______o~C~ E=A~N~O~L~O~G~'~C~A_A~C~T_A~. _, _98~,~. _N_.~S~P_1~------
Gulf of Mc:o:lco Seismic data Rifling phase Subsidence A model for the early evolution Drif! Golfe du Me"iqu(' Données sismiques r hase de di s ( en~ion Subsidence of the Gulf of Mexico basin W placcmenl
R. T . Buffler ". F. J. Shaub ", R. Huerta ' , A. Il . K. Ibrahim ", J . S. Walkins ~ • Universit y of Te'las Marine Science InstiWle. 700 The Strand , Galveston . Te:< ••5 77550. USA. b Gulf Science and Technology Company. Explonttion llnd Production Di vision , 1'.0 . 80x 20]8, Pinsburgh. Pennsylvania 15230. USA.
ABSTRAC T Seismic rdlcelion and refraC lio" data from the deep Gulf of Mexico basin conslrain the early geologic evolution o f the b
Oeel/lloi. Acw. 1981. '>roceedings 26, h International Geologklll Congre!> s. Gcolog )' of continental margins symposium . Paris. luly 7- 17. 19HO. 129-136.
129 A T. BUFFlER el si.
RESUME Modèle de révolution initiale du bassin du Golfe du Mexique
Les données do: si~ mique réneClion el réfraclion du bassin profond du GoJf~ du ~fcxique fourni~senl des guides pour le modèle d 'évolmion géologique initiale du bassin. Le ~ princi palcs observatio ns. incluant ];1 géométrie dc la croûte. la nature el la répartition de ~ sédiments inil iaux sont les suivanlelo: 1) la crOÛ le océanique supposée dans la partie cenlmle profonde du Golfe (5 à 6 km d·épai ~~c ur : 6.8 à 7,1 kl1tls) est bordée ~ymét riq ucmcn t au Nord el au Sud par unc croûle supposée de transition (8 à 15 km d ·épai~~eur : 6.4 à 6.8 kmfs) : 21 le substratum acousliquc visible sur les données de s i ~ miquc réflection dans le Gol(e eenlral eSI un réflecteur irrégul ier représenwnt probable ment le toit d·un niveau volcanique (basaltique) correspondant à la couche 2 : 3) au nord de rescarpcmenl de Campêche. le toil de la croûle tran~ilionnelle est représenté par un fort réflecteurldilocontinuilé régulier qui limite le~ bassins du rift ct cst surmonté Ù·llne é pai s~e couche de sel et de sédiments: 4) dan~ Ic Gol fe méridional. la crOU le de Iran'i ilion csl constituée de blocs de substratum basculé~ corrcspon dant probilble ment li une croûte continenlale amincie et effondrée: Ic s crcux enlre les blocs sont comblés par le s sédiments ~y n - rift : 5) rhorizon salifèrc borde symétriquement le~ cx trémités nord CI sud de III croûte océanique : 6) la slratigraphie sismique montre que 'e~ sédiments du Golfe duns les zoncs de croûte transitionnelle rcprésentent une évolution verti C.tle depuis Ics volcanÎtes el les sédiments continentllux, y compris le .c; évaporites. vers des faciès marins de faible profondcur. pui ~ de ~ ~éd imcn u à grande profondeur de dépôt. ces dcrniers reposant directement sur la croûte océanique dans le Golfe central. Ces données suggèrcnt le schéma d ·évolution suivllnt : le balo~ in initial currespondant au Golfe du Mexique a évolué comme une marge continenHlle di vergente (passivc). il la même époque et ~ ui vant Ics mêmes m écanis me ~ que la marge nord-atlantique. y compris la formation de croûte océanique. Nous suppoMlns qu·une "hase d~ distension ("·rif"·) Trillssico-Jurassiquc a élé accomp:lgnée d·un bombement générali sé. une distcnsion el un comblemcnt de bassins de rift par dc ~ volcaniles CI des roches d·origine terre ~ lre lor~ que I"Amérique du No rd s·esl séparée de I"enscmblc Afrique-Amérique d u Sud. La formaI ion de la croûtc de transition amincie a eu lieu ~ndanc celle période. Pendant rultime ét,lpc de celle phase (Jurassiq ue moyen-supérieur) le Golfe ~ ·cst morcelé cn une sérÎe de bassins subsidencs séparés: dans ce rlain ~ d·entre eux. un épaiS horizon de sel s·e,t déposé dans un environnement marin de faible profondeur. Une phase dt s'Ibsidence el de dlpfacemeJ/l C·drif"" IL débuté au Jurassique supérieur. iJ y a environ 150 millions d·annécs avec une di~ten s i on majeure du Golfe ce ntral et la formation d'une croûle océanique nou velle. Le bloc Vucatnn 5·eu probablement déplH cé " c("Illi, rAmérique du Nord suivant une dircction NO-S É parallèle aux failles trllnsformantes de l' Atlantiquc Nord. en séparant le ba s~in salifère de chaque côté de la nouvelle croûtC océaniquc. L·accrétion fut aplXlremmenl brèvc. mais le bassin a continué à subsider rapidcment du fait du refr o id i~~emcnl de la lit hosphère. La s ub~idenee IL été accompagnée d ·un fluage par gr:t\'ilé du sel daM le bassin et de la mise en place d·une marge carbonatée toul le long d·une Lo nc tectonique charnière à la périphérie du bassin. Des sédiments d ·eau peu profonde il profonde sc sonl dépo~és le long des margcs du bassin . landis que seuls des sédiments de grande profondeur se sont déposés dans le bassin océanique nouvcllcmcnl créé. L ·évolu tion initiale du ba~s in s·cst tcrminée au Créweé moyen. °lur, d·un ,hénement dïmport;;nce majeure ~ouligné par unc forte discontinuité à I"échelle du Golfe lout entier. L3 sub~idence 101:'llc du bassin depuis la formai ion de la croÎlle océanique a été supérieure à 10 km. cn partie à cause de la charge ~é dimenl;tire et en pilrlic à cause du refroidissement de ln lithosphère.
