suggested that some of the sediments in these basins are similar lithologically and stratigraphically. This relation-. 'Geology Department, Melbourne University.
Second South-Eastem Australia Oil Exploration Symposium, Clenie, R. C. (ed. ), 1986. Technical papers presented at PESA Symposium, 14-15 November, 1985, Melbourne, 469 pp. ©Petroleum Exploration Society ofAustralia.
65
Regional Evaluation of the Tectonic and Thermal History of the Gippsland Basin K.A. Hegarty\ I.R. Duddy, 1 P.F. Green, 1 A.J.W. Gleadow, 1 I. Fraser2 and J.K. WeisseP
Abstract The subsidence history at several locations within the Gippsland Basin is reconstructed using biostrati• graphic and lithostratigraphic data from onshore and offshore wells. Four distinct Zones which correspond to different patterns of subsidence are identified. Each Zone shares an early relatively rapid phase of sub• sidence which may be caused by extensional tectonics driving basin subsidence. This rapid phase is followed by a period of slower subsidence which is modelled as simple thermal cooling. A stretching factor of 1.4 is used to best approximate the observed subsidence in the central axis of the basin. Similar amounts of extension were determined for wells in the Otway Basin. The 'mismatch' between observed and modelled subsidence curves in Zone 2 (central basin axis) is attributed to the effects of palaeo-water depths and eustatic sealevel fluctuations. The magnitude of palaeo-water depths are believed to be greater than sealevel changes, but the relative contributions from each effect are difficult to resolve. Fission-track analysis was undertaken on several outcrop and downhole samples. The palaeo-tempera• ture history determined from the analysis shows that the downhole samples from the basin axis were simply and continuously buried. Exposed Early Cretaceous samples have fission-track characteristics (age and track lengths) which suggest that these rocks have never been hotter than about 70°C. Nearby outcrop• ping Devonian granite samples show evidence of heat• ing effects possibly related to the tectonics which led to the formation of the Gippsland Basin.
Introduction Three large sedimentary basins lie along the southern perimeter of Victoria; these basins are the Otway, Bass and Gippsland Basins (Fig. 1). Several researchers have suggested that some of the sediments in these basins are similar lithologically and stratigraphically. This relation'Geology Department, Melbourne University. Office of Minerals and Energy, Oil and Gas Division 3 Lamont-Doherty Geological Observatory, Columbia University, NY 2 Victorian
ship may imply that each basin experienced approximately the same tectonic and thermal history. However thus far, at least one clear distinction exists between the Gippsland Basin and the other two basins. That is, only the Gipps• land Basin has been demonstrated as a rich reservoir for economic levels of oil and gas. In this study, we apply some of the thermo-mechanical principles determined in earlier studies in the Otway Basin (Gleadow & Duddy, 1981; Hegarty, 1985) to understand the tectonic history of the Gippsland Basin. Quantitative studies have shown that a lithospheric stretching model can account for much of the observed subsidence in the Otway Basin. It is recognized that subsidence predictions are not unique; that is, distinct basin-forming models, such as lithospheric extension (Jarvis & McKenzie, 1980) and deep-crustal metamorphism (Falvey, 1974), may correspond to similar predicted patterns of subsidence (Hegarty, 1985). Therefore, in this study we have assumed that, like the Otway Basin, the Gippsland Basin formed initially as a result of lithospheric extension. When predictions of subsidence from the extension model are compared to observed subsidence patterns (derived from well data within the Gippsland Basin), we can resolve valuable information about the amount of crustal exten• sion, palaeo-water depths, and the thermal and tectonic history of the region. Subsidence patterns from several wells drilled within the Gippsland Basin are determined and compared with theoretical predictions from a lithospheric extensional model. In addition, fission-track analysis of outcrop and downhole samples is used to constrain models of the regional development of the basin. A discussion of the method of analysis used in subsidence calculations and in fission-track analysis follows a brief review of the tectonic setting and geology of the Gippsland Basin .
