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Rotational displacements in Southeastern Australia and their influence on hydrocarbon occurrence
Tectonophysics, 63 (1980) 139-153 @ Elsevier Scientific Publishing Company,
139 Amsterdam
- Printed
in The Netherlands.
ROTATIONAL DISPLACEMENTS IN SOUTHEASTERN AUSTRALIA AND THEIR INFLUENCE ON HYDROCARBON OCCURRENCE
JOHN K. DAVIDSON Exso Australia
Ltd., GPO Box 4047, Sydney
2001 (Australia)
ABSTRACT Davidson, J.K., 1980. Rotational displacements in southeastern Australia and their influence on hydrocarbon occurrence. In: M.R. Banks and D.H. Green (Editors), Orthodoxy and Creativity at the Frontiers of Earth Sciences (Carey Symposium). Tectonophysics, 63: 139-153. From Late Jurassic through Middle Eocene times, southeastern Australia experienced a deformation which created the Gippsland, Bass, Otway and West Tasmania Basins. This basin-forming deformation can be ascribed to a “dextral divergent rotation” of Antarctica relative to Australia. Detailed strain and fracture patterns resulting from dextral divergent rotational displacement can be studied in a homogeneous medium such an ice. However, the geometry of southeastern Australian sedimentary bask resulta from streamer acting on a heterogeneous medium. Thus pre-Late Jurassic zonea of basement weakness were rejuvenated and influenced basin geometry. Sinistral divergent rotation of the Lord Howe-New Zealand continental masa relative to Australia occurred, possibly in the Late Jurassic. This separation created the narrow East Tasmania Basin but does not appear to have structurally influenced Gippsland Basin. By latest Eocene, the structural environment in southeartem Australia had changed dramatically, responding to a “dextral convergent rotation”. Late Eocene through Recent compresaional features resulted mainly in Gippsland, with minor features in northern Bass Basin and in various parta of northern Otway Basin. Hydrocarbon entrapment in Gippeland Basin is directly related to traps associated with anticlinical growth of Late Eocene through Recent movements.
INTRODUCTION
Total sediment thickness contours indicate in excess of 10,000 m of post mid Jurassic sediments in the Gippsland and Otway Basins (Fig. 1) slightly less in Bass Basin and approximately 5000 m in the West Tasmania Basin. Isolated pockets of sediments constitute the East Tasmania Basin. The earliest reference to global tectonic causes of the Late Mesozoic to Recent structural evolution of southeastern Australia (Carey, 1958, p. 273) was followed by a more detailed map (Fig. 2) which was drawn by Prof. S. Warren Carey for Dr. Lewis G. Weeks in 1959. Although presented to ANZAAS in 1962, the map has not been published and is reproduced here
--- 200 METRE ISOBATH -to- SEDIMENT THl&KNESS 1M METRESI
Fig. 1. Southeastern
Australia:
sediment
thickness.
by kind permission of Prof. Carey. This paper will investigate the hypothesis that the GambierCabo Lineament was a zone of dextral shear but will suggest that the dispiacement had a divergent component initially, which was followed by a convergent component and that these movements were prime controls of hydrocarbon entrapment.
Fig. 2. Southeastern Carey, 1969).
Australia:
structural
trends,
Late Mesozoic-Recent
(By S. Warren
141 CONTINENTAL
ROTATIONAL
SEPARATION
While describing relative motions of continents on an asymmetrically expanding earth, Carey (1958) recorded several continental rotations, seventeen of which have since been confirmed palaeomagnetically (Carey, 1976, p. 5). Carey (1958, p. 182) described the motion of Arabia relative to North Africa (Fig. 3) by two vectors; vector s which was equal in magnitude along the Red Sea and Gulf of Aden and which expressed sinistral motion of the Dead Sea Fault, and by a rotational vector r about the Dead Sea as Euler Pole. The orientation of r to the coasts of Red Sea and Gulf of Aden is indicative of a dextral component to the divergent rotational displacement separating Arabia and North Africa. Thus, the displacement could be described as a “dextral divergent rotation”. If Sinai is moving dextrally relative to North Africa, dextral divergence along the Gulf of Suez might be similar to the early motion on the Gambier-Gabo Lineament thereby suggesting that in Late Mesozoic times, Tasmania, Australia and Antarctica may have been arranged similarly to Sinai, North Africa-Somaliland and Arabia, respectively. The initial divergent rotational displacement between continents is particularly important for the occurrence of hydrocarbons as it is basin-forming. An ice model will be used in order to gain details of strain resulting from such a stress acting on a homogeneous medium.
Fig. 3. Red Sea-Gulf
of Aden sphenochasm
(from Carey, 1958).
142 ICE MODEL
OF SINISTRAL
DIVERGENT
ROTATION
The Filchner Ice Shelf lies southeast of the southern tip of South America at the head of the Weddell Sea. On a southerly reconnaissance flight across the shelf, Fuchs (in Fuchs and Hilary, 1958, p. 31) recorded a “tremendous chasm extending westward for at least 20 miles before being obscured by low cloud” (Fig. 4a). “To the east, wide curving crevasses replaced the deep trench of the chasm” (Fig. 4b). The “Great Ice Chasm” as the feature in Fig. 4 came to be known, floated on approximately 1100 m of water, 50 km south of the shelf edge (Neuberg et al., 1959). The “Chasm” reached a maximum width of 5 km, was approximately 2 km in width 32 km from its eastern termination, where it was 0.3 km wide. Thus it was rotationally divergent. Wilson (1960) removed the obliquity of Fig. 4 and found that the trends of the crevasses on the south side of the chasm (Fig. 4a) lay at an angle of 15” to the trend of the chasm. However, the crevasses curved towards the Chasm and approached 30” near its edge. Wilson concluded that the enechelon orientation of the crevasses was indicative of sinistral shear failure. In addition this paper reports a “sinistral divergent rotation” with an Euler Pole east, but also north of the axis of the chasm, i.e., below and to the right of Fig. 4a. Just below Fig. 4b, between the Great Ice Chasm and its Euler Pole, the shelf ice was grounded along a north-south trend. This interruption of the easterly propagating Chasm may have created the strain of Fig. 4b which can be divided into two areas: (a) lower left, where extensional fractures were strongly rotated along a trend which was slightly southeast from the easterly trending chasm; and (b) lower centre and right, where the crevasses on the side of the chasm extend and trend east-northeasterly (after Wilson, 1960) but with much less rotation. Both areas are crossed by a northerly trending set of slightly divergent dextral shears. Figure 4 will be compared with Late Mesozoic strain in southeastern Australia. Evolution of the chasm is not immediately apparent as Fig. 4 depicts only one instant in strain development. Two stages of strain can be depicted diagrammatically (Fig. 5). After elastic and viscous strain, extensional failure will follow (A of Fig. 5a). This event occurred slightly earlier at B where sinistrally induced anticlockwise rotation has caused the fracture to open while it extended at each end, together inducing a sigmoidal form. When the A event occurred much earlier at the extreme right, it was orientated at a much lower angle to the horizontal trend of the “active” zone of strain. In fact, these extensional fractures were initiated parallel or almost parallel to radii about the Euler Pole. The obliquity of Fig. 4a obscures this divergent rotationally induced fracture-angle change. Continued rotation of a B extensional fracture will induce a component of conjugate dextral shear motion, antithetic to the major sinistral motion (C of Fig. 5a). Further rotation might cause a second generation of extensional failure (D) as indicated by Carey (1976, p. 173, and others). Rotation of the active zone will induce local extensional stress between successive extensional fractures of the active
Fig. 4. a, b. Great Ice Chasm, Filchner Ice Shelf, Antarctica (Fuchs and Hilary, 1958. Reproduced by permission of the Commonwealth Trans Antarctic Expedition).
