悉尼“婚礼蛋糕石”白色形成分析及其意义

Analysis on White Origin of Sydney “Wedding Cake Stone” and Its Significances

澳大利亚悉尼附近广泛分布着三叠纪陆相冲洪积砂岩层。该砂岩颜色以黄色为主,此外还有红色和褐色等。在南郊的皇家国家公园内,有一处岩层水平的白色砂岩被当地人称为婚礼蛋糕石(Wedding Cake Rock)。白色蛋糕石耸立在海边悬崖,与周围不同程度褐黄色砂岩层形成鲜明对比。野外观察和室内磁性矿物研究认为,白色的蛋糕石是普通黄色砂岩历经了频繁的干湿交替,使得铁质不断流失而形成的。该蛋糕石地形平坦略显低洼,雨季能够局部汇水。在此环境中,岩层长期经历了雨季的湿润和旱季水分不足的交替过程,导致铁质不断被溶解流失。当旱季水分不足时,铁矿物为针铁矿和赤铁矿,是不溶于水的三价铁。而当雨季水分充足时,岩层充水,空气被隔绝,处于还原环境,铁质在此条件下可以变成可溶的氢氧化物被溶解并能够随流水迁移。如此长期干湿交替的过程就使得岩层中的铁质不断被溶解流失,最后形成局部白色层。这种过程可能是导致岩层次生白化和退色的重要原因。此外,悉尼的砂岩普遍发育有高角度的交错层,而且常见倾斜的交错层是由褐黄色/褐红色纹理交互而成(或者由不同深浅的褐红/褐黄色纹理交互而成),表明褐黄色与褐红色是与层理,交错层层理近于同期形成的,是砂岩的原生颜色。磁性矿物研究表明,褐黄色为针铁矿所致,褐红色为赤铁矿所致,2种矿物分别为三价铁的氢氧化物和氧化物,指示着原生氧化环境,因此可能暗示着悉尼广泛分布的褐黄色/褐红色砂岩是其长期处于空气中干旱氧化环境而不是水下还原环境的陆相冲洪积物。

Well bedded Triassic sandstone is widely distributed around Sydney, Australia. High angle cross-bedding is common in the sandstone and the common stone colors are grey, brown-yellow, and brown-red. There is a very stark white or bleached-looking sandstone outcrop known as the ＂Wedding Cake Rock＂ in the Royal National Park, south of Sydney. This is atop the sea cliff and is surrounded by the normal brown and yellow colored sandstone layers. And investigation by thorough field observation and laboratory magnetic mineral measurements has been carried out. It concluded that the white or bleached color of Wedding Cake Rock has replaced the ordinary yellow color of the sandstone due to a very localized slightly lowered terrain factor. This is thought to be forming a small shallow reservoir during the rainy season. In such environment, the rock suffers environmental change frequently between dry oxidation and wet reduction. This alternating process results in the iron being dissolved and gradually draining off the sandstone at the top of the cliff. When the environment is dry, both goethite and hematite are insoluble minerals in water. However during the rainy season, the upper part of the sandstone has pore spaces saturated by water. This pore water becomes a layer isolating from air, to become a reducing environment. Iron within the rock under such environmental conditions becomes a soluble hydroxide and drains away with the water. Such rock will be finally changed to a stark white color when all the original iron in the rock has been completely dissolved and drained away. This process of iron loss and rock bleaching is likely to be one of the important causes leading to widespread phenomena of color change in geology. High angle cross-bedding is common in the Triassic sandstone around Sydney, and there are various brown yellow, brown and red colors which may further texturally highlight the ＂cross-bedding＂ in the sandstone. These color-texture features vary with bedding （horizontal bedding and cross-bedding）, indicating that these colors （brown yellow： goethite and brown red： hematite） formed more or less at the same time as bedding. The original oxidizing color indicated by goethite and hematite therefore likely suggests that the Triassic sandstones arround Sydney were deposited as continental diluvium in arid area, where the sediments were mainly exposed under air, rather than being mostly under water.