Lord Howe Rise

Basin Details and Geological Overview

The Lord Howe Rise (LHR) is a major submerged plateau that extends almost 2000km from southwest of New Caledonia to the Challenger Plateau, west of New Zealand (Location Map). Much of the LHR lies within the Australian marine jurisdiction, at water depths of 1000–3000m. It is a vast frontier region of about 1 500 000km² with little previous scientific or resource exploration. The sparse data coverage consists of regional 2D seismic, gravity, magnetic and bathymetry data. DSDP drill holes (e.g. DSDP 207 and 208) provide the only existing well data for the uppermost part of the sedimentary succession. No petroleum exploration wells have yet been drilled. A few dredge samples provide information on the nature and age of basement and magmatism in the LHR, the Dampier Ridge and the Challenger Plateau. Recent work by Geoscience Australia in the Capel and Faust basins has significantly improved the data coverage and geological knowledge over the northern LHR. This study has also confirmed that the LHR contains sedimentary basins that are potentially capable of generating and accumulating petroleum.

The LHR is interpreted to be a continental fragment that rifted from eastern Australia during the Cretaceous break-up of Eastern Gondwana margin and the Tasman Sea opening (85–52 Ma). Refraction seismic, gravity and magnetic data suggest that rifting extended the continental crust beneath the LHR to varying degrees. The crustal thickness generally decreases westward, indicating an increase in crustal stretching toward the Tasman Sea. Rock samples and tectonic reconstructions suggest that the LHR is underlain by offshore continuations of the Paleozoic to Mesozoic orogens and basins of eastern Australia, New Zealand and New Caledonia.

ProvExplorer

• Capel Basin Details and Geological Overview
• Faust Basin Details and Geological Overview
• Gower Basin Details and Geological Overview
• Kenn Plateau Details and Geological Overview
• Mellish Rise Details and Geological Overview

Structural Elements

The LHR comprises four sub-parallel geological provinces that extend along its length: the Lord Howe Platform, the Central Rift Province, the Western Rift Province, and the Dampier and Monowai ridges.

The Lord Howe Platform is a shallow, planated ridge forming the eastern flank of the LHR. It is composed of pre-rift basement rocks that underwent little stretching during Cretaceous rifting. It is thinly mantled by a few hundred metres of mainly Cenozoic siliceous and carbonate oozes and Cretaceous–Cenozoic volcanic rocks. Its boundary with the Fairway, New Caledonia and Aotea basins to the east is marked by a scarp and an abrupt deepening of the basement surface.

The Central Rift Province, west of the Lord Howe Platform, consists of basement blocks and normally faulted basin depocentres with up to 4km of Cretaceous to Cenozoic syn- and post-rift sediments. The underlying continental crust is moderately extended (about 20km thickness). This province includes the Faust Basin in the north and the Moore Basin in the south.

The Western Rift Province to the west of the Central Rift Province contains large normally faulted depocentres with up to 6km of Cretaceous to Cenozoic syn- and post-rift sediments. Sediment thickness and water depths generally increase westward across the Central and Western Rift provinces. The Western Rift Province includes the Capel Basin in the north and the Monowai Basin in the south. In the central LHR, the Central and Western Rift provinces cannot be separated and the combined province has been named the Gower Basin.

The Dampier Ridge in the north and the Monowai Ridge in the south form the boundary between the LHR and the oceanic crust of the Tasman Sea Basin to its west. The ridges are composed of continental crust; the Dampier Ridge is partly composed of Permian granite. The Dampier Ridge is separated from the Western Rift Province by the Lord Howe and Middleton basins, These basins are underlain by highly extended continental or transitional crust.

A series of SW–NE fracture zones extending from the oceanic crust of the Tasman Sea fragment the Lord Howe Rise into discrete microcontinents. These include the Barcoo–Elizabeth–Fairway Lineament separating the Capel and Faust basins from the Gower Basin. In addition, an extension of the Vening–Meinesz Fracture Zone appears to extend into the eastern flanks of the Lord Howe Rise from the southeast.

Lord Howe Rise Structural Aspects

Stratigraphy

One pre-rift, two syn-rift and one post-rift seismic megasequences have been identified in the Capel and Faust basins of the northern LHR. Further south in the Gower Basin, these megasequences have been subdivided into 11 seismic sequences. The top of the (mega-) sequences are bounded by unconformities and their correlative conformities. Regional correlations indicate that the stratigraphy is broadly similar over most of the LHR. Many of the unconformities correspond to regional unconformities that may be related to major plate tectonic or oceanographic events. The age, composition and depositional environment of the (mega-) sequences have been inferred from regional tectonic reconstructions, DSDP well ties, and correlations with analogue basins in eastern and southern Australia, New Zealand and New Caledonia.

The pre-rift succession (Pre-rift in the Capel-Faust basins, GB11 and GB10 in the Gower Basin) is likely to comprise Paleozoic and Mesozoic rocks that correlate with the Tasman Orogen of eastern Australia and the accretionary terranes of New Zealand and New Caledonia. The Paleozoic rocks may include the equivalents of the New England Orogen in the northern LHR and the Lachlan Orogen in the southern LHR. Mesozoic sedimentary rocks that are probably equivalent to the Maryborough, Clarence-Moreton and Sydney basin successions are also present. Volcanic, intrusive and volcaniclastic rocks, either of pre-rift or earliest syn-rift age, are widespread in some parts of the LHR. These rocks may correlate with the earliest phases of the latest Jurassic to Early Cretaceous regional magmatism along the former eastern Gondwana margin, e.g. the lower Grahams Creek Formation in the Maryborough Basin and the Casterton Formation of the Crayfish Supersequence in the Otway Basin. Continued development of the eastern Gondwanan magmatic rift led to syn-rift deposition across the LHR region during the Early Cretaceous.