144
. EULER POLE (a)
StNlSTRAL SPHENOCHASM
if SPhENOCHASM
INTRUSION OF OCEANIC CRUST
Y l
EULER POLE
(a)
Fig. 5a. Fault pattern resulting from einistral divergent rotation. ing from sinistral divergent rotation.
b. Fault pattern result-
zone and the undeformed “passive” areas (E). As a result, the active zone will collapse and a rift valley develop. The Great Ice Chasm was at this stage (Fig. 4a). Sinistral shear motion could be relieved along either margin of the rift, or by continued deformation of the rift floor. If the latter occurs, failure in shear can proceed as considerable transcurrent motion occurs right to the surface. This was prevented from occurring ea#er as tensile failure occurs at a lower threshold (Carey, 1976, p. 173). This failure (F) wilI be conjugate to dextral motion on the earlier extensional fractures. F might develop its own extensional fracture set and the gross rotation wiIl cause F fractures to diverge away from the Euler Pole in a like manner as the rift valley. In the case of continental rotational separation, F is the zone along which initial intrusions of oceanic crust will take place as F becomes the focus of most of the divergence (Fig. 5b). It must be emphasized that the total strain of either Fig. 5a or 5b is an instantaneous strain. A consequence of rotational divergence (or convergence) is that any particular type of fault will
145
occur later nearer the Euler Pole, as strain-rate decreases in that direction. The “V” shape of the rift valley of Fig. 5 can be described as a sphenochasm as Carey (1958, p. 173) used the term to describe “the triangular gap of oceanic crust separating two cratonic blocks with fault margins converging to a point, and interpreted as having originated by the rotation of one of the blocks with respect to the other”. The writer suggests that this term be applied to rotationally divergent rifts which parallel radii about the Euler Pole (lower left of Fig. 5b) and that “sinistral (or dextral) spenochasm” by applied to rotationally divergent rifts which traverse radii (Fig. 5b). A modern example of strain resulting from dextral divergent rotation lies in the Gulf of Aden. Le Pichon et al. (1973) considered a six-plate model in the area while calculating the positions of Euler Poles. They showed that 4.4” of rotation has taken place about a pole located at 26.5”N, 21.5”E. Fig. 6a (after Beydoun, 1970, p. 281) is a reconstruction of the shorelines of the Gulf of Aden. The above pole lies off the figure to the northwest. The bulk of Beydoun’s north-northwesterly to west-northwesterly orientated “suggested correlations lines” can be seen to trace trends of normal faults. Several points of comparison can be made with Fig. 6b, an inversion of Fig. 5b: (a) The angle between normal faults and rift axis increases to the left as the. Euler Pole is approached. (b) The sigmoidal form of normal faults is consistent with a component of dextral shear. (c) The dotted 200 m isobath is composed of two trends, A and B, also marked on the model (Fig, 6b). These data suggest that the Gulf of Aden can be described as a dextral sphenochasm resulting from a dextral divergent rotation.
(b) Fig. 6. a. Reconstruction-Gulf rotational stress.
/
of Aden (After Beydoun,
1970).
b. Dextral divergent
146 LATE
JURASSIC-MIDDLE
SOUTHEASTERN
EOCENE
DEXTRAL
DIVERGENT
ROTATION
IN
AUSTRALIA
Figure 7 shows major normal fauft trends of Late Jurassic-Early CretaceAustralia. The onshore expression of pre-late Jurassic structural trends is one of dominantly north-south trends. The figure shows only the major Late Jurassic-Early Cretaceous faults which were derived almost exclusively from mapping of seismic reflection data by EssoB.H.P. It is very difficult to map faults of this age in the Otway Basin due to the thick sedimentary section. However, detailed mapping of fault trends was possible in the Gambia Embayment, in the northwestern part of Otway Basin. The more major faults and trends have been reproduced in Fig. 8a. The sigmoidal form of the prominent west-northwesterly trending normal faults indicates a strong component of dextral shear during basin initiation, e.g., the Chama Fault. This fault is identified in Fig. 8b which is an inversion of Fig. 5b. The approximate area of Fig. 8a with which analogy is made to Fig. 8b is shown on the latter. The Trumpet Low and Crayfish High display southwesterly divergence which is analogous to the rotationally induced divergence on the similarly marked trends of Fig. 8b. The northern rift margin faults of the Gambier Embayment traverse faults of the Chama trend as do the similarly marked faults of Fig. 8b. These data provide evidence for the creation of Otway Basin rift valley by dextral divergent rotation. Just west of Cape Otway (Fig. 7) there is a distinct change of fault trends to one composed of a prominent trend oriented a little west of north and an ous age in southeastern
Fig. 7. Southeastern Australia: structural trends, Jurassic-Early
Cretaceous.
