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Abstract

The Ceduna subbasin forms part of the under-explored but highly prospective frontier Bight Basin located on the southern margin of South Australia. Structural mapping of the Ceduna subbasin reveals two separate delta lobes/systems deposited in the late Albian-Santonian and late Santonian-Maastrichtian. Each system is comprised of an updip delta top linked via a shale detachment to a down-dip delta toe or deep-water fold-thrust belt. These delta lobes are separated by a transgressive sequence of Turonian-Santonian age that deposited a thick marine mud, up to 2000 m in places. Like the Niger Delta, this marine mud forms the detachment for the overlying Santonian-Maastrichtian delta—deep-water fold-thrust belt system and is also a proposed source rock. An Albian marine mud forms the detachment for the older Cenomanian system and is also thought to exhibit source rock potential.

We examine the differences in structural style between the western and eastern parts of the basin, the west being dominated by the Cenomanian lobe and the east by the Santonian-Maastrichtian lobe. Recent work on the sedimentary provenance of the basin suggests two different mechanisms were responsible for deposition of the delta lobes in the west and east, as well as significant changes in regional tectonics dominating the basin fill history. This has resulted in the deposition of two delta—deep-water fold-thrust belt systems rather than a continuous system as is commonly observed elsewhere in Cenozoic analogues such as the Niger Delta. Evidence from the drilling of Gnarlyknots-1A on the delta-top suggests excellent reservoir quality in the Santonian-Maastrichtian system; the potential for seal and source development increases farther offshore toward the deep-water fold-thrust belts. The abundant availability of deep-water contractional targets, combined with modeled increase in source and sealing potential farther offshore, results in a highly prospective system in both the Cenomanian and Santonian-Maastrichtian deep-water fold-thrust belts.

Introduction

The Ceduna subbasin forms part of the larger Bight Basin located along the southern margin of Australia, between Kangaroo Island and Cape Leeuwin (Western Australia), covering a distance in excess of 3000 km (Fig. 1). The Ceduna subbasin is significantly underexplored, yet it is a prospective hydrocarbon province that extends along the coast of South Australia over a distance of ~ 700 km from west to east and offshore for hundreds of kilometers. Only two exploration wells have been drilled in the Ceduna subbasin, which covers an area of approximately 90,000 km2 and water depths ranging from 200 m to more than 5,000 m (Fig. 1).

Figure 1.

Location map of the Great Australian Bight (A) showing extent of Bight Basin (top right inset in green), the position of the Ceduna, Recherche, Polda, Eyre and Duntroon subbasins and the position of the two wells drilled in the Ceduna subbasin (Potoroo-1 and Gnarlyknots-1; Figure modified with permission from South Australian Geological Survey). (B) A digital elevation model of South Australia shows the broad bathymetry of the Ceduna Terrace and position of 3D Trim seismic data (black rectangle), 2D regional seismic lines (white dashed), White Pointer (light grey) and Hammerhead (dark grey) deep-water fold-thrust belt limits. Approximate position of delta lobe limits are outlined (yellow dashed line) and overlap zone on delta-top is labeled. Thick yellow arrows represent inferred sediment input direction based on geometry of deep-water fold-thrust belts (modified with permission after MacDonald et al., 2012).

Figure 1.

Location map of the Great Australian Bight (A) showing extent of Bight Basin (top right inset in green), the position of the Ceduna, Recherche, Polda, Eyre and Duntroon subbasins and the position of the two wells drilled in the Ceduna subbasin (Potoroo-1 and Gnarlyknots-1; Figure modified with permission from South Australian Geological Survey). (B) A digital elevation model of South Australia shows the broad bathymetry of the Ceduna Terrace and position of 3D Trim seismic data (black rectangle), 2D regional seismic lines (white dashed), White Pointer (light grey) and Hammerhead (dark grey) deep-water fold-thrust belt limits. Approximate position of delta lobe limits are outlined (yellow dashed line) and overlap zone on delta-top is labeled. Thick yellow arrows represent inferred sediment input direction based on geometry of deep-water fold-thrust belts (modified with permission after MacDonald et al., 2012).

The Ceduna subbasin contains two spatially and temporally separate Cretaceous delta—deep-water fold-thrust belt (DDWFTB) systems: the late Albian-Santonian White Pointer and the late Santonian-Maastrichtian Hammerhead. These systems detach above the mud-rich Blue Whale and Tiger supersequences, respectively (Fig. 2). The total thickness of Middle Jurassic to Cenozoic sedimentary rocks exceeds 12 km (Struckmeyer et al., 2001; Krassay and Totterdell, 2003).

Figure 2.

Litho- and sequence stratigraphic framework of the Bight Basin showing supersequences described in text and interpreted basin phases and sea level curve from Totterdell et al., 2000. (Modified with permission from Totterdell et al., 2008.)

Figure 2.

Litho- and sequence stratigraphic framework of the Bight Basin showing supersequences described in text and interpreted basin phases and sea level curve from Totterdell et al., 2000. (Modified with permission from Totterdell et al., 2008.)

Previous work completed by Geoscience Australia (in 1999–2008) and Woodside (including drilling of the most recent well, Gnarlyknots–1A) resulted in an improved understanding of the hydrocarbon prospectivity of the Bight Basin, particularly the Ceduna subbasin. This work focused on the sequence stratigraphy, tectonics, and petroleum systems of the Bight Basin to identify organic-rich supersequences, understand the regional maturity of these rocks, and identify suitable traps and seals for hydrocarbon accumulation (Blevin et al., 2000; Totterdell et al., 2000; Somerville, 2001; Ruble et al., 2001; Struckmeyer et al., 2001; Sayers et al., 2001; Struckmeyer et al., 2002; Krassay and Totterdell, 2003; Totterdell and Krassay, 2003; Totterdell and Bradshaw, 2004; King & Mee, 2004; Tapley et al., 2005; Totterdell et al., 2008). Although significant work as been completed, many questions remain unanswered such as the source of the delta lobes, why is there an ~8 m.y. hiatus between deposition of the lobes, the relationship between the extensional, transitional and compressional provinces of the delta lobes, and how the overlapping delta tectonics affect prospectivity in the region.

In this paper, we use recently acquired 2D seismic reflection data to investigate the change in structural style from west to east in the basin, mainly by examining the structure of the two delta lobes, particularly their individual deep-water fold-thrust belt geometries. Previous work completed on the provenance of the delta lobes indicates a significant change in tectonic setting along the margin in the ~8 million years (m.y.) period between deposition of the two lobes (MacDonald et al., in review). This has a significant impact on the distribution of sediment in the basin between the Cenomanian and Maastrichtian, resulting in switching of lobe orientation and concentration of sediment from west to east throughout the late Cretaceous. The impact on the delta lobe structural geometry is discussed and implications for prospectivity are addressed given the increased understanding of tectonic history along the margin and availability of new seismic data to examine the interaction between the two delta lobes.

Tectono-Stratigraphic Framework

The Bight Basin developed during Late Jurassic to Early Cretaceous rifting and continued to develop during the breakup of Gondwana, separating Australia and Antarctica during the Late Cretaceous (Totterdell et al., 2000; Sayers et al., 2001). Santonian-Campanian break-up resulted in the separation of the basin into three structurally controlled subbasins: the Eyre, Ceduna, and Recherche (Fig. 1A). The Ceduna subbasin is the largest depocenter and the focus of this study (Totterdell et al., 2000; Krassay and Totterdell, 2003). The subbasins are separated by northwest striking accommodation zones (right-lateral), which were formed by the rifting and crustal thinning of the Australian Plate (Stagg et al., 1990; Wilcox and Stagg, 1990; and Totterdell et al., 2000).

The Ceduna subbasin is bound to the north by Proterozoic and older terrains and to the east by the Proterozoic of the Gawler Craton (Krassay and Totterdell, 2003). The Eyre subbasin defines the western limit of the Ceduna subbasin while the Duntroon Basin forms the southeast boundary, and to the south it tapers out onto the thin oceanic crust of the South Australian Abyssal Plain (Fraser and Tilbury, 1979; Bein and Taylor, 1981; Willcox and Stagg, 1990; Hill, 1995; Norvick and Smith, 2001; Totterdell and Krassay, 2003; Fig. 1).

The stratigraphy of the Ceduna subbasin is comprised of 10 supersequences, which relate to the four Bight Basin phases first described by Totterdell et al. (2000; Fig. 2). The subbasin evolution involved two successive periods of extension and thermal subsidence that commenced in the Late Jurassic (Totterdell et al., 2000; Totterdell and Krassay, 2003). Basin Phase 1 marks the onset of sedimentation in the Bight basin during the Middle–Late Jurassic extension and the contemporaneous formation of a series of extensional and transtensional half-graben structures. Pre-existing basement trends appear to have focused the deformation during this phase (Totterdell et al., 2000). Basin Phase 2 records the slow thermal subsidence during the Early Cretaceous, which ended abruptly with the onset of rapid subsidence (Basin Phase 3) during the Late Albian (Totterdell et al., 2000). Sea-floor spreading commenced during the Late Santonian, resulting in breakup between Australia and Antarctica marking the end of Basin Phase 3. A period of thermal subsidence (Basin Phase 4) followed, which marks the initiation of this region as a true passive margin (Totterdell and Krassay, 2003). These four Bight basin phases are observed in the Ceduna subbasin (Fig. 2).

