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
boundary in the onshore and nearshore portions of the Otway, Port
Phillip–Torquay, and Gippsland basins of southeast Australia. The unconformity is angular (generally , 1–58 angularity), with the underlying Miocene units having been deformed (gentle folding and reverse
faulting) and eroded prior to deposition of the Pliocene succession. The
unconformity also marks a change from Oligocene–Miocene deposition
of cool-water carbonate sediments and brown coal-bearing successions
to the accumulation of more siliciclastic-rich sediments in Pliocene
time. The Miocene–Pliocene boundary therefore represents an interval
of significant regional uplift in the southeast Australian basins. The
amount of section removed is greatest around the Otway and Strzelecki
ranges in Victoria, where up to a kilometer of section may have been
removed. In most onshore sections of the Victorian basins hundreds of
meters of section have been eroded. In distal offshore locations the
boundary becomes conformable. The timing of uplift and erosion is
best constrained in the Otway and Port Phillip basins, where late Miocene (N16 ; 10 Ma) sediments underlie the unconformity and earliest
Pliocene (; 5 Ma) sediments overlie it. This timing coincides with a
change in the dynamics of the Australian plate, beginning at around
12 Ma. Southeast Australia is currently under a NW–SE compressional
regime, and this has probably persisted since the late Miocene. In the
basins (as opposed to the basement-dominated highland areas), the late
Miocene uplift event is more significant than later Pliocene–Recent uplift.
INTRODUCTION
Determining the relative importance of eustasy versus tectonic phenomena in controlling regression and transgression within sedimentary basins
has proved problematic. The Exxon sequence stratigraphic approach, developed during the 1970s and 1980s, provides a framework for deriving
relative sea level by analysis of the geometry of sedimentary packages.
Regional and global correlation of such data should distinguish between
tectonically and eustatically controlled sea-level change. However, widespread, short-lived tectonic events can easily be confused with eustatic
events, particularly when the additional problem of age resolution is considered. The Miocene–Pliocene boundary is an example of an event (or
events) where tectonism is apparently widespread across the globe (e.g.,
Mercier et al. 1987; Dewey et al. 1989; Amano and Taira 1992; Walcott
1998; Hill and Raza 1999), and potential exists for considerable confusion
between eustatic and tectonic events.
There has been much discussion of the tectonic or eustatic significance
of the Miocene–Pliocene unconformity in the southeast Australian basins
(Gippsland, Port Phillip, and Otway basins; Fig. 1; e.g., Mallett 1978; Carter 1979). Unconformities of late Miocene–early Pliocene age (commonly
associated with phosphatic clasts) from several localities across Victoria
have long been recognized as being stratigraphically significant markers
(Coulson 1932; Keble 1932; Singleton 1941; Gill 1957; Bowler 1963).
However, little research has been carried out on the regional significance
of this Miocene–Pliocene unconformity.
In this paper, we attempt to synthesize paleontologic, stratigraphic, and
structural data from the various Tertiary basins across southeast Australia
in order to assess the timing and origin of these Miocene–Pliocene unconformities. Through this we highlight the importance of independent interpretation of sequences rather than systematic correlation with templates for
JOURNAL OF SEDIMENTARY RESEARCH, VOL. 72, NO. 2, MARCH, 2002, P. 288–303
Copyright q 2002, SEPM (Society for Sedimentary Geology) 1527-1401/02/072-288/$03.00
a sea-level curve (e.g., Haq et al. 1988). We suggest that a major interval
of regional uplift and exhumation began in southeast Australia during the
late Miocene. Exhumation in localized areas was on the order of several
hundreds of meters to a kilometer or more (e.g., Otway Ranges). This and
later Pliocene uplift is probably largely responsible for the present topographic relief of the southern Cretaceous Highlands (Otway and Strzelecki
ranges; Fig. 2) of Victoria.
The recognition of late Miocene uplift in southeast Australia has some
important economic implications. These include the erosion of cover sequences from the giant Oligocene–Miocene brown coal deposits of the
Latrobe Valley, making large volumes of coal economic for mining, and
structures offshore produced in the late Miocene provide favorable targets
for petroleum accumulation (e.g., the Nerita structure, Torquay sub-basin).
REGIONAL GEOLOGY
The southeast Australian basins occupy an area along the southern margin of the Australian continent. The basins developed during the rifting of
the Australian continent from Antarctica, Lord Howe Rise, and New Zealand during the Cretaceous to early Tertiary (Etheridge et al. 1987), resulting in a common structural and stratigraphic history. Consequently, all
the basins contain a largely nonmarine Lower Cretaceous rift–fill succession overlain by an Upper Cretaceous to Recent nonmarine and marine
post-rift deposits.
The margins of the basins are partly defined by blocks of Lower Cretaceous sediment (forming the southern Cretaceous Highlands), bounded
by reverse faults that form structural highs and define the present limit of
Tertiary sediments. The Otway basin is separated from the Torquay Embayment and Port Phillip basin by the Otway Ranges, which is in turn
separated from the Gippsland basin by the Mornington Peninsula High and
Strzelecki Ranges (Fig. 2). The northern margin of the basins is defined by
the Western and Eastern Highlands, whilst the southern margin extends
offshore (Fig. 1).
The Oligocene to Recent marine sediments of the Otway, Port Phillip
and Gippsland basins are dominated by cool-water and temperate-water
carbonates. The sedimentology and diagenesis of these nontropical carbonates has been the focus of several recent studies (e.g., James et al. 1992;
Boreen et al. 1993). The Oligocene–Miocene carbonates that make up the
bulk of the onshore marine sections are characterized by high-energy bryozoal grainstones with little or no siliciclastic content. Nonmarine Oligocene–Miocene onshore sediments are characterized by extensive accumulation of brown coal, again with little or no siliciclastic content. Conversely,
the Pliocene succession is typified by calcarenites and unconsolidated sand
deposits which contain a significant proportion of siliciclastic material, including quartz-rich pebble formations. Deposition of brown coal largely
ended in the late Miocene, and there have been no significant Pliocene
coals discovered to date. Pliocene–Quaternary basalts, known as the Newer
Volcanics (Douglas and Ferguson 1988), generally overlie the marine Pliocene.