The Early Cretaceous rifting event in the LHR region (Syn-rift 1 in the Capel-Faust basins, GB9 and 8 in the Gower Basin) is inferred to have taken place during the Early Cretaceous. In the northern LHR, a pre-existing NW–SE basement structural trend appears to have interacted with an oblique extensional vector to form NNE–SSW trending depocentres. Seismic character and analogue basin studies suggest that Early Cretaceous syn-rift deposition was dominated by volcaniclastic, fluvial and fan sediments, volcanics and intrusives, and localised coal and lacustrine sediments. The succession probably correlates with the upper Grahams Creek Formation in the Maryborough Basin, Strzelecki Group in the Gippsland Basin and the Eumeralla Supersequence in the Otway Basin. Deposition ended with the Cenomanian uplift, erosion and basin inversion that is widely documented from eastern and southern Australia, New Zealand and New Caledonia. Previous work attributes the tectonic event to a major reorganisation of the relative motions between Australian, Pacific and Antarctic plates.

A second rifting event (Syn-rift 2 in the Capel-Faust basins, GB7 in the Gower Basin) affected the LHR during the Late Cretaceous. Rifting was focused in the Western Rift Province and appears to be related to the opening of the Tasman Sea to the west of the LHR. In the Capel and Faust basins, the earlier formed NNE–SSW trending depocentres were overprinted by NW–SE trending structures. Sediments deposited during this phase are likely to be predominantly fluvial. As the Tasman Sea Basin opened, marine conditions spread northwards along the LHR resulting in marginal and shallow marine deposition by the Campanian. The succession is likely to correlate with the Emperor and Golden Beach subgroups of the Gippsland Basin and the Taniwha and Rakopi formations of the Taranaki Basin. Volcanism appears to have been common over the LHR during the Late Cretaceous. Trachyte and latite samples recovered during the AUSFAIR survey to the southeast of the Faust Basin have been U-Pb dated by ion microprobe (SHRIMP) at 97 and 74 Ma, while rhyolite from the DSDP 207 drilling site on the southern Lord Howe Rise was previously dated at c. 94 Ma.

Thermal subsidence of the region from the Late Cretaceous to the present has resulted in marine deposition under progressively deepwater conditions. In the earlier part of this post-rift phase, marginal marine to mid-bathyal clastic sediments were deposited. Seismic data have revealed features that may be deltaic, shoreline and turbidite sand bodies. Since the Late Maastrichtian, bathyal conditions have dominated, resulting in the deposition of chalk, marl and calcareous ooze as seen at DSDP 207 and 208 drill holes. The lateral continuity and thickness make these fine-grained sediments a potential regional seal for petroleum traps. Pulses of igneous and tectonic activity are recorded during the Maastrichtian–Paleocene/Eocene, the Late Oligocene–Miocene and the Pliocene. Magmatism associated with these events has produced widespread small volcanic cones, flows and sills. The northwestern part of the LHR is also traversed by the Lord Howe seamount chain. Post-rift magmatism in the LHR region appears to have triggered fluid migration, which has extensively affected the post-rift sediments and continues to shape features on the seafloor today. Major regional unconformities also occur during the Paleocene–Eocene and Eocene–Oligocene, which may be related to a reorganisation of plate tectonic motion (e.g. the Hawaiian–Emperor bend) and/or regional oceanic circulation.

Petroleum Systems and Hydrocarbon Potential

In terms of conventional hydrocarbons, the deeper sedimentary depocentres of the Central and Western Rift provinces have sufficient sediment thickness for the potential generation and expulsion of hydrocarbons.

Coaly potential petroleum source rocks may be present in the pre-rift sedimentary rocks, if the succession is a correlative of the Maryborough, Clarence-Moreton and Sydney basins. Coaly and lacustrine potential source rocks may also occur in the Cretaceous syn-rift megasequences, by analogy with the eastern Australian and New Zealand basins. 1D basin modelling has indicated that potential source rocks within the pre-rift and lower syn-rift sections would be capable of generating and expelling oil and gas. Potential reservoir rocks may be found in fluvial sandstones within the syn-rift succession and the deltaic, shoreline and shallow marine sandstones in the upper syn-rift and lower post-rift successions. The upper post-rift deep marine sediments may act as a regional seal.

Expected major trapping styles include fault-related plays, stratigraphic pinch-outs, unconformity plays, and drape and anticlinal structures. 3D geological modelling indicates the existence of several large potential anticlinal structures with likely four-way closure.

Majority of the petroleum generation and migration is expected to have occurred before most trapping structures and the potential regional seal were in place by the Oligocene. However, a significant amount of generation is also likely after this time. Cenozoic igneous activity may have enhanced late-stage petroleum generation. On the other hand, the widespread fluid migration triggered by the Cenozoic magmatism is a major risk for seal integrity of the upper post-rift sediments.

Much of the previous petroleum exploration interest in the LHR region related to the possibility of vast gas hydrate occurrences within the post-rift sediments. This interpretation was based on the widely distributed bottom simulating reflectors (BSRs) seen on regional seismic data. However, an appraisal of recently acquired high-resolution seismic data and shallow sediment cores collected during the AUSFAIR MD-153 survey has indicated that the BSRs are most likely to be an Opal-A/Opal-CT diagenetic boundary.

Key References