陕西神木白垩纪恐龙足迹沉积环境与时代研究

2017年,陕西省地质调查中心在陕西神木市中鸡镇公格沟白垩系紫红色砒砂岩中发现多处恐龙足迹,这在鄂尔多斯盆地东北部尚属首次。含恐龙足迹地层岩石组合为一套典型沙漠相"红色砒砂岩"系:岩石组合以紫红色—暗紫红色厚—块状粉砂质泥岩、泥质粉砂岩与薄层中—粗粒长石石英砂岩、石英砂岩为主,构成典型的丹霞地貌景观。发现识别出恐龙足迹3处,小型四足动物足迹2处。根据足迹形态对比,推断恐龙为兽脚类实雷龙类、恐爪龙类足迹,小型四足足迹为哺乳动物足迹类,恐龙足迹分布在水深更浅的滨岸线附近。由于恐龙足迹产地地层研究程度较低,因此,恐龙足迹的研究在地层年代确定以及恢复岩相古环境方面具有重大意义。西部邻区鄂尔多斯盆地鄂托克旗查布地区中生代植物化石组合以银杏类为主,次为真蕨类,属有利于植物快速生长的温暖潮湿—干燥温带—亚热带气候,适合恐龙生活的有利地区。神木市栏杆堡镇邱井沟侏罗系曾经发现恐龙足迹,表明神木市恐龙足迹化石从早、中侏罗世到早白垩世均有分布。下白垩系洛河组沙漠相湖泊沉积中有少量的火山灰层,表明气候炎热干燥。早、中侏罗世与早白垩世恐龙足迹形成时代、岩相古地理、沉积环境明显不同。

Study on the sedimentary environment and stratigraphic age of dinosaur tracks in Shenmu City, Shaanxi Province

Several track fossil sites were found in the Cretaceous purple-red feldspathic sandstone at Gonggegou of Zhongji Town, Shenmu City, Shaanxi Province, by Shaanxi Geological Survey Center in 2017, which makes it the first discovery in northeastern Ordos basin. The rock sequence bearing track fossils is composed of a suite of typical desert facies red feldspathic sandstone; it is characterized by purple-red, dark purple-red thick massive silty mudstone and pelitic siltstone, and is mixed with thin medium- and coarse-grained feldspar quartz sandstone, constituting the representative Danxia Landform. Of these five track fossil sites, three dinosaur footprint sites and two small-sized tetrapod footprint sites were identified. Based on the comparison in footprint morphology, we infer that the dinosaur footprints belong to Eubrontes and Deinonychus, while the small-sized tetrapod footprints belong to primitive mammals; the dinosaur footprints were found in the environment that was nearby the shallow lakeshore line. Due to the limited research on the footprint bearing stratigraphic sequence, hence these newly found dinosaur footprints was of great importance to ascertain the stratigraphic age and recover the paleoenvironment. The Mesozoic fossil plants assemblage in the Chabu area of Otog Qi in Western Ordos basin was dominated by ginkgoales, followed by filicinae, indicating a warm humid temperate-subtropical climatic zone that was facilitated to plant growth and dinosaur living. Combined with dinosaur trackways found in Langanbao town of Shenmu city, it is indicated that dinosaurs lived continuously here from Early Jurassic to Early Cretaceous. There was a thin volcanic ash layer contained in the desert facies-lake sediment from the Luohe Formation of Early Jurassic, indicating a hot and arid climate. The geological age, lithofacies paleogeography and sedimentary environment of dinosaur trackways during Early and Middle Jurassic were distinguished greatly from that during Early Cretaceous.