Oligocene to Recent sediments in the onshore areas of the Otway and
Gippsland basins are variously folded and faulted. Oligocene–Miocene sections commonly have gentle dips (generally , 108) with monoclines and
open folds dominating. Reverse faults are also common in some regions,
particularly surrounding the Cretaceous Highlands. Tertiary sediments in
close proximity to reverse faults rarely have steep dips (up to vertical).
Pliocene and younger sections are generally less deformed, but some faults
and monoclines do affect these sediments. In the offshore portions of these
MIOCENE–PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA
289
FIG. 1.—Digital 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
is generally less intense than in the onshore sections. In the offshore Gippsland basin, Oligocene–Recent deformation has produced large-scale structural traps that play host to giant oil and gas fields.
There are numerous structures with evidence of relatively young (Neo-
gene) activity in southeast Australia. In the southern parts of Victoria,
northeast oriented reverse faults of Neogene age bound the Otway Ranges,
the Mornington Peninsula, and the Strzelecki Ranges, which are similarly
oriented (Fig. 2). Farther north, in the Murray basin, Neogene faults have
a NNE orientation. In South Australia, Neogene reverse faults are present
FIG. 2.—Digital elevation model for southern Victoria (from Australian Geological Survey Organisation) showing the Southern Highlands and major faults.
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J.A. DICKINSON ET AL.
FIG. 3.—Generalized chronostratigraphy of the southeastern basins during the Neogene. T.p. 5 Tubuliforidites pleistocenicus, M.l. 5 Meyeripollis lipsis, C.b. 5
Cingulolatisporites bifurcatus, T.b. 5 Triporollenites bellus, P.t. 5 Proteacidites tuberculatus. Modified from Abele et al. (1988).
in the Flinders Ranges, Mount Lofty Ranges, and St. Vincent basin and
they trend N–NNE. This is consistent with the predominantly W–WNW
oriented compressional stress field to which southeast Australia appears to
be currently subject (Coblentz et al. 1995).
The onshore parts of the southeast Australian basins contain a major
stratigraphic break within the late Miocene–early Pliocene at the change
from carbonate-rich to more clastic-rich sediments (Fig. 3). Phosphatic
clasts and vertebrate remains are commonly present at this unconformity
(Coulson 1932; Keble 1932). This regional unconformity has previously
been interpreted as being due to tectonic, eustatic, or climatic affects (e.g.,
Bowler 1963; Carter 1978a; Mallett 1978). In this paper, we discuss the
nature and significance of this stratigraphic break at various localities in
the onshore and nearshore parts of the southeast Australian basins.
METHODS
Stratigraphic data were obtained through detailed sections taken at locations in the Otway, Port Phillip, and Gippsland basins. Borehole data
were used from Sorrento (Mallett and Holdgate 1985) and from coal bores
in the Gippsland basin (Bolger 1991; unpublished Rural Water Commission
TABLE 1.—Measured 87Sr/86Sr ratios of calcitic carbonate material from stratigraphic sections used in this study.
Location
Hamilton
Batesford
Coghills
Beaumaris
Stratigraphic
height (m)
2.90
2.90
2.90
1.60
0.80
0.80
7.00
6.60
6.00
0.50 m below unconformity
4.30
2.80
2.50
Age (Ma)
4.94
5.03
3.52
10.86
10.86
11.13
4.86
4.21
4.45
10.26
4.52
5.24
5.67
87
Sr/86Sra Std errorb
0.709039
0.709036
0.709061
0.708865
0.708865
0.708844
0.709041
0.709052
0.709049
0.708882
0.709048
0.709028
0.709011
612
612
612
612
612
612
612
612
614
612
612
612
614
All results are normalized to the SRM-987 standard 5 0.710260 and to 86Sr/87Sr 5 0.1194.
b Analytical uncertainty represents 2 standard errors of the mean of ; 100 individual measurements and
refers to the last two digits of the ratios.
a
data). Paleontological samples were disaggregated by hydrogen peroxide
and foraminifera picked, sorted, and identified for qualitative biostratigraphic purposes.
All 87Sr/86Sr measurements were made on calcitic and unaltered aragonitic molluscs. Samples were cleaned with concentrated hydrochloric acid
to remove any potential surface overgrowths and rinsed with distilled water.
Dissolution of powdered samples was carried out in 10% acetic acid.
Chemical separations were performed using 1N HCl and 2.5N HCl as eluents through AG 50W-X8 ion-exchange resin in 15 ml quartz columns
for Sr separation. The Sr isotope composition of the separate was measured
at the University of Adelaide with mass fractionation normalized to a 86Sr/
88Sr ratio of 0.1194 and to a SRM 987 5 0.710260. Analytical reproducibility of samples is 12 3 1026 (2s of the mean). Calcite ages were calculated using Howarth and McArthur’s (1997) Sr isotope Look-Up Table
Version 3:10/99 (Table 1). Time–depth conversion of interpreted seismic
sections from the Torquay basin was carried out using Seismic Plugin v.
3.0 (Jay Lieske, Jr.) for Adobe PhotoshopTM. Sonic velocities used were
1500 m/s, 1800 m/s, and 2200 m/s for sea water, Pliocene–Quaternary
sediments, and Miocene Torquay Group sediments, respectively.
MIOCENE–PLIOCENE BOUNDARY IN THE OTWAY BASIN
Where the boundary between Miocene and Pliocene sediments is exposed in the Otway basin, it is generally an unconformity (Fig. 3). The
unconformity is marked by a planar erosional surface with no evidence of
oxidation and a significant change in the lithology. The underlying Miocene
strata exhibit a shallowing-upward (more calcareous upward) character.
