134Thefrequencyseismicattribute(Figure20C)isagoodindicatorofcomplexreflectorgeometriesthatarebelowseismicresolution(RobertsonandNogami,1984;Taneretal.,1994;ChenandSydney,1997;Taner,2001).Allslicesshowrapidlychang-ingfrequenciesinthenortheastatthetoeofslopeoftheGoldenLaneplatform.Suchfrequencytun-ingmayindicatecomplexreflectorgeometries,suchasslumps,largeblocks,andcoarsedebrisflows.SeismicArchitecture,SantaAguedaandPozaRicaFields,Mexico

Figure21.Comparisonofdip-parallelcore-basedcrosssections(A,B)withseismicamplitude(C)andinvertedacousticimpedance(D)sections.Core-basedcrosssectiontakenfromLoucksetal.(inpress).CrosssectioninpanelBwasscaledtohaveadimensioncomparabletothatoftheseismicsectionshowninpanelsCandD.Bothcrosssectiondataareflattenedonthemaximumfloodingsurface(MFS)oftheAlbian2sequence(topF).Green,yellow,andbluehorizonsonbothcrosssectionsandseismicsections=topofAlbian4(topTamabraFormation),topofAlbian2(topBC),andMFSofAlbian2(topF),respectively.ForlocationofthecoredwellandseismicsectionseeFigure3.TWT=two-waytraveltime.

Jansonetal.135

Moredistally,thefrequencydisplayshowsseverallobateandarchedshapesofrelativelyconstantfre-quencythatcorrespondtothelobesidentifiedontheamplitudeslices.Therectilinearfeaturecuttingoneofthesouthernlobesisparticularlyclear,withitsfrequencyvalueshigherthanthoseofthead-jacentlobatearea.Thedeepestslicedisplaysamorechaoticdistributionoffrequency,indicatingcom-plexreflectordistribution.

Coherence

Thecoherenceattributeindicatestheamountofsimilaritybetweenadjacenttraces.Itisagoodin-dicatorofthecomplexitiesofreflectiongeome-triesandenhancessmall-scaleandsubtlefeatures(BahorichandFarmer,1995;ChenandSydney,1997;Taner,2001;Massaferroetal.,2004).Sev-erallinearfeaturesvisibleoncoherenceslices(Figure20D)areinterpretedasfaults.Inallthreeslices,theareaclosetotheGoldenLaneplatformhasalowcoherence,indicatingcomplexreflectiongeometries,whereasthedistalpartshowsmorelaterallycontinuousreflections(lightercolorinthecoherencydisplay).SeverallobatefeaturesthatmatchtheoneobservedintheotherattributemapcanbeobservedintheAlbian2,Albian3,andAlbian4slices.

InterpretationofSeismicSlicesareinterpretedaschannelsthatfeedorerodelo-batedeposits.Theselobateshapesare1to5km(0.6–3.1mi)wide,andtheirgeometriesarecon-sistentwithaplatformsourcetotheeast.Themorediscontinuouscomplexreflectionareaisinterpretedasslumpswithlargeblocksandcoarsedebris-flowdepositsthatcreateacomplexdistributionofimpedanceanddiffractionofseis-micwaves,resultingincomplexdiscontinuousreflectiongeometries.ThisreflectioncharacterisseenmostlyintheproximalareaneartheGoldenLaneplatformandintheAlbian1sequence,whichisdominatedbydebris-flowdeposits(Figure4)(Loucksetal.,inpress).ComparisonofSeismicandLithofaciesCore-BasedSectionFigure21comparesaseismiccrosssectionwithacore-basedgeologiccrosssectionalongthegeneraldipdirectionofPozaRicafieldofLoucksetal.(inpress)(Figure21A,B).Theseismicamplitudesection(Figure21C),theinvertedacousticimped-ancesection(Figure21D),andthecoresectionareflattenedonthetopAlbian2sequence(topBC)horizon.Characteristicsanddistributionoftheseismicfaciescanbecomparedwiththoseofthelitho-faciesmappedonthecore-basedcrosssection.TheupperAlbiansequence4,whichconsistsofathinstackofhyperconcentrateddensityflows,concen-