Of the ten supersequences defined in Totterdell et al. (2000) the supersequences of most importance to this study are the stratigraphic units that comprise the two DDWFTB systems and their respective detachments. At the northern margin of the Ceduna subbasin these include: the Albian Blue Whale supersequence, Cenomanian White Pointer supersequence, Turonian-Santonian Tiger supersequence, Santonian-Maastrichtian Hammerhead supersequence, and the Cenozoic rocks above the intra-Maastrichtian regional unconformity (Fig. 2). Basin modeling of the distal parts of the subbasin indicates that the Blue Whale, White Pointer, and Tiger supersequences are most prospective in terms of source rocks (Blevin et al., 2000).

The supersequence lithology descriptions below come from Totterdell et al. (2000), who used well data from the Bight, Eyre, and Duntroon subbasins (Fig. 1). The oldest unit relevant to this study is the Middle Albian–Cenomanian Blue Whale supersequence, which records the first major marine flooding event with the deposition of restricted marine siltstones nearest the paleo-shelf and an inferred thick package of marine mudstones in the basin. This package of marine mudstone forms the detachment for the overlying Cenomanian White Pointer DDWFTB, comprising the White Pointer supersequence (Totterdell and Bradshaw, 2004). Rapid (~5 m.y.) deposition of the aggradational White Pointer supersequence likely produced overpressure in the underlying ductile marine mudstone enabling gravity-driven deformation, which resulted in the formation of growth faults and toe thrusts that sole out or are rooted in the Blue Whale mudstone and form the White Pointer DDWFTB (Totterdell and Krassay, 2003). The White Pointer supersequence is composed primarily of fluvial to lagoonal siltstone and mudstone intercalated with minor sandstone and coal units. Dewatering of the underlying shale has resulted in a loss of overpressure (Totterdell et al., 2000; Totterdell and Krassay, 2003) and provides the likely mechanism for the cessation of all gravity-driven deformation.

Overlying the White Pointer DDWFTB is the Turonian– Santonian Tiger supersequence, containing an aggradational package of what is suggested to be marine shale based on seismic sequence stratigraphy and extrapolation of well data from Potoroo–1 in the northern Ceduna subbasin (Totterdell et al., 2000). The unit is heavily faulted in the Ceduna bub-basin due to reactivation of older faults (Cenomanian), controlled by the underlying ductile shales of the Blue Whale supersequence. The timing of this fault reactivation is well constrained to the Late Santonian as this faulting had ceased before deposition of the overlying Hammerhead supersequence (Totterdell et al., 2000).

The Santonian–Maastrichtian Hammerhead supersequence has an overall progradational to aggradational character and is believed to be almost entirely composed of channel sandstones near the present-day shelf in the Ceduna subbasin (Totterdell et al., 2000). The 19 m.y. period of progradation of the Hammerhead DDWFTB across the paleo-shelf has resulted in the initiation of gravity-driven deformation with the top of the underlying Tiger supersequence forming the detachment surface (Totterdell and Krassey, 2003; King and Backé, 2010; MacDonald et al., 2010). The gravitational tectonics initiated at the paleo-shelf margin, where the Hammerhead supersequence is the thickest (~5,000 m at this location; Totterdell and Krassay, 2003).

The Hammerhead supersequence is overlain by the Paleocene–early Eocene Wobbegong supersequence, which consists of marginal marine-to-deltaic sandstone and siltstone that was deposited on a hiatus, which is believed to represent 5–7 m.y. Above this, the middle Eocene–Pleistocene Dugong supersequence is present, consisting of a basal coarse sandstone and thick uniform cold water carbonate succession, which is related to the development of a stable carbonate shelf. This regional unconformity can be traced across the Ceduna subbasin and the thin Cenozoic sediments are incised by deep submarine canyons in some areas allowing for dredging of late Cretaceous reservoir and source rocks by Geoscience Australia (Totterdell et al., 2008; Fig. 1B).

Background: Delta—Deep-Water Fold-Thrust Belt Systems

Delta—deep-water fold-thrust belt systems have been the subject of extensive research over the last two decades given the increase in data availability and significant hydrocarbon reserves they contain. Two such examples are the Niger (e.g., Doust and Omatsola, 1990; Morley and Guerin, 1996; Bilotti and Shaw, 2005; Briggs et al., 2006; and Cobbold et al., 2009) and Baram DDWFTBs (e.g., Tingay et al., 2009; King et al., 2010; Morley et al., 2011). Delta—deep-water fold-thrust belts typically form at continental margins where rapid progradation of deltaic sediments over salt or water-saturated mud results in overpressure development (in water-saturated mud) and deformation under gravitational forces (Morley, 2003; Morley and Guerin, 1996; Rowan et al., 2004). The result is a broad segregation of the delta into extensional and compressional provinces, whereby margin-parallel gravitational extensional stresses on the delta top drive down-dip, margin-normal, compressional stresses in the deep-water fold-thrust belt (Yassir and Zerwer, 1997; Corredor et al., 2005; King et al., 2009).

The delta-top is typically characterized by regional and counter-regional listric, normal, growth faults that sole out at the level of the underlying prodelta sediments (salt or shale; Mandl and Crans, 1981; Morley and Guerin, 1996; Rowan et al., 2004). These extensional structures are in theory balanced by the deep-water fold-thrust belt, which is composed of imbricate thrust sheets and associated fault-propagation folds rooted at the basal detachment (Morley and Guerin, 1996; McClay et al., 2003; King et al., 2009). The extensional and compressional provinces are commonly, but not always, separated by a transitional province that is characterized by detachment folds, salt diapirs, and sometimes progressively rotated and abandoned near-vertical dipping thrust faults (Jackson et al., 1994; Morley and Guerin, 1996; Briggs et al., 2006).

Shale detachment horizons are dependent on the development and maintenance of overpressure, and rapid progradation of delta sediments over the water-saturated mud contributing to the initial disequilibrium compaction overpressure of the muds (Morley et al., 2008; Morley et al., 2011). Overpressure is not required for salt as it is naturally weak and mobile, whereas shale requires overpressure to induce weakness and mobility (Davis and Engelder, 1985; Morley and Guerin, 1996; Costa and Vendeville, 2002; Rowan et al., 2004; Bilotti & Shaw, 2005). Subsequent inflationary overpressures can also develop, most often by dewatering during the smectite-illite transition and volume increase due to hydrocarbon generation; these serve to maintain overpressure (Morley and Guerin, 1996; Osborne and Swarbrick, 1998; Morley et al., 2011). Variations in sediment supply and detachment parameters, such as lithology and thickness of the detachment horizon(s), and development of overpressure are some of the key factors that contribute to the varied structural styles observed in DDWFTBs worldwide.

Delta Tectonics of the Ceduna Subbasin

The Late Jurassic to Cenozoic Ceduna subbasin contains two Late Cretaceous DDWFTB systems: the Albian-Cenomanian White Pointer and the Santonian-Maastrichtian Hammerhead. This basin provides a unique opportunity to study two separate systems that are independent in size, shape, and structural geometry. Their geometry of these extensive DDWFTBs was first recognized by Boeuf and Doust (1975) and Fraser and Tilbury (1979) and have since been investigated by several authors including Totterdell and Krassay (2003), Totterdell and Bradshaw (2004), Espurt et al., (2009), King & Backé (2010), and MacDonald et al., (2010; 2012). Although the delta-top setting has been the subject of increased research due to its easier accessibility for exploration with shallower water depths and closer proximity to shore, the deep-water fold-thrust belts have been only briefly investigated using seismic data due to their extreme frontier nature. A very limited data set exists for the basin as only two exploration wells drilled (Potoroo-1 and Gnarlyknots-1A; Fig. 1) and there is sparse 2D seismic reflection data over most of the basin as well as a very limited 3D seismic survey.

A recent regional 2D seismic dataset has been acquired by Ion Geophysical (BightSPAN©), and BP Developments Australia Pty Ltd has just finished shooting an extensive 3D seismic dataset in the southwestern part of the basin. The deep-water fold-thrust belts currently lie in ~1500-5000 m of water making them high risk exploration targets in a currently unproven potential petroleum system. Here we focus primarily on the structural interpretation of the White Pointer and Hammerhead deep-water fold-thrust belts. The interpretation is based on recently acquired 2D Bight-SPAN© and existing vintage seismic data.

The Cenomanian White Pointer DDWFTB system

The aggradational White Pointer delta system was rapidly deposited above the regional Albian marine mud of the Blue Whale supersequence, resulting in development of a linked system of extension and compression (Totterdell et al., 2000). Figure 3 is a regional BightSPAN© 2D seismic line located in the western Ceduna subbasin, which illustrates the regional geometry of the White Pointer system, as well as its relationship to the younger Hammerhead delta system. In the western part of the basin the Hammerhead supersequence is present; however, it lacks the deformation required to classify it as a DDWFTB system (MacDonald et al., 2010). Thus, we classify this as purely chronostratigraphic Hammerhead Supersequence rather than part of the Hammerhead DDWFTB system for the purpose of understanding and dividing the delta based on gravitationally-driven tectonic controls. In the eastern Ceduna subbasin the Hammerhead supersequence is of sufficient thickness to initiate gravitational collapse on the delta-top driving down-dip compression in the delta toe (Fig. 4). This results in a well established DDWFTB system in the eastern part of the subbasin, and will be discussed in detail below.

Figure 3.