Gray fossiliferous marls (Gellibrand and Muddy Creek marls) and bryozoan-rich fine-grained white limestones (Port Campbell and Bochara limestones) with a nominal siliciclastic content typify the sequence. The overlying Pliocene succession represents a prograding quartz-carbonate barrier
system with the initial marine incursion marked at the base by quartz-rich
shallow marine silts and shelly sands (Whalers Bluff and Grange Burn
formations) and subsequent deposition of yellow flaggy calcarenites (Werrikoo Limestone).
Glenelg River Section
In the Dartmoor region (Fig. 4) along the Glenelg River valley, Pliocene–
Pleistocene quartzose calcarenites of the Werrikoo Limestone unconfor-
MIOCENE–PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA
291
strontium isotope dates of 11.13 6 0.5 and 10.86 6 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 phosphatic 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.—Location map for the Otway basin showing the location of stratigraphic
sections studied.
amably overlie Eocene Knight Group and Miocene limestones of the Glenelg Group (Fig. 5). The youngest age of strata preserved beneath the
unconformity is placed in the early Miocene (planktonic foraminiferal zone
N5; Figs. 3, 5). The lower Werrikoo Limestone is here placed within the
Werrikooian (upper Pliocene) on the basis of the molluscan fauna found
towards the base of the unit, typical of Singleton’s (1941) Werrikooian
stage, and the absence of planktonic foraminifera Globorotalia truncatulinoides. G. truncatulinoides does not become apparent until higher in the
section, placing the upper part in the Pleistocene (Fig. 5). In the vicinity
of Jones Ridge basalt dated at 2.24 Ma by K–Ar (Singleton et al. 1976)
overlies the lower Miocene sediments marking the unconformity, with deposition of the Werrikoo Limestone above this.
A regional cross section produced by Kenley (1971) based on outcrop
and well stratigraphy in the western part of the Otway basin (Fig. 5) indicates that the Knight and Glenelg groups have undergone folding following their deposition. This folding occurred prior to the deposition of the
Werrikoo Limestone, which overlies the angular unconformity. The folding
is focused around the Merino High (as defined by Kenley 1971), an area
of outcropping Otway Group (Lower Cretaceous) sediments (Fig. 4) that
has been elevated and dissected.
The Port Phillip and Torquay basins lie between the Otway and Gippsland basins and are bordered by uplifted areas of Lower Cretaceous sediment and Paleozoic basement (Fig. 6). The Miocene–Pliocene boundary is
well exposed in coastal and river sections and is characterized by a planar
unconformity. The unconformity is typically represented by a sharp lithological change from marl to quartz-rich carbonate sand (Figs. 7, 8A), with
no evidence of karstification or oxidation. There is commonly a discontinuous horizon of coarse quartz gravel containing distinctive phosphatic
clasts immediately overlying the unconformity. These phosphatic clasts
commonly have borings (Fig. 8B). There has been much early work on the
Miocene–Pliocene boundary in this region, largely because of the abundance of vertebrate remains (e.g., Coulson 1932; Keble 1932).
The strata underlying the Miocene–Pliocene unconformity range in age
from early (e.g., Shelford) through to mid- (e.g., Geelong) and late Miocene
(e.g., Beaumaris and Point Nepean; Fig. 7), which is detailed below. It is
a succession of gray-buff calcareous clay-rich sediment with cemented ferruginous horizons and layers of carbonate nodules and sporadic phosphate
concretions (Gellibrand Marl and Fyansford Formation). The overlying
units are characterized by basal marine quartz-rich calcareous sandstones
and molluscan-rich sandy calcarenites that grade upwards into a nonmarine
sand (Moorabool Viaduct Formation and Brighton Group). The abundance
of quartz sand in these units contrasts with the quartz-poor clayey carbonates of the underlying Miocene.
Portland
Shelford
In the coastal cliffs north of Portland (Fig. 4), Pliocene Whalers Bluff
Formation and a younger subaerial basalt unconformably overlie Miocene
Port Campbell Limestone (Fig. 5). The top of the limestone is dated as late
Miocene by the presence of the planktonic foraminifera Globorotalia miotumida (planktonic foraminiferal zone N16; Fig. 3). The age of the initial
deposition of the Whalers Bluff Formation is determined to be early Pliocene (Zone N19) by the presence of Globorotalia punticulata. Boutakoff
(1963) noted that the Pliocene sediments infilled a karst topography developed 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
constraint on the age of the Whalers Bluff Formation.
West of Geelong, on the Leigh River near Shelford (Fig. 6), the underlying Miocene sequence is attributed to the Gellibrand Marl (Fig. 7; Edwards et al. 1996), and the upper part of the unit is dated as early Miocene
(planktonic foraminiferal zone N7) on the basis of the presence of the
planktonic foraminifera Globigerinoides trilobus. Quartz sands of the overlying Pliocene Moorabool Viaduct Formation are poorly exposed.
Hamilton
To the west of Hamilton in the Muddy Creek–Grange Burn area, the
Miocene–Pliocene sequence unconformably overlies mid-Paleozoic rhyolite and is capped by basalt. The Miocene Muddy Creek Marl contains a
foraminiferal assemblage derived by Mallett (1977) that suggests that the
unit spans the interval from foraminiferal zone N7 to N16. Directly beneath
the Miocene–Pliocene boundary a distinctive limonitic and glauconitic
green marl is present and contains abundant erosional lag beds yielding
Geelong
In the vicinity of Geelong, along the Moorabool River (old Batesford
Quarry, Coghills section; Coulson 1932; Bowler 1963), the Miocene–Pliocene succession is represented by the Fyansford Formation, which is unconformably overlain by the Moorabool Viaduct Formation (Fig. 8C). The
upper part of the Fyansford Formation, where exposed beneath the unconformity, varies in age from mid- to late Miocene (Fig. 3). A strontium
isotope date from just below the unconformity at Coghills (Fig. 6) gives a
date of 10.26 6 0.5 Ma (Table 1).
