美国Chesapeake湾撞击坑研究进展及其启示
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摘要
位于美国弗吉尼亚东海岸直径85 km的Chesapeake湾撞击坑,是十几年前发现的由一颗陨星撞击形成的一个复杂撞击坑.该坑的研究经历四个阶段:地下水调查、撞击坑的发现、美国多学科多部门的综合研究和即将进行的国际钻探取心项目.钻井岩心中的角砾成份和微体化石,提示撞击坑的存在,并确定撞击发生在35 Ma前,即始新始晚期.地震反射剖面资料帮助寻找到撞击坑的具体位置,确定撞击坑的结构和形态特征.Chesapeake湾撞击坑埋藏在新生界沉积层之下,是全球已知最大的、保存最好的撞击坑之一.Chesapeake湾撞击坑主要形态像一顶倒置的宽边大草帽,包括外缘、环状洼地、峰环(内缘)、内盆和中央峰.撞击坑的形成破坏了原来的含水层,撞击坑当时即被富含咸水的抛射角砾岩和海啸角砾岩充填,再被后来的沉积层覆盖.Chesapeake湾撞击坑导致地面沉降、河流变向、海岸含水层的中断、内陆咸水楔的出现、地震,决定Chesapeake湾本身的位置,至今仍然影响当地居民的生活.了解Chesapeake湾撞击坑对我国撞击坑研究具有借鉴作用.
The Chesapeake Bay impact crater is a complex crater that was found in the eastern coast,Virginia,USA,a decade years ago,and that formed due to the impact of a meteorite.The history of studies of the crater might be divided into 4 phases: ground water research,discovery of the crater,U.S.multi-agency research program and future international coring program.The composition of breccia and microfossils in core samples suggest the existence of a impact crater and indicate that the meteorite collided with the Earth 35 million years ago,the late Eocene Epoch.Seismic reflection profiles help find the position of the crater and detect the structures and morphology of the crater.The Chesapeake Bay impact crater buried under the Cenozoic deposit is among the largest and best preserved of the known impact craters on Earth.The crater is like an inverted "sombreo", and the principal morphological features include the outer rim,annular trough, peak ring(inner rim),inner basin and central peak.When the impact occurred,the original groundwater aquifers were truncated and excavated by the crater,which was instantly filled by ejective breccia and tsunami breccia.The crater has covered by the post-impact deposit since.The Chesapeake impact crater still affects citizens around the bay today: land subsidence,river diversion,disruption of coastal aquifers,emergence of inland saltwater wedge,earthquakes and location of Chesapeake Bay.Knowing the Chesapeake crater is useful for our Chinese study of impact craters.
The Chesapeake Bay impact crater is a complex crater that was found in the eastern coast,Virginia,USA,a decade years ago,and that formed due to the impact of a meteorite.The history of studies of the crater might be divided into 4 phases: ground water research,discovery of the crater,U.S.multi-agency research program and future international coring program.The composition of breccia and microfossils in core samples suggest the existence of a impact crater and indicate that the meteorite collided with the Earth 35 million years ago,the late Eocene Epoch.Seismic reflection profiles help find the position of the crater and detect the structures and morphology of the crater.The Chesapeake Bay impact crater buried under the Cenozoic deposit is among the largest and best preserved of the known impact craters on Earth.The crater is like an inverted "sombreo", and the principal morphological features include the outer rim,annular trough, peak ring(inner rim),inner basin and central peak.When the impact occurred,the original groundwater aquifers were truncated and excavated by the crater,which was instantly filled by ejective breccia and tsunami breccia.The crater has covered by the post-impact deposit since.The Chesapeake impact crater still affects citizens around the bay today: land subsidence,river diversion,disruption of coastal aquifers,emergence of inland saltwater wedge,earthquakes and location of Chesapeake Bay.Knowing the Chesapeake crater is useful for our Chinese study of impact craters.
引文
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[3]Earth Impact Database.
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[6]Poag C W,Powars D S,Poppe L J,.et al.Deep sea drillingproject site 612 bolide event:new evidence of a late eoceneimpact-wave deposit and a possible impact site,U S eastcoast[J].Geology,1992,20:771~774.
[7]Powars D S,Poag C W,Mixon R B.The Chesapeake Bay"impact crater"———Stratigraphic and seismic evidence[abs.]:Geological Society of America Abstracts withPrograms,1993,25(6):A-378.
[8]Poag C W,Powars D S,Poppe L J,et al.Meteoroid mayhemin ole virginny:source of the north american tektite strewnfield[J].Geology,1994,22:691~694.
[9]Powars D S.The effect of Chesapeake bay impact crater on thegeological framework and correlation of hydrological units ofsoutheastern Virginia,South of James River[M].U SGeological Survey Professional Paper 1622,1999,61.
[10]Powars D S,Bruce T S.The effects of the Chesapeake Bayimpact crater on the geological framework and correlation ofhydrogeologic units of the lower York-James peninsula,Virginia[M].USGS Professional Paper 1612,2000.
[11]Sanford W E,Gohn G S,Powars D S,et al.Drilling thecentral crater of the Chesapeake Bay impact structure:Afirst look[J].EOS,2004,85(39):369,377.
[12]Gohn G S.ICDP workshop report:Deep drilling in thecentral of the Chesapeake Bay impact structure,Virginia,USA[J].Int Cont Sci Drill Program Newsl,2004,6:30~35.