Background Thus far, the Gippsland Basin supports eleven major oil and gas fields, all of which lie offshore. The distribution of wellsites in the Gippsland Basin (Fig . 2) indicates that the basin has been extensively explored offshore. More than 80% of the basin lies offshore. In plan view, the basin is approximately wedge-shaped such that the basin width onshore is about half the width of the basin at the
Copyright © 2016 by Petroleum Exploration Society of Australia (PESA)
66
Tectonic and Thermal History
FRACTURE ZONE MARINE MAGNETIC ANOMALIES
e
32°8
DSDP SITE 4500m ISOBATH APPROXIMATE BASIN BOUNDARIES
----...
" "-
----12
Otway Basin
" "\
"•
_:[) \
-10
-a ---7
---6
--58 138°E
SOUTHERN OCEAN
\
"•
,.
\282
\
\ \
\
\
I I \ \
\
150°
Figure 1: Tectonic setting of the Gippsland Basin. Most of the Gippsland Basin lies offshore where relatively large accumulations of oil and gas exist. Some of the offshore Gippsland structures (Fig. 2) can be traced onshore.
continental shelf edge . Much of the Gippsland Basin is bounded to the north by Australia's Palaeozoic Southeas• tern Highlands, and to the south by the Bassian Rise which extends from Wilsons Promontory to Flinders Island. The southeastern limit of the Gippsland Basin coincides with the continent-ocean boundary between the Australian continental lithosphere and oceanic lithosphere of the Tasman Sea. Hayes and Ringis (1973) showed that sea• floor spreading began in the Tasman Sea at Anomaly-33 time (about 80 Ma ago) and ceased at Anomaly-24 time (about 55 Ma ago). Unlike the oldest marine magnetic anomalies of the Southern Ocean south of Australia, the magnetic anomalies of the Tasman Sea exhibit relatively high fidelity and are more easily recognised. For studies of the thermo-mechanical control on basin formation, the most important lithotectonic units in the Gippsland Basin include the Early Cretaceous sediments (Strzelecki Group), Late Cretaceous sediments (Latrobe Group) and Tertiary sediments. For the purpose of discus• sion herein, there is limited reference to detailed stratigra• phic relationships in the basin . The stratigraphy of the Gippsland Basin is described in detail by several authors (James & Evans, 1971; Hocking, 1972; Partridge, 1976). Much of the offshore stratigraphy has been determined by workers at Esso Australia, the most active company in the basin .
Structures within the offshore Gippsland Basin are dominated by NW- SE trending anticlines and normal faults (Fig. 2). Most of the drillsites in the basin were selected at proposed structural traps associated with nor• mal faulting or anticlinal closure. The main axis of the basin is bounded by two E- W trending faults or fault zones referred to as the Rosedale Fault (north-bounding fault) and the Foster Fault (south-bounding fault). Beyond the axis of thick sediment-fill to the north and south, the flanks of the basin are characterised by relatively thin sedimentary cover and fewer faults (James & Evans, 1971; Stainforth, 1984).
Subsidence History of the Gippsland Basin Method of Analysis The method used to calculate the cumulative subsidence history at a wellsite is similar to the technique called 'geohistory analysis' proposed by Van Hinte (1978). The technique called 'backstripping' (Watts & Ryan, 1976) results in the calculation of the tectonic subsidence history . Tectonic subsidence is the amount of subsidence which cannot be accounted for by sedimentary loading effects, and results from thermo-mechanical processes that drive basin subsidence.
K.A. Hegarty, I.R. Duddy, P.F. Green, A.J.W. Gleadow, I. Fraser and J.K. Weissel
67
·.
