Basement Rocks and Younger Sediments on the Southeast Australian Continental Margin : RV Franklin cruise FR3/01

Overall, the cruise met its objectives of studying rift and drift sedimentation, and obtaining cores for palaeo-oceanography. The east Tasmanian seismic program was completely successful. The planned sampling program was somewhat curtailed by bad weather, equipment failures and other factors. It was least successful off east Tasmania.

A total of about 1300 km of 8-fold multichannel seismic data were acquired along 8 transects across the east Tasmanian margin. The quality of the seismic profiles was excellent, with good resolution and penetration, given the bad weather and the limitations of the acquisition system. The seismic source comprised 2 GI airguns (each 45/105 cu. in. capacity) giving a penetration of 2-2.5 s twt (2.5-3 km) in places. The seismic profiles indicate a structurally complex margin with rugged basement relief that includes large-scale horst/graben structures and volcanic intrusions. The sedimentary section on the continental slope is at least 1.5 s twt thick in some graben and includes Campanian-Paleocene early sag-phase deposits, which are 0.5-1.0 s twt thick. Regional compressive tectonism in the Late Paleocene-Early Eocene has produced widespread inversion (folding/faulting) in this succession. A wedge of Neogene shallow-water carbonates underlies the continental shelf. It shows seaward progradation and attains a maximum thickness of ~700 m beneath the shelf edge. Oceanic basement (?Campanian) adjacent to the margin lies at a depth of 7.0-7.5 s twt. The continental rise and Tasman Abyssal Plain in this zone are underlain by 1.5-2.0 s twt of post-breakup sedimentary section. The East Tasman Saddle is underlain by `transitional? basement and contains a sedimentary section of similar thickness.

During the sampling program 58 of 86 stations were successful: 38 gravity cores (21 successful), 4 piston cores (3), 16 dredges (7) and 28 grabs (28). Total core recovery was 81.4 metres from the 16 successful cores taken in soft sediments, an average recovery of about 5 metres. The fairly low success rate with the gravity corer can be ascribed to problems with foram sand east of Tasmania, and shelly sand in Storm Bay. The low success rate with the dredge was related to the lightness of the gear. The deployment of the heavy piston corer for the first time on Franklin was successful. However, we did not attempt to piston core in deep water.

East of Tasmania we recovered 8 gravity cores, and 7 dredge hauls. Deepwater dredging and coring were surprisingly unsuccessful. The upper slope stations, designed to sample older rocks, were reasonably successful. From these results and some existing information, general conclusions can be drawn about changes along the margin with increasing water depth.

The shelf and upper slope wedge of Neogene, seaward-prograding sediments was sampled out to 1640 m. The sediments recovered include muddy sand, clayey sandstone with siliceous nodules, siliceous sandstone and calcarenite. The calcarenite is presumably part of the Middle Miocene shelf limestone sequence that is widespread off St Helens. Somewhat deeper on the upper slope, basement outcrops occur in steep slopes: granite, arkose, metasediments, conglomerate, quartz sandstone and gritty mudstone. The granites are probably from Devonian batholiths like those onshore up the east coast. Volcanic rocks and conglomerate form a basement block in deeper water on a ridge off northeast Tasmania at ~3750m. Deepwater outcrop ridges support manganese nodules and crusts. Nannofossil oozes cling to the slope, particularly in local basins, and are ubiquitous in deep water. The East Australian Current apparently winnows many of the oozes to form a blanket of foram sand.

本次科考航次圆满完成了裂谷沉积与漂移沉积研究以及获取古海洋学研究用岩芯的既定目标。塔斯曼海东部地震勘探项目完全达标。原定的取样计划因恶劣天气、设备故障及其他因素受到一定缩减，其中塔斯曼海东部海域的取样效果最差。

本次勘探沿横跨塔斯曼海东部陆缘的8条测线，累计采集了约1300千米的8次覆盖多道地震数据。尽管遭遇恶劣天气且采集系统存在一定局限，但地震剖面质量仍属上乘，分辨率与穿透深度俱佳。震源采用2台GI气枪（GI airguns，单枪容量45/105立方英寸），局部区域的穿透深度可达2~2.5秒双程旅行时（two-way travel time，twt，对应2.5~3千米）。地震剖面显示该陆缘构造复杂，基底起伏剧烈，发育大规模地垒（horst）-地堑（graben）构造与火山侵入体。部分地堑内的大陆坡沉积地层厚度至少达1.5秒twt，其中包含厚度0.5~1.0秒twt的坎潘期-古新世早期裂陷期沉积。古新世晚期-始新世早期的区域挤压构造作用使得这套地层发生了广泛的反转构造（褶皱/断裂）。大陆架下方发育一套新近纪浅水碳酸盐岩楔体，该楔体向海方向进积，在陆架边缘下方的最大厚度约700米。陆缘毗邻的洋壳（推测为坎潘期）埋深达7.0~7.5秒twt。该区域的大陆隆与塔斯曼深海平原下方发育厚度1.5~2.0秒twt的张裂后沉积地层。东塔斯曼海鞍部以过渡型基底为基底，其上方沉积地层厚度与之相当。

本次取样计划共布设86个站位，其中58个站位取样成功：重力取芯器（gravity corer）作业38次，成功21次；活塞取芯器（piston corer）作业4次，成功3次；拖网取样（dredge）16次，成功7次；抓斗取样器（grab）28次，全部成功。在软沉积物区域获取的16个成功岩芯中，总岩芯回收率为81.4米，平均回收率约5米。重力取芯器成功率偏低的原因，可归结为塔斯曼海东部海域存在有孔虫砂（foram sand）以及风暴湾（Storm Bay）内的介壳砂层带来的作业难题。拖网取样成功率偏低则与采样装置自重过轻有关。重型活塞取芯器在“富兰克林”号科考船上的首次布放作业取得成功，但本次航次未尝试在深水区开展活塞取芯工作。

在塔斯曼海东部海域，我们共获取8个重力岩芯以及7次拖网样品。深水区的拖网与取芯作业结果出人意料地差。原本用于采集古老岩石的陆坡上部站位取样效果尚可。结合本次取样结果与已有资料，可以总结出该陆缘区域随水深增加的沉积与构造变化规律。

本次取样覆盖了陆架与陆坡上部的新近纪向海进积沉积楔体，最大取样水深达1640米。采集到的沉积物包括泥质砂、含硅质结核的黏土质砂岩、硅质砂岩以及钙砂屑岩。该钙砂屑岩应属于广泛分布于圣海伦斯（St Helens）外海的中中新世陆架灰岩序列。在陆坡上部稍深区域，陡坡处可见基岩露头，岩性包括花岗岩、长石砂岩、变质沉积岩、砾岩、石英砂岩以及含砂砾岩。这些花岗岩可能源自东海岸陆上的泥盆纪岩基。在塔斯曼海东北部外海约3750米水深的海岭上，深水区的基底地块由火山岩与砾岩构成。深水区的露头海岭上发育有锰结核与锰结壳。超微化石软泥（nannofossil ooze）附着于陆坡之上，尤其在局部盆地中更为富集，且在深水区广泛分布。东澳洋流（East Australian Current）显然对多数软泥进行了分选，形成了一层有孔虫砂覆盖层。