Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes
Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes
复制标题
DOI：
10.1111/sed.12280
复制
发表时间：
2016-10-01
期刊：
影响因子：
3.5
通讯作者：
Marshall, John E. A.
中科院分区：
文献类型：
--
作者：
Bennett, Carys E.;Kearsey, Timothy I.;Marshall, John E. A.
洪水生成的桑迪粉砂岩是未被充分认识的沉积物，保存了关键的脊椎动物（放线菌、根齿类和较罕见的肺鱼、软骨鱼类和四足动物）、无脊椎动物和植物化石。首次记录于苏格兰下密西西比统Ballagan组，在490米厚的岩心序列中出现了140多个岩层，其特征是来自海岸冲积平原环境的河流砂岩、古土壤、粉砂岩、白云岩胶结岩和石膏。桑迪粉砂岩被描述为Ballagan组（英国苏格兰）的一种独特的出露相;它们是基质支撑的粉砂岩，含有毫米级粉砂岩和非常细的砂岩岩屑碎屑。常见的破生物体包括植物和大孢子、鱼类、介形类、广翅类和双壳类。与在其他含化石沉积物中发现的化石相比，例如在河流河道砂岩底部的砾岩滞后物，化石具有高度的清晰度。地层厚度和分布在整个地层中变化，没有地层趋势。基质沉积物和碎屑来源于洪泛平原沉积物的改造，包括干燥的表面和古土壤。次生成土改造影响了30%的桑迪粉砂岩层，大部分（71%）覆盖在古土壤或干燥裂缝上。桑迪粉砂岩被解释为粘性泥石流沉积物，其起源于河流的漫滩洪水，以及高降雨时局部洪泛区沉积物的运输;其与古土壤和干燥裂缝的关系表明整个图尔奈阶的季节性干湿循环。来自洪泛平原湖泊和陆地表面的四足动物和鱼类化石通过局部侵蚀和改造而集中，并通过沉积到洪泛平原上的临时湖泊中而保存下来;它们的分布表明了当地的起源，在季节性降雨期间，沉积物被运送到洪泛平原。这些矿床是重要的新地点，可用于保护稀有的非海洋化石材料，并为早期陆地生态系统的演变提供独特的见解。
Flood-generated sandy siltstones are under-recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone cementstones' and gypsum from a coastal-alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix-supported siltstones with millimetre-sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil-bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non-marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.