ZONE 2=
-
ACCUMULATION
39°
14?0
A
1111 11111-1 148°
149°
Figure 2: Well location map showing exploratory and production wells drilled onshore and offshore in the Gippsland Basin. Many of these wells were used to define Zones 1 to 4 which are characterised by diagnostic patterns of subsidence. Several of the major fault trends and structural features are shown. (Base map from Victorian Department of Minerals and Energy, R. Christ.)
To accurately determine the cumulative and tectonic subsidence at a well, one must consider the effects of: 1) the compaction history of the sediments, 2) worldwide eustatic sealevel fluctuations, 3) the mechanical behaviour of continental lithosphere in response to loading, and 4) water depths during basin development. It is necessary to correct for the amount of sediment compaction through time. Pore water is progressively squeezed out of basin sediments as younger sediments are deposited, and the amount of pore-water loss may represent a significant vertical section of the sedimentary unit (as much as 40%). That is , the present-day thickness of the deepest strata in a 3 km-deep hole may be only 40% of its original thickness when the strata was deposited at the surface . If the porosity- depth relation is known for each lith• ology encountered in a well, then the sedimentary thick• ness for each unit can be reconstructed through time. By incrementally sliding homogeneous lithologic units up a known porosity-depth curve, the original thickness and depth to each stratigraphic interval can be determined. The sediment types in the Gippsland and Otway Basins are very similar in chemical composition and palaeo• environmental habitat. Thus, it was assumed that porosity- depth relations determined for sediments in the Otway Basin (Hegarty, 1985) closely approximate the
porosity-depth dependence in the Gippsland Basin . Systematic errors resulting from this assumption will probably be less than 5% at the deepest stratigraphic horizons. Duddy (1983) has shown that the Strzelecki Group (Early Cretaceous) underwent early diagenesis and cementation soon after deposition. If the Strzelecki Group did not mechanically compact according to the porosity• depth curves used in this study, then errors in the subsidence calculations for the Early Cretaceous may be as much as 25% . Six abundant and distinct sediment types were iden• tified in the Otway Basin and porosity-depth curves were determined for each using downhole log data. These sediments include shale, sandstone, siltstone, micrite, calcarenite, and limey-silt. Although it is assumed that the porosity function corresponding to each lithology remains constant through time, the overall porosity-depth rel.:•;on for the basin necessarily varies because the type of accumulating sediment is changing. If a compaction correction is not made: 1) calculations of tectonic subsidence can be significantly underes• timated, and 2) calculations of cumulative subsidence will usually indicate depths to stratigraphic horizons that are systematically too shallow . Accurate decompaction schemes are especially critical for calculations of palaeo• heatflow and maturation history of the sediments.
68
Tectonic and Thermal History
Calculations of porosity for the Otway Basin sediments were made using downhole sonic and density logs. It was assumed that porosity varies exponentially with depth according to the simple relationship first proposed by Athy ( 1930):
=
exp HICK) where, 0(z) = porosity at depth, z 00 initial porosity CK = characteristic depth. 0(z)
0
0
·
A least-squares exponential curve was fitted to the downhole porosity data, and the compaction constants (initial porosity and characteristic depth) for each lith• ology were calculated. Corrections for worldwide changes in sealevel (eustacy) were not included in calculations of cumulative and tectonic subsidence in this study. The effects of palaeo• water depth in the central axis of the Gippsland Basin are probably greater than eustatic changes in sealevel (discus• sed below). In contrast, sediment accumulation and sub• sidence in the Otway Basin generally occurred in very shallow water environments, so that subsidence histories in the Otway Basin are much more sensitive to eustatic sealevel fluctuations . Calculations of tectonic subsidence must include a correction for the mechanical behaviour of continental lithosphere as it is loaded by sediments. In this study, it is assumed that local Airy-type isostatic compensation oc• curs when the lithosphere is loaded. This approximation is justified in the central axis of the Gippsland Basin where relatively large amounts of loading have occurred and where the crust is presumed to lack considerable finite strength. However, on the northern and southern flanks of the central axis where the lithosphere has not undergone significant extension and exhibits relatively high finite strength, the lithosphere appears to respond flexurally to the incoming sedimentary load (Watts et al., 1982).