Determination of the age of the overlying Moorabool Viaduct Formation
has been based largely on molluscan fauna. It is consequently considered
to be very late Miocene to early Pliocene (Abele et al. 1988), but there is
the suggestion that the unit is as young as late Pliocene (Bowler 1963).
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J.A. DICKINSON ET AL.
FIG. 5.—Measured stratigraphic sections across the Miocene–Pliocene unconformity in the Otway basin. Cross section is located on Figure 4 (modified from Kenley 1971).
Strontium isotope analysis of marine molluscs at Batesford quarry places
initial deposition at 4.5 6 0.5 Ma. Basalts that overlie the sequence are
dated at 2.0 Ma using K–Ar (Whitelaw 1989), constraining the upper age
of deposition.
Beaumaris
At Beaumaris, on the coast southeast of Melbourne, the age of the underlying Fyansford Formation is poorly defined. However the upper part is
dated as mid-Miocene where it outcrops at Mornington farther to the south
(Mallett and Holdgate 1985). The phosphatic nodule bed, which marks the
unconformity and consists of quartz pebbles, phosphatic clasts, and a rich
fauna of teeth, bones, and shell material (Fig. 7; Singleton 1941), is located
0.5 m beneath beach level.
The exposure of the Pliocene (Black Rock Sandstone) at Beaumaris constitutes the type section of the Cheltenhamian Stage (Singleton 1941), placing the unit in the upper Miocene (planktonic foraminiferal zones N17–
18). This is supported by the foraminiferal assemblage, which, although
MIOCENE–PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA
FIG. 6.—Location map of the central coastal basins showing the sections studied and the position of offshore seismic profiles.
FIG. 7.—Measured stratigraphic sections across the Miocene–Pliocene unconformity in the Port Phillip basin.
293
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J.A. DICKINSON ET AL.
FIG. 8.—A) 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 fields beneath the unconformity (black arrows) and the Haunted Hill Formation.
poor, places deposition of the unit within the same time (Mallett 1977).
However, strontium isotope analysis of molluscs within the unit (Fig. 7)
suggests that the deposit formed during the early Pliocene (5.5–4.5 Ma).
The Black Rock Sandstone grades upward into terrestrial sands and clays
that are regarded as Pliocene (Abele et al. 1988).
Sorrento
The Nepean Peninsula extends northwest into the center of the Port Phillip basin from the southern end of the Mornington Peninsula. A number
of bores are situated at Sorrento, where sediments of the Port Phillip basin
are thickest. The Fyansford Formation is here present at its youngest, ranging up to upper Miocene. Elsewhere in the basin, the strata above the
unconformity are Pliocene. At Sorrento, however, the overlying Brighton
Group is considered to be upper Miocene (Mallett 1977). In contrast, the
Wannaeue Formation (Fig. 7), as proposed by Mallett and Holdgate (1985),
contains a rich faunal component that indicates an early to mid Pliocene
age. Both the age and the lithology are comparable to the Whalers Bluff
Formation at Portland (Mallett and Holdgate 1985). However, the unit is
only apparent in the subsurface and has not been recorded in any other part
of the Port Phillip area, reaching a thickness of 88 m in the center of the
Sorrento Graben and becoming thinner towards the margins.
TORQUAY SUB-BASIN
The Torquay sub-basin, offshore from the Port Phillip basin (Fig. 6),
contains an upper Tertiary succession similar to that exposed onshore. Car-
bonate-rich marls of the Torquay Group are offshore equivalents of the
Oligocene–Miocene sequence. The sub-basin has been the subject of petroleum exploration, and although there is little sample material for study,
much seismic and well log data are available (Trupp et al. 1994). Several
shallow seismic lines have also been run across the sub-basin. Using paleontological data from Nerita #1 (Fig. 6) and the nearby Nepean bores, it
appears that the youngest Torquay Group (Trupp et al. 1994) ranges in age
from early to late Miocene. Well-log data from Wild Dog #1 (Fig. 6) also
indicate that the succession becomes less clay-rich upwards, as does the
section onshore.
On industry seismic sections, the Torquay Group and underlying successions are folded and faulted (Fig. 9). Reverse faults in the underlying
Otway Group and Paleozoic basement generally grade into monoclinal
structures in the Torquay Group sediments. Folding and faulting affects the
entire section up to the upper Miocene. Furthermore, much erosion of section is evident at the sea floor. On the Nerita #1 structure 400 m of Miocene
section has been eroded and outcropping sediments at the sea floor are
lower Miocene (Fig. 9; Shell 1967).
Near the Otway coastline and the Mornington Peninsula, the entire Torquay Group and underlying successions (Angahook Formation and Boonah
Formation) have been removed by erosion, indicating more than 600 m of
erosion (Fig. 10). No thinning of the Torquay Group towards the Otway
Ranges or the Mornington Peninsula is visible on seismic, and this suggests
that large thicknesses of Torquay Group have been stripped from these
highs. Deformation and erosion must postdate the upper Miocene Torquay
MIOCENE–PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA
295
FIG. 9.—Seismic profile (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
and is uniformly affected by deformation.
On shallow seismic sections (Fig. 11), a spectacular angular unconformity is visible (also just visible on industry seismic, Fig. 9). From correlation with the Nepean bores and Nerita #1, the unconformity postdates the
upper Miocene Torquay Group sediments and appears to be the lateral
equivalent of the Fyansford Formation–Brighton Group unconformity in
the Nepean bores (i.e., the Miocene–Pliocene unconformity). This would
indicate that the overlying undeformed sequence is the equivalent of the
Brighton Group and Wannaeue Formation described by Mallett and Holdgate (1985) from the Nepean bores.
The Miocene–Pliocene unconformity on seismic (at Nerita #1 and on
shallow seismic) indicates that most deformation and erosion of the underlying Torquay Group took place during the late Miocene. The Nerita
#1 structure, for example, appears to be almost entirely due to late Miocene
deformation. On shallow seismic near the Otway Ranges, however, some
folding is evident within the overlying Pliocene sediments, indicating continued deformation into the Pliocene.
