Study on the Geo-Environmental Evolution of the Laolonggou Lagoon Under the Impacts of the Caofeidian Reclamation Project in Hebei Province

The Caofeidian Reclamation Project has been the biggest reclamation project in China so far, in which 310 km^2 sea areas were reclaimed along the coast of Hebei Province, and it also bring about many problems and defects for large reclaiming area. The study focuses on the influences of the engineering exerted on evolution of the Laolonggou Lagoon with the methods of topographical measurement and surface sediment analysis. The results demonstrate that the topographical changes in the Laolonggou Lagoon had been controlled obviously by the engineering at three stages. Besides, blocking and reopening of the shoal tidal channel also affected the geological environment of the lagoon area. In the aspect of topographical change, the Laolonggou Inlet first experienced deposition after the shoal tidal channel was blocked, followed by short-time eroding for quarrying and cofferdam construction in the east, then depositing slowly after the reclaiming activity ceased, and finally eroding after the shoal tidal channel was reopened. The project, particularly cofferdam construction led to the movement of the Laolonggou Inlet axis from west to east for 50 – 100 m. In the aspect of sediment variation, the reclamation project and hydrodynamic change resulted in the variation in compositions and distribution pattern. The western lagoon area has become land mainly constituted by silt, while the sediments in the eastern area have turned finer in size. After the shoal tidal channel was reopened, the current velocity in the Laolonggou Inlet has been enhanced, making the sediments at the bottom become coarser. The sediments around the Caofeidian foreland went through a process of changing in grain size from fine to coarse and back to fine again, and the sediments are mainly constituted by silt at present.

Late Mesozoic topographic evolution of western Transbaikalia:Evidence for rapid geodynamic changes from the Mongol-Okhotsk collision to widespread rifting

The Mesozoic geodynamic evolution of Transbaikalia has been largely controlled by the scissors-like closure of the Mongol-Okhotsk Ocean that separated Siberia from Mongolia-North China continents.Following the oceanic closure,the tectonic evolution of that region was characterized by collisional uplift and subsequent extension that gave rise to the formation of metamorphic core complexes.This complex tectonic setting prevailed simultaneously between 150 Ma and 110 Ma both in Transbaikalia,North Mongolia,and within the North China Craton.Published paleobotanical and paleontological data show that the oldest Mesozoic basins had formed in western Transbaikalia before the estimated age of extension onset.However no precise geochronological age is available for the onset of extension in Transbaikalia.The Tugnuy Basin,as probably the oldest Mesozoic basin in western Transbaikalia,is a key obj ect to date the onset of extension and following changes in tectonic setting.In this study,U-Pb(LA-ICP-MS)dating of detrital zircons from three key Jurassic sediment formations of the Tugnuy Basin are used to identify the potential source areas of the sediments,understand the changes in sediment routing and provide insights on the topographic evolution of western Transbaikalia.Our results show several significant changes in tectonic regime after the closure of the Mongol-Okhotsk Ocean.A wide uplifted plateau formed during the closure of the Mongol-Okhotsk Ocean,determining the Early Jurassic drainage system reaching the AngaraVitim batholith to the north and shedding sediments to the continental margin to the South.The following collisional event at the end of the Early Jurassic led to the uplift of the collision zone,which partially inverted the drainage system toward the North.A strike-slip displacement induced by the oblique collision initiated some of the early Transbaikalian depressions,such as the Tugnuy Basin at about 168 Ma.A phase of basin inversion,marked by folding and erosion of the Upper Jurassic sediments,could correspond to the short-term collision event that took place during the latest Jurassic-earliest Cretaceous in the eastern Central Asian Orogenic Belt.The following inversion in tectonic regime from compression to extension is consistent with the mid-lower-crustal extension that led to the formation of the numerous metamorphic core complexes throughout northeastern continental Asia during the Early Cretaceous.

Ice flux of alpine glaciers controls erosion and landscape in the Nianbaoyeze Shan,northeastern Tibetan Plateau

The evolution of mountainous topography is the result of the interactions of climate,topography,and erosion.Glaciers are one of the most active erosional forces sculpting mountainous regions.However,our ability to understand this erosional force is limited by the controversy regarding the major factors controlling glacial erosion.We selected the Nianbaoyeze Shan,which is occupied by subcontinental glaciers(likely cold-based or polythermal glaciers)in the northeastern Tibetan Plateau,to study how erosion by alpine glaciers sculpts the landscape.We first applied a power law model(y=axb)fit to the cross-profiles of glacial valleys and analyzed the controlling factor of glacial erosion;we then identified the morphometric characteristics of the cirques to analyze the influencing factors,and investigated the development patterns of glacial valleys and cirques.Finally,we used hypsometry and the slopealtitude curves and reconstructed the palaeo-glacier surfaces and palaeo-Equilibrium Line Altitudes(ELAs)to determine if the glacial buzzsaw effect had impacted this glaciated region with subcontinental glaciers.Our findings show that ice flux is the main factor controlling glacial erosion.In this region,decreasing temperatures promote an increase in glacier mean size(i.e.,ice flux),which enhances the rate of glacial erosion.Cirque morphology is mainly influenced by aspect and the ELA.The development patterns of glacial valleys and cirques were dominated by a widening process.The ELAs of Marine Isotope Stage(MIS)-3 derived from the cirque floor altitudes or the Accumulation Area Ratio(AAR)and Area-Altitude Balance Ratio(AABR)coincide with the hypsometric maximum and the reduction in slopealtitude curves,which indicates that glacial buzzsaw has impacted this region.Furthermore,the glacial buzzsaw effect has profoundly influenced the mountain heights and topographic evolution in this subcontinental glaciated region.