Results of Analysis The tectonic and cumulative subsidence were calculated for more than 25 wells in the Gippsland Basin. Three distinct patterns of cumulative subsidence were identified and correlate with four tectonic zones within the Gipps• land Basin (Fig . 2) . The pattern of cumulative subsidence characterising each of these four zones is shown in Figure 3. The subsidence pattern in Zone 1 is similar to the subsidence portrayed at many of the wells in the Otway Basin (Mutter et al., 1985). Very rapid subsidence (about 100--150 m/Ma) commencing about 100--120 Ma ago is followed by a phase of relatively slow subsidence which may often be interrupted by periods of non-deposition or erosion. The central portion of Zone 1 is often associated with a thick sedimentary section (i.e. in excess of 2000 m). Subsidence patterns within Zone 2 are distinguished by a later period of rapid subsidence usually beginning
between 20 and 40 Ma ago (two examples in Fig. 3). Zone-2 subsidence patterns exhibit two distinct phases of apparent rapid subsidence which are separated by an often prolonged period (about 40--80 Ma) of slower subsidence. The most recent phase of rapid subsidence, which begins during the Miocene, correlates with thick prograding carbonate sequences comprising the Gippsland Limestone Formation. Zones 3 and 4 which lie on the northern and southern flanks of the central axis of the basin (Fig. 2) are associated with similar patterns of subsidence. Unlike Zone 2, Zones 3 and 4 do not unequivocally exhibit the second later phase of rapid subsidence. However, sub• sidence patterns on the basin flanks show that during the early stages of basin development, a restricted period of rapid subsidence occurred and a moderate thickness of rift-related sediments (800--2000 m) accumulated. The four zones of subsidence identified in this study correlate closely with structural regions defined by James and Evans (1971) . Their North and South Platforms are approximately equivalent in position to Zones 3 and 4 respectively. James and Evans used the term 'Central Deep' to describe the central axis of the basin which coincides with our Zone 2. James and Evans do not offer an equivalent to Zone 1; however their study was largely restricted to offshore regions.
A Thermo-Mechanical Model for Basin Subsidence It is generally accepted that the process of sedimentary basin formation consists of at least two distinct tectonic stages which in turn may be represented by two distinct phases of subsidence. The initial stage is the 'Rift Stage', a period of active tectonism which is followed by the 'Drift Stage' (or Post-rift Stage), a relatively quiescent tectonic interval. These two distinct phases are most readily apparent in subsidence patterns from Zone 1. (1) Rift Stage - The rift stage is marked by brittle failure of the upper continental crust and the formation of grabens or half-grabens. The grabens are often termed rift valleys and may be infilled with a combination of volcanic material, clastics, and marginally marine sequences. Normal faulting and local folding may affect the sediments deposited during this stage. Along a passive continental margin, the rift stage generally results in the formation of a series of en echelon, interconnected rift valleys. The rift 'zone' may be abandoned at any time during its development as , for instance, the North Sea's central graben (Sclater & Christie, 1980). Generally, rifting culmin• ates in the formation of a mature passive continental margin like the east coast of North America. However, when the margin-formation process is abor• ted during the rift stage, as in the case of the Gippsland Basin and possibly the Bass Basin, then a largely intracratonic basin results. (2) Drift Stage- The drift stage follows rifting, and may begin either when oceanic crust is first emplaced adjacent to rifted or altered continental lithosphere, or
K.A. Hegarty, I.R. Duddy, P.F. Green, A.J.W. Gleadow, I. Fraser and J.K. Weissel
69
CUMULATIVE SUBSIDENCE HISTORY
120
100
80
TIME (Ma ago)
60
40
20 500 1000
ZONE 1
1500 2000
c m "tl -i
::t
[
2500 3000
ZONE 2
ZONE 3
1\L____I I -1 I ~:00
120
100
80
60
40
20
ZONE 4
0
1000 1500
Figure 3: Representative subsidence patterns from each of the four Zones delimited in Figure 2. Biostratigraphic and lithostratigraphic data are used for the analysis. Lines within each subsidence pattern represent individual stratigraphic horizons in a well. An early accelerated phase of subsidence is identified in all zones; however rapid subsidence within the past 20-40 Ma is diagnostic of Zone 2 behaviour only.
