Geology of the Exmouth and Wallaby Plateaus off northwest Australia: sampling of seismic sequences

In this paper we present the first detailed geological information about the margins of the Exmouth and Wallaby Plateaus. New single-channel seismic profiles helped to select sampling targets, which were related to the established seismic stratigraphy. We obtained pre-Quaternary rocks, mainly by dredging, from 29 stations on the Exmouth Plateau, and from 12 stations on the Wallaby Plateau and the Cuvier Abyssal Plain. The results have led to major changes in our understanding of the northern margin of the Exmouth Plateau, which behaved more as part of the offshore Canning Basin than of the Carnarvon Basin. Middle and Late Triassic paralic, detrital sediments are generally overlain by an average of 2500 m of Early Jurassic shallow-water carbonates and mid-Jurassic coal measures and ferruginous sediments. The coal-measure sequence (lithofacies association A) includes carbonaceous silty claystones with seams of an immature sub-bituminous vitreous coal and coaly mudstone, carbonaceous quartz siltstones and very fine sandstones, medium to coarse-grained quartz arenites, and pyrite concretions. The barren ferruginous sediments (B) consist of brown clayey ironstones, sandy ironstones, ferruginous concretions, and ironstones breccias. The Liassic transgressive shallow-water carbonates (C) are a heterogenous group of micritic limestones, biocalcarenites, very coarse crinoidal biosparites, calcareous quartz arenites, recrystallised sparry limestones and dolosparites. On the northern margin of the, Wombat Plateau (a sub-plateau of the Exmouth), at least 300 m of Late Triassic to earliest Jurassic flows of early-rift alkali rhyolite and undersaturated trachyte s.l. (213-192 m.y.) underlie the early Jurassic carbonate sequence. Above the main unconformity, which corresponds to the Callovian breakup of this margin, Early Cretaceous shallow-marine claystones (facies D) indicate the formation of a juvenile ocean. The mature ocean stage is indicated by a condensed sequence of pelagic foraminiferal nanno chalks of Aptian, Albian and Tertiary age (facies E). On the northwestern Exmouth Plateau we sampled Aptian and Miocene chalks. On the southern margin Mesozoic sandstone and shale, latest Jurassic or earliest Cretaceous marine shale, and Cainozoic chalks were recovered. On the eastern and southern Wallaby Plateau, and on the Sonne Ridge which extends northward into the Cuvier Abyssal Plain, the layered sequence beneath the main Neocomian unconformity consists of interbedded weathered tholeiitic and differentiated alkali basalts, tuffs, basalt breccias and thick volcaniclastic sandstones and conglomerates. A minimum mid-Cretaceous age (K/ Ar age: 89 m.y.) was determined for a somewhat altered basalt from the southern Wallaby Plateau. This suggests that intense volcanism and associated deposition of volcaniclastic debris flows formed the plateau, during or after the Neocomian breakup of this region. Therefore, the plateau appears to have no petroleum potential. Quaternary cores in the central Exmouth Plateau contain methane in very small amounts; the u13C isotope results (-14 to -40%) and the absence of higher hydrocarbons tend to downgrade the petroleum potential of this part of the plateau. Manganese nodules from the southern and eastern margins of the Wallaby Plateau have combined Cu + Ni + Co values of about 0.76%, and are of no commercial interest.

本文首次呈现了埃克斯茅斯高原（Exmouth Plateau）与瓦勒比高原（Wallaby Plateau）边缘的详细地质信息。新的单道地震剖面（single-channel seismic profile）助力筛选了与已建立地震地层学（seismic stratigraphy）相关的采样目标。我们主要通过拖网取样（dredging）获取了第四纪前（pre-Quaternary）岩石，样本来自埃克斯茅斯高原的29个站点、瓦勒比高原及库维耶深海平原（Cuvier Abyssal Plain）的12个站点。

研究结果极大地改变了我们对埃克斯茅斯高原北缘的认知——该区域更应被视为坎宁盆地（Canning Basin）的近海部分，而非卡那封盆地（Carnarvon Basin）的一部分。中-晚三叠世（Middle and Late Triassic）海陆交互相（paralic）碎屑沉积物（detrital sediment）之上，普遍覆盖着平均厚度达2500米的早侏罗世（Early Jurassic）浅水碳酸盐岩、中侏罗世（mid-Jurassic）煤系地层（coal measure）及含铁沉积物（ferruginous sediment）。

煤系地层序列（岩相组合A，lithofacies association A）包含含碳粉砂质黏土岩（其中夹有未成熟次烟煤玻璃质煤层及煤质泥岩）、含碳石英粉砂岩、极细砂岩、中至粗粒石英砂屑岩（quartz arenite）以及黄铁矿结核（pyrite concretion）。无矿含铁沉积物（B）由棕色黏土质铁岩、砂质铁岩、含铁结核及铁岩角砾岩组成。里阿斯统（Liassic）海侵浅水碳酸盐岩（C）为一套异质性组合，包括微晶灰岩（micritic limestone）、生物砂屑灰岩（biocalcarenite）、极粗粒海百合生物亮晶灰岩（crinoidal biosparite）、钙质石英砂屑岩、重结晶亮晶灰岩及白云石亮晶灰岩（dolosparite）。

在埃克斯茅斯高原的次级高原——袋熊高原（Wombat Plateau）北缘，早裂谷期碱性流纹岩（early-rift alkali rhyolite）与不饱和粗面岩（undersaturated trachyte s.l.）的熔岩流（形成于晚三叠世至早侏罗世早期，年龄213-192百万年）厚度至少达300米，伏于早侏罗世碳酸盐岩序列之下。主不整合面之上（对应该边缘的卡洛夫期（Callovian）破裂事件），早白垩世（Early Cretaceous）浅海黏土岩（相D）标志着新生洋盆的形成。阿普第期（Aptian）、阿尔必期（Albian）及第三纪（Tertiary）的远洋有孔虫纳米白垩岩（pelagic foraminiferal nanno chalk）凝缩序列（facies E）标志着洋盆的成熟阶段。

在埃克斯茅斯高原西北部，我们采集了阿普第期及中新世（Miocene）白垩岩样本；在南缘，获取了中生代（Mesozoic）砂岩与页岩、晚侏罗世或早白垩世海相页岩及新生代（Cainozoic）白垩岩。在瓦勒比高原的东部及南部，以及向北延伸至库维耶深海平原的索恩脊（Sonne Ridge），新康统（Neocomian）主不整合面之下的层状序列由互层的风化拉斑玄武岩（weathered tholeiitic basalt）、分异碱性玄武岩（differentiated alkali basalt）、凝灰岩、玄武岩角砾岩及厚层火山碎屑砂岩与砾岩组成。

来自瓦勒比高原南部的一块轻微蚀变玄武岩的K/Ar年龄测定为89百万年，这确定了其最小年龄为中白垩世（mid-Cretaceous）。这表明，在该区域新康统破裂期间或之后，强烈的火山活动及相关火山碎屑泥石流沉积形成了该高原。因此，该高原似乎不具备石油潜力。

埃克斯茅斯高原中部的第四纪岩芯含微量甲烷；碳13同位素（u¹³C isotope）结果（-14至-40‰）及高级烃类的缺失，进一步降低了该区域的石油潜力。瓦勒比高原南部及东部边缘的锰结核（manganese nodule）中Cu+Ni+Co总含量约为0.76%，无商业价值。