147
APPROX. AREA OF FIG.Ga
(b)
Fig, 8. a. Structural trends in Gambier Embayment.
EULER
POLE ’
b. Dextral divergent rotational stress.
almost northeasterly cross trend. The trend a little west of north is not significantly different from the west-northwesterly Chama Fault trend common to the greater part of the Otway Basin. Indeed both these trends have large Late Cretaceous and Tertiary normal displacements indicative of separation of Australia and Antarctica. The northeasterly trend and the one a little west of north persist from Cape Otway to south of West Tasmania. The fault patterns of the Otway and West Tasmania Basins appear different due to this northeasterly trend. However, a similar difference is shown on the left half of Fig. 9, an inverted and obliquity-corrected version of the eastern extremity of the Great Ice Chasm (Fig. 4b). Clearly Figs. 7 and 9 are not identical as ice is a homogeneous medium while West Tasmania basement trends are heterogeneous. The rotation inferred for formation of the Late Jurassic-Early Cretaceous Otway-West Tasmania developing sphenochasm is also expressed sedimentologically. Quilty (1973) said that separation of the originally juxtaposed Kerguelen and Broken Ridge was advanced by Cenomanian-Turonian times as sediments have been found 500 km north of Kerguelen which were
148
Fig. 9. Great Ice Chasm (inverted) with obliquity corrected.
deposited in water “deeper than expected on continental shelves but not under abyssal conditions”. About 2500 km east along the rift valley in the Otway Basin similarly dated Belfast Mudstone was deposited under shallow marine conditions, while further east deposition in the King Island Sub-Basin was nearshore to sub-aerial. These data might suggest that at any instant during rotational displacement, the rift was wider off the southwestern coast of Australia than off the southern coast. The northeasterly trend of normal faulting at Cape Otway extends eastward to the northern Baas Basin and probably to the onshore Gippsland Basin (Fig. 7). In the northern Bass Basin, there is an abrupt change to a southeasterly orientation. These two trends can be identified in the rightcentral part of Fig. 9. This fiie shows that the extensional features east of the Great Ice Chasm (“Otway”) were formed by a component of dextral shear. The attitude of the northwesterly basin-forming faults in Gippsland Basin (Fig. 7) is indicative of a component of dextral shear (Threlfall et al., 1976). The change from an easterly trend of the Great Ice Chasm to the fault trends at its eastern termination was attributed above to the influence of north-trending grounded ice which lay in the path of the Chasm and its Euler Pole. Similarly, as the dextral divergent rotational rift valley of the Otway Basin approached its Euler Pole, perhaps located southeast of Tasmania, it encountered the dominantly north-south grain of the Tasman Orogen (Fig. 7) thereby giving rise to a fracture pattern very similar to that of Fig. 9. As mentioned above, basement heterogeneities are locally very important modifiers of strain, e.g., Precambrian and Palaeozoic trends seen onshore, clearly control some of the offshore West Tasmania basin trends. Hanington et al. (1973) cannot conclusively demonstrate pre-Late Jurassic movement of the
149
“Gippsland Kink Zone” or the “Sorell Lineament” (West Tasmania Basin). However, they did suggest 1100 km of Late Ordovician-Middle Devonian sinistral motion along the “GambierBeaconsfield Lineament” (Otway Basin-Bass Basin). Harrington et al. (1972) extended this lineament westward, parallel to the offshore extension of the southern Australian coast, to southwestern Western Australia. This might imply that the Otway Basin rift valley and its westerly extension were located on pre-existing zone of basement weakness. The fault pattern off the east coast of Tasmania, although poorly controlled suggests a sinistral component to the motion of Lord Howe Rise and New Zealand from this area. Tapering of the Tasman Sea sphenochasm northwards is also indicative of divergent rotation, thereby suggesting a sinistral divergent rotation. This movement does not seem to have affected structures in east Gippsland as the East Gippsland High could be attributed to activation of offshore extensions of southwesterly trending, Palaeozoic structures by dextral divergent motion associated with the movement of Antarctica. Perhaps the departure of Lord Howe Rise in Late Cretaceous times (Hayes and Ringis, 1973 and JOIDES 283) allowed several kilometres of dextral motion on the Cambier-Gabo Lineament. LATE EOCENE-RECENT
DEXTRAL
CONVERGENT
ROTATION
The bulk of the oil and gas in the Gippsland Basin is reservoired in sediments of the Late Cretaceous-Eocene Latrobe Group beneath shales of the Oligocene Lakes Entrance Formation. The unconformity at the top of the Latrobe Group was formed by erosion and/or nondeposition, partly contemporaneous with latest Eocene anticlinal growth. These folds were superimposed by folds of Late Miocene to Recent age (Fig. 10a). If parallel dextral shear was the only factor which controlled fold orientation, one would expect all fold axes to be parallel, and perhaps due to rotation, oriented at a little less than 45” to the direction of shear (A-A). Changes in senses of fault throw along A-A together with adjacent intense deformation provide evidence for east-west directed shear movements. As suggested by Threlfall et al. (1976), this motion is considered to be dextral due to the alignments of Barracouta-Snapper and Marlin-Tuna Anticlines (Fig. 10a) to the zone. Motion on fault A-A and fold growth was contemporaneous. The east-west orientation of Kingfish Anticline is not consistent with easterly directed parallel dextral shear. Rather, it implies that the compressional axis lay somewhat south of southeast. This indicates the deforming stress was a convergent dextral shear. Onshore the Gippsland fold and monoclinal trends (Fig. lob, from Abele et al., 1976) involve rocks as young as Pliocene. Northeasterly to east-northeasterly orientated fold axes are consistent with dextral shear. However, the easterly trending fold axes again indicate a convergent dextral shear. The Kingfish Oilfield (Figs. lla and 10a) shows pronounced folding of
‘%S
SHEAR FAULT ANTICLINAL
TREN#
BALOOK
BLO
Fig. 10. a. Offshore Gipprland rtructure map (After Threlfall et al., 1976). Gippsland structure map (From Abele et al., 1976).
b. Onshore
151
1
a@#
SEtSMIC
TIME SECTION
SE.
