A clastic dike containing unusually large clasts occurs in the Quaternary deposits that unconformably cover the Mesoproterozoic sediments in the Fangshan District,Beijing area,China.The material into which the dike in...A clastic dike containing unusually large clasts occurs in the Quaternary deposits that unconformably cover the Mesoproterozoic sediments in the Fangshan District,Beijing area,China.The material into which the dike intruded is also uncommon because it consists mainly of loess-type silts that were deposited by braided rivers.The intrusion of the dike is explained as the result of the expulsion of pore water into the coarse,gravel-containing layers of a braided river system.The large size of the clasts in the dike is explained by an exceptionally strong upwards directed flow which owed its high energy to a high hydrostatic pressure that had been built up because pore water could not gradually seep through the impermeable silt-sized material during ongoing burial.This uncommon dike is compared with a second example,in similar Quaternary sediments covering the Mesozoic rocks in the Huairou District.展开更多
The Mesoproterozoic Wumishan Formation,composed of dolomite is a widely distributed stratigraphic unit in the Beijing area.It was formed over a long period of time in the Yan-Liao aulacogen,a stable peritidal environm...The Mesoproterozoic Wumishan Formation,composed of dolomite is a widely distributed stratigraphic unit in the Beijing area.It was formed over a long period of time in the Yan-Liao aulacogen,a stable peritidal environment that was ideal for recording earthquakes in the form of soft-sediment deformation structures(SSDS).Numerous examples occur in the upper part of the Wumishan Formation,along the Yongding River Valley.In addition,brittle structures include intrastratal fault and seismically cracked breccias.The soft-sediment deformation structures include liquefied features(diapirs,clastic dykes,convolute bedding),compressional deformation features(accordion folds,plate-spine breccias,mound-and-sag structures),and extensional plastic features(loop-bedding).Based on the regional geological setting and previous research,movements along the main axial fault of the Yan-Liao aulacogen are considered as the triggers for earthquakes since the Early Mesoproterozoic.The number and distribution of the SSDS suggest the major earthquake frequency in the Wumishan Formation of 20 to 32 thousand years.展开更多
The Mesoproterozoic Wumishan Formation consists mainly of dolomites that represent diagenetically transformed limestones deposited in a shallow peritidal environment.Although the environment was tectonically stable,th...The Mesoproterozoic Wumishan Formation consists mainly of dolomites that represent diagenetically transformed limestones deposited in a shallow peritidal environment.Although the environment was tectonically stable,the study area in the Changping District(north of the Beijing area) was affected by synsedimentary earthquakes.Later,the dolomites in the study area were intruded by a granite.This resulted in contact metamorphism of the dolomites,changing them in a halo around the intrusion into marble.This marble,which is exposed near Taoyukou Village,shows some levels with well-developed stromatolites.The stromatolites contain structures that point at deformation when the stromatolites had not yet been lithified,and thus represent soft-sediment deformation structures.The stromatolites apparently built up during a time-span when repeated deformational processes affected the still unlithified sediment,from the sedimentary surface down to maximally a few decimeters.The deformed stromatolites thus represent seismites,which feature is exceptionally rarely recognizable in marble.展开更多
文摘A clastic dike containing unusually large clasts occurs in the Quaternary deposits that unconformably cover the Mesoproterozoic sediments in the Fangshan District,Beijing area,China.The material into which the dike intruded is also uncommon because it consists mainly of loess-type silts that were deposited by braided rivers.The intrusion of the dike is explained as the result of the expulsion of pore water into the coarse,gravel-containing layers of a braided river system.The large size of the clasts in the dike is explained by an exceptionally strong upwards directed flow which owed its high energy to a high hydrostatic pressure that had been built up because pore water could not gradually seep through the impermeable silt-sized material during ongoing burial.This uncommon dike is compared with a second example,in similar Quaternary sediments covering the Mesozoic rocks in the Huairou District.
文摘The Mesoproterozoic Wumishan Formation,composed of dolomite is a widely distributed stratigraphic unit in the Beijing area.It was formed over a long period of time in the Yan-Liao aulacogen,a stable peritidal environment that was ideal for recording earthquakes in the form of soft-sediment deformation structures(SSDS).Numerous examples occur in the upper part of the Wumishan Formation,along the Yongding River Valley.In addition,brittle structures include intrastratal fault and seismically cracked breccias.The soft-sediment deformation structures include liquefied features(diapirs,clastic dykes,convolute bedding),compressional deformation features(accordion folds,plate-spine breccias,mound-and-sag structures),and extensional plastic features(loop-bedding).Based on the regional geological setting and previous research,movements along the main axial fault of the Yan-Liao aulacogen are considered as the triggers for earthquakes since the Early Mesoproterozoic.The number and distribution of the SSDS suggest the major earthquake frequency in the Wumishan Formation of 20 to 32 thousand years.
文摘The Mesoproterozoic Wumishan Formation consists mainly of dolomites that represent diagenetically transformed limestones deposited in a shallow peritidal environment.Although the environment was tectonically stable,the study area in the Changping District(north of the Beijing area) was affected by synsedimentary earthquakes.Later,the dolomites in the study area were intruded by a granite.This resulted in contact metamorphism of the dolomites,changing them in a halo around the intrusion into marble.This marble,which is exposed near Taoyukou Village,shows some levels with well-developed stromatolites.The stromatolites contain structures that point at deformation when the stromatolites had not yet been lithified,and thus represent soft-sediment deformation structures.The stromatolites apparently built up during a time-span when repeated deformational processes affected the still unlithified sediment,from the sedimentary surface down to maximally a few decimeters.The deformed stromatolites thus represent seismites,which feature is exceptionally rarely recognizable in marble.