simply when the rifting phase ceases . At Australia's southern margin (e.g. the Otway Basin), the begin• ning of the drift stage coincides with the age of the oldest adjacent oceanic crust (Mutter et al., 1985). Sediments deposited during the drift stage are usually fully marine sequences, and facies are largely controlled by a strong interplay between changes in sediment supply and rate of subsidence.
These two phases of subsidence can be quantitatively modelled as a result of horizontal extension of continental lithosphere . McKenzie's (1978) stretching model showed that sedimentary basins may form as a result of litho• spheric extension, which in turn results in crustal thinning and increased temperatures at depth. According to McKenzie, the continental lithosphere and crust are stret• ched by a fac tor ~ during an instantaneous rifting event.
70
Tectonic and Thermal History TECTONIC SUBSIDENCE (ZONE 2)
80
60
TIME (Mal
40
20
0
500 DEPTH
(ml
1000
--CALCULATED - - - · MODELLED
1500 400
~·2000~~1H rf
80
--~~~LLLLLLLL~~~~~~LLLL~O
60
40
TIME (Mal
20
0
PALAEO-WATER DEPTHS (predicted)
Figure 4: Comparison of modelled (predicted) and calculated (observed) tectonic subsidence patterns for one well within Zone 2. A stretching factor of 1.4 and rifting time of 15 Ma were used to generate the modelled subsidence curve. The apparent discrepancy between calculated and modelled curves is attributed mainly to palaeo• water depths which are shown below the compar• ison. Maximum palaeo-water depths (about 375 m) occurred about 22 Ma ago.
Immediately following this event, lithospheric and crustal thicknesses are reduced by a factor ~, and the temperature gradient in the lithosphere and crust is increased by a factor ~- McKenzie computed the tectonic subsidence patterns resulting from one-dimensional cooling of the lithosphere, for times following the rifting event. At equilibrium, the thickness of the lithosphere returns to the original value, while the crust remains 1/~ of its original value. A similar extensional model is applied in this study; however, several important variations have been included: 1) an extended rifting event (about 15 Main duration), and 2) the effects of lateral heatflow. The extended rifting model was first proposed by Jarvis and McKenzie (1980); however, they did not include the effect of lateral heat conduction which has been shown to contribute significan• tly to subsidence modelling (Steckler, 1981). The lateral heatflow effect was combined with the finite-duration rifting model by Cochran (1983). For a more complete description of the modelling techniques and parameters used in this study, the reader is referred to Cochran (1983) and Hegarty ( 1985). Figure 4 shows a comparison of observed tectonic subsidence from a Zone 2 well with a theoretical sub• sidence pattern based on the finite-duration rifting model. The model includes a 15 Ma rifting event and a stretching factor (~) of 1.4 which is similar to the amount of
extension determined for the Otway Basin (Hegarty, 1985) . Subsidence analyses from about 15 wells at Aus• tralia's southern margin showed that stretching factors describing this margin varied between 1.2 and 1.5. In addition, the subsidence analyses indicated that at various positions along the southern margin, the rifting event was between 20 and 30 Main duration. The difference between the tectonic subsidence cal• culated from well data and that theoretically modelled is attributed to palaeo-water depths (bottom of Figure 4). Our predictions of palaeo-water depths compare favorably with crude estimates based on limited biostratigraphic analyses. The relationship shown in Figure 4 shows that from about 55 to 22 Ma ago subsidence outpaced sediment supply to the basin, resulting in increasing water depths (from about 100 to 375m). However near the beginning of the Miocene, the rate of sediment supply (prograding carbonate sequences--Gippsland Limestone Formation) was greater than the slowing pace of basin subsidence. The sediment supply has essentially filled in the previously water-filled Gippsland Basin which has resul• ted in present-day water depths of less than 75 m at this well site. Using detailed palynological zonations with lithological and seismic data, Partridge (1976) has suggested that water-depth variation during the Tertiary in the