Interpreted regional 2D seismic line through the western Ceduna subbasin showing well developed extensional and compressional provinces of the White Pointer delta—deep-water fold-thrust belt system (blue). The Hammerhead supersequence (purple) does not form a DDWFTB system here due to insufficient thickness to initiate gravitational sliding. Cenozoic strata (yellow) form a thin veneer above the intra-Maastrichtian unconformity (between purple and yellow; modified with permission after MacDonald et al., 2012).

Figure 3.

Interpreted regional 2D seismic line through the western Ceduna subbasin showing well developed extensional and compressional provinces of the White Pointer delta—deep-water fold-thrust belt system (blue). The Hammerhead supersequence (purple) does not form a DDWFTB system here due to insufficient thickness to initiate gravitational sliding. Cenozoic strata (yellow) form a thin veneer above the intra-Maastrichtian unconformity (between purple and yellow; modified with permission after MacDonald et al., 2012).

Figure 4.

Interpreted regional 2D seismic line through the eastern Ceduna subbasin showing preserved extensional province of the White Pointer DDWFTB (blue) and well developed extensional, transitional and compressional provinces of the Hammerhead delta—deep-water fold-thrust belt system (purple) detaching above the Turonian-Santonian Tiger supersequence (green). The Hammerhead supersequence (purple) forms a classical DDWFTB system nearest the paleoshelf edge due to increased sediment deposition in the southeastern Ceduna subbasin. A significant fault scarp or slump is exposed along the Eocene extensional fault which dissects the delta-toe and detachment of the Hammerhead DDWFTB system and likely forms a boundary between the continental and oceanic crust. A significant unconformity at the top of the White Pointer extensional province may indicate pre-Turonian exhumation of a paleodeep-water fold-thrust belt that is no longer visible. Projected position of Gnarlyknots-1A is indicated at top of section on the delta-top.

Figure 4.

Interpreted regional 2D seismic line through the eastern Ceduna subbasin showing preserved extensional province of the White Pointer DDWFTB (blue) and well developed extensional, transitional and compressional provinces of the Hammerhead delta—deep-water fold-thrust belt system (purple) detaching above the Turonian-Santonian Tiger supersequence (green). The Hammerhead supersequence (purple) forms a classical DDWFTB system nearest the paleoshelf edge due to increased sediment deposition in the southeastern Ceduna subbasin. A significant fault scarp or slump is exposed along the Eocene extensional fault which dissects the delta-toe and detachment of the Hammerhead DDWFTB system and likely forms a boundary between the continental and oceanic crust. A significant unconformity at the top of the White Pointer extensional province may indicate pre-Turonian exhumation of a paleodeep-water fold-thrust belt that is no longer visible. Projected position of Gnarlyknots-1A is indicated at top of section on the delta-top.

The White Pointer DDWFTB can be divided into two tectonic provinces: an extensional province, associated with the delta top and a compressional province, referred to as a delta-toe or deep-water fold-thrust belt (Fig. 3). As mentioned beforehand, delta systems often exhibit a transitional province dominated by detachment folding, diapir growth, or less often no deformation at all (Morley and Guerin, 1996; Bilotti and Shaw, 2005). Figure 3 illustrates the linked White Pointer delta system in its entirety from the present day shelf break to the Australian abyssal plain (Fig. 1B). The transitional province is poorly developed but is indicated in the figure nonetheless.

The extensional province of the White Pointer DDWFTB is dominated by a series of northwest-southeast striking, seaward dipping, strongly listric, normal, growth faults that were active in the Cenomanian (Totterdell and Krassay, 2003). This fault system is widespread, displays up to 1500-2500m of vertical displacement, are spaced 5-10 km apart (as determined from depth-converted seismic of Totterdell and Krassay, 2003; Fig. 3), and control the depocenters for deltaic sediments of the White Pointer supersequence. These large-scale faults sole out in the underlying detachment within the Blue Whale supersequence (Fig. 3); interestingly, there are very few counter-regional faults developed in the extensional province of the White Pointer DDWFTB (Fig. 3). Totterdell and Krassay (2003) suggest that the often thin substrate of the Blue Whale (average 2 km thickness) inhibited the formation of these counter-regional faults that are commonly observed where DDWFTBs overlie thick shales (up to 6 km thick) such as in the Niger DDWFTB (Morley et al., 2011).

The compressional province of the White Pointer DDWFTB (Figs. 3, 5, 6, and 7) is best imaged and most impressive in the farthest southwest extent of the Ceduna subbasin (Fig. 1B), where the deep-water fold-thrust belt is nearly 100 km wide and contains abundant imbricate thrust sheets (Figs. 5, 6). The thrust faults and associated folding illustrated in Figures 3, 5, and 7 are slightly skewed due to the oblique angle of the regional seismic line with respect to the transport direction of the delta lobe thrust sheets (south-southwest versus southwest; Fig. 1B). The result is a steeper apparent dip of the thrust faults and poor seismic resolution nearest the faults as these structures are best imaged when the seismic line is oriented orthogonal to the thrust vergence (i.e., Fig. 6). Therefore, fault trajectories on oblique seismic lines have been interpreted based on geometry of hanging wall and footwall strata that bound the faults (Figs. 3, 5, and 7).

Figure 5.

Seismic section through the southwestern White Pointer DDWFTB showing extensive volcanic intrusions exploiting deep-water fold-thrust belt structures and skewing the seismic data. Forced folds, saucer shaped sills, and volcanic vents interpreted after Holford et al., 2012, with permission.

Figure 5.

Seismic section through the southwestern White Pointer DDWFTB showing extensive volcanic intrusions exploiting deep-water fold-thrust belt structures and skewing the seismic data. Forced folds, saucer shaped sills, and volcanic vents interpreted after Holford et al., 2012, with permission.

Figure 6.

Seismic section through the southwestern White Pointer DDWFTB showing regularly spaced thrust faults in the deep-water fold-thrust belt and thrust related features such as synkinematic onlap, ponding in thrust hanging-wall, and position of seaward dipping detachment zone within the Albian Blue Whale supersequence. Chaotic seismic reflections may indicate the presence of deep-water marine muds as suggested by Totterdell et al., 2000.

Figure 6.

Seismic section through the southwestern White Pointer DDWFTB showing regularly spaced thrust faults in the deep-water fold-thrust belt and thrust related features such as synkinematic onlap, ponding in thrust hanging-wall, and position of seaward dipping detachment zone within the Albian Blue Whale supersequence. Chaotic seismic reflections may indicate the presence of deep-water marine muds as suggested by Totterdell et al., 2000.

The relatively chaotic and poorly imaged transitional province illustrated in Figures 5 and 7 appears to be highly deformed by steeply dipping thrust faults, possibly due to sequential thrust stacking and steepening landward and/or subseismic scale shear folding/ faulting. Totterdell and Krassay (2003) suggest that possible mobile shales and diapiric structures in the transition zone of the White Pointer Delta indicate excessive overpressure similar to the Niger Delta. However, we suggest that although some shale mobilization may have occurred, these structures are more suitably attributed to reverse/thrust faulting and detachment folding, as fold-limb reflectors are observed throughout (Fig. 5). It is likely that the poorly imaged transition zone in the White Pointer DDWFTB is an artifact of minor offset thrust splays coupled with minor detachment folding working to accommodate strain seaward of the extensional faults rather than forming a large domal detachment fold. In addition to this, Figures 5 and 7 clearly image Cenozoic volcanic intrusions that are manifesting themselves within the detachment of the Albian shales and even exploiting thrust faults and particular stratigraphic units within the DDWFTB system.

Figure 7.

Seismic section through the southern White Pointer deep-water fold-thrust belt showing thick detachment zone in the Albian Blue Whale supersequence where listric, normal growth faults of the extensional province and down-dip thrust faults detach. Crestal collapse faulting is observed above a single fault bend fold and volcanic intrusions/vents are indicated. Dashed white lines represent inferred thrust faults where seismic resolution is insufficient for confident interpretation.

Figure 7.

Seismic section through the southern White Pointer deep-water fold-thrust belt showing thick detachment zone in the Albian Blue Whale supersequence where listric, normal growth faults of the extensional province and down-dip thrust faults detach. Crestal collapse faulting is observed above a single fault bend fold and volcanic intrusions/vents are indicated. Dashed white lines represent inferred thrust faults where seismic resolution is insufficient for confident interpretation.

Farther outboard of the poorly imaged transitional zone, the compressional province tends to manifest well illustrated and relatively uncomplicated thrust faulting and associated fault bend folding (Figs. 5, 6, 7). The imbricate thrust sheets are spaced 2-10 km apart, variability being controlled by the dip and thickness of the detachment horizon. Position of thrust ramps also has an effect on thrust dip and fault spacing, as sudden changes in detachment geometry are reflected in adjacent structures (Fig. 5). Footwall fault panels rarely display minor back-thrusting between thrust ramps (Fig. 5) and are gently folded, rarely with small antiforms that show limited evidence for crestal collapse faulting (Fig. 7). Ponded minibasins in the footwall synclines are common, where growth strata slump off the anticline forelimb with ongoing deformation. These minibasins are also present on the back limbs of major hanging-wall anticlines, the synkinematic sedimentary package typically sealing the fault tip of the preceding thrust fault (Figs. 6, 7).

