LESSON7自翻

LESSON7DEPOSITIONALBASINSANDPLATETECTONICSFromanalysisofstratigraphicdataaccumulatedduringthepast100years,itisclearthatpreservedsedimentaryrocksaredominantlymarine.Thereareseveralreasonsforthedominanceofmarinesediments:(分析过去100年的地层学数据，明显可知残留下来的沉积岩主要在海里。海洋沉积物占大部分主要有几个原因)Thelightsialicmaterialthatformsalargeproportionofcontinentalmassesislimitedinvolume,withtheresultthatcontinentalareasconstituteonlyabout30%oftheearth'ssurface.Themarineareasaresedimenttrapsthatcover70%oftheearth'ssurface.（构成大陆块一大部分的硅铝质物质体积有限，导致陆地区域只占地球表面约30%，海洋由沉积物构成，占地球表面的70%）Becauseofthemovementandsubductionofoceaniclithosphereattheedgesofsomecontinentalblocks,topographicallydepressedareas(trenches)withadjacenteasilyerodedhighlandareasexistatactivecontinentalmargins.（因为板块运动和一些大陆板块边缘发生的洋壳俯冲，临近的负地形（海沟）很容易侵蚀掉活动大陆边缘的高地）Therehasbeenapronouncedtendencythroughtimeforbroadareasofthecontinentalblockstobeinvadedbyshallowmarinewaters(epicontinentalseas).Theresultingshallow-watermarinesedimentsarelaterallyextensiveonthecraton,althoughtypicallytheyarethin.Figure11-2showsthenumberandextentofthemajorglobaltransgressionsandregressionsoftheseseas,asdeterminedfromseismicrecords;butmanymoreminortransgressive-regressivesequencesareknowntobepresent.Thetectonicexplanationforatleastthemajorcyclesisthechangingvolumeoftheoceanicriftsystem.Anincreaseinvolumeoftheridgesresultsinadecreaseinvolumeoftheoceanbasinsandsubsequentfloodingofthelow-lyingpartsofcontinents.Itisclearthattherehavebeentwofirst-ordertransgressions,thefirstextendingfromEarlyCambriantoLateCambriantimeandthesecondfromEarlyCretaceoustoLateCretaceoustime.ExtensivecontinentalfloodingalsowasprominentfromCambrianthroughMississippiantime.Thecausesofhigherordercyclesmightbeeitherthewaxingandwaningoficesheetsorregionaltectonicmovements.PhanerozoiccontinentalglaciationsareknowntohaveoccurredineverygeologicperiodexceptCambrian,Triassic,Jurassic,andCretaceous.（随着时间推移，大陆板块的广大地区存在明显会被浅海侵蚀的趋势（陆缘海）。结果浅海沉积物在克拉通边缘广泛分布，尽管通常很薄。插图11-2展示了主要的全球性海侵和海退的次序和程度，这些都是从地震记录得来的；但是许多更小的一连串的海侵海退现在也被人们所知。主要的沉积旋回的构造学解释是其由大洋裂谷体积改变引起的。大洋体积扩张导致海洋盆地体积减少，随后导致大陆低洼部分被海水淹没。很明显全球有两期一级海侵，第一期从早寒武世到晚寒武世，第二期从早白垩世到晚白垩世。寒武世到密西西比世也主要以大陆广泛遭受海侵为主。次级海进海退的原因可能是冰席的消融和凝结或者是区域性的构造运动。显生宙大陆冰期已知发生在除了寒武纪、三叠纪、侏罗纪和白垩纪的每一个地质时期。）Continentaldepositsare,bydefinition,formedabovesealevelandhencearesubjecttoremovalshouldtherateofaccumulationfallbehindtheerosionrate.Itisnoaccidentthatstratigraphicsectionsonthecontinentalblockscontainmanymorelargeunconformitiesthandeepermarineoroceanicsections.