Mesozoic sedimentary and volcanic rocks dredged from the northern Exmouth Plateau: petrography and microfacies

The deeply incised northern margin of the Exmouth Plateau has been dredged extensively along seismic reflection profiles, in water 2000- 5600 m deep, by R. V. Sonne (Cruise SO-8) and R. V. Rig Seismic (BMR Cruise 56). Geological samples obtained have greatly increased our understanding of the Late Triassic- Recent history of the margin. Detailed petrography and microfacies analysis have enabled us to define seven major lithofacies associations. Three Late Triassic to Middle Jurassic associations were laid down roughly coevally on this southeastern margin of Tethys: (1) a Late Triassic- early Liassic volcanic and volcaniclastic association of early rift volcanics, (2) a Late Triassic- Middle Jurassic shallow water carbonate association, and (3) a ?Late Triassic- Middle Jurassic coal measure association. The coal measures were uplifted and weathered to form a ?Jurassic ferruginous sediment and ironstone association. We distinguish 14 Late Triassic- Callovian microfacies types of shallow water carbonates, which can be correlated with the facies of coeval platform carbonates in the Alps and Mediterranean area of the Tethys ocean. During Late Triassic times intertidal to shallow-subtidal carbonates were deposited in the Swan Canyon area close to the palaeo-coastline in the east, and deeper subtidal and shelf lithologies in the Wombat Plateau area in the west. During the latest Triassic and earliest Jurassic, the carbonate platform subsided and was structured into shoals with red biomicrites, and basinal areas with hemipelagic autochthonous micrites and redeposited calcarenitic turbidites. Locally, uplifted blocks, such as the Wombat Plateau horst, were subaerially eroded during Jurassic or earliest Cretaceous times. Carbonate platform deposition continued in places until Middle Jurassic time. Following breakup to form the Argo Abyssal Plain in the earliest Cretaceous, the margin started to subside and a Lower Cretaceous marginal-marine claystone association was deposited, followed by a hemipelagic late Lower Cretaceous radiolarian claystone. As subsidence continued, from Turonian times onwards, there was increasingly pelagic deposition of a Late Cretaceous to Cainozoic association of hemipelagic to eupelagic variably silicified marls and chalks. Complex diagenetic transformations involve silica, silicates, carbonates, and phosphates.

埃克斯茅斯高原（Exmouth Plateau）深切的北缘曾在水深2000~5600米的区域沿地震反射剖面（seismic reflection profiles）开展了大规模拖网采样工作，由桑讷号科考调查船（R. V. Sonne，SO-8航次）与里格地震号科考调查船（R. V. Rig Seismic，BMR 56航次）执行。本次采样获取的地质样品极大地增进了我们对该陆缘晚三叠世至今（Late Triassic-Recent）演化历史的认知。详细的岩石学（petrography）与微相分析工作使我们得以明确7种主要岩相组合（lithofacies associations）。其中晚三叠世至中侏罗世的3种岩相组合在特提斯洋（Tethys）东南缘大致同期形成：（1）晚三叠世-早里阿斯期火山及火山碎屑岩组合，对应早期裂谷火山活动；（2）晚三叠世-中侏罗世浅水碳酸盐岩（shallow water carbonate）组合；（3）成因存疑的晚三叠世-中侏罗世煤系（coal measure）组合。煤系地层经抬升与风化作用，形成了一套成因存疑的侏罗纪铁质沉积物（ferruginous sediment）与铁矿岩（ironstone）组合。本次研究识别出14种晚三叠世至卡洛维阶（Callovian）的浅水碳酸盐岩微相类型，这些微相可与特提斯洋阿尔卑斯及地中海区域同期台地碳酸盐岩相进行对比。晚三叠世时期，潮间带至浅潮下带碳酸盐岩沉积发育于东部紧邻古海岸线（palaeo-coastline）的天鹅峡谷（Swan Canyon）区域，而西部的袋狼高原（Wombat Plateau）区域则沉积了较深水的潮下带与陆架岩性。晚三叠世末期至早侏罗世初期，碳酸盐岩台地发生沉降，被分割为发育红色生物泥晶灰岩（biomicrites）的浅滩，以及沉积半远洋原地泥晶灰岩（hemipelagic autochthonous micrites）与再沉积钙砂粒浊积岩（redeposited calcarenitic turbidites）的盆地区域。局部区域诸如袋狼高原地垒（Wombat Plateau horst）一类的抬升地块，在侏罗纪至早白垩世初期遭受了陆上剥蚀。部分区域的碳酸盐岩台地沉积持续至中侏罗世。早白垩世初期张裂形成阿尔戈深海平原（Argo Abyssal Plain）后，该陆缘开始沉降，先后沉积了早白垩世边缘海黏土岩（marginal-marine claystone）组合与晚早白垩世半远洋放射虫黏土岩（radiolarian claystone）。随着沉降持续，自土仑阶（Turonian）时期起，晚白垩世至新生代的半远洋至远洋不同程度硅化泥灰岩与白垩（hemipelagic to eupelagic variably silicified marls and chalks）组合的远洋沉积愈发发育。复杂的成岩转化作用涉及二氧化硅、硅酸盐、碳酸盐岩及磷酸盐。