x
I
(a)
SW
NI
Fig. 11. a. Kingfish Anticline, Gippsland Basin (From Threlfalf et al,, 19761, b, Seismic time section, Bass Basin. Miocene age, and thinning over the structure near the “Top Eocene” seismic reflector is of latest Eocene-Early Oligocene age. Similar relationships can be demonstrated over the other major oil and gas field such as Halibut, Barracouta, Marlin and Snapper. The northern Bass Basin also lies along the Gambier-Gabo Lineament. ~~~t~~~~~l~
152
The Cormorant Anticline (Fig. lib) is located in a pre-Ohgocene basin which trended a little east of southerly. Thinning of Lower Oligocene and post-Middle Miocene sediments over the structure suggests reversal of stress from extensional to compressional at these times which is identical to the two pulses recognized in Gippsl~d Basin. South-southeasterly trending normal faults moved during the folding pulses and some acted as conduits for intrusive and extrusive igneous activity, This deformation is compatible with south-southeasterly compression. Compression of post-Early Tertiary age is evidenced on numerous seismic lines near the western end of Gambier-Gab0 Lineament at the northern edge of Gambier Embayment. The areal extent and amplitude of compressional structures in Gambier Embayment is much less than in Gippsland. These relationships suggest convergent rotational movement with an Euler Pole west of Gambier Embayment. If this is true, one could expect an intermediate amount of compression between the two areas, viz. in northern Bass Basin. Not only the Cormorant area experienced compression contemporaneously with Gippsland as Abele et al. (1976) noted Early Oligocene upfift in the Torquay Basin and the Otway Ranges probably underwent a major compression in late Early Miocene. The sum total of compression in the general North Bass area is probably a little less than in Gippsland and certainly greater than in Gambier Embayment. The south-southeasterly directed compression imposed a component of dextral shear on the almost east-trending Gambier-Gabo Lineament which was a Late Eocene-Early Oligocene and Late MioceneRecent zone of dextral convergent rotation, i.e., it was a sphenopeism or a “compression wedge” (Carey, 1958). In the terminology of this paper the component of dextral rotation indicates that it was a “dextral sphenopeism”. INFLUENCE OF STRUCTURAL CONTROL ON HYDROCARBON OCCURR3ZNCE Significant accumulations of hydrocarbons in Gippsland Basin are related to compressional anticlines. Late Jurassic-Middle Eocene dextral divergent rotation created the sedimentary basins and Late Eocene-Recent dextral convergent rotation created structures which trap the bulk of these hydrocarbons. Although anticlines of similar age are present in the Bass Basin, the lack of significant accumulations could be attributed to other factors such as maturity of the hydrocarbon source rocks. The lack of compressional anticlines in the Otway Basin should not be thought of as being the only cause for the lack of signifieant accumulations. Rather the inadequate trapping mechanisms of many of the normal fault closures could be a contributing factor. Analysis of an ice model enabled an Euler Pole for Late Jurassic-Middle Eocene dextral divergent rotation to be postulated southeast of Tasmania. It can only be said that the Euler Pole for Late Eocene-Early Oligocene and Late Miocene-Recent dextral convergent rotation lay west of Gambier Embayment.
153
It would seem appropriate to use Carey’s (1958) terms of “sphenochasm” when the zone of divergent or convergent rotation and “sphenopeism” parallels a radius about the Euler Pole and the terms “dextral or sinistral sphenochasm” or “dextral or sinistral sphenopeism” when the axis crosses radii about the Euler Pole. ACKNOWLEDGEMENTS
While at the University of Tasmania, the writer was fortunate to study under such an inspiring person as Professor S. Warren Carey with whom warm personal ties have been maintained. Esso Australia Ltd., has benefited from its contact with Professor Carey, and together with the writer, wish him every success with his numerous endeavours planned for his retirement. This paper utilizes the work of many people of Esso Australia Ltd. Special thanks are due to Andrew J. Rigg who helped with compilation of the paper. The paper is presented with the permission of Esso Australia Ltd., and the Broken Hill Proprietary Company. REFERENCES Abele et al., 1976. Tertiary. In: J.C. Douglas and J.A. Ferguson (Editors), Geology of Victoria. Geol. Sot. Aust. Spec. Pub., 5: 528. Beydoun, Z.R., 1970. Southern Arabia and Northern Somalia: comparative geology. Philos. Trans. R. Sot. London, Ser. A., 267: 267-292. Carey, S.W., 1958. The tectonic approach to continental drift. In: Continental Drift -A Symposium. University of Tasmania, Hobart (1958), pp. 177-363. Carey, S.W., 1976. The Expanding Earth, Developments in Geotectonics, 10. Elsevier, Amsterdam-Oxford-NewYork, N.Y., 488 pp. Fuchs, Sir Vivian and Hilary, Sir Edmund, 1958. The Crossing of Antarctica. The Commonwealth Trans-Antarctic Expedition 1955-58. Cassell, London, 338 pp. Harrington, H.J., Burns, K.L. and Thompson, B.R., 1973. Gambier-Beaconsfield and Gambiei-Sore11 fracture zones and the movement of plates in the Australia-Antarctica-New Zealand region. Nature Phys. Sci., 245: 109-112. Hayes, D.E. and Ringis, J., 1973. Seafloor spreading in the Tasman Sea. Nature, 243: 454-458. Le Pichon, X., Francheteau, J. and Bonnin, J., 1973. Plate Tectonics. Developments in Geotectonics, 6, Elsevier, Amsterdam-Oxford-New York, N-Y., 300 pp. Neuberg, H-AC., Thiel, E., Walker, P.T., Behrendt, J.C. and Aughenbaugh, N.B., 1969. The Filchner Ice Shelf. Ann. Assoc. Amer. Geogr., 49: 110-119. Quilty, P.G., 1973. Cenomanian~uronian and Neogene sediments from northeast of Kerguelan Ridge, Indian Ocean. J. Geol. Sot. Aust., 20 (3): 361-367. Threlfall, W.F., Brown, B.R. and Griffith, B.R., 1976. Gippsland Basin offshore. In: Economic Geology of Australia and New Guinea, 3. Petroleum. Australas. Inst. Min. Metall., pp. 41-67. Wilson, G., 1960. The tectonics of the “Great Ice Chasm”, Filchner Ice Shelf, Antarctica. Proc. Geol. Assoc., 71: 130-138.