Gippsland Basin is entirely a eustatic effect. He clearly identifies channelling events and several transgressive and regres• sive cycles which occurred during the Tertiary . Partridge (1976) suggests that many of these cycles are not recog• nised in the Bass Basin because during this time the Bass Basin was 'barred' from the encroachment of the sea. However, it is not clear why these cycles are often not recognised in the Otway Basin which was open to marine incursions at this time. If the Gippsland and Otway basins formed by the same physical mechanism at approximately the same time as we suggest, then any differences in the stratigraphy of the two basins must be attributed to differences in sediment supply. The influence of climate, eustacy, and tectonic subsidence are expected to have the same effect in each basin. Hence, we suggest that the difference in patterns of tectonic subsidence between the Gippsland and Otway basins is dominantly controlled by the interplay of sediment supply and basin subsidence. Unlike the Otway Basin, basement subsidence in the Gippsland Basin during the Early and Middle Tertiary outpaced sediment supply resulting in significant water depths. Later in the Tertiary, the water column was infilled by either prograding marine sequences or margin• ally marine sediments. Sediment supply to the Otway Basin during this time was sufficiently rapid to keep pace with basin subsidence. Clearly, not until reliable estimates of palaeo-water depths(± 100m) are available will subsidence patterns in Zone 2 of the Gippsland Basin prove to be a useful technique with which to test models of basin formation and to determine the relative magnitude of eustatic effects. Future research effort will be directed toward determina• tion of regions or zones where restricted palaeo-water depths are present in order to rigorously test basin models.
K.A. Hegarty, I.R. Duddy, P.F. Green, A.J.W. Gleadow, I. Fraser and J.K. Weisse!
71
14
TRACK LENGTH (f!m)
Figure 5: Cumulative subsidence history for the Wellington Park-1 well in the eastern onshore Gippsland Basin (for location see Fig. 6). Seven downhole samples were obtained and prepared for fission-track analysis (see text). For each sample, the corresponding fission-track age and track length distribution are shown. Track lengths along the abscissa are shown to the nearest micron. In general, the fission-track age and average track length decrease with depth (and increasing temperature).
Fission-Track Analysis: Constraints on Basin Thermal History Method of Analysis Fisson-track dating and track length analysis of detrital apatites from sedimentary rocks are new methods for evaluating the thermal history of sedimentary basins. The methods utilise the naturally occurring process of spontan• eous fission decay of trace quantities of U-238 present in apatite crystals. The radiation damage results in small linear defects, called fission tracks, which are visible under an optical microscope when the tracks are chemically etched. The rate at which U-238 fissions is a constant. In the absence of elevated temperatures (i.e. greater than about 60°C), the number of fission tracks in apatite will progressively increase with geological time. Thus, the age of the apatite grain can be determined by
counting the fission tracks and independently measuring the uranium concentration of the sample . In addition, fission-track analysis provides valuable information in reconstructing the palaeo-temperature history of a basin. At elevated temperatures, the lengths of fission tracks begin to decrease. That is, the tracks become unstable at higher temperatures and fade in a predictable manner. The natural process of fading is called 'track annealing' and has been studied in detail in the laboratory (Green et al., in press). Annealing leads to a reduction in the observed length of the fission tracks, and a correspon• ding reduction in the fission -track age (when the sample is heated to temperatures in the track-annealing zone). The annealing process of fission tracks is dominantly a func • tion of temperature, and though the variable time cannot be neglected, it is of secondary importance. As a general rule, the temperature range bounding the annealing
72
Tectonic and Thermal History
108± 5 •
SAMPLE LOCALITY AND F ISSION - TRACK AGE (Ma)
S~LE
Q
38'
1 1 2+8.