trateddensityflowsandturbiditedeposits(MulderandAlexander,2001),andunderlyinglimemudandsuspensiondeposits,iscontainedwithinthetopTamabraFormationstrongpositivereflection(be-lowthegreenhorizon).TheAlbian3sequencecorrespondstothereflectionabovethetopoftheAlbian2sequence(yellowhorizon).Thesereflec-tionsarerelativelycontinuousandalmostparallelthetopoftheTamabraFormation.Thetwointer-valsshowasimilaracoustic-impedancesignature.Albian2sequencesTSTandHSTarelocatedbe-tweentheAlbian2sequence(yellowhorizon)andtheMFSoftheAlbian2sequence(bluehorizon).Thisintervalischaracterizedbyanupperpartwithahigheramplitudesemicontinuousreflectionthanitsbase.Similarly,acousticimpedanceisloweratthetopofthisintervalthannearthebase,althoughthedifferenceismorepronouncedintheproximalSeismicslicesfromvariousseismicattributesshowacommonpattern:?TheproximalareaclosetotheGoldenLaneplatformhasdiscontinuouscomplexreflections,whereasthemoredistalareashowsmorecon-tinuoushigheramplitudereflections.?IntheAlbian2,Albian3,andAlbian4slices,morecontinuousreflectionsformlargelobateshapes.?TheAlbian1slicehasconsistentlymorechaotic,discontinuous,complexreflectiongeometries.Giventhetoe-of-slopeandbasinalsettingsofthePozaRicaarea(Enos,1977),weinterpretedlo-bateshapeswithmorecontinuousreflectionsaslargeprobablybasin-floor-fandepositsofhyper-concentrateddensityflowsandturbidites.Recti-linearfeaturesassociatedwiththelobateshape

136SeismicArchitecture,SantaAguedaandPozaRicaFields,Mexico

Figure22.Comparisonofstrike-parallelcore-basedcrosssections(A,B)withseismicamplitude(C)andinvertedacousticimpedance(D)sections.Thecore-basedcrosssectiontakenfromLoucksetal.(inpress).ThecrosssectioninpanelBwasscaledtohaveadimensioncomparabletothatofseismicsectionshowninpanelsCandD.Bothcrosssectiondataareflattenedonthemaximumfloodingsurface(MFS)ofAlbian2sequence(topF).Green,yellow,andbluehorizonsonbothcrosssectionandseismicsections=topofAlbian4(topTamabraFormation),topofAlbian2(topBC),andMFSofAlbian2(topF),respectively.ForlocationofthecoredwellandseismicsectionseeFigure3.partofthesectionthaninthedistalpart.Thisseismic

reflectionseemstocorrespondtothedifferencebe-

tweenahyperconcentrateddensityflow–dominated

HSTandatighter,debris-flow–dominatedTST.The

Albian1sequenceandtheAlbian2LSTcorrespondtoseismicreflectionsbetweentheupperTamaulipasFormationhorizon(orange)andtheMFSoftheAlbian2sequence(bluehorizon).Incore,thisin-tervalisdominatedbystackeddebris-flowdeposits.Thereflectionsaremorecontinuousandhave

a

Jansonetal.137

higheramplitudeinthecentralpartofthesectionthaninthemoredistalandproximalparts.Instrike-orientedsections(Figure22),thenorthwesternpartofthecrosssectionshowsthesameseismicsigna-tureastheonedescribedforthedip-orientedsection.However,theupperpartoftheTamabraFormation(Albian2throughAlbian4sequences)showsade-creaseinacousticimpedancetowardthesoutheastthatcannotbeexplainedbythegeologicinforma-tiondepictedonthecore-basedcrosssection.RESULTSANDDISCUSSION