GIPPSLAND BASIN
The onshore Gippsland basin is characterized by an Eocene to Miocene
coal-dominated sequence that is laterally equivalent to the Oligocene–Miocene cool-water carbonates of the Seaspray Group in marine sections of
the basin. A number of coal seams—Traralgon, Morwell, and Yallourn—
make up the Tertiary brown coal measures, which constitutes the bulk of
the Latrobe Valley Group. This is unconformably overlain by a siliciclastic
sand- and gravel-dominated unit (Pliocene Haunted Hills Formation) and
laterally equivalent marine sandy calcarenites and coquinas (Jemmys Point
Formation). In the Latrobe Valley, the Miocene–Pliocene unconformity is
expressed as an angular difference accompanied by erosion and weathering
(oxidation) of coal seams, together with pronounced burn holes in the top-
of-coal surface. The unconformity has been well documented by Bolger
(1984, 1991).
The major structures of the onshore Gippsland basin are illustrated in
Figure 12. Faulting associated with the folding tends to be NE-trending
and is both extensional and compressional, resulting in horsts or tilted horst
blocks (Smith 1982; Hocking et al. 1988). Many of the blocks and monoclines are now bounded by and overlie reverse faults at depth, and appear
to be inverted structures. Tertiary drape over these reverse faults defines
monocline warping (Barton 1981), which formed largely after Miocene
coal-measure deposition.
Latrobe Valley Depression
The Tertiary coal measures of the Latrobe Valley Depression occupy an
elongated asymmetric east-pitching syncline known as the Latrobe and
Traralgon synclines (Fig. 12). Within these synclines over 700 m of Oligocene–Miocene coal measures are present. Peripheral to the central synclines 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 458. The coal seams exposed in open cuts on the Morwell–Yallourn and Loy Yang blocks (Fig. 12) are also strongly jointed
because of this folding. Most joints trend NNW to NW, swinging more
northerly at Morwell because of regional stress accompanied by shear on
the Morwell Monocline/Fault (Fig. 13; Barton 1979). The structures postdate coal-seam deposition, inasmuch as they are unconformably overlain
by flat-lying Haunted Hill Formation sediments, which are also seen to fill
the joints.
Post-coal measure folding, uplift, and subaerial erosion of the Latrobe
Valley Group and equivalents dominates the near-surface geology of the
Latrobe Valley (Thomas and Baragwanath 1949, 1951; Gloe 1960, 1976;
Bolger 1984, 1991). Evidence for this erosion can be seen in all the anticlinal structures adjacent to the Balook Block, and along the northern and
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FIG. 10.—Section of the seismic profile OS88A–1 projected up onto the Otway Ranges (Fig. 6) with reconstruction of the eroded pre-Pliocene succession.
FIG. 11.—Shallow seismic profile 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.—Location 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
well exposed in all the brown coal open-cut exposures, where the Pliocene
Haunted Hill Formation rests unconformably on coal seams (Fig. 8D). Areas of maximum uplift have largely been stripped of their Tertiary cover,
and this stripping may exceed 200 m in places (e.g., the Baragwanath
Anticline and Loy Yang Dome; Fig. 12; Dickinson et al. 2001). In the
adjacent synclinal areas, evidence for the unconformity is provided by subsurface data. Post-Yallourn Seam truncation on the major bounding mono-
clines is evident on cross sections (Fig. 14), but in the center of the synclines more continuous sedimentation appears to have occurred between
coal measures, post-Yallourn Seam clays, and Haunted Hill Formation
(Bolger 1991).
The timing of structural uplift for the Tertiary coal measures in Gippsland is closely related by their peripheral occurrence to the Balook Block
and Eastern Highlands uplift:
(1) The isopachs of the Oligocene to lower Miocene Morwell 2 coal
FIG. 13.—Correlation 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.—Cross 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
form the Yallourn North and Extension coal fields (Holdgate 1985). The
original extent of the Morwell 2 Seam is conjectural, but the present extent
is clearly seen to be controlled by later uplift and erosion. The Morwell 2
(Latrobe) Seam is also cut and offset by reverse faulting along the Yallourn
Monocline at Yallourn Power Station (Fig. 15) and is present on the upthrown Haunted Hill Block.
(2) The earliest appearance in the Traralgon Syncline of the lower Cretaceous Strzelecki Group detritus (taken to indicate erosion from the Balook
and Narracan blocks) is observed in upper Miocene clays above the Yallourn Seam (Bolger 1984).
In the Traralgon and Latrobe synclines, clayey sediments of the Hazelwood Formation, reaching over 100 m thickness, conformably overlie the
Yallourn coal seam (Fig. 13). Both are folded and unconformably overlain
by sands and gravels of the Haunted Hills Formation. Constraints from
palynology dates suggest that the Hazelwood Formation extends through
the Upper T. bellus and lower part of the C. bifurcatus zones (Fig. 3),
whereas the basal units of the Haunted Hill Formation include the upper
part of the M. lipsis Zone (Fig. 13; Dawson 1983). This effectively indicates a hiatus during which folding took place spanning the time interval
recorded between the upper C. bifurcatus and lower M. lipsis zones, i.e.,
between 7 and 4 Ma (Fig. 3).
The correlation of stratigraphy across the onshore Gippsland basin (Fig.
13) is consistent with the other southeast Australian basins. Here the best
constraint, by palynology dates, microfossil biostratigraphy, and structural
control, confines the timing of unconformity development to the latest Miocene.
Alberton Depression
The structural pattern of the pre-middle Miocene Alberton Coal Measures in the Alberton Depression of South Gippsland is broadly similar to
that of the Latrobe Valley Depression. The coal measures are folded and
unconformably overlain by flat-bedded Haunted Hill Formation clastic sediments and Pliocene coquinas of the Jemmys Point Formation (Hocking
1976; Greer and Smith 1982; Holdgate 1982; Thompson and Walker 1982).