The detachment zones in the White Pointer deep-water fold-thrust belt dip moderately basinward and occur at all levels within the Albian shales; however, it is difficult to place the detachment exactly due to the amount of compaction, dewatering, and uplift that the DDWFTB systems have witnessed post Maastrichtian (i.e., continental crust uplift in Fig. 3). Volcanic sills have almost certainly exploited the basal detachment in some areas as the impedance contrast in the seismic data and position of thrust faults clearly demonstrates this (Fig. 5). The seaward dip of the detachment surface may be an artifact of regional uplift post deposition or it may be an intact geometry, it is impossible to tell with the current dataset.

In the center of the Ceduna subbasin, the White Pointer deep-water fold-thrust belt is no longer discernible on the seismic data, and only one seismic line in the basin clearly images the overlap of both the White Pointer and Hammerhead deep-water fold-thrust belts (Fig. 8). Mapping of all available 2D seismic data for the Ceduna subbasin allows us to map the extent of each deep-water fold-thrust belt, with the White Pointer concentrated in the west and the Hammerhead in the east (Fig. 1B); however, the extensional overlap zone is geographically extensive across the delta top (Fig. 1B). Both the White Pointer and Hammerhead supersequences are mapable across the entire Ceduna subbasin.

Figure 8.

Seismic line through the central Ceduna subbasin which illustrates the only known example of two stacked DDWFTB systems in the region. The White Pointer DDWFTB system remains intact below the younger Hammerhead DDWFTB which was deposited after a ~8 m.y. hiatus. The Ablian Blue Whale supersequence (pink) forms the detachment zone for the White Pointer DDWFTB while the Turonian-Santonian Tiger supersequence forms the detachment zone for the Hammerhead DDWFTB system. The Hammerhead DDWFTB system displays a clear detachment fold province whereas the White Pointer DDWFTB system does not. This region represents the only area where the two deep-water fold-thrust belts overlap in the Ceduna subbasin.

Figure 8.

Seismic line through the central Ceduna subbasin which illustrates the only known example of two stacked DDWFTB systems in the region. The White Pointer DDWFTB system remains intact below the younger Hammerhead DDWFTB which was deposited after a ~8 m.y. hiatus. The Ablian Blue Whale supersequence (pink) forms the detachment zone for the White Pointer DDWFTB while the Turonian-Santonian Tiger supersequence forms the detachment zone for the Hammerhead DDWFTB system. The Hammerhead DDWFTB system displays a clear detachment fold province whereas the White Pointer DDWFTB system does not. This region represents the only area where the two deep-water fold-thrust belts overlap in the Ceduna subbasin.

We recognize two distinct delta systems (White Pointer and Hammerhead) due to their differing age, seismic character, structural styles, and difference in thrust vergence between the two systems (MacDonald et al., 2010). Figure 8 illustrates the existence of a White Pointer deep-water fold-thrust belt, albeit the section is again oblique to the thrust vergence, so the faults appear steeper and lower resolution then in Figure 6. From west to east in the subbasin, the Hammerhead supersequence increases in thickness and develops a linked system with the formation of the deep-water fold-thrust belt (Fig. 8). The Hammerhead deep-water fold-thrust belt is developed directly above the White Pointer deep-water fold-thrust belt forming an overlapping system in this unique “overlap zone” (Fig. 1B). In Figure 8 the thickness of the Tiger supersequence, which forms the detachment for the Hammerhead DDWFTB, increases to the north and east, accommodating the increased deformation in the Hammerhead to the east (Fig. 4). In this section, the entire outboard part of the DDWFTB systems appears to have been uplifted post Maastrichtian, resulting in bending and warping of the detachment causing it to dip to the north and south.

The Santonian-Maastrichtian Hammerhead DDWFTB system

In the central-eastern Ceduna subbasin there is regionally extensive linked system of extension and compression within the Hammerhead supersequence, forming a classical DDWFTB system (Totterdell et al., 2000; King et al., 2009; MacDonald et al., 2010; Fig. 9). Unlike the White Pointer system, which dominates the western part of the subbasin, the Hammerhead DDWFTB contains a well-developed extensional, transitional and compressional province (Fig. 10). The regional seismic line through the eastern part of the Ceduna subbasin (Fig. 4) illustrates the unstable nature of the DDWFTB in this region, where subsequent extensional faulting related to continued Eocene sea-floor spreading has dissected the deep-water fold-thrust belt entirely. The Hammerhead deep-water fold-thrust belt in this region is directly overlying the continent-ocean transition zone (defined by Totterdell and Krassay, 2003), where uplifted continental crust inboard of the regional extensional fault (Figs. 4, 11) is responsible for uplift of the transitional detachment fold province in the Hammerhead DDWFTB. On the basinward side of the same fault, possible graben reactivation is a likely mechanism for the observed uplift of the deep-water fold-thrust belt (Figs. 10, 11) and outboard of the ocean-continent boundary Eocene volcanic cones dominate the Australian abyssal plain (Fig. 11).

Figure 9.

Seismic section through the eastern Ceduna subbasin showing extremely well developed Hammerhead DDWFTB system as well as up-dip extension, a detachment fold province, and down-dip compression in the delta-toe. Relict White Pointer extensional faults have been interpreted below the detachment fold province, but there is no evidence for a White Pointer deep-water fold-thrust belt present in the section.

Figure 9.

Seismic section through the eastern Ceduna subbasin showing extremely well developed Hammerhead DDWFTB system as well as up-dip extension, a detachment fold province, and down-dip compression in the delta-toe. Relict White Pointer extensional faults have been interpreted below the detachment fold province, but there is no evidence for a White Pointer deep-water fold-thrust belt present in the section.

Figure 10.

Seismic section through the eastern Ceduna subbasin imaging the Hammerhead deep-water fold-thrust belt; an extensive detachment fold is visible on the north side of the section. The detachment fold exhibits a well imaged back thrust and is most likely cored by marine mud of the Tiger supersequence, which forms the regional detachment zone (green) for the extensional and compressional provinces. Possible uplift by graben reactivation has affected the fold-thrust belt in this region.

Figure 10.

Seismic section through the eastern Ceduna subbasin imaging the Hammerhead deep-water fold-thrust belt; an extensive detachment fold is visible on the north side of the section. The detachment fold exhibits a well imaged back thrust and is most likely cored by marine mud of the Tiger supersequence, which forms the regional detachment zone (green) for the extensional and compressional provinces. Possible uplift by graben reactivation has affected the fold-thrust belt in this region.

Figure 11.

Seismic section through the complexly faulted eastern Ceduna subbasin showing the Hammerhead DDWFTB system and extensional growth faults on the delta-top linked via a detachment within the Tiger supersequence (green) to the down-dip deep-water fold-thrust belt. As in Figure 4, the deep-water fold-thrust belt has been dissected by a post-Maastrichtian extensional fault, causing in a large slump complex in the detachment fold zone and resulting in parasitic folding throughout. An isolated thrust fault is present on the north side of the regional fault, which is either a remnant of the original compression in the deep-water fold-thrust belt or formed post faulting due to remaining updip extensional forces driving the sediments into the regional extensional fault. Eocene volcanic cones are illustrated to form in the oceanic crust on the basinward side of the image. The thrust belt has again been uplifted by graben reactivation and the buried White Pointer extensional province is visible beneath the Hammerhead extensional province.

Figure 11.

Seismic section through the complexly faulted eastern Ceduna subbasin showing the Hammerhead DDWFTB system and extensional growth faults on the delta-top linked via a detachment within the Tiger supersequence (green) to the down-dip deep-water fold-thrust belt. As in Figure 4, the deep-water fold-thrust belt has been dissected by a post-Maastrichtian extensional fault, causing in a large slump complex in the detachment fold zone and resulting in parasitic folding throughout. An isolated thrust fault is present on the north side of the regional fault, which is either a remnant of the original compression in the deep-water fold-thrust belt or formed post faulting due to remaining updip extensional forces driving the sediments into the regional extensional fault. Eocene volcanic cones are illustrated to form in the oceanic crust on the basinward side of the image. The thrust belt has again been uplifted by graben reactivation and the buried White Pointer extensional province is visible beneath the Hammerhead extensional province.

In the central-eastern Ceduna subbasin, the Cenomanian White Pointer supersequence is purely an extensional system that is lacking a downdip deep-water fold-thrust belt (Espurt et al., 2010; MacDonald et al., 2010; Figs. 4, 11). Therefore, as with the Hammerhead supersequence in the west, the White Pointer supersequence illustrated here cannot be classified as a linked DDWFTB system at this location (MacDonald et al., 2010). The strong evidence for an angular unconformity at the top of the White Pointer supersequence, adjacent to the uplifted continental crust (?), suggests there may have been a deep-water fold-thrust belt down-dip of the White Pointer delta-top in the east; however, either it has been uplifted and eroded prior to deposition of the Turonian-Santonian section or it never has formed in the first place, acting purely as an extensional seaward dipping system in the eastern part of the subbasin (Espurt et al., 2010; MacDonald et al., 2010; Figs. 4, 11).

The > 5000 m thick Hammerhead supersequence in the central-east Ceduna subbasin (Totterdell and Krassay, 2003) exhibits a continuous seaward-dipping basal detachment at the base of and within the Tiger supersequence marine mud detachment zone (Figs. 4, 8-11). As with the White Pointer system in the west this seaward dip may be partially due to subsequent uplift or it could be the original geometry inherited from the presence of an existing extensional province underlying this system (i.e., White Pointer delta-top).

