratigraphy is apparent. The clastic- Australian and Paci®c plates, with pressure along the New Zealand, Papua rich upper Miocene to lower Pliocene Bookpurnong Beds unconformably New Guinea, and Himalayan collisional boundaries (Coblentz et al. 1995). overlie the carbonate-dominated mid-Miocene sediments of the Duddo The onset and effect of the associated forces is related to the timing of Limestone, which changes laterally to the Winnambool Formation and plate boundary development. Geera Clay to the east (Brown and Stephenson 1989). Although the general The inception of the Australia±Paci®c plate boundary through New Zea- view is that this unconformity re¯ects eustatic cycles (e.g., Brown 1983; land is considered to have occurred during the Oligocene at about 25 Ma Brown and Stephenson 1989), there is evidence of late Miocene uplift (Kamp 1986). At this time, strike-slip movement was dominant along the events resulting in tilting of strata (Stephenson 1986). Alpine Fault section of the Australia±Paci®c plate boundary. Between 12 Literature on the Eastern Highlands has been dominated by the concept and 5 Ma the plates became slightly compressive across the Alpine Fault, of youthful uplift (e.g., Andrews 1910; Browne 1969; Hills 1975; Douglas which initiated the uplift of the Southern Alps (Kamp et al. 1992; Suth- and Ferguson 1988). The period is referred to as the ``Kosciusko Uplift'' erland 1996). To the north of the Alpine Fault, this change in stress is and culminated during the late Pliocene to Pleistocene. Our work shows evident as the structural inversion of normal to reverse faults and resulted that in the basins, there have been two successive pulses of tectonism ini- in late Miocene uplift across the Taranaki basin (Kamp and Green 1990). tiated under the same stress regime. Of these two pulses late Pliocene to At around 6.4 to 5 Ma, the boundary underwent a marked increase in Pleistocene uplift is less signi®cant than late Miocene uplift and exhuma- convergence, which resulted in very rapid uplift of the Southern Alps, MIOCENE±PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA 301

mentation during the Miocene±Pliocene is plausible. The sea-level curve of Haq et al. (1988) indicates a signi®cant interval of regression occurring through the late Miocene with subsequent transgression in the early Plio- cene. More recent work (e.g., Miller et al. 1998) indicates, however, that the magnitude of sea-level change presented by Haq et al. (1988) is grossly exaggerated. It is also well established that the southeast Australian climate experienced gradually increasing seasonality and aridity during the Tertia- ry. The vegetation changed from a temperate, closed rain forest dominated by Nothofagus sp. to open sclerophyll forest (e.g., Eucalyptus), as indicated by plant microfossils and macrofossils (Bowler 1982). The change was a re¯ection of pronounced global cooling at the end of the Miocene brought about by the onset of glaciation and the formation of ice sheets in the Northern Hemisphere (Kemp 1978). The humid conditions that prevailed in the Oligocene±Miocene were replaced by the arid conditions experienced during the Pliocene±Recent. The more seasonal climate is thought to have contributed to an increase in runoff and with that a higher sediment yield (Bolger 1991). This mechanism has been suggested as bringing more sil- iciclastic material into the marine basins and so could be used as an ex- planation for increased siliciclastic sediment ¯ux in early Pliocene succes- sions overlying the Miocene±Pliocene unconformity. However, the unconformity is typically angular, with Miocene strata hav- ing been subjected to deformation (folding and faulting) and erosion prior to deposition of Pliocene sediments. This is the case in all of the basins FIG. 17.ÐSummary of the tectonic events and unconformity development across that we have examined from southeast Australia (e.g., Figs. 9, 10, 11, 14 the southeast Australian margin along with the tectonic history of the Indo±Austra- lian plate (Walcott 1988; Hill and Raza 1999). Modi®ed from Dickinson et al. and 15). Therefore, a change in eustatic sea level or climate cannot be used (2001). as a viable mechanism for the origin of the Miocene±Pliocene unconformity in southeast Australia. Similarly, the increased siliciclastic content of Pli- ocene sediments is best explained by tectonic uplift and consequent in- which continued through to the Quaternary (Tippett and Kamp 1993; Suth- creased erosion of older siliciclastic successions (like the