•MORW ELL
113 ± 14
W ELLI NG T ON PARK-: 1
.
WON TH AGGI
0
15
30km
39'
147°
Figure 6: General location map of four outcrop samples and Wellington Park-1 used for fission-track analysis. Fission-track ages and corresponding standard errors are shown at each sample site. At Wellington Park-1, the age of the highest (coolest) stratigraphic sample is shown.
window is similar to those temperatures required for hydrocarbon maturation, i .e. about 75-125°C . At temperatures greater than about 125°C, all tracks fade within about one million years . For more complete descriptions of sample preparation , methods of analysis and geological applications, the reader is referred to Gleadow et al. (1983), Gleadow and Duddy (1981), Harrison and McDougall (1980), Duddy et al. (1984), and Naeser (1979) .
Results of Analysis The distribution of track lengths is important in inter• preting fission-track ages because each individual track is actually a different age, and therefore has experienced a different sector of the total thermal history. Some tracks will be shorter than others, although all tracks originally formed at approximately the same length (about 16.3 microns) . As part of an ongoing program to determine the thermal history of the Gippsland Basin, we have collected outcrop and downhole samples within several Zones of the basin. The cumulative subsidence history and sample depths for one well, Wellington Park- ! (Zone I) are shown in Figure 5. Seven samples were collected within the apatite• rich Strzelecki Group in Wellington Park- 1 and used for fission-track analysis . Sample depths vary from about 1150 m to 3650 m which correspond to uncorrected downhole temperatures of 58°C to 157°C (Lindsay, 1982).
The stratigraphic age of the Strzelecki Group is Early Cretaceous (James & Evans, 1971) . The shallowest Wel• lington Park-1 sample has a fission -track age of 112 Ma (Fig. 5) which is indistinguishable from the stratigraphic age, suggesting that this sample has not experienced elevated temperatures. The track-length distribution for this sample exhibits a narrow range of lengths with a mean length of 13 .6 microns (Fig. 5). By comparison with results from the Otway Basin reference wells (Green et al., in press), the narrow distribution and mean track length (13. 6 microns) suggest that the uppermost Wellington Park-! sample has never been hotter. That is, the present temperature of 58°C is the maximum temperature encountered by this shallow sample. Similarly, the thermal history of each downhole sample can be recovered. The mean track length and fission-track age for each sample are progressively smaller downhole (Fig. 5). The sample at 1591 m has a slightly broader track-length distribution and younger age than the shall• ower sample at 1146 m. Ages and lengths are correspon• dingly altered at all underlying samples . The complete destruction of tracks occurs between 3051 m and 3642 m where current temperatures are in excess of l20°C . Measurable mean track lengths vary downhole from 13 .6 microns to 8.3 microns. The distribution pattern in some of the samples show a slightly skewed pattern (1591 m) and a marginally bimodal pattern (1631 m). Though these types of distribution patterns can often be used to resolve
K.A. Hegarty, I.R. Duddy, P.F. Green, A.J.W. Gleadow, I. Fraser and J.R. Weissel
NORTH
SOUTH
.....' - - - - - - - - -- - -FLE XURA L LOADING
73
-----
-----
HINGE ZON E
ZON E 4
FLEXURAL LOADING
EXTENSION & BRITT LE FAI LURE HI NGE ZONE
ZONE 2
ZON E 3
e
;!; ...J
w > w
.