The extensional province in the Hammerhead DDWFTB varies in fault geometry across the shelf to shelf edge, where it is composed of mainly regional planar faults in the northwest and steep, seaward-dipping listric, normal, growth faults in the south and southwest (Figs. 4, 9, 11). The extensional stresses in the delta-top are inferred to drive the compressional stresses responsible for the structures observed downdip in the transitional and compressional provinces (Yassir and Zerwer, 1997; Corredor et al, 2005; King et al., 2009). Overlap occurs between the lobes on the delta top (Fig. 1B; yellow dashed line) due to proximity to the sediment source.

The deeper Cenomanian fault system which developed in the White Pointer supersequence, is shared at this location (MacDonald et al., 2010). Linkage [of potential fluid migration pathways?] between the White Pointer, Tiger and Hammerhead supersequences on the delta top is achieved via the Cenomanian fault system (MacDonald et al., 2012), whereby optimally oriented faults have been reactivated as the Hammerhead supersequence prograded over the existing White Pointer system. This selective reactivation is evidenced by the growth strata in the Tiger and Hammerhead supersequences (Fig. 4) and is localized to areas where the sedimentary wedge is thickest. In areas where the Mulgara Faults are observed to propagate through the Tiger and Hammerhead supersequences but do not demonstrate evidence for growth, it is interpreted that these faults are a result of ongoing compaction of the existing White Pointer DDWFTB below. To the west (Fig. 3) and east (Fig. 4) of the overlap zone between the DDWFTB systems the reactivated faults become less prominent and the extensional faults that belong to each respective system become more abundant. This is observed in Figures 4 and 8, where the outboard growth faults, within the Hammerhead DDWFTB, detach at the level of the Tiger supersequence, rather than connecting to faults within the Cenomanian section.

In the Hammerhead DDWFTB the transitional province is characterized by large scale detachment folding bounded by growth strata to the north and the hanging wall of the first thrust to the south. The province ranges from ~ 15 to 25 km in width and contains a large scale open concentric detachment anticline having gently dipping limbs, especially in the west (Fig. 9). In the east it becomes somewhat more periclinal (MacDonald et al., 2010). The anticline has been thrusted by a large back-thrust in the southeast limb that is rooted at the basal detachment and may act as an accommodation structure within the Tiger supersequence detachment zone occupying the core of the anticline (Fig. 10). Four of the five seismic lines (Figs. 4, 8-10) presented herein display prominent detachment folding in the Hammerhead supersequence which could provide excellent potential hydrocarbon trapping mechanisms (MacDonald et al., 2010).

The compressional province of the Hammerhead DDWFTB is composed of a series of northeast dipping imbricate thrust sheets that are generally steepest in the hinterland and shallow out in the foreland, separated into 2-10 km wide panels (Figs. 4, 8-11). Thrust faults control the hanging-wall anticlines, which are commonly paired with an asymmetric footwall syncline and less commonly with a footwall anticline (Fig. 9). Hanging-wall anticlines are commonly tight to open symmetric concentric folds that exhibit intact limbs with proportionally steep dips on forelimbs and back limbs. There are variations in fold geometry, given the significant variation if some of the thrust fault geometries due to: (1) changes in detachment thickness and dip (Figs. 4 vs. 8); (2) Hammerhead supersequence thickness; (3) amount of up-dip extension to drive the downdip compression (Figs. 8 vs. 9); (4) subsequent uplift altering fault geometry (Fig. 11); and, (5) slumping due to gravitational collapse (Figs. 4, 11). There is significant evidence for slumping and growth strata shedding from both the forelimbs and back limbs resulting in ponded minibasin infilling (Fig. 9). This suggests the upper Hammerhead supersequence is synkinematic. Thrust tips are commonly invisible as they end within trishear zones which are characterized by upward thickening wedges of reduced seismic amplitude caused by shearing nearest the thrust tips, a feature which is often recorded in outcrop (Erslev, 1991; Briggs et al., 2006). These trishear zones are often overlapped by growth strata that thicken with ongoing deformation and deposition (Fig. 10).

There are notable similarities between the White Pointer DDWFTB and the Hammerhead DDWFTB, in particular in the fold-thrust belt. These similarities include the geometry of listric normal growth faults, thrust sheet spacing, and variation in dip of thrust faults, hanging-wall anticline geometry, ponded basin development adjacent to the forelimb and back limbs of folds, and the initiation of structures in an underlying ductile detachment unit. Axial traces/planes of the folds are likely curved for both DDWFTBs, an artifact of the lobate nature of thrust sheets. However, due to the spacing of the available seismic lines it is not possible to trace the folds/faults across the deep-water fold-thrust belt with certainty. Both systems appear to have seaward-dipping detachments that occur at multiple levels within their respective detachment units (Tiger and Blue Whale supersequences). As noted previously, these geometries may be original or the result of regional uplift post deposition.

Major differences in the two systems include size, age, thrust vergence, detachment lithology and thickness, deltaic lithology/depositional mechanism, thickness, and the position of the delta lobes. One of the more significant differences between the White Pointer and Hammerhead DDWFTBs is the thickness of the underlying supersequences, which form the detachment horizons. The Blue Whale supersequence is on average 2 km thick (and up to 4 km in places) while the Tiger supersequence is on average 1 km thick (rarely up to 4.5 km in the central Ceduna subbasin; Totterdell et al., 2000), thus restricting the amount of secondary overpressure that can form (i.e., from hydrocarbon generation and conversion of illite to montmorillonite; Morley, 2003; Osborne and Swarbrick, 1998) and limiting the initial volume of mobile material to accommodate the structures above (i.e., detachment folds).

The expectation that detachment folds form where the lithology of the detachment horizon(s) is thickest and mobile is not met for the White Pointer DDWFTB, where thrusting takes precedence over detachment folding despite a thick (>2 km) section of Blue Whale supersequence (Totterdell and Krassay, 2003). This may be influenced by the shallowly seaward-dipping detachment, driving the system to deform by thrusting rather than folding (i.e., Orange Delta offshore Namibia; Butler and Paton, 2010) or it may be attributed to the Blue Whale supersequence having a different lithology in this particular area. In contrast, the Hammerhead DDWFTB shows excellent evidence for shale mobilization into the core of the detachment antiform, thus conforming to the models proposed by Stewart (1996) and Simpson (2009) (Figs. 4, 11).

Finally, the most important difference between the two systems is related to the regional tectonic setting of the margin. A recent study on the South Australian margin thermo-chronological history and delta provenance indicates the two delta lobes which constitute Australia’s largest delta system formed during a major shift in tectonic setting and are likely sourced from different regions on the Australian continent (MacDonald et al., in press).

Source of the DDWFTB Systems

A tectono-stratigraphic framework study of the Ceduna subbasin completed by Totterdell et al. (2000) recognizes that the aggradational Cenomanian White Pointer DDWFTB system was deposited above a thick Albian marine mud (Blue Whale supersequence; Fig. 2). Upon cessation of Cenomanian DDWFTB deposition, the Turonian-Santonian Tiger supersequence, a proposed source rock in the subbasin (Blevin et al., 2000; Fig. 2), was deposited during a regional transgression. This non-deltaic deposition was followed by the rapid progradation of the Hammerhead DDWFTB system concentrated in the southeastern portion of the Ceduna subbasin (Figs. 1B, 4). The significant hiatus in deltaic deposition between the two systems (~8 m.y.) is enigmatic, and this has been proposed to be related to a significant change in tectonic style along the South Australian margin during the late Cretaceous (MacDonald et al., in press).

Presently, four possible sources are being considered for the Ceduna subbasin deltaic sediments, of which only one is based on a data set from the delta itself (MacDonald et al., in press). The sediment source originally proposed for the delta system by Veevers (1984) suggested an Australian Eastern Highlands provenance (Fig. 12). This model requires an extensive continent-scale river system similar in size to the Niger or Mississippi Rivers to transport eroded material from the uplifted highlands to the Ceduna subbasin. In contrast, King and Mee (2004) suggest that uplift and erosion of the Eromanga basin in the late Cretaceous supplied the bulk of the sediment responsible for building the Ceduna delta systems (Fig. 12).

Figure 12.

Digital elevation map of the Australian continent showing the position of the Australian Eastern Highlands, the Eromanga basin and the Flinders Ranges which have been proposed as the source regions for the Ceduna delta system, offshore South Australia.

Figure 12.

Digital elevation map of the Australian continent showing the position of the Australian Eastern Highlands, the Eromanga basin and the Flinders Ranges which have been proposed as the source regions for the Ceduna delta system, offshore South Australia.

A third model postulates that a late Cretaceous exhumation event in the Flinders Ranges (Fig. 12) as evidenced in an apatite fission track analysis (AFTA) study by Tingate et al. (2007) has resulted in cooling from 60-80° C and removal of up to 2 km of Permian-Jurassic sediments from this area. Tingate et al. (2007) suggest that the removed sediment may have contributed to building the Ceduna subbasin delta systems.

The most recent model proposed by MacDonald et al. (in press) is based on an extensive delta provenance study involving detrital zircon, apatite fission track, and zircon fission track analysis from Gnarlyknots-1A and previously published AFTA results from the South Australian margin. The AFTA results from previous exhumation studies along the margin (Gibson and Stuwe, 2000; Tingate et al., 2007; Tingate and Duddy, 2002) show a late Cretaceous cooling and exhumation pulse along an arcuate-shaped corridor on the South Australian southern margin, which has resulted in exhumation amounts of approximately 2 km in the late Cretaceous. The timing of this exhumation pulse is coincident with the onset of rapid progradation in the Hammerhead DDWFTB. Additionally, the AFTA data clearly show a shut off in exhumation in the late Cretaceous-Eocene, coincident with the cessation of delta building (MacDonald et al., in press).

