With the objective of establishing a distinction between deformation structures caused by freeze/thaw cycles and those resulting from seismic activity, we studied three well–exposed alluvial deposits in a section at ...With the objective of establishing a distinction between deformation structures caused by freeze/thaw cycles and those resulting from seismic activity, we studied three well–exposed alluvial deposits in a section at Dogai Coring, northern Qiangtang Basin, Tibetan Plateau. Deformation is present in the form of plastic structures(diapirs, folds and clastic dykes), brittle structures(micro–faults) and cryogenic wedges. These soft–sediment deformation features(except the micro–faults) are mainly characterized by meter–scale, non–interlayered, low–speed and low–pressure displacements within soft sediments, most commonly in the form of plastic deformation. Taking into account the geographic setting, lithology and deformation features, we interpret these soft–sediment deformation features as the products of freeze/thaw cycles, rather than of earthquake–induced shock waves, thus reflecting regional temperature changes and fluctuations of hydrothermal conditions in the uppermost sediments. The micro–faults(close to linear hot springs) are ascribed to regional fault activity;however, we were unable to identify the nature of the micro–faults, perhaps due to disturbance by subsequent freeze/thaw cycles. This study may serve as a guide to recognizing the differences between deformation structures attributed to freeze/thaw cycles and seismic processes.展开更多
The authors introduced two kinds of newly found soft-sediment deformation-synsedimentary extension structure and syn-sedimentary compression structure, and discuss their origins and constraints on basin tectonic evolu...The authors introduced two kinds of newly found soft-sediment deformation-synsedimentary extension structure and syn-sedimentary compression structure, and discuss their origins and constraints on basin tectonic evolution. One representative of the syn-sedimentary extension structure is syn-sedimentary boudinage structure, while the typical example of the syn-sedimentary compression structure is compression sand pillows or compression wrinkles. The former shows NW-SE-trendlng contemporaneous extension events related to earthquakes in the rift basin near a famous Fe-Nb-REE deposit in northern China during the Early Paleozoic （or Mesoproterozoic as proposed by some researches）, while the latter indicates NE-SW-trending contemporaneous compression activities related to earthquakes in the Middle Triassic in the Nanpanjiang remnant basin covering south Guizhou, northwestern Guangxi and eastern Yunnan in southwestern China. The syn-sedimentary boudinage structure was found in an earthquake slump block in the lower part of the Early Paleozoic Sailinhudong Group, 20 km to the southeast of Bayan Obo, Inner Mongolia, north of China. The slump block is composed of two kinds of very thin layers-pale-gray micrite （microcrystalline limestone） of 1-2 cm thick interbedded with gray muddy micrite layers with the similar thickness. Almost every thin muddy micrite layer was cut into imbricate blocks or boudins by abundant tiny contemporaneous faults, while the interbedded micrite remain in continuity. Boudins form as a response to layer-parallel extension （and/or layer-perpendicular flattening） of stiff layers enveloped top and bottom by mechanically soft layers. In this case, the imbricate blocks cut by the tiny contemporaneous faults are the result of abrupt horizontal extension of the crust in the SE-NW direction accompanied with earthquakes. Thus, the rock block is, in fact, a kind of seismites. The syn-sedimentary boudins indicate that there was at least a strong earthquake belt on the southeast side of the basin during the early stage of the Sailinhudong Group. This may be a good constraint on the tectonic evolution of the Bayan Obo area during the Early Paleozoic time. The syn-sedimentary compression structure was found in the Middle Triassic flysch in the Nanpanjiang Basin. The typical structures are compression sand pillows and compression wrinkles. Both of them were found on the bottoms of sand units and the top surface of the underlying mud units. In other words, the structures were found only in the interfaces between the graded sand layer and the underlying mud layer of the flysch. A deformation experiment with dough was