Lake Wellington Depression
Down-basin towards the Sale area, the timing of folding is further constrained. Here the latest age for the underlying folded Miocene (Tambo
River and Boisdale formations; Fig. 13) is dated as planktonic foraminifera
zone N17 (Fig. 3), whereas the oldest part of the overlying Jemmys Point
Formation is confined to planktonic foraminifera zone N20. Therefore the
age of folding, given the constraints on the dating, must have taken place
at the end of the Miocene (an interval of 8–4 Ma). Farther east towards
the Bairnsdale–Lakes entrance area, the same relationship and ages are
observed between the Tambo River Formation and the Jemmys Point Formation (Carter 1964, 1978a, 1978b; Mallett 1978), where horizons of phosphatic clasts are widespread along the unconformity.
DISCUSSION
The stratigraphic relationship summarized above points to a major
change having taken place in the sedimentological regimes of the southeast
Australian basins during the late Miocene to early Pliocene:
1. An unconformity of approximately late Miocene–early Pliocene age is
present in every onshore succession from the Otway, Port Phillip, and
Gippsland basins. Indeed, very few sections onshore from any of these
basins have uppermost Miocene sediments preserved, and in many cases
subsequent deposition did not occur until much later in the Pliocene.
2. Where the Miocene–Pliocene boundary is preserved in marine successions, the unconformity is typically sharp and planar, with no evidence
of oxidation or karstification of the underlying strata (the only exception
being Portland). Furthermore, the earliest sediments overlying the unconformity are of marine origin. This indicates that the unconformity
was produced by marine erosion, or that any evidence of subaerial erosion has been removed by subsequent marine erosion.
3. The Miocene–Pliocene break is generally an angular unconformity, with
differences in dip ranging from 0.58 to nearly 908 (most commonly in
the range 0.58 to 58). The underlying Miocene and older successions
are folded, whereas the overlying Pliocene succession is generally not.
4. The Oligocene–Miocene carbonates of southeast Australia are characteristically clastic-poor successions. In contrast, the majority of onshore
Pliocene to Recent marine sequences are characterized by mixed clasticcarbonate facies.
5. Accumulation of clastic-poor Oligocene–Miocene brown coal seams
ceased during late Miocene–early Pliocene time. Nonmarine Pliocene to
Recent sediments overlying the coals are characterized by high-energy
siliciclastic fluvial successions.
The timing of the unconformity, which marks this change in sedimentation, is confined by the deposition of sediments above and below the
unconformity. In many cases the youngest deposits beneath the boundary
are lower to middle Miocene. Precise dating of the overlying units is made
difficult owing to a lack of diagnostic fauna. The timing is best constrained
by the hiatus that is apparent in the stratigraphy at Portland and Hamilton.
At both locations upper Miocene sediments are preserved beneath the unconformity (; 11 Ma from 87Sr/86Sr marine carbonate dating at Hamilton).
Sequences above the unconformity are earliest Pliocene at Portland by
MIOCENE–PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA
299
FIG. 15.—Reverse 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
dating at Hamilton. This places the commencement of the interval of unconformity development during the late Miocene (11–5 Ma), with cessation
of the event by the Pliocene. Similar age relationships exist at Beaumaris
(Mallett and Holdgate 1985), and in the Gippsland basin, a range of 8–4
Ma is determined for the unconformity.
Uplift and Exhumation of the Southern Highlands
Blocks of Lower Cretaceous sediments are present across the southeast
Australian basins as structural highs. These blocks consist of sediments that
are principally feldspathic sandstone, mudstone, and shale, with conglomerate in places prominent at the base. The relationship of Oligocene–Miocene sediments to these Cretaceous highs is important in determining their
timing of uplift and exhumation and consequently the origin of the Miocene–Pliocene unconformity.
Where Oligocene–Miocene sediments flank the shoulders of these Mesozoic highs (e.g., Otway Ranges–Torquay basin, Strzelecki Ranges–Latrobe Valley) the stratigraphy is dominated by the deposition of carbonaterich sediments (e.g., Port Campbell Limestone, Gellibrand Marl, Fyansford
Formation) or nonmarine brown coal seams (e.g., Latrobe Valley Group).
The siliciclastic component is everywhere low or nonexistent in these flanking Oligocene–Miocene sediments. This suggests that the Lower Cretaceous blocks had not been exhumed and exposed at this time, inasmuch as
the erosion of the feldspathic beds would cause a significant clastic input
to the surrounding depositional regimes and be evident in the resulting
strata. Such a clastic component within the Tertiary succession is not apparent until the Pliocene, as reflected in the Brighton Group, the Haunted
Hill Formation and the Jemmys Point Formation.
Further evidence for the Early Cretaceous highs not being as significant
during the Oligocene–Miocene comes from seismic profiles in the Torquay
sub-basin (Figs. 9–11). The Oligocene–Miocene Torquay Group overlying
the Otway Group sediments in the subsurface shows no indication of thinning or onlap onto what would have been a topographic high had the Otway
Ranges been present. Instead, the uniform thickness of strata indicates that
deposition occurred on a relatively low-relief basin floor that has subsequently been uplifted. Furthermore, the Torquay Group sediments are re-
moved by erosion at the sea floor as the Otway Ranges are approached
(Fig. 10).
Very similar relationships exist around the Strzelecki Ranges in the Latrobe Valley. Near the margins of the ranges, Oligocene–Miocene brown
coals are completely removed by erosion, with little or no evidence of
stratigraphic thinning towards the ranges (Fig. 14). The significant emergence of the Balook Block above depositional base level and into an active
denudation zone did not occur until post-Yallourn Seam times in the late
Miocene. The stratigraphic and structural relationships around the Cretaceous highlands indicate several hundred meters or more of erosion from
the Cretaceous blocks during the late Miocene–Pliocene. The offshore seismic profiles and regional cross sections in the vicinity of the Early Cretaceous highs show that folding and faulting is most strongly developed in
regions adjacent to these uplifted areas (Fig. 11; Skeats 1935).
