Geology and Geophysics of the Offshore Maryborough, Capricorn and Northern Tasman Basins: Results of AGSO Survey 91

The Australian Geological Survey Organisation (AGSO) completed a major marine multichannel seismic (MCS) reflection survey off southeast Queensland in December 1989 using RV Rig Seismic. The research cruise (AGSO Survey 91) was designed to investigate the structure, stratigraphy and evolution of offshore basins along this sector of the Australian continental margin. About half of the 2900 km of seismic data collected was recorded over the offshore Maryborough Basin; the remainder was collected over the Capricorn Basin and margins of the northern Tasman Basin. The survey included seismic ties to the only deep offshore wells in the region, Aquarius 1 and Capricorn 1A located in the northern Capricorn Basin. It was the first MCS survey shot in the region since 1974.\nSonobuoy refraction, gravity, magnetic and bathymetric data were also collected.\n\nThe Maryborough Basin developed in the earliest Triassic - mid Jurassic as an epicratonic downwarp, probably a foreland depression. Neocomian volcanism and rifting, perhaps generated by transtensional movements, led to a second phase of deposition that produced thick elastic shallow marine and deltaic sequences. Subsequent basin inversion in the mid Cretaceous produced major folding along northwest/NNW axes as well as normal and reverse faulting. Peneplanation following uplift has eroded up to several kilometres of section from the basin, leaving it as a remanent basin. The main causes of the uplift are believed to be (i) the mid Cretaceous deformation episode, (ii) rift flank uplift as the adjacent northern Tasman Sea opened (-63 Ma ago), and (iii) rise of the Eastern Highlands in the Late Cretaceous - Cainozoic due to crustal underplating associated with detachment\nfaulting and transit over mantle plumes.\n\nUp to 5 km of the Early Cretaceous Maryborough Formation (shallow marine elastics) and Burrum Coal Measures are preserved in a NNW-trending syncline beneath the western side of Hervey Bay, just off the coast northeast of Bundaberg. Depth to basement may be as much as 9-10 km. No major anticlines in the Early Cretaceous units are present beneath Hervey Bay. However, small culminations may exist in the more flat-lying strata of the basin deep northeast of Bundaberg.\n\nReinterpretation of onshore seismic data in the Maryborough Basin has provided new insight into the structural configuration of this part of the basin. \n\nSurvey 91 established that the Maryborough Basin does extend beneath the continental shelf southeast of Fraser Island. The section is folded and faulted, with broad anticlinal structures present. Seismic data quality in the Cretaceous and older section is poor in this area, perhaps a consequence low acoustic impedance contrast (?tight formations). Much of the shelf is underlain by about 2.0 s twt (-4 km) of Maryborough Basin section. A major syncline in Wide Bay contains up to 5 km of Early Cretaceous sediments. Basement here may lie at 8-9 km depth.\n\nYou can also purchase hard copies of Geoscience Australia data and other products at http://www.ga.gov.au/products-services/how-to-order-products/sales-centre.html

澳大利亚地质调查组织（Australian Geological Survey Organisation, AGSO）于1989年12月使用“‘Rig Seismic”号科考船，在昆士兰州东南部海域完成了一项大型海洋多道地震（marine multichannel seismic, MCS）反射勘探。本次科考航次（AGSO航次91）旨在探究澳大利亚大陆边缘该段近海盆地的构造、地层特征与演化历程。采集的2900公里地震数据中，约一半覆盖了近海玛丽伯勒盆地（Maryborough Basin），剩余数据则覆盖了摩羯座盆地（Capricorn Basin）与塔斯曼海盆（Tasman Basin）北部陆缘。本次调查还与该区域仅有的两口深海近海钻井——位于摩羯座盆地北部的Aquarius 1与Capricorn 1A完成了地震联测。这是该区域自1974年以来首次开展的MCS勘探。

本次科考还同步采集了声呐浮标折射（Sonobuoy refraction）、重力、磁法与水深数据。

玛丽伯勒盆地（Maryborough Basin）形成于早三叠世至中侏罗世，属于克拉通内部拗陷，大概率为前陆坳陷。新康尼亚期（Neocomian）火山活动与裂谷作用（可能由走滑拉张运动引发）带来了第二套沉积期，形成了厚层碎屑浅海相及三角洲相地层。中白垩世的盆地反转作用引发了沿北西/北北西向轴部的大规模褶皱，同时伴随正断层与逆断层发育。抬升后的准平原化作用剥蚀了盆地内可达数千米的地层，使其成为残留盆地。本次抬升的主要成因被认为包括：（i）中白垩世构造事件；（ii）伴随北部塔斯曼海扩张（约63 Ma前）引发的裂谷侧翼抬升；以及（iii）晚白垩世至新生代东部高地隆升，该隆升源于与拆离断层作用（detachment faulting）及地幔柱（mantle plumes）过境相关的地壳底侵作用（crustal underplating）。

在班达格（Bundaberg）东北近海的赫维湾（Hervey Bay）西侧下方的北北西向向斜中，保存了最厚可达5公里的早白垩世玛丽伯勒组（Maryborough Formation）与伯勒姆煤系（Burrum Coal Measures）。基底埋深可达9至10公里。赫维湾下方的早白垩世地层中未发育大型背斜构造，但在班达格东北部更深位置的盆地平缓地层中，可能存在小型构造高点。

对玛丽伯勒盆地上陆域地震数据的重新解译，为该区域盆地的构造格局提供了全新认识。

航次91证实，玛丽伯勒盆地确实延伸至弗雷泽岛（Fraser Island）东南部的大陆架下方。该区域地层发生褶皱与断裂，发育有宽缓的背斜构造。本区域内白垩系及更老地层的地震数据质量较差，可能源于低声波阻抗反差（推测为致密地层）所致。该大陆架大部分区域下方的玛丽伯勒盆地地层厚度约为2.0秒双向旅行时（two-way travel time, twt），约合4公里。宽湾（Wide Bay）区域的大型向斜中，沉积了最厚可达5公里的早白垩世沉积物。该区域基底埋深可达8至9公里。

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