conducted, showing that the tectonic deformation must have been instantaneous one accompanied by earthquakes. The compression sand pillows or wrinkles showed uniform directions along the bottoms of the sand layer in the flysch, revealing contemporaneous horizontal compression during the time between deposition and diagenesis of the related beds. The Nanpanjiang Basin was affected, in general, with SSW-NNE compression during the Middle Triassic, according to the syn-sedimentary compression structure. The two kinds of syn-sedimentary tectonic deformation also indicate that the related basins belong to a rift basin and a remnant basin, respectively, in the model of Wilson Cycle.展开更多
Detailed fieldwork carried out in the southern part of Bida Basin, Nigeria, allowed the documentation of soft sediment deformation structures (SSDS) in the Maastrichtian Patti Formation. The aim of this study is to ex...Detailed fieldwork carried out in the southern part of Bida Basin, Nigeria, allowed the documentation of soft sediment deformation structures (SSDS) in the Maastrichtian Patti Formation. The aim of this study is to examine the sedimentary successions, describe and analyse these deformation features, discuss their deformation mechanisms and potential triggers. The Maastrichtian Patti Formation is composed of lithofacies interpreted to have been deposited in tidal and fluvial sedimentary environments. Soft sediment deformation structures recognised in the tidal sediments were clastic dykes, load cast, isolated sand balls, dish-and-pillar structures, convolute lamination, diapiric structures and recumbent folds. Severely deformed cross beds, ring structures, associated sand balls, normal folds and recumbent folds were identified in the fluvial sediments. SSDS recognised were interpreted to have been caused by effects of liquefaction and fluidization. Field observations, facies analysis and morphology of the SSDS indicate that there are relationship between the depositional environments and SSDS. Endogenic processes are considered as the trigger agents and they are represented by rapid sedimentation and overloading, impact of breaking waves, pressure fluctuations caused by turbulent water flow, cyclic stress and current generated by storm waves and changes in water table. The present study did not identify exogenic processes as trigger agent. The occurrence of SSDS in southern Bida Basin strongly favoured a non-tectonic origin but a clear relationship high energy processes in tidal and fluvial depositional environments.展开更多
基金supported by projects from the National Natural Science Foundation of China (41807298, 41702372, 41672211)the China Postdoctoral Science Foundation (2019M650788)+2 种基金National Key Research and Development Project of China (2016YFC0600310)the China Geological Survey (DD20160022, DD20190059)the Basic Research Funds (JYYWF201810) of the Institute of Geology, CAGS.
文摘With the objective of establishing a distinction between deformation structures caused by freeze/thaw cycles and those resulting from seismic activity, we studied three well–exposed alluvial deposits in a section at Dogai Coring, northern Qiangtang Basin, Tibetan Plateau. Deformation is present in the form of plastic structures(diapirs, folds and clastic dykes), brittle structures(micro–faults) and cryogenic wedges. These soft–sediment deformation features(except the micro–faults) are mainly characterized by meter–scale, non–interlayered, low–speed and low–pressure displacements within soft sediments, most commonly in the form of plastic deformation. Taking into account the geographic setting, lithology and deformation features, we interpret these soft–sediment deformation features as the products of freeze/thaw cycles, rather than of earthquake–induced shock waves, thus reflecting regional temperature changes and fluctuations of hydrothermal conditions in the uppermost sediments. The micro–faults(close to linear hot springs) are ascribed to regional fault activity;however, we were unable to identify the nature of the micro–faults, perhaps due to disturbance by subsequent freeze/thaw cycles. This study may serve as a guide to recognizing the differences between deformation structures attributed to freeze/thaw cycles and seismic processes.
基金This paper was sponsored by the National Natural Science Foundation of China(grant No.40272049)Doctor Research Foundation of China University of Petroleum(Project No.Y020109).