The history and type of tectonism is illustrated best on shallow seismic
sections from the Torquay sub-basin. Here, the bulk of folding, faulting,
and erosion occurred during the late Miocene. This has largely taken the
form of doming (e.g., line OS88A–1, Fig. 9) and reverse faulting in the
Otway Group (producing monoclines in the Torquay Group) along the margins of the Otway Ranges and Mornington Peninsula. The Miocene–Pliocene unconformity overlies much of the eroded and folded Miocene section, indicating a pre-Pliocene origin for most deformation and erosion.
However, the Miocene–Pliocene unconformity in the Torquay sub-basin
has a gentle synformal morphology with dips of less than 18 on the limbs
(Fig. 11). The initial surface is likely to have been a relatively planar feature
(particularly because it is almost certainly of marine origin, discussed
above) and the synformal geometry must be a result of Pliocene and younger deformation. Pliocene and younger sediments appear to have filled the
syncline as it was formed. Most deformation and erosion therefore took
place during the late Miocene, with significant, but less intense uplift occurring during the Pliocene–Quaternary. Similarly, in the Gippsland basin,
tilting and faulting of the Haunted Hill Formation across the more active
monoclines indicates that compressional uplift continues to the present day.
Late Tertiary Uplift in Southeast Australia
The evidence for a late Miocene tectonic uplift event is not confined to
the southeastern margin. In the St. Vincent basin, which flanks the western
300
J.A. DICKINSON ET AL.
FIG. 16.—Tectonic setting of the Indo–
Australian plate and stress orientation for
southeast Australia. H 5 Himalaya, S 5
Sumatra Trench, J 5 Java Trench, B 5 Banda
Arc, PNG 5 Papua New Guinea, SM 5
Soloman Trench, NH 5 New Hebrides, TK 5
Tonga–Kermadec trench, NZ 5 New Zealand,
MOR 5 Mid Ocean Ridge. The plate
boundaries are characterized as cb (collisional
boundary), sz (subduction zone), and ia (island
arc). Modified from Coblentz et al. (1995).
margin of the Mount Lofty Ranges (Fig. 1), the sedimentary succession
shows siliciclastic Pliocene–Recent deposits lying upon an angular unconformity over a mid-Eocene to mid-Miocene dominantly cool-water carbonate sequence (Lindsay and Allen 1995; Tokarev et al. 1998). Evidence from
morphological features and drainage systems in the Mount Lofty Ranges
in conjunction with the neighboring stratigraphy suggests that two phases
of tectonism took place (Tokarev et al. 1998), the first in the mid-Eocene
and the second, of more relevance here, around 5 Ma. This second phase
is associated with reverse faulting, as demonstrated by Bourman and Lindsay (1989).
Similarly, on the western side of the Flinders Ranges, there is evidence
of young uplift. At Wilkatana North Creek, Pleistocene fanglomerates are
affected by reverse faulting (Williams 1973). A series of uplifted and dissected ‘‘pediments’’ are also present on the western side of the Flinders
Ranges, and these appear to be due to late Tertiary reverse faulting (Twidale and Bourne 1996).
In the Murray basin, a comparable stratigraphy is apparent. The clasticrich upper Miocene to lower Pliocene Bookpurnong Beds unconformably
overlie the carbonate-dominated mid-Miocene sediments of the Duddo
Limestone, which changes laterally to the Winnambool Formation and
Geera Clay to the east (Brown and Stephenson 1989). Although the general
view is that this unconformity reflects eustatic cycles (e.g., Brown 1983;
Brown and Stephenson 1989), there is evidence of late Miocene uplift
events resulting in tilting of strata (Stephenson 1986).
Literature on the Eastern Highlands has been dominated by the concept
of youthful uplift (e.g., Andrews 1910; Browne 1969; Hills 1975; Douglas
and Ferguson 1988). The period is referred to as the ‘‘Kosciusko Uplift’’
and culminated during the late Pliocene to Pleistocene. Our work shows
that in the basins, there have been two successive pulses of tectonism initiated under the same stress regime. Of these two pulses late Pliocene to
Pleistocene uplift is less significant than late Miocene uplift and exhuma-
tion. It is clear that the late Miocene uplift event we have documented is
part of the same major episode of tectonism as the late Pliocene–Pleistocene
Kosciusko Uplift. However, the Kosciusko Uplift has historically always
been considered to be late Pliocene–Pleistocene in age, and to extend this
terminology to a much older event may cause considerable confusion in
the literature.
Correlation of the Miocene–Pliocene unconformity across the southeast
margin indicates that the uplift event occurred over a regional area. Several
datasets (in situ stress measurements, seismicity, and young faults) indicate
that a compressional regime now characterizes the Australian continent
(Coblentz et al. 1995), resulting in the reactivation of older structures as
features of compression. In southeast Australia the regional stress field is
orientated E–W to WNW–ESE (Fig. 16; Tokarev et al. 1998), which is
consistent with the ENE–WSW orientation of the Early Cretaceous highs
and the reverse faults that bound them. The stress field has been attributed
to changes in the relative motion and forces at the boundary between the
Australian and Pacific plates, with pressure along the New Zealand, Papua
New Guinea, and Himalayan collisional boundaries (Coblentz et al. 1995).
The onset and effect of the associated forces is related to the timing of
plate boundary development.
The inception of the Australia–Pacific plate boundary through New Zealand is considered to have occurred during the Oligocene at about 25 Ma
(Kamp 1986). At this time, strike-slip movement was dominant along the
Alpine Fault section of the Australia–Pacific plate boundary. Between 12
and 5 Ma the plates became slightly compressive across the Alpine Fault,
which initiated the uplift of the Southern Alps (Kamp et al. 1992; Sutherland 1996). To the north of the Alpine Fault, this change in stress is
evident as the structural inversion of normal to reverse faults and resulted
in late Miocene uplift across the Taranaki basin (Kamp and Green 1990).