139 Amsterdam
- Printed
in The Netherlands.
ROTATIONAL DISPLACEMENTS IN SOUTHEASTERN AUSTRALIA AND THEIR INFLUENCE ON HYDROCARBON OCCURRENCE
JOHN K. DAVIDSON Exso Australia
Ltd., GPO Box 4047, Sydney
2001 (Australia)
ABSTRACT Davidson, J.K., 1980. Rotational displacements in southeastern Australia and their influence on hydrocarbon occurrence. In: M.R. Banks and D.H. Green (Editors), Orthodoxy and Creativity at the Frontiers of Earth Sciences (Carey Symposium). Tectonophysics, 63: 139-153. From Late Jurassic through Middle Eocene times, southeastern Australia experienced a deformation which created the Gippsland, Bass, Otway and West Tasmania Basins. This basin-forming deformation can be ascribed to a “dextral divergent rotation” of Antarctica relative to Australia. Detailed strain and fracture patterns resulting from dextral divergent rotational displacement can be studied in a homogeneous medium such an ice. However, the geometry of southeastern Australian sedimentary bask resulta from streamer acting on a heterogeneous medium. Thus pre-Late Jurassic zonea of basement weakness were rejuvenated and influenced basin geometry. Sinistral divergent rotation of the Lord Howe-New Zealand continental masa relative to Australia occurred, possibly in the Late Jurassic. This separation created the narrow East Tasmania Basin but does not appear to have structurally influenced Gippsland Basin. By latest Eocene, the structural environment in southeartem Australia had changed dramatically, responding to a “dextral convergent rotation”. Late Eocene through Recent compresaional features resulted mainly in Gippsland, with minor features in northern Bass Basin and in various parta of northern Otway Basin. Hydrocarbon entrapment in Gippeland Basin is directly related to traps associated with anticlinical growth of Late Eocene through Recent movements.
INTRODUCTION
Total sediment thickness contours indicate in excess of 10,000 m of post mid Jurassic sediments in the Gippsland and Otway Basins (Fig. 1) slightly less in Bass Basin and approximately 5000 m in the West Tasmania Basin. Isolated pockets of sediments constitute the East Tasmania Basin. The earliest reference to global tectonic causes of the Late Mesozoic to Recent structural evolution of southeastern Australia (Carey, 1958, p. 273) was followed by a more detailed map (Fig. 2) which was drawn by Prof. S. Warren Carey for Dr. Lewis G. Weeks in 1959. Although presented to ANZAAS in 1962, the map has not been published and is reproduced here
--- 200 METRE ISOBATH -to- SEDIMENT THl&KNESS 1M METRESI
Fig. 1. Southeastern
Australia:
sediment
thickness.
by kind permission of Prof. Carey. This paper will investigate the hypothesis that the GambierCabo Lineament was a zone of dextral shear but will suggest that the dispiacement had a divergent component initially, which was followed by a convergent component and that these movements were prime controls of hydrocarbon entrapment.
Fig. 2. Southeastern Carey, 1969).
Australia:
structural
trends,
Late Mesozoic-Recent
(By S. Warren
141 CONTINENTAL
ROTATIONAL
SEPARATION
While describing relative motions of continents on an asymmetrically expanding earth, Carey (1958) recorded several continental rotations, seventeen of which have since been confirmed palaeomagnetically (Carey, 1976, p. 5). Carey (1958, p. 182) described the motion of Arabia relative to North Africa (Fig. 3) by two vectors; vector s which was equal in magnitude along the Red Sea and Gulf of Aden and which expressed sinistral motion of the Dead Sea Fault, and by a rotational vector r about the Dead Sea as Euler Pole. The orientation of r to the coasts of Red Sea and Gulf of Aden is indicative of a dextral component to the divergent rotational displacement separating Arabia and North Africa. Thus, the displacement could be described as a “dextral divergent rotation”. If Sinai is moving dextrally relative to North Africa, dextral divergence along the Gulf of Suez might be similar to the early motion on the Gambier-Gabo Lineament thereby suggesting that in Late Mesozoic times, Tasmania, Australia and Antarctica may have been arranged similarly to Sinai, North Africa-Somaliland and Arabia, respectively. The initial divergent rotational displacement between continents is particularly important for the occurrence of hydrocarbons as it is basin-forming. An ice model will be used in order to gain details of strain resulting from such a stress acting on a homogeneous medium.
Fig. 3. Red Sea-Gulf
of Aden sphenochasm
(from Carey, 1958).
142 ICE MODEL
OF SINISTRAL
DIVERGENT
ROTATION
The Filchner Ice Shelf lies southeast of the southern tip of South America at the head of the Weddell Sea. On a southerly reconnaissance flight across the shelf, Fuchs (in Fuchs and Hilary, 1958, p. 31) recorded a “tremendous chasm extending westward for at least 20 miles before being obscured by low cloud” (Fig. 4a). “To the east, wide curving crevasses replaced the deep trench of the chasm” (Fig. 4b). The “Great Ice Chasm” as the feature in Fig. 4 came to be known, floated on approximately 1100 m of water, 50 km south of the shelf edge (Neuberg et al., 1959). The “Chasm” reached a maximum width of 5 km, was approximately 2 km in width 32 km from its eastern termination, where it was 0.3 km wide. Thus it was rotationally divergent. Wilson (1960) removed the obliquity of Fig. 4 and found that the trends of the crevasses on the south side of the chasm (Fig. 4a) lay at an angle of 15” to the trend of the chasm. However, the crevasses curved towards the Chasm and approached 30” near its edge. Wilson concluded that the enechelon orientation of the crevasses was indicative of sinistral shear failure. In addition this paper reports a “sinistral divergent rotation” with an Euler Pole east, but also north of the axis of the chasm, i.e., below and to the right of Fig. 4a. Just below Fig. 4b, between the Great Ice Chasm and its Euler Pole, the shelf ice was grounded along a north-south trend. This interruption of the easterly propagating Chasm may have created the strain of Fig. 4b which can be divided into two areas: (a) lower left, where extensional fractures were strongly rotated along a trend which was slightly southeast from the easterly trending chasm; and (b) lower centre and right, where the crevasses on the side of the chasm extend and trend east-northeasterly (after Wilson, 1960) but with much less rotation. Both areas are crossed by a northerly trending set of slightly divergent dextral shears. Figure 4 will be compared with Late Mesozoic strain in southeastern Australia. Evolution of the chasm is not immediately apparent as Fig. 4 depicts only one instant in strain development. Two stages of strain can be depicted diagrammatically (Fig. 5). After elastic and viscous strain, extensional failure will follow (A of Fig. 5a). This event occurred slightly earlier at B where sinistrally induced anticlockwise rotation has caused the fracture to open while it extended at each end, together inducing a sigmoidal form. When the A event occurred much earlier at the extreme right, it was orientated at a much lower angle to the horizontal trend of the “active” zone of strain. In fact, these extensional fractures were initiated parallel or almost parallel to radii about the Euler Pole. The obliquity of Fig. 4a obscures this divergent rotationally induced fracture-angle change. Continued rotation of a B extensional fracture will induce a component of conjugate dextral shear motion, antithetic to the major sinistral motion (C of Fig. 5a). Further rotation might cause a second generation of extensional failure (D) as indicated by Carey (1976, p. 173, and others). Rotation of the active zone will induce local extensional stress between successive extensional fractures of the active
Fig. 4. a, b. Great Ice Chasm, Filchner Ice Shelf, Antarctica (Fuchs and Hilary, 1958. Reproduced by permission of the Commonwealth Trans Antarctic Expedition).