文摘The authors introduced two kinds of newly found soft-sediment deformation-synsedimentary extension structure and syn-sedimentary compression structure, and discuss their origins and constraints on basin tectonic evolution. One representative of the syn-sedimentary extension structure is syn-sedimentary boudinage structure, while the typical example of the syn-sedimentary compression structure is compression sand pillows or compression wrinkles. The former shows NW-SE-trendlng contemporaneous extension events related to earthquakes in the rift basin near a famous Fe-Nb-REE deposit in northern China during the Early Paleozoic （or Mesoproterozoic as proposed by some researches）, while the latter indicates NE-SW-trending contemporaneous compression activities related to earthquakes in the Middle Triassic in the Nanpanjiang remnant basin covering south Guizhou, northwestern Guangxi and eastern Yunnan in southwestern China. The syn-sedimentary boudinage structure was found in an earthquake slump block in the lower part of the Early Paleozoic Sailinhudong Group, 20 km to the southeast of Bayan Obo, Inner Mongolia, north of China. The slump block is composed of two kinds of very thin layers-pale-gray micrite （microcrystalline limestone） of 1-2 cm thick interbedded with gray muddy micrite layers with the similar thickness. Almost every thin muddy micrite layer was cut into imbricate blocks or boudins by abundant tiny contemporaneous faults, while the interbedded micrite remain in continuity. Boudins form as a response to layer-parallel extension （and/or layer-perpendicular flattening） of stiff layers enveloped top and bottom by mechanically soft layers. In this case, the imbricate blocks cut by the tiny contemporaneous faults are the result of abrupt horizontal extension of the crust in the SE-NW direction accompanied with earthquakes. Thus, the rock block is, in fact, a kind of seismites. The syn-sedimentary boudins indicate that there was at least a strong earthquake belt on the southeast side of the basin during the early stage of the Sailinhudong Group. This may be a good constraint on the tectonic evolution of the Bayan Obo area during the Early Paleozoic time. The syn-sedimentary compression structure was found in the Middle Triassic flysch in the Nanpanjiang Basin. The typical structures are compression sand pillows and compression wrinkles. Both of them were found on the bottoms of sand units and the top surface of the underlying mud units. In other words, the structures were found only in the interfaces between the graded sand layer and the underlying mud layer of the flysch. A deformation experiment with dough was conducted, showing that the tectonic deformation must have been instantaneous one accompanied by earthquakes. The compression sand pillows or wrinkles showed uniform directions along the bottoms of the sand layer in the flysch, revealing contemporaneous horizontal compression during the time between deposition and diagenesis of the related beds. The Nanpanjiang Basin was affected, in general, with SSW-NNE compression during the Middle Triassic, according to the syn-sedimentary compression structure. The two kinds of syn-sedimentary tectonic deformation also indicate that the related basins belong to a rift basin and a remnant basin, respectively, in the model of Wilson Cycle.
文摘Detailed fieldwork carried out in the southern part of Bida Basin, Nigeria, allowed the documentation of soft sediment deformation structures (SSDS) in the Maastrichtian Patti Formation. The aim of this study is to examine the sedimentary successions, describe and analyse these deformation features, discuss their deformation mechanisms and potential triggers. The Maastrichtian Patti Formation is composed of lithofacies interpreted to have been deposited in tidal and fluvial sedimentary environments. Soft sediment deformation structures recognised in the tidal sediments were clastic dykes, load cast, isolated sand balls, dish-and-pillar structures, convolute lamination, diapiric structures and recumbent folds. Severely deformed cross beds, ring structures, associated sand balls, normal folds and recumbent folds were identified in the fluvial sediments. SSDS recognised were interpreted to have been caused by effects of liquefaction and fluidization. Field observations, facies analysis and morphology of the SSDS indicate that there are relationship between the depositional environments and SSDS. Endogenic processes are considered as the trigger agents and they are represented by rapid sedimentation and overloading, impact of breaking waves, pressure fluctuations caused by turbulent water flow, cyclic stress and current generated by storm waves and changes in water table. The present study did not identify exogenic processes as trigger agent. The occurrence of SSDS in southern Bida Basin strongly favoured a non-tectonic origin but a clear relationship high energy processes in tidal and fluvial depositional environments.