At around 6.4 to 5 Ma, the boundary underwent a marked increase in
convergence, which resulted in very rapid uplift of the Southern Alps,
MIOCENE–PLIOCENE UNCONFORMITY IN SOUTHEAST AUSTRALIA
FIG. 17.—Summary of the tectonic events and unconformity development across
the southeast Australian margin along with the tectonic history of the Indo–Australian plate (Walcott 1988; Hill and Raza 1999). Modified from Dickinson et al.
(2001).
which continued through to the Quaternary (Tippett and Kamp 1993; Sutherland 1996; Walcott 1998).
At the northern margin of the Australian continent along the Papua New
Guinea collisional boundary a similar change in plate dynamics is observed.
At 12–10 Ma a change to oblique convergence between the Australian and
Pacific plates resulted in kilometers of uplift throughout northern Papua
New Guinea (Hill and Raza 1999).
Considering the relative proximity of these plate boundaries to the Australian continent and the timing of compressional movement along them,
it appears likely that the change in plate dynamics, beginning at 12 Ma
and culminating at 6–5 Ma, is largely responsible for uplift and exhumation
in southeast Australia during the late Miocene (Fig. 17). The propagation
of stress across the plate appears to have been focused around older structures at depth, which have consequently become reactivated resulting in
the inversion of Lower Cretaceous blocks and associated localized folding.
301
mentation during the Miocene–Pliocene is plausible. The sea-level curve
of Haq et al. (1988) indicates a significant interval of regression occurring
through the late Miocene with subsequent transgression in the early Pliocene. 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 Tertiary. 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
reflection 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 siliciclastic material into the marine basins and so could be used as an explanation for increased siliciclastic sediment flux in early Pliocene successions overlying the Miocene–Pliocene unconformity.
However, the unconformity is typically angular, with Miocene strata having been subjected to deformation (folding and faulting) and erosion prior
to deposition of Pliocene sediments. This is the case in all of the basins
that we have examined from southeast Australia (e.g., Figs. 9, 10, 11, 14
and 15). Therefore, a change in eustatic sea level or climate cannot be used
as a viable mechanism for the origin of the Miocene–Pliocene unconformity
in southeast Australia. Similarly, the increased siliciclastic content of Pliocene sediments is best explained by tectonic uplift and consequent increased erosion of older siliciclastic successions (like the Eocene clastic
successions, the Lower Cretaceous volcaniclastics, and the Paleozoic basement rocks).
It does, however, seem likely that a period of eustatically driven transgression is responsible for the marine erosion, deposition of earliest Pliocene sediments, and widespread phosphatization evident at the Miocene–
Pliocene unconformity. Evidence from deep-sea isotope records (Shackleton et al. 1995) indicates that the early Pliocene was a warm (possibly icefree) period. This would explain the occurrence of a rapid and synchronous
flooding event across the southeast Australian margin made more invasive
by regional subsidence following the compressional uplift. From this it
would appear that the preservation of the late Miocene tectonic event is a
result of a eustatic cycle being superimposed on an extended period of
tectonism in southeast Australia.
CONCLUSIONS
Climate and Eustasy at the Miocene–Pliocene Boundary
In the past, the unconformity that is present between the Miocene and
Pliocene successions has been attributed to both eustatic and climatic
changes (e.g., Carter 1978a), with little importance placed on the role of
tectonics in producing the stratigraphy of the sedimentary basins. The role
of tectonics has generally been invoked only when discussing the timing
of uplift of the Eastern Highlands, which is dominated by ideas of late
Pliocene to Pleistocene uplift (e.g., Douglas and Ferguson 1988). Although
some researchers have suggested a late Miocene inversion event along the
southeastern margin (e.g., Duddy 1994; Hill et al. 1995; Cooper and Hill
1997), most thermochronologic studies have constrained older events (e.g.,
Duddy 1994; Hegarty et al. 1988) showing substantial exhumation of the
Otway Group in the mid-Cretaceous. The preference for eustasy and climate change over tectonism as an explanation for the Miocene–Pliocene
unconformity reflects the influence of seismic and sequence stratigraphy in
the literature and a general belief that the Australian continent has been
tectonically stable through the Neogene–Recent.
The possibility of a climatic and/or eustatic event influencing the sedi-
An unconformity is observed close to the Miocene–Pliocene boundary
(between 10 and 5 Ma) in the onshore and nearshore parts of the Otway,
Port Phillip–Torquay, and Gippsland basins of southeast Australia. The
unconformity is generally angular, with the underlying Miocene units having been deformed (gentle folding and reverse faulting) and eroded prior
to deposition of the Pliocene succession. It also marks the change from
Oligocene–Miocene cool-water carbonate and brown-coal-dominated successions to more siliciclastic-rich Pliocene sediments.
The change to Pliocene siliciclastic-rich sediments and the angular nature
of the Miocene–Pliocene unconformity indicate that a significant regional
uplift event has occurred in all southeast Australian basins. The timing of
this uplift coincides with a change in the dynamics of the Australian plate
at around 12 to 5 Ma, most evident in New Zealand, where the Southern
Alps underwent an increase in the rate of uplift. The southeast Australian
margin is currently under a compressional stress regime, and this has probably persisted since the late Miocene, when the change in plate dynamics
occurred. The preservation of this tectonic event as an unconformity in the
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 phosphatization.
ACKNOWLEDGMENTS
We thank Jim Bowler for many stimulating discussions about this work and Mike
Sandiford for a thoughtful review. Thanks also to Paul Green, Garry Karner, and
Nicholas Christie-Blick for their constructive and positive reviews. The project was
funded by the Australian Research Council as part of ongoing research on the southeast Australia basins and supported by a grant from the Australian Institute of Mining and Metallurgy.
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