（陆相沉积物，根据定义讲，形成于海平面之上，因此与消除作用有关，即沉积的速率应该小于剥蚀的速率。无疑大陆板块的地层相比深海或者海洋中有更多大型不整合界面。）ItisclearfromstudiesofsedimentdistributionintheoceansthatthethickestaccumulationsofCenozoicsedimentoccuradjacenttothecontinentalmargins;andpaleogeographicreconstructionsbasedonstratigraphy,paleontology,andfaciesanalysisindicatethatthesamepatternhasexistedthroughoutgeologichistory.Figure11-3showstheextentofaccretionontothePrecambriannucleusoftheUnitedStatesduringPhanerozoictime.AlsoshownisthepositionoftheUnitedStateswithrespecttotheequatorduringeachgeologicperiod;itisapparentthattheareawaslocatedwithin30°oftheequatorfromCambrianthroughTriassictime.Thispositionimpliesacontinualtropicaltosubtropicalclimate,andclimateisanimportantcontrolofthemineralcompositionofsediments(seeChapter12).（通过对大洋中沉积物分布的研究，显然最厚的新生代沉积物在邻进大陆边缘出现。并且基于地层学，古生物学，沉积相分析对古地理进行重建表明相同的（沉积）模式在地质历史中一直存在。图11-3展示了美国前寒武纪结晶基地在显生宙时期增长的规模。同样也展示了在各个地质时期美国相对于赤道的位置。显然美国地区从寒武纪到三叠纪处于赤道三十度以内。这个位置意味着连续的热带亚热带气候，而且气候是沉积矿产的一个重要的控制因素（见12章））Auniversallyacceptedclassificationschemeforsedimentarybasinshasyettoappear,butthoseproposedsofararebasedonthreefactors:whetherthebasinisunderlainbycontinentaloroceaniccrust;whetherthebasinwasformedalongadivergent,convergent,orstrike-slipplatemargin;andthepositionofthebasinontheplate.Thegeologichistoryofeachbasinmaythenbesubdividedintocyclesonthebasisofthreeparameters:basin-formingtectonics,depositionalsequences,andbasin-modifyingtectonics.Alltypesofcomplexitiescanbeaccommodatedwithinthisframework.（现在还没有一个被广泛接受的沉积盆地的分类方案，但是到目前为止那些提议主要是基于三个方面：盆地是否下伏在大陆或者洋壳之下；盆地是否沿着分离，聚合或者是走滑板块边缘产出；盆地在板块上的位置。每个盆地的地质历史可以细分至旋回，基于三个因素：形成盆地的大地构造，沉积旋回，和使盆地发生变化的大地构造。在分类框架中，各种复杂的因素都应该被考虑。）Inthepresentdiscussionwegroupthemajortypesofsedimentarybasinsintosixcategories,eachofwhichhasdistinctivestructuralandsedimentarypetrologiccharacteristics.（现有讨论中，我们将主要的沉积盆地分为六类，每一种都有各自的构造和沉积岩石学特征。）Oceanicbasinsareareasofdepositionunderlainbyoceaniclithosphere,forexample,thebulkoftheAtlanticandPacificOceanbasins.Mesozoicexamplesofoceanicdepositsnowaccretedtothecontinentalblocksarewellknowninthecircum-Pacificregion.PaleozoicexamplesoccurineasternCanada.（大洋盆地是海洋岩石圈上的沉积区域，比如大西洋盆地和太平洋盆地的大部分区域。现产出于大陆板块附近的中生代海洋沉积矿产的例子众所周知在环太平洋带。显生宙的例子在加拿大东部。）Arc-trenchsystembasins,themostcomplexzoneofbasindevelopment,forminconvergent-marginareasandposeverydifficultproblemsofinterpretationbecauseoftheintensetectonismthataccompaniessubduction.ExampleshavebeendescribedfromaroundthePacificrim.(弧沟体系盆地，是盆地发展最复杂的区域，形成于聚合边界地区并且因为强烈的构造运动伴随着洋壳俯冲，导致有多问题难以解释。例子已经在环太平洋带提到。)Continental-collisionbasinsdevelop