Eleven major seismic sequences have been identified within the Gower Basin and are probably present though most of the LHR region. They have been labelled from seabed downwards as GB1, GB2.....etc and are overlain by unconformities or correlative conformities, U1 = water bottom, U2, U3 etc...... U11 = pre-rift sediments/volcanics. Their ages and environments of deposition have been deduced from seismic character correlations between the western LHR and the Bass Strait basins, the kinematic evolution of the region and from ties to the DSDP sites.
| Age | Event | Environment | Correlation | Mega sequence | Comment |
|---|---|---|---|---|---|
| Palaeozoic and Early Mesozoic | Pre-Lord Howe Rise; pre rift | Partially cratonised basement with folding, faulting and volcanism; includes some areas with residual stratification | Tasman Geosyncline and possibly some older Mesozoic basins such as Esk Trough equivalent | GB11 | Eroded pre-rift complex of variable age. Areas of more volcanic basement appear to be prevalent towards the west and becomes indistinguishable from what may be Cretaceous 'oceanic crust' along the western flank of the LHR. |
| ?Latest Jurassic | Extension | Unconformity U11 | Detachment sub-branch appears to dip W or SW from the LH Platform, under the western LH (Gower Basin) province. | ||
| ?Latest Jurassic - earliest Cretaceous | Initial extension and commencement of syn-rift deposition | Unknown, but possibly a mix of alluvial fans with substantial volcanism in many grabens | Could be broadly equivalent to Casterton Beds in Otway Basin | GB10 | In many places the synrift volcanics are difficult to distinguish from volcanic basement; also,some sills may have been injected later in the rift phase. |
| Earliest Cretaceous | ?Further block faulting in some graben | Unconformity U10 | The termination of sills is difficult to distinguish from the edges of fault-blocks in some areas. | ||
| Early Cretaceous | Major syn-rift sedimentary phase | Unknown, but possibly a mix of alluvial fan, fluvial and lacustrine; subordinate volcanics | Probably equivalent to the Otway Gp in Otway/Bass Basins and Strzelecki Gp in Gippsland Basin | GB9/GB8 | Reasonable character correlation with Otway/Strzelecki Groups (see Figure 7); may comprise volcanogenic sediments. Mid Strzelecki unconformity U9 also recognised in Gippsland Basin. |
| Mid Cretaceous
(?Cenomanian) |
Crustal shortening; major episode of basin inversion:
(Probably the final episode of subduction to east of LHR) |
Unconformity U8 | Reverse faulting, wrenching, & some overthrusting; either due to W to E compression, particularly adjacent to LH Platform, and/or left-lateral oblique extension between LHR and mainland Australia. Minimal subsequent erosion. (Extension in Durroon Sub-basin, Bass Strait, at this time). | ||
| Late Cretaceous
(?Cenomanian-Santonian) |
Substantial topographic infill with some growth against faults; onlaps Cenomanian folds | High reflection continuity indicates that this megasequence may include marine units | Probably equivalent to the older part of the Golden Beach Formation in Gippsland Basin | GB7 | The Golden Beach Formation is usually considered to extend from Cenomanian to Campanian; however, the effect of Tasman Sea breakup in the Santonian may have been delayed in more elevated areas such as Gippsland. |
| Campanian | End of syn-rift and infill phases; onset of 'Southern Margin' & Tasman Basin seafloor spreading (A33 | Unconformity U7 | |||
| ?Approx. Campanian - Maastrichtian | ?Thermal subsidence (sag) phase; some infill and compaction drape | Unknown; but seismic characteristics and DSDP results suggest fluvial-lacustrine, coastal and marginal marine conditions | None | GB6 | Age equivalent to early Latrobe Group of Gippsland; however, no direct correlation between Gippsland and LHR following breakup. At DSDP 207 (southern LHR) this unit comprises glauconitic sandy siltstone indicating marginal marine; at 208 (northern LHR) indicators are for mid-bathyal water depths. |
| ?Maastrichtian | Unconformity U6 | Poorly-defined unconformity at DSDP sites | |||
| largely Palaeocene | ?Latter part of thermal subsidence (sag) phase; some infill and compaction drape | Unknown; although seismic characteristics suggest fluvial-lacustrine and marginal marine as for GB6, the DSDP results indicate that bathyal conditions more likely | DSDP208 (Unit 2) - nanno chalk and radiolarite
DSDP 207 (Unit 2) - foram ooze/clay |
GB5 | |
| Early or Middle Eocene | ?Plate readustments 52 Ma; or end spreading 42 Ma. | Unconformity U5 | 42 Ma end of Tasman spreading, but more likely 52 Ma plate readjustment (Hawaiian-Emperor bend) | ||
| L. Eocene - E.Oligocene | ?Lowstand
Broad folding Start Southern Ocean fast spreading. New convergent plate boundary to east. |
Marine, possibly high terrigenous input | Hiatus at DSDP 207 and 208 | GB4 | ?Erosion of platform/horsts during lowstand. |
| Mid Oligocene | Australian and Antarctic Plates clear south of Tasmania | Unconformity U4 | Unconformity encompasses entire Middle Eocene-Late Oligocene in some areas, particularly over the platforms and horsts. | ||
| Oceanic phase | Bathyal marine | DSDP (Unit 1) - foram/nanno ooze from this period onwards | GB3/GB2 | Deep sea oozes and clays, with chalk and siliceous content. ?Late Oligocene Unconformity U3 not recognised in all areas. | |
| Middle Miocene | Compression and broad folding continue. Active convergence to east. | Overlies fractured siliceous competent beds (?cherts) | Unconformity U2 | Unconformity characterised by numerous bedding fractures. Extended period of folding probably due to crustal shortening near plate margins. | |
| Bathyal marine | Deep sea ooze | GB1 | |||
| Recent | Formation of gas hydrates on New Caledonia Basin flank of the LHR | ||||
|
(after Willcox
et al., 2001, 2002)
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