Eocene clastic erland 1996; Walcott 1998). successions, the Lower Cretaceous volcaniclastics, and the Paleozoic base- At the northern margin of the Australian continent along the Papua New ment rocks). Guinea collisional boundary a similar change in plate dynamics is observed. It does, however, seem likely that a period of eustatically driven trans- At 12±10 Ma a change to oblique convergence between the Australian and gression is responsible for the marine erosion, deposition of earliest Plio- Paci®c plates resulted in kilometers of uplift throughout northern Papua cene sediments, and widespread phosphatization evident at the Miocene± New Guinea (Hill and Raza 1999). Pliocene unconformity. Evidence from deep-sea isotope records (Shackle- Considering the relative proximity of these plate boundaries to the Aus- ton et al. 1995) indicates that the early Pliocene was a warm (possibly ice- tralian continent and the timing of compressional movement along them, free) period. This would explain the occurrence of a rapid and synchronous it appears likely that the change in plate dynamics, beginning at 12 Ma ¯ooding event across the southeast Australian margin made more invasive and culminating at 6±5 Ma, is largely responsible for uplift and exhumation by regional subsidence following the compressional uplift. From this it in southeast Australia during the late Miocene (Fig. 17). The propagation would appear that the preservation of the late Miocene tectonic event is a of stress across the plate appears to have been focused around older struc- result of a eustatic cycle being superimposed on an extended period of tures at depth, which have consequently become reactivated resulting in tectonism in southeast Australia. the inversion of Lower Cretaceous blocks and associated localized folding. CONCLUSIONS Climate and Eustasy at the Miocene±Pliocene Boundary An unconformity is observed close to the Miocene±Pliocene boundary In the past, the unconformity that is present between the Miocene and (between 10 and 5 Ma) in the onshore and nearshore parts of the Otway, Pliocene successions has been attributed to both eustatic and climatic Port Phillip±Torquay, and Gippsland basins of southeast Australia. The changes (e.g., Carter 1978a), with little importance placed on the role of unconformity is generally angular, with the underlying Miocene units hav- tectonics in producing the stratigraphy of the sedimentary basins. The role ing been deformed (gentle folding and reverse faulting) and eroded prior of tectonics has generally been invoked only when discussing the timing to deposition of the Pliocene succession. It also marks the change from of uplift of the Eastern Highlands, which is dominated by ideas of late Oligocene±Miocene cool-water carbonate and brown-coal-dominated suc- Pliocene to Pleistocene uplift (e.g., Douglas and Ferguson 1988). Although cessions to more siliciclastic-rich Pliocene sediments. some researchers have suggested a late Miocene inversion event along the The change to Pliocene siliciclastic-rich sediments and the angular nature southeastern margin (e.g., Duddy 1994; Hill et al. 1995; Cooper and Hill of the Miocene±Pliocene unconformity indicate that a signi®cant regional 1997), most thermochronologic studies have constrained older events (e.g., uplift event has occurred in all southeast Australian basins. The timing of Duddy 1994; Hegarty et al. 1988) showing substantial exhumation of the this uplift coincides with a change in the dynamics of the Australian plate Otway Group in the mid-Cretaceous. The preference for eustasy and cli- at around 12 to 5 Ma, most evident in New Zealand, where the Southern mate change over tectonism as an explanation for the Miocene±Pliocene Alps underwent an increase in the rate of uplift. The southeast Australian unconformity re¯ects the in¯uence of seismic and sequence stratigraphy in margin is currently under a compressional stress regime, and this has prob- the literature and a general belief that the Australian continent has been ably persisted since the late Miocene, when the change in plate dynamics tectonically stable through the Neogene±Recent. occurred. The preservation of this tectonic event as an unconformity in the The possibility of a climatic and/or eustatic event in¯uencing the sedi- stratigraphic record is probably due to the effects of a major early Pliocene 302 J.A. DICKINSON ET AL. eustatic transgression, which may also be responsible for widespread phos- histories in Finlayson, D.M. 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