The fission track based two-stage provenance model detailed by MacDonald and colleagues (in press) suggests there are two separate depositional mechanisms responsible for the two delta lobes. This idea is supported by a major shift in tectonic style between the Turonian and Santonian resulting, in widespread exhumation and significant erosion and recycling of Permian to Jurassic strata from the South Australian southern margin. This model also provides an explanation for the concentration of each DDWFTB system in a different depocenter (White Pointer in the west and Hammerhead in the east) and the difference in lithology between the two systems described by Totterdell et al. (2000).

The rapid progradation of the Hammerhead system fits the rapid margin exhumation model and can also explain the sand rich nature of the Hammerhead supersequence (Totterdell et al., 2000; Baker, 2003). This sand is likely sourced from the predominantly Permian glacial tillites and sandstones that were eroded from the margin, based on their remnant abundance in South Australian basins (Veevers et al., 2000; MacDonald et al., in press). The unconsolidated nature and high porosity (~ 27%) and permeability observed in the Gnarlyknots-1A sidewall cores (Baker, 2003) supports the rapid deposition model. The low fraction of clay minerals (< 10%; Baker, 2003) suggests only a minor volcanogenic component in the source sediment, which is in agreement with the glacial derived Permian tillite found in South Australia. This widespread tillite would have been subjected to extensive weathering and leaching of clay minerals between the Permian and Cretaceous erosion and deposition into the Ceduna subbasin. Based on this observation from Gnarlyknots-1A it is not inconceivable that this high porosity and permeability exists throughout the basin for the Hammerhead supersequence.

Prospectivity of the White Pointer and Hammerhead DDWFTB Systems

The prospective targets for hydrocarbon traps in the White Pointer and Hammerhead DDWFTBs are presented from seismic data only. They are located primarily in the transitional and compressional provinces of the White Pointer and Hammerhead DDWFTBs. Previous work on the prospectivity of the deep-water fold-thrust belts using vintage seismic data is presented in MacDonald et al. (2010); however, the recent availability of the BightSPAN© 2D seismic data and the Trim 3D seismic data sets (Fig. 1B), parts of which are presented herein, have warranted a second look at the region.

Potential hydrocarbon traps in the extensional and shelf provinces of the two DDWFTBs have been presented by previous authors in detail and are therefore not the focus of this study (e.g., Blevin et al., 2000; Totterdell et al., 2000; Somerville, 2001; Ruble et al., 2001; Struckmeyer et al., 2001; Struckmeyer et al., 2002; King and Mee, 2004; Tapley et al., 2005; Totterdell et al., 2008). However, recent mapping of the Trim 3D seismic dataset located on the delta-top (Fig. 1B) has resulted in the identification of potential four-way closure trap geometry within the extensional province of the overlap zone.

Figure 13 illustrates two listric normal faults on the delta-top, which are reactivated Cenomanian faults that detach within the Blue Whale supersequence. A correlatable horizon within the Hammerhead supersequence has been mapped across the two faults. Its contoured surface provides evidence for four-way (two-way time) closure on this localized fault geometry. The smaller structure to the east (Fig. 13) displays three-way closure; however, due to the fault shadow where the two extensional faults are linked it could possibly have some degree of four-way closure which is skewed by fault shadow. Pre-stack depth migration of the data will be required to confirm the prospectivity these features.

Figure 13.

Chair view of the 3D Trim seismic cube (Fig. 1B) showing a small fault network of reactivated Cenomanian listric growth faults (amplitude displayed on faults) which host a four-way closed (left) and three-way closed (right) potential hydrocarbon traps. These structures are within the Hammerhead supersequence and are interpreted to be related to a mild inversion pulse in the Santonian (MacDonald et al., 2012).

Figure 13.

Chair view of the 3D Trim seismic cube (Fig. 1B) showing a small fault network of reactivated Cenomanian listric growth faults (amplitude displayed on faults) which host a four-way closed (left) and three-way closed (right) potential hydrocarbon traps. These structures are within the Hammerhead supersequence and are interpreted to be related to a mild inversion pulse in the Santonian (MacDonald et al., 2012).

These two structures as seen in the time data could have been formed by one of three possible mechanisms. MacDonald et al. (2012) attribute this fault geometry to a mild inversion pulse created by a ridge push force in the late Cretaceous, resulting in reverse movement and the prominent drag fold observed on the 3D seismic data. The structures however may also be the result of a unique fault bend fold geometry forming a roll over anticline on the bounding listric fault. Finally, the structure may be the result of wrench faulting which could have produced a localized pop-up in this small network of extensional faults. This form of fault reactivation, which results in four-way closure being created prior to (potential) charge, provides a new and potentially lower risk trap geometry in the basin. Whatever the cause of this unique four-way closed fault-independent trap geometry, the presence of such structures bodes well for future exploration initiatives as post Cretaceous strike-slip to strike-slip normal fault reactivation has been demonstrated in the basin (Reynolds et al., 2003; MacDonald et al., 2012), which significantly increases exploration risk in areas of the Ceduna subbasin that rely on sealed fault-depended trap geometries. Seismic stratigraphy indicates this structure formed in the late Cretaceous when hydrocarbon generation was active (Tapley et al., 2005), and thus there exists potential for charge from all available source rocks along the bounding Cenomanian listric fault network including the Albian marine shale, Cenomanian deltaic sequence and the Santonian-Turonian marine shale. If charge did occur at this time it is likely any hydrocarbons would remain given the four-way fault-independent closure, even with the high net-to-gross and poor fault seal potential of the Hammerhead supersequence (Reynolds et al., 2003).

In this section we present a number of compressional structures that have the potential, based on position, quality of fold closure and presence of seals, to contain hydrocarbons provided they are continuous and close along strike. As the seismic lines that cover the transitional and compressional provinces are broadly spaced (average of 25-50 km; Fig. 1B), it is very difficult to predict with any certainty how structures will continue along strike. MacDonald et al. (2010) present two regional seismic lines that cross-cut the Hammerhead DDWFTB and provide a good visualization of the structures along strike indicating strong evidence for four-way closures both in the detachment folds (as in Fig. 10) within the transitional province and within the lobate thrust sheets of the compressional province. The existence of these structures provides the basis for identifying similar structures between seismic lines and describing them as potential hydrocarbon traps.

White Pointer DDWFTB prospectivity

Because no intersecting seismic lines are available to provide evidence for four-way closures within the White Pointer DDWFTB system, the following interpretations can only be based on the assumption that thrust sheets in DDWFTBs have curved axial traces and are commonly lobate as observed in other DDWFTBs, such as the Niger (e.g., Cobbold et al., 2009) and Baram deltas (e.g., Van Rensbergen and Morley, 2003) and in analog models of DDWFTBs (McClay et al., 2003). This geometry appears to be essential for four-way closure of both fault-propagation folds and hanging-wall anticlines, where, assuming a credible topseal, potential hydrocarbon accumulations may be isolated in the crests of folds within the fold-thrust belt (Cobbold et al., 2009).

Based on the available seismic data, we interpret the transitional province of the White Pointer DDW-FTB to be lacking suitable fold structures, possibly due to the influence of subseismic faulting/folding and the presence of volcanic sills and dykes distorting the seismic data and exploiting potential trap structures (MacDonald et al., 2012; Holford et al., 2012; Fig. 5, 7). The strong influence of volcanic overprinting and likely diagenesis in this area can pose significant problems for exploration (Holford et al., 2012), especially in such a structurally complex transitional zone, and may preclude this region (Fig. 1B; i.e., area surrounding Figs. 5-7) from exploration initiatives; their location in ultra-deep water only adds to the enormous risk involved.

We interpret the fold-thrust belt of the White Pointer DDWFTB to contain a number of structures that provide a realistic geometry for hydrocarbon traps (Figs. 3, 5-7), based on comparison with similar structures in hydrocarbon producing DDWFTBs such as the Baram (e.g., Van Rensbergen and Morley, 2003) and Niger DDWFTBs (e.g., Cobbold et al., 2009). We suggest that the traps having the greatest potential in this system are related to the fault-dependent and fault-independent hanging-wall anticlines which are generally manifest as open to tight concentric folds having limited evidence of seal destroying crestal collapse (Fig. 7). On the plus side, such structures are largely fault independent, mitigating the risk of fault breach.

There are two possibilities for trap seal mechanisms for the White Pointer DDWFTB based on both well data (the Potoroo-1 well only, as Gnarlyknots 1-A did not intersect the White Pointer; Fig. 1) and seismic data interpreted by Totterdell et al. (2000). The first potential seal mechanism is the intercalated strata of the White Pointer supersequence (Fig. 2), which is composed primarily of lagoonal siltstone and mud, and minor sandstone and coal, as it is observed in wells of both the Bight and Duntroon basins (Totterdell et al., 2000). In addition to the potential intra-stratal seals, the entirety of the White Pointer DDWFTB is draped with shales of the transgressive Tiger supersequence, which unconformably overlie the delta in the southwest and likely act as a regional seal (Totterdell et al., 2000).

In terms of potential reservoir, the White Pointer supersequence contains sandstone intervals, which are interbedded with muds, siltstones and coals providing potential reservoir-seal pairs. The Platypus-1 well in the Duntroon basin (Fig. 1A) contains high-energy fluvial sandstone facies which may also act as potential reservoirs, provided they are represented to the west in the Ceduna subbasin (Totterdell et al., 2000).