Results

TheGoldenLaneplatformtopinSantaAguedafieldshowsindicationsofpervasiveandmaturekarst.Theupperslopeshowsnumerousreentrantsthatcanbeinterpretedasmargincollapse,whereasthelowerslopeshowsnumerouschannelsprob-ablycreatedbythecontinuousexportofsedimentfromtheplatformtoptothebasinalarea.Becausethetoe-of-slopeareaconsistentlyshowsachaoticseismicfacies,itisinterpretedasslumpandcoarsedebris-flowdeposits.Inthedistalarea,thebaseoftheTamabraFormationshowssimilarseismicfa-cies,whereastheupperparthasmorecontinuouslobatetolensoidreflectionsthathaveavaguefan-likedistributionontheseismicslice.Theisochoremapofthetwouppersequencesalsoshowsirreg-ularthicknessdistributionassociatedwithincreas-ingamountsofhyperconcentrateddensityflowde-posits.Seismicreflectionarchitecturematchescoreandwell-loginterpretationrelativelywell.Albian1andAlbian2sequencesaredominatedbymassivedebris-flowdepositsandshowlow-amplitudedis-continuousreflections,whereastheAlbian3andAlbian4sequencesdisplayfrequentthinnerhyper-concentrateddensityflowdeposits,aswellasloweramplitudemorecontinuousreflections.

Discussion

PozaRicaToeofSlopeVersusLowstandIsolatedPlatformsThedepositionalenvironmentoftheTamabraFor-mationhashistoricallybeenthetopicofdiscussion

138ontheshallow-ordeep-waterdepositionalsettingoftheTamabraFormation(BarnetcheandIlling,1956;Beboutetal.,1969;ViniegraandCastillo-Tejero,1970;Cooganetal.,1972;Enos,1977;WilsonandWard,1993).SincetheEnos(1977)studyanditsinterpretationasdeep-waterdeposits,theTamabraFormationiscommonlyregardedastheclassicdeep-waterredepositedcarbonatesystem.However,re-cently,theshallow-wateroriginoftheTamabraFor-mationwasreintroducedbyHorburyetal.(2004)onthebasisofcoreobservationandanalysisof3-DseismicdataoverPozaRicafield.SeismicdataareunambiguousinshowingthelowstructuralpositionofPozaRicafield,some700to1200m(2297–3937ft)belowthetopoftheGoldenLaneplatform(RockwellandGarciaRojas,1953).Noindicationexistsontheseismicdataofmajorpre–TamabraFormationorsyn–TamabraFormationupliftofthebasementblockunderlyingPozaRicafieldwithamagnituderequiredforbringingthesedepositsintheshallowwater.AsecondunambiguousrelationshipistheformationoftheAlbiancarbonatewestward-taperingwedge,whichisthickestadjacenttotheplatformmargin,graduallybecoming0.25timesthethicknessatthecrestofthefield(Figures7,8,18).Thistrendinthicknesssupportsaninterpre-tationofaneasterlysourceofsedimentsatPozaRica.Suchathicknesstrendcouldbecreatedbybacksteppingtheshallow-watercarbonateplatformontotheunderlyingbasementhigh,althoughitwouldthenconflictwiththeHorburyetal.(2004)interpretationofshinglingreflectionasplatformprogradation.Moreover,ifashallow-watercarbon-ateplatformhadinitiatedonthebasementhighandwasbacksteppingand/ordrowning,thereshouldbeasymmetricalgeometryonbothsidesofthebasementhigh.Inaddition,circularseismicanomaliesthatareinterpretedaskarstbyHorburyetal.(2004)arenumerousonthecoherencemapthroughoutthePozaRica–Tamabrainterval.How-ever,theseanomaliesareconcentratednearthetoeofslopeoftheGoldenLaneplatform(Figure20)andnotonthebasementhigh,whereshallowwaterhasbeeninvokedintheHorburyetal.(2004)model.Weinterprettheseanomaliesastheresultofseismicdiffractioninsteadofkarstfeatures.Inaddition,thefan-shapedfeatureidentifiedontheSeismicArchitecture,SantaAguedaandPozaRicaFields,Mexico