144
. EULER POLE (a)
StNlSTRAL SPHENOCHASM
if SPhENOCHASM
INTRUSION OF OCEANIC CRUST
Y l
EULER POLE
(a)
Fig. 5a. Fault pattern resulting from einistral divergent rotation. ing from sinistral divergent rotation.
b. Fault pattern result-
zone and the undeformed “passive” areas (E). As a result, the active zone will collapse and a rift valley develop. The Great Ice Chasm was at this stage (Fig. 4a). Sinistral shear motion could be relieved along either margin of the rift, or by continued deformation of the rift floor. If the latter occurs, failure in shear can proceed as considerable transcurrent motion occurs right to the surface. This was prevented from occurring ea#er as tensile failure occurs at a lower threshold (Carey, 1976, p. 173). This failure (F) wilI be conjugate to dextral motion on the earlier extensional fractures. F might develop its own extensional fracture set and the gross rotation wiIl cause F fractures to diverge away from the Euler Pole in a like manner as the rift valley. In the case of continental rotational separation, F is the zone along which initial intrusions of oceanic crust will take place as F becomes the focus of most of the divergence (Fig. 5b). It must be emphasized that the total strain of either Fig. 5a or 5b is an instantaneous strain. A consequence of rotational divergence (or convergence) is that any particular type of fault will
145
occur later nearer the Euler Pole, as strain-rate decreases in that direction. The “V” shape of the rift valley of Fig. 5 can be described as a sphenochasm as Carey (1958, p. 173) used the term to describe “the triangular gap of oceanic crust separating two cratonic blocks with fault margins converging to a point, and interpreted as having originated by the rotation of one of the blocks with respect to the other”. The writer suggests that this term be applied to rotationally divergent rifts which parallel radii about the Euler Pole (lower left of Fig. 5b) and that “sinistral (or dextral) spenochasm” by applied to rotationally divergent rifts which traverse radii (Fig. 5b). A modern example of strain resulting from dextral divergent rotation lies in the Gulf of Aden. Le Pichon et al. (1973) considered a six-plate model in the area while calculating the positions of Euler Poles. They showed that 4.4” of rotation has taken place about a pole located at 26.5”N, 21.5”E. Fig. 6a (after Beydoun, 1970, p. 281) is a reconstruction of the shorelines of the Gulf of Aden. The above pole lies off the figure to the northwest. The bulk of Beydoun’s north-northwesterly to west-northwesterly orientated “suggested correlations lines” can be seen to trace trends of normal faults. Several points of comparison can be made with Fig. 6b, an inversion of Fig. 5b: (a) The angle between normal faults and rift axis increases to the left as the. Euler Pole is approached. (b) The sigmoidal form of normal faults is consistent with a component of dextral shear. (c) The dotted 200 m isobath is composed of two trends, A and B, also marked on the model (Fig, 6b). These data suggest that the Gulf of Aden can be described as a dextral sphenochasm resulting from a dextral divergent rotation.
(b) Fig. 6. a. Reconstruction-Gulf rotational stress.
/
of Aden (After Beydoun,
1970).
b. Dextral divergent
146 LATE
JURASSIC-MIDDLE
SOUTHEASTERN
EOCENE
DEXTRAL
DIVERGENT
ROTATION
IN
AUSTRALIA
Figure 7 shows major normal fauft trends of Late Jurassic-Early CretaceAustralia. The onshore expression of pre-late Jurassic structural trends is one of dominantly north-south trends. The figure shows only the major Late Jurassic-Early Cretaceous faults which were derived almost exclusively from mapping of seismic reflection data by EssoB.H.P. It is very difficult to map faults of this age in the Otway Basin due to the thick sedimentary section. However, detailed mapping of fault trends was possible in the Gambia Embayment, in the northwestern part of Otway Basin. The more major faults and trends have been reproduced in Fig. 8a. The sigmoidal form of the prominent west-northwesterly trending normal faults indicates a strong component of dextral shear during basin initiation, e.g., the Chama Fault. This fault is identified in Fig. 8b which is an inversion of Fig. 5b. The approximate area of Fig. 8a with which analogy is made to Fig. 8b is shown on the latter. The Trumpet Low and Crayfish High display southwesterly divergence which is analogous to the rotationally induced divergence on the similarly marked trends of Fig. 8b. The northern rift margin faults of the Gambier Embayment traverse faults of the Chama trend as do the similarly marked faults of Fig. 8b. These data provide evidence for the creation of Otway Basin rift valley by dextral divergent rotation. Just west of Cape Otway (Fig. 7) there is a distinct change of fault trends to one composed of a prominent trend oriented a little west of north and an ous age in southeastern
Fig. 7. Southeastern Australia: structural trends, Jurassic-Early
Cretaceous.