...J
w
"';::0 ...J
w
"'.... J:
ll.
w
0
4
FAU L
0
20
4 0 km
[IT] N EOGENE T O RECEN T
~
UPPER CRE TACEOUS
r;o.-1
~
LOWER CRETACEOUS
~
PALAEOGENE
a
T
PALAEOZOIC BASEMENT WELL CONTROL
~
Figure 7: Interpreted seismic reflection profile across the axis of offshore Gippsland Basin (for location see Fig. 2). Three of the four zones are depicted in this section and related to distinct tectonic environments associated with the basin's extensional and loading history. (After James & Evans, 1971.)
complicated thermal histories, at Wellington Park-1 these patterns are believed to be not significant. Fission-track ages from four outcrop samples are shown in Figure 6. Two of the samples were collected in exposed sections of Strzelecki sediments near Foster and southeast of Wonthaggi . The corresponding fission-track ages are indistinguishable from their stratigraphic age (Fig. 6). These samples have fission-track ages (124 and 108 Ma) similiar to the oldest age determined at Wellington Park-1 in Strzelecki sediments (112 Ma) which suggests that each of these samples were exposed to temperatures not ex• ceeding 60°C . The sample near Foster which has a relatively younger age was collected within a local shear zone and may have been thermally affected. Fission-track ages were determined for two Devonian granite samples collected from Wilsons Promontory and Cape Woolamai (Fig. 6). The granite at Wilsons Promon• tory corresponds to a fission-track age of 246 Ma. This apparent age represents a partially altered age as shown by length distribution data. That is, Devonian tracks may have been only partially annealed during a moderate thermal event which occurred after Devonian time. This post-Devonian thermal event may be related to Cretaceous tectonism which led to the formation of the Gippsland Basin. The granitic sample at Cape Woolamai has a fission• track age of 113 Ma (Fig . 6). The heating event near Cape Woolamai was apparently sufficiently intense to almost entirely overprint the Palaeozoic age. Length-distribution data for this sample suggests that the maximum age of this event is close to its fission-track age of I 13 Ma. We believe that the same heating event during the Cretaceous affected both the Cape Woolamai and Wilsons Promon• tory samples.
Summary and Conclusions The following observations and conclusions are presen• ted herein: (I) Four distinct Zones can be identified in the Gippsland Basin region . Each Zone is distinguished by its pattern of subsidence . The approximate structural position of three of the four Zones and their relation to the mechanics of basin formation is shown in Figure 7. (2) Differences in the pattern of subsidence between Zones is attributed to the palaeo-water depth history and to the amount of extension. (3) The thermo-mechanical processes controlling the formation of the Gippsland Basin are assumed to be similar to the process controlling the formation of the Otway Basin. Thus , by applying the extensional model which successfully accounts for Otway subsi• dence, we can predict the history of water depths in the Gippsland Basin. (4) The subsidence history of the Gippsland Basin exhibits two phases-rapid, mechanically-controlled subsidence followed by slower, thermally-controlled subsidence. Thus the Gippsland Basin is not a composite basin (i .e. the Gippsland Basin overlying the Strzelecki Basin), but rather it is one basin with a two-part tectonic history . (5) Apparently rapid subsidence during the Miocene is an artifact of not including palaeo-water depth correc• tions in the initial subsidence analysis. During the Miocene, sediments began to infill a relatively deep• water basin, such that sediment supply exceeded the rate of basin subsidence. The relative contributions to palaeo-water depth from basin subsidence and eustatic effects are difficult to resolve.
74
Tectonic and Thermal History
(6) Fission-track ages and length distributions from downhole samples in Wellington Park- 1 are consistent with a model of simple burial and heating. (7) Fission-track ages from two outcropping Strzelecki samples are approximately coincident with the stratigraphic age of the formation. This relationship suggests that these samples have never been hotter than about 70°C or buried to great depths. (8) Two samples of Devonian granite correspond to fission-track ages which are considerably younger than the established emplacement age of the granite. These ages represent partially reset ages and may be related to thermal events which controlled the Early Cretaceous development of the Gippsland Basin.
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