[13]Poag C W.A review of marine seismic reflection studies ofthe Chesapeake Bay impact crater[A].Edwards L E,Horton J W,Gohn G S(Compilers).ICDP-USGS workshopon deep drilling in the central crater of the Chesapeake Bayimpact structure,Virginia,USA[C]:Proceedings Volume:U S Geol Surv Open File Rep,2004~1016,2004,40~41.
[14]Koeberl C,Poag C W.Reimold W U,Brandt D.Impactorigin of the Chesapeake Bay structure and the source of theNorth American Tektites[J].Science,1996,271:1263~1266.
[15]Poag,C.W.The Chesapeake Bay bolide impact:Aconvulsive event in Atlantic Coastal Plain evolution[J].Sedimentary Geology,1997,108:45~90.
[16]Morgan J,Warner M,Chicxulub Working Group,et al.Sizeand Morphology of the Chicxulub impact crater[J].Nature,1997,390:472~476.
[17]Johnson G H,Kruse S E,Vaughn A W,et al.Postimpactdeformation associated with the late Eocene Chesapeake Bayimpact structure in southeastern Virginia[J].Geology,1998,26(6):507~510.
[18]Melsoh H J.Impact Cratering[M].Oxford UniversityPress,New York,1989,245.
[19]向缉熙.陨星撞击地球将是21世纪的研究热门[J].中国地质,1998,3:26~29.
[20]覃功炯,欧强,常旭.国内外对天体撞击地球的撞击构造研究的新进展[J].地学前缘,2001,8(2):345~352.
[21]Grieve R A F,Pesonen L J.The terrestrial impact crateringrecord[J].Tectonophysics,1992,216:1~30.
[2]Poag C W,Koeberl C,Reimold W U.Chesapeake Baycrater[M].Springer-Verlag,New York,2004,522.
[3]Earth Impact Database.
[4]Cederstrom D J,Geology and ground-water resources of theCoastal Plain in southeastern Virginia[M].VirginiaGeological Survey Bulletin 63,1945,384.
[5]Cederstrom D J.Geology and ground-water resources of theYork-James Peninsula[M].U S Geological SurveyWater-Supply Paper 1361,1957,237.
[6]Poag C W,Powars D S,Poppe L J,.et al.Deep sea drillingproject site 612 bolide event:new evidence of a late eoceneimpact-wave deposit and a possible impact site,U S eastcoast[J].Geology,1992,20:771~774.
[7]Powars D S,Poag C W,Mixon R B.The Chesapeake Bay"impact crater"———Stratigraphic and seismic evidence[abs.]:Geological Society of America Abstracts withPrograms,1993,25(6):A-378.
[8]Poag C W,Powars D S,Poppe L J,et al.Meteoroid mayhemin ole virginny:source of the north american tektite strewnfield[J].Geology,1994,22:691~694.
[9]Powars D S.The effect of Chesapeake bay impact crater on thegeological framework and correlation of hydrological units ofsoutheastern Virginia,South of James River[M].U SGeological Survey Professional Paper 1622,1999,61.
[10]Powars D S,Bruce T S.The effects of the Chesapeake Bayimpact crater on the geological framework and correlation ofhydrogeologic units of the lower York-James peninsula,Virginia[M].USGS Professional Paper 1612,2000.
[11]Sanford W E,Gohn G S,Powars D S,et al.Drilling thecentral crater of the Chesapeake Bay impact structure:Afirst look[J].EOS,2004,85(39):369,377.
[12]Gohn G S.ICDP workshop report:Deep drilling in thecentral of the Chesapeake Bay impact structure,Virginia,USA[J].Int Cont Sci Drill Program Newsl,2004,6:30~35.
[13]Poag C W.A review of marine seismic reflection studies ofthe Chesapeake Bay impact crater[A].Edwards L E,Horton J W,Gohn G S(Compilers).ICDP-USGS workshopon deep drilling in the central crater of the Chesapeake Bayimpact structure,Virginia,USA[C]:Proceedings Volume:U S Geol Surv Open File Rep,2004~1016,2004,40~41.
[14]Koeberl C,Poag C W.Reimold W U,Brandt D.Impactorigin of the Chesapeake Bay structure and the source of theNorth American Tektites[J].Science,1996,271:1263~1266.
[15]Poag,C.W.The Chesapeake Bay bolide impact:Aconvulsive event in Atlantic Coastal Plain evolution[J].Sedimentary Geology,1997,108:45~90.
[16]Morgan J,Warner M,Chicxulub Working Group,et al.Sizeand Morphology of the Chicxulub impact crater[J].Nature,1997,390:472~476.
[17]Johnson G H,Kruse S E,Vaughn A W,et al.Postimpactdeformation associated with the late Eocene Chesapeake Bayimpact structure in southeastern Virginia[J].Geology,1998,26(6):507~510.
[18]Melsoh H J.Impact Cratering[M].Oxford UniversityPress,New York,1989,245.
[19]向缉熙.陨星撞击地球将是21世纪的研究热门[J].中国地质,1998,3:26~29.
[20]覃功炯,欧强,常旭.国内外对天体撞击地球的撞击构造研究的新进展[J].地学前缘,2001,8(2):345~352.
[21]Grieve R A F,Pesonen L J.The terrestrial impact crateringrecord[J].Tectonophysics,1992,216:1~30.
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