147
APPROX. AREA OF FIG.Ga
(b)
Fig, 8. a. Structural trends in Gambier Embayment.
EULER
POLE ’
b. Dextral divergent rotational stress.
almost northeasterly cross trend. The trend a little west of north is not significantly different from the west-northwesterly Chama Fault trend common to the greater part of the Otway Basin. Indeed both these trends have large Late Cretaceous and Tertiary normal displacements indicative of separation of Australia and Antarctica. The northeasterly trend and the one a little west of north persist from Cape Otway to south of West Tasmania. The fault patterns of the Otway and West Tasmania Basins appear different due to this northeasterly trend. However, a similar difference is shown on the left half of Fig. 9, an inverted and obliquity-corrected version of the eastern extremity of the Great Ice Chasm (Fig. 4b). Clearly Figs. 7 and 9 are not identical as ice is a homogeneous medium while West Tasmania basement trends are heterogeneous. The rotation inferred for formation of the Late Jurassic-Early Cretaceous Otway-West Tasmania developing sphenochasm is also expressed sedimentologically. Quilty (1973) said that separation of the originally juxtaposed Kerguelen and Broken Ridge was advanced by Cenomanian-Turonian times as sediments have been found 500 km north of Kerguelen which were
148
Fig. 9. Great Ice Chasm (inverted) with obliquity corrected.
deposited in water “deeper than expected on continental shelves but not under abyssal conditions”. About 2500 km east along the rift valley in the Otway Basin similarly dated Belfast Mudstone was deposited under shallow marine conditions, while further east deposition in the King Island Sub-Basin was nearshore to sub-aerial. These data might suggest that at any instant during rotational displacement, the rift was wider off the southwestern coast of Australia than off the southern coast. The northeasterly trend of normal faulting at Cape Otway extends eastward to the northern Baas Basin and probably to the onshore Gippsland Basin (Fig. 7). In the northern Bass Basin, there is an abrupt change to a southeasterly orientation. These two trends can be identified in the rightcentral part of Fig. 9. This fiie shows that the extensional features east of the Great Ice Chasm (“Otway”) were formed by a component of dextral shear. The attitude of the northwesterly basin-forming faults in Gippsland Basin (Fig. 7) is indicative of a component of dextral shear (Threlfall et al., 1976). The change from an easterly trend of the Great Ice Chasm to the fault trends at its eastern termination was attributed above to the influence of north-trending grounded ice which lay in the path of the Chasm and its Euler Pole. Similarly, as the dextral divergent rotational rift valley of the Otway Basin approached its Euler Pole, perhaps located southeast of Tasmania, it encountered the dominantly north-south grain of the Tasman Orogen (Fig. 7) thereby giving rise to a fracture pattern very similar to that of Fig. 9. As mentioned above, basement heterogeneities are locally very important modifiers of strain, e.g., Precambrian and Palaeozoic trends seen onshore, clearly control some of the offshore West Tasmania basin trends. Hanington et al. (1973) cannot conclusively demonstrate pre-Late Jurassic movement of the
149
“Gippsland Kink Zone” or the “Sorell Lineament” (West Tasmania Basin). However, they did suggest 1100 km of Late Ordovician-Middle Devonian sinistral motion along the “GambierBeaconsfield Lineament” (Otway Basin-Bass Basin). Harrington et al. (1972) extended this lineament westward, parallel to the offshore extension of the southern Australian coast, to southwestern Western Australia. This might imply that the Otway Basin rift valley and its westerly extension were located on pre-existing zone of basement weakness. The fault pattern off the east coast of Tasmania, although poorly controlled suggests a sinistral component to the motion of Lord Howe Rise and New Zealand from this area. Tapering of the Tasman Sea sphenochasm northwards is also indicative of divergent rotation, thereby suggesting a sinistral divergent rotation. This movement does not seem to have affected structures in east Gippsland as the East Gippsland High could be attributed to activation of offshore extensions of southwesterly trending, Palaeozoic structures by dextral divergent motion associated with the movement of Antarctica. Perhaps the departure of Lord Howe Rise in Late Cretaceous times (Hayes and Ringis, 1973 and JOIDES 283) allowed several kilometres of dextral motion on the Cambier-Gabo Lineament. LATE EOCENE-RECENT
DEXTRAL
CONVERGENT
ROTATION
The bulk of the oil and gas in the Gippsland Basin is reservoired in sediments of the Late Cretaceous-Eocene Latrobe Group beneath shales of the Oligocene Lakes Entrance Formation. The unconformity at the top of the Latrobe Group was formed by erosion and/or nondeposition, partly contemporaneous with latest Eocene anticlinal growth. These folds were superimposed by folds of Late Miocene to Recent age (Fig. 10a). If parallel dextral shear was the only factor which controlled fold orientation, one would expect all fold axes to be parallel, and perhaps due to rotation, oriented at a little less than 45” to the direction of shear (A-A). Changes in senses of fault throw along A-A together with adjacent intense deformation provide evidence for east-west directed shear movements. As suggested by Threlfall et al. (1976), this motion is considered to be dextral due to the alignments of Barracouta-Snapper and Marlin-Tuna Anticlines (Fig. 10a) to the zone. Motion on fault A-A and fold growth was contemporaneous. The east-west orientation of Kingfish Anticline is not consistent with easterly directed parallel dextral shear. Rather, it implies that the compressional axis lay somewhat south of southeast. This indicates the deforming stress was a convergent dextral shear. Onshore the Gippsland fold and monoclinal trends (Fig. lob, from Abele et al., 1976) involve rocks as young as Pliocene. Northeasterly to east-northeasterly orientated fold axes are consistent with dextral shear. However, the easterly trending fold axes again indicate a convergent dextral shear. The Kingfish Oilfield (Figs. lla and 10a) shows pronounced folding of
‘%S
SHEAR FAULT ANTICLINAL
TREN#
BALOOK
BLO
Fig. 10. a. Offshore Gipprland rtructure map (After Threlfall et al., 1976). Gippsland structure map (From Abele et al., 1976).
b. Onshore
151
1
a@#
SEtSMIC
TIME SECTION
SE.