Hammerhead DDWFTB prospectivity

The transitional province of the Hammerhead DDWFTB is well imaged in the five lines that were interpreted for this paper. They display detachment folding (Figs. 4, 8-11) that is represented across the >350 km strike length of the DDWFTB (Fig. 1B). We interpret the fold-thrust belt structures to include numerous fault bend folds in the form of hanging-wall anticlines, many of which display fault dependent and/ or independent geometry, and have fold crests that appear unbreached (i.e., Fig. 9). Cobbold et al. (2009) provide evidence in the Niger DDWFTB for hydrocarbons migrating along existing thrust faults, and charging associated anticlines. This could provide a mechanism for fluid migration from the potential source rocks of the Tiger supersequence, where thrust faults are rooted, to the overlying structures in the Hammerhead DDWFTB (Totterdell et al., 2000; Totterdell et al., 2008).

Potential seal mechanisms for the Hammerhead DDWFTB would likely be provided by the intra-stratal muds that are present in several Duntroon basin wells (Blevin et al., 2000; Totterdell et al., 2000; Krassay and Totterdell, 2004). Totterdell and Krassay (2003) indicate that the Hammerhead supersequence is comprised of basinward-thinning wedges of marine shale at the toe of slope. Interpretation of seismic data, however, indicates that the Hammerhead supersequence may become more shale prone closer to to the basin. This is based on seismic reflections near the terminus of the DDWFTB, which suggest that intra-stratal packages are becoming increasingly chaotic, characteristic of shale rather than the thick sands representing the supersequence closer to the shelf (Totterdell et al., 2000).

In terms of reservoir, the Hammerhead supersequence itself displays the best potential. The Gnarlyknots-1A idrilled through a blocky sandstone having an overall porosity of ~ 27% and a seismic character indicative of strongly prograding stratal geometries nearest the shelf (Totterdell et al., 2000; Totterdell et al., 2008). Based on the interpretation of Hammerhead provenance by Tingate et al. (2007) and MacDonald et al. (in press) it is likely that there was little time for significant muds to deposit near the delta top, and thus sealing potential of the poorly consolidated, un-cemented (Baker, 2003) Hammerhead supersequence is likely limited. Potential seals may be found in Cenozoic carbonates above the intra-Maastrichtian unconformity, which may prove effective, provided the Cenozoic fault reactivation has not breached the seals (Reynolds et al., 2003; MacDonald et al., 2012)

The Ceduna subbasin is certainly a viable target for exploration as it has many different play types, although a majority of the structural plays rely on some degree of fault seal, aside from those four-ways presented in Figure 13 and the fault independent traps suggested for the detachment fold and deep-water fold-thrust belts. As the fault reactivation potential in the Ceduna subbasin poses a considerable risk to delta-top extensional structures, it would be better to explore in an area of the basin that provides some significant Cenozoic burial, which is a proven exploration model in the Gippsland and Otway basins (Duddy, 2003). The Greenly-1 well located in the eastern Ceduna subbasin or western Duntroon subbasin (Fig. 1A) encountered minor oil shows in a region that has up to 2 km of Cenozoic burial, mostly Eocene sediments (Totterdell et al., 2000). Without this Cenozoic burial, it is impossible to generate hydrocarbons today. Thus, any generation and charge that may have occurred in the late Cretaceous Ceduna subbasin would require an excellent seal to remain in place and not succumb to trap breach from fault reactivation or significant capillary action resulting in leak off. Unfortunately, most of the Ceduna subbasin does not have a significant cover of Cenozoic strata; only a thin veneer of Cenozoic carbonates are present over much of the basin (e.g., Figs. 3, 4).

Conclusions

The Ceduna subbasin provides an excellent opportunity to study the interaction of two delta—deep-water fold-thrust belt systems from the extensional regime of the delta-top to the compressional regime of the delta-toe. A Cenomanian age DDWFTB system dominates the western portion of the subbasin. Detaching above a thick Albian marine shale, it exhibits a well developed extensional and compressional provinces separated by a poorly developed transitional province. The presence of numerous volcanic intrusions that exploit thrust faults and detachment surfaces are common and will likely be detrimental for future exploration initiatives. A Santonian-Maastrichtian DDWFTB is present in the eastern Ceduna subbasin, also having a well developed extensional, transitional and compressional provinces.

Potential fluid dynamic communication between the two systems on the delta-top is likely given the proximity to the paleo-drainage; however, the deep-water fold-thrust belts remain almost exclusively isolated and independent aside from a restricted zone of overlap imaged in one seismic line. These two systems likely are sourced from different parts of the Australian continent, and a well evidenced change in tectonic style along the South Australian margin provides a possible explanation for the ~8 m.y. hiatus between deposition of the two delta lobes.

There are numerous petroleum exploration targets in the subbasin in the form of delta-top extensional structures, stratigraphic traps, regional detachment folds having demonstrated four-way closure (MacDonald et al., 2010), and delta-toe thrust faults and associated folds. Modeling by Geoscience Australia has identified numerous high quality source rocks suggesting excellent reservoir potential in the Hammerhead supersequence. There is potential for seal development within several intervals throughout the subbasin. Absence of a thick Cenozoic cover requires generation and migration to have occurred in the late Cretaceous and for hydrocarbons to remain in place. This poses a significant exploration risk given the demonstrated potential for fault seal breach such as has been observed in Jerboa-1 in the neighboring Eyre subbasin. Unfortunately, the challenging water depths (> 1500-5000 m) covering the deep-water fold-thrust belts would exclude a majority of the potential targets using current technology. Additional exploratory data is required to support the existence of a viable hydrocarbon system on the delta-top.

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Acknowledgments

The authors would like to thank the University of Adelaide and the Australian Research Council for financial assistance. We thank Ion Geophysical for access to the BightSPAN© seismic data and Geotrack International Pty Ltd for discussions on hydrocarbon prospectivity in the Ceduna subbasin. This article forms TRaX # 261.

Figures & Tables

Figure 1.

Location map of the Great Australian Bight (A) showing extent of Bight Basin (top right inset in green), the position of the Ceduna, Recherche, Polda, Eyre and Duntroon subbasins and the position of the two wells drilled in the Ceduna subbasin (Potoroo-1 and Gnarlyknots-1; Figure modified with permission from South Australian Geological Survey). (B) A digital elevation model of South Australia shows the broad bathymetry of the Ceduna Terrace and position of 3D Trim seismic data (black rectangle), 2D regional seismic lines (white dashed), White Pointer (light grey) and Hammerhead (dark grey) deep-water fold-thrust belt limits. Approximate position of delta lobe limits are outlined (yellow dashed line) and overlap zone on delta-top is labeled. Thick yellow arrows represent inferred sediment input direction based on geometry of deep-water fold-thrust belts (modified with permission after MacDonald et al., 2012).

Figure 1.

Location map of the Great Australian Bight (A) showing extent of Bight Basin (top right inset in green), the position of the Ceduna, Recherche, Polda, Eyre and Duntroon subbasins and the position of the two wells drilled in the Ceduna subbasin (Potoroo-1 and Gnarlyknots-1; Figure modified with permission from South Australian Geological Survey). (B) A digital elevation model of South Australia shows the broad bathymetry of the Ceduna Terrace and position of 3D Trim seismic data (black rectangle), 2D regional seismic lines (white dashed), White Pointer (light grey) and Hammerhead (dark grey) deep-water fold-thrust belt limits. Approximate position of delta lobe limits are outlined (yellow dashed line) and overlap zone on delta-top is labeled. Thick yellow arrows represent inferred sediment input direction based on geometry of deep-water fold-thrust belts (modified with permission after MacDonald et al., 2012).

Figure 2.

Litho- and sequence stratigraphic framework of the Bight Basin showing supersequences described in text and interpreted basin phases and sea level curve from Totterdell et al., 2000. (Modified with permission from Totterdell et al., 2008.)

Figure 2.

Litho- and sequence stratigraphic framework of the Bight Basin showing supersequences described in text and interpreted basin phases and sea level curve from Totterdell et al., 2000. (Modified with permission from Totterdell et al., 2008.)

Figure 3.

Interpreted regional 2D seismic line through the western Ceduna subbasin showing well developed extensional and compressional provinces of the White Pointer delta—deep-water fold-thrust belt system (blue). The Hammerhead supersequence (purple) does not form a DDWFTB system here due to insufficient thickness to initiate gravitational sliding. Cenozoic strata (yellow) form a thin veneer above the intra-Maastrichtian unconformity (between purple and yellow; modified with permission after MacDonald et al., 2012).

Figure 3.

Interpreted regional 2D seismic line through the western Ceduna subbasin showing well developed extensional and compressional provinces of the White Pointer delta—deep-water fold-thrust belt system (blue). The Hammerhead supersequence (purple) does not form a DDWFTB system here due to insufficient thickness to initiate gravitational sliding. Cenozoic strata (yellow) form a thin veneer above the intra-Maastrichtian unconformity (between purple and yellow; modified with permission after MacDonald et al., 2012).

Figure 4.