x
I
(a)
SW
NI
Fig. 11. a. Kingfish Anticline, Gippsland Basin (From Threlfalf et al,, 19761, b, Seismic time section, Bass Basin. Miocene age, and thinning over the structure near the “Top Eocene” seismic reflector is of latest Eocene-Early Oligocene age. Similar relationships can be demonstrated over the other major oil and gas field such as Halibut, Barracouta, Marlin and Snapper. The northern Bass Basin also lies along the Gambier-Gabo Lineament. ~~~t~~~~~l~
152
The Cormorant Anticline (Fig. lib) is located in a pre-Ohgocene basin which trended a little east of southerly. Thinning of Lower Oligocene and post-Middle Miocene sediments over the structure suggests reversal of stress from extensional to compressional at these times which is identical to the two pulses recognized in Gippsl~d Basin. South-southeasterly trending normal faults moved during the folding pulses and some acted as conduits for intrusive and extrusive igneous activity, This deformation is compatible with south-southeasterly compression. Compression of post-Early Tertiary age is evidenced on numerous seismic lines near the western end of Gambier-Gab0 Lineament at the northern edge of Gambier Embayment. The areal extent and amplitude of compressional structures in Gambier Embayment is much less than in Gippsland. These relationships suggest convergent rotational movement with an Euler Pole west of Gambier Embayment. If this is true, one could expect an intermediate amount of compression between the two areas, viz. in northern Bass Basin. Not only the Cormorant area experienced compression contemporaneously with Gippsland as Abele et al. (1976) noted Early Oligocene upfift in the Torquay Basin and the Otway Ranges probably underwent a major compression in late Early Miocene. The sum total of compression in the general North Bass area is probably a little less than in Gippsland and certainly greater than in Gambier Embayment. The south-southeasterly directed compression imposed a component of dextral shear on the almost east-trending Gambier-Gabo Lineament which was a Late Eocene-Early Oligocene and Late MioceneRecent zone of dextral convergent rotation, i.e., it was a sphenopeism or a “compression wedge” (Carey, 1958). In the terminology of this paper the component of dextral rotation indicates that it was a “dextral sphenopeism”. INFLUENCE OF STRUCTURAL CONTROL ON HYDROCARBON OCCURR3ZNCE Significant accumulations of hydrocarbons in Gippsland Basin are related to compressional anticlines. Late Jurassic-Middle Eocene dextral divergent rotation created the sedimentary basins and Late Eocene-Recent dextral convergent rotation created structures which trap the bulk of these hydrocarbons. Although anticlines of similar age are present in the Bass Basin, the lack of significant accumulations could be attributed to other factors such as maturity of the hydrocarbon source rocks. The lack of compressional anticlines in the Otway Basin should not be thought of as being the only cause for the lack of signifieant accumulations. Rather the inadequate trapping mechanisms of many of the normal fault closures could be a contributing factor. Analysis of an ice model enabled an Euler Pole for Late Jurassic-Middle Eocene dextral divergent rotation to be postulated southeast of Tasmania. It can only be said that the Euler Pole for Late Eocene-Early Oligocene and Late Miocene-Recent dextral convergent rotation lay west of Gambier Embayment.
153
It would seem appropriate to use Carey’s (1958) terms of “sphenochasm” when the zone of divergent or convergent rotation and “sphenopeism” parallels a radius about the Euler Pole and the terms “dextral or sinistral sphenochasm” or “dextral or sinistral sphenopeism” when the axis crosses radii about the Euler Pole. ACKNOWLEDGEMENTS
While at the University of Tasmania, the writer was fortunate to study under such an inspiring person as Professor S. Warren Carey with whom warm personal ties have been maintained. Esso Australia Ltd., has benefited from its contact with Professor Carey, and together with the writer, wish him every success with his numerous endeavours planned for his retirement. This paper utilizes the work of many people of Esso Australia Ltd. Special thanks are due to Andrew J. Rigg who helped with compilation of the paper. The paper is presented with the permission of Esso Australia Ltd., and the Broken Hill Proprietary Company. REFERENCES Abele et al., 1976. Tertiary. In: J.C. Douglas and J.A. Ferguson (Editors), Geology of Victoria. Geol. Sot. Aust. Spec. Pub., 5: 528. Beydoun, Z.R., 1970. Southern Arabia and Northern Somalia: comparative geology. Philos. Trans. R. Sot. London, Ser. A., 267: 267-292. Carey, S.W., 1958. The tectonic approach to continental drift. In: Continental Drift -A Symposium. University of Tasmania, Hobart (1958), pp. 177-363. Carey, S.W., 1976. The Expanding Earth, Developments in Geotectonics, 10. Elsevier, Amsterdam-Oxford-NewYork, N.Y., 488 pp. Fuchs, Sir Vivian and Hilary, Sir Edmund, 1958. The Crossing of Antarctica. The Commonwealth Trans-Antarctic Expedition 1955-58. Cassell, London, 338 pp. Harrington, H.J., Burns, K.L. and Thompson, B.R., 1973. Gambier-Beaconsfield and Gambiei-Sore11 fracture zones and the movement of plates in the Australia-Antarctica-New Zealand region. Nature Phys. Sci., 245: 109-112. Hayes, D.E. and Ringis, J., 1973. Seafloor spreading in the Tasman Sea. Nature, 243: 454-458. Le Pichon, X., Francheteau, J. and Bonnin, J., 1973. Plate Tectonics. Developments in Geotectonics, 6, Elsevier, Amsterdam-Oxford-New York, N-Y., 300 pp. Neuberg, H-AC., Thiel, E., Walker, P.T., Behrendt, J.C. and Aughenbaugh, N.B., 1969. The Filchner Ice Shelf. Ann. Assoc. Amer. Geogr., 49: 110-119. Quilty, P.G., 1973. Cenomanian~uronian and Neogene sediments from northeast of Kerguelan Ridge, Indian Ocean. J. Geol. Sot. Aust., 20 (3): 361-367. Threlfall, W.F., Brown, B.R. and Griffith, B.R., 1976. Gippsland Basin offshore. In: Economic Geology of Australia and New Guinea, 3. Petroleum. Australas. Inst. Min. Metall., pp. 41-67. Wilson, G., 1960. The tectonics of the “Great Ice Chasm”, Filchner Ice Shelf, Antarctica. Proc. Geol. Assoc., 71: 130-138.