Interpreted regional 2D seismic line through the eastern Ceduna subbasin showing preserved extensional province of the White Pointer DDWFTB (blue) and well developed extensional, transitional and compressional provinces of the Hammerhead delta—deep-water fold-thrust belt system (purple) detaching above the Turonian-Santonian Tiger supersequence (green). The Hammerhead supersequence (purple) forms a classical DDWFTB system nearest the paleoshelf edge due to increased sediment deposition in the southeastern Ceduna subbasin. A significant fault scarp or slump is exposed along the Eocene extensional fault which dissects the delta-toe and detachment of the Hammerhead DDWFTB system and likely forms a boundary between the continental and oceanic crust. A significant unconformity at the top of the White Pointer extensional province may indicate pre-Turonian exhumation of a paleodeep-water fold-thrust belt that is no longer visible. Projected position of Gnarlyknots-1A is indicated at top of section on the delta-top.

Figure 4.

Interpreted regional 2D seismic line through the eastern Ceduna subbasin showing preserved extensional province of the White Pointer DDWFTB (blue) and well developed extensional, transitional and compressional provinces of the Hammerhead delta—deep-water fold-thrust belt system (purple) detaching above the Turonian-Santonian Tiger supersequence (green). The Hammerhead supersequence (purple) forms a classical DDWFTB system nearest the paleoshelf edge due to increased sediment deposition in the southeastern Ceduna subbasin. A significant fault scarp or slump is exposed along the Eocene extensional fault which dissects the delta-toe and detachment of the Hammerhead DDWFTB system and likely forms a boundary between the continental and oceanic crust. A significant unconformity at the top of the White Pointer extensional province may indicate pre-Turonian exhumation of a paleodeep-water fold-thrust belt that is no longer visible. Projected position of Gnarlyknots-1A is indicated at top of section on the delta-top.

Figure 5.

Seismic section through the southwestern White Pointer DDWFTB showing extensive volcanic intrusions exploiting deep-water fold-thrust belt structures and skewing the seismic data. Forced folds, saucer shaped sills, and volcanic vents interpreted after Holford et al., 2012, with permission.

Figure 5.

Seismic section through the southwestern White Pointer DDWFTB showing extensive volcanic intrusions exploiting deep-water fold-thrust belt structures and skewing the seismic data. Forced folds, saucer shaped sills, and volcanic vents interpreted after Holford et al., 2012, with permission.

Figure 6.

Seismic section through the southwestern White Pointer DDWFTB showing regularly spaced thrust faults in the deep-water fold-thrust belt and thrust related features such as synkinematic onlap, ponding in thrust hanging-wall, and position of seaward dipping detachment zone within the Albian Blue Whale supersequence. Chaotic seismic reflections may indicate the presence of deep-water marine muds as suggested by Totterdell et al., 2000.

Figure 6.

Seismic section through the southwestern White Pointer DDWFTB showing regularly spaced thrust faults in the deep-water fold-thrust belt and thrust related features such as synkinematic onlap, ponding in thrust hanging-wall, and position of seaward dipping detachment zone within the Albian Blue Whale supersequence. Chaotic seismic reflections may indicate the presence of deep-water marine muds as suggested by Totterdell et al., 2000.

Figure 7.

Seismic section through the southern White Pointer deep-water fold-thrust belt showing thick detachment zone in the Albian Blue Whale supersequence where listric, normal growth faults of the extensional province and down-dip thrust faults detach. Crestal collapse faulting is observed above a single fault bend fold and volcanic intrusions/vents are indicated. Dashed white lines represent inferred thrust faults where seismic resolution is insufficient for confident interpretation.

Figure 7.

Seismic section through the southern White Pointer deep-water fold-thrust belt showing thick detachment zone in the Albian Blue Whale supersequence where listric, normal growth faults of the extensional province and down-dip thrust faults detach. Crestal collapse faulting is observed above a single fault bend fold and volcanic intrusions/vents are indicated. Dashed white lines represent inferred thrust faults where seismic resolution is insufficient for confident interpretation.

Figure 8.

Seismic line through the central Ceduna subbasin which illustrates the only known example of two stacked DDWFTB systems in the region. The White Pointer DDWFTB system remains intact below the younger Hammerhead DDWFTB which was deposited after a ~8 m.y. hiatus. The Ablian Blue Whale supersequence (pink) forms the detachment zone for the White Pointer DDWFTB while the Turonian-Santonian Tiger supersequence forms the detachment zone for the Hammerhead DDWFTB system. The Hammerhead DDWFTB system displays a clear detachment fold province whereas the White Pointer DDWFTB system does not. This region represents the only area where the two deep-water fold-thrust belts overlap in the Ceduna subbasin.

Figure 8.

Seismic line through the central Ceduna subbasin which illustrates the only known example of two stacked DDWFTB systems in the region. The White Pointer DDWFTB system remains intact below the younger Hammerhead DDWFTB which was deposited after a ~8 m.y. hiatus. The Ablian Blue Whale supersequence (pink) forms the detachment zone for the White Pointer DDWFTB while the Turonian-Santonian Tiger supersequence forms the detachment zone for the Hammerhead DDWFTB system. The Hammerhead DDWFTB system displays a clear detachment fold province whereas the White Pointer DDWFTB system does not. This region represents the only area where the two deep-water fold-thrust belts overlap in the Ceduna subbasin.

Figure 9.

Seismic section through the eastern Ceduna subbasin showing extremely well developed Hammerhead DDWFTB system as well as up-dip extension, a detachment fold province, and down-dip compression in the delta-toe. Relict White Pointer extensional faults have been interpreted below the detachment fold province, but there is no evidence for a White Pointer deep-water fold-thrust belt present in the section.

Figure 9.

Seismic section through the eastern Ceduna subbasin showing extremely well developed Hammerhead DDWFTB system as well as up-dip extension, a detachment fold province, and down-dip compression in the delta-toe. Relict White Pointer extensional faults have been interpreted below the detachment fold province, but there is no evidence for a White Pointer deep-water fold-thrust belt present in the section.

Figure 10.

Seismic section through the eastern Ceduna subbasin imaging the Hammerhead deep-water fold-thrust belt; an extensive detachment fold is visible on the north side of the section. The detachment fold exhibits a well imaged back thrust and is most likely cored by marine mud of the Tiger supersequence, which forms the regional detachment zone (green) for the extensional and compressional provinces. Possible uplift by graben reactivation has affected the fold-thrust belt in this region.

Figure 10.

Seismic section through the eastern Ceduna subbasin imaging the Hammerhead deep-water fold-thrust belt; an extensive detachment fold is visible on the north side of the section. The detachment fold exhibits a well imaged back thrust and is most likely cored by marine mud of the Tiger supersequence, which forms the regional detachment zone (green) for the extensional and compressional provinces. Possible uplift by graben reactivation has affected the fold-thrust belt in this region.

Figure 11.

Seismic section through the complexly faulted eastern Ceduna subbasin showing the Hammerhead DDWFTB system and extensional growth faults on the delta-top linked via a detachment within the Tiger supersequence (green) to the down-dip deep-water fold-thrust belt. As in Figure 4, the deep-water fold-thrust belt has been dissected by a post-Maastrichtian extensional fault, causing in a large slump complex in the detachment fold zone and resulting in parasitic folding throughout. An isolated thrust fault is present on the north side of the regional fault, which is either a remnant of the original compression in the deep-water fold-thrust belt or formed post faulting due to remaining updip extensional forces driving the sediments into the regional extensional fault. Eocene volcanic cones are illustrated to form in the oceanic crust on the basinward side of the image. The thrust belt has again been uplifted by graben reactivation and the buried White Pointer extensional province is visible beneath the Hammerhead extensional province.

Figure 11.

Seismic section through the complexly faulted eastern Ceduna subbasin showing the Hammerhead DDWFTB system and extensional growth faults on the delta-top linked via a detachment within the Tiger supersequence (green) to the down-dip deep-water fold-thrust belt. As in Figure 4, the deep-water fold-thrust belt has been dissected by a post-Maastrichtian extensional fault, causing in a large slump complex in the detachment fold zone and resulting in parasitic folding throughout. An isolated thrust fault is present on the north side of the regional fault, which is either a remnant of the original compression in the deep-water fold-thrust belt or formed post faulting due to remaining updip extensional forces driving the sediments into the regional extensional fault. Eocene volcanic cones are illustrated to form in the oceanic crust on the basinward side of the image. The thrust belt has again been uplifted by graben reactivation and the buried White Pointer extensional province is visible beneath the Hammerhead extensional province.

Figure 12.

Digital elevation map of the Australian continent showing the position of the Australian Eastern Highlands, the Eromanga basin and the Flinders Ranges which have been proposed as the source regions for the Ceduna delta system, offshore South Australia.

Figure 12.

Digital elevation map of the Australian continent showing the position of the Australian Eastern Highlands, the Eromanga basin and the Flinders Ranges which have been proposed as the source regions for the Ceduna delta system, offshore South Australia.

Figure 13.

Chair view of the 3D Trim seismic cube (Fig. 1B) showing a small fault network of reactivated Cenomanian listric growth faults (amplitude displayed on faults) which host a four-way closed (left) and three-way closed (right) potential hydrocarbon traps. These structures are within the Hammerhead supersequence and are interpreted to be related to a mild inversion pulse in the Santonian (MacDonald et al., 2012).

Figure 13.

Chair view of the 3D Trim seismic cube (Fig. 1B) showing a small fault network of reactivated Cenomanian listric growth faults (amplitude displayed on faults) which host a four-way closed (left) and three-way closed (right) potential hydrocarbon traps. These structures are within the Hammerhead supersequence and are interpreted to be related to a mild inversion pulse in the Santonian (MacDonald et al., 2012).

Contents

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