Ocea nol. A ("((j, t981 . Acles 26'COnllrès Jnternation'II de Géologie. colloque GéOlogie des marge ~ conlÎnentalcs. Paris. 7-17 juil. 19S0. 120,1·136.
INTRODUCT ION Reccnt plaie re eons tru ç .ion ~ ba~ed o n paleomagnctic data suggest various Triassic-Early Jurassic ovcrlap~ o f South The Universil y of Tc xa~ Marine Scicnce Instilute America with ccntral Mexico. Vuca",". and portion~ o f Ihe GalveMon Geo("lhysics Laboratory - (UTMSI-GGL) began Gul f (Ladd , 1976 ; Va n der Voo el al. . 1976 : Pilger. 1978 : in 1974 a Jong-Ierm project 10 study the geologic hi ~ t ory of Gosc et ai .. 19S0). These reconstructions pl us olher struelU the deep Gulf o f Mexicu basin a nd adjacent margins using rai and stratigraphie data, mainly from the periphery of the new seismic rdleClio n and refraction techniquclo. Since Gulf. ha ve led va ri ou~ a Ulhor ~ 10 propose models for the then. UTM SI-GGL has collected nearly 30.000 km of multi origin of the Gulf that involvc mo\·ement of Ihe Vucalan fold (t2·fold and 24·(0Id) ~ebmic rdlection data and block away from North Amerie;1 wilh an alo~oc i 130 A MODEL FOR THE EARL Y EVOLUTION OF THE GULF OF MEXICO BASIN menlS look pl vroro Fisure 2 Crll slOl SUI;OIlS IICroSS tlle Grllf <1/ MtXiCfJ boscd rI" UT.WS I D IJ S N'fractiOIl profiles. Sto/lo" 12 S W ' om1 21 W ' aN' from e,..illget al. (1960. 19(2). VII/oeities are i/l km/ue. SrI' Figllfl! 1 for /ocalio" of ons profiles. A nmp showing the generalized distribut ion of infcrrcd oceanic vs. transitÎonal I;rust i:. incl udcd as Figure 3. This map is b.1sed both on the published and O OS refraction datlt (1x1TS on Fig. 3) as weil as the scismi c reflcction data (Fig. 1). This dil\ tribution is l.tirly weil controlled over most ol the basin excepl in the norlhern part benellth the Tc:xas-LouisÎana slope. Hcre li thi ck dcformed salt and Figure 1 sedime nHtr y sectio n masks any deep seismi c rcflection and Mllp of Oulf of Mexico IIfeo slrol<'ing /ocMimr uf UTMSI·GGL rcfracti on data. The presence and distribution of transit io· mrllt/fuld sei.mric Un es IInd OHS refroeliQlr profiles. Hea~y lirre alurr g Line G1'2-/O is foell/i/r (If Figure j. l.e/t ers A· /J IlIdleou nul crust, therefore. is hi ghly tcntollivc. It is based mainly on co"rro/ poillts used ill Figure 7. the assumption that the salt underlying the slope is 13 1 A T. BUFFLEA 6/ al. somewhat lIutochthonous and was cleposited on transitional w E crus!. The former assumption is Slillsomcwhal specul athe. 0- 10 Km while the la tter is ba~ed on analogy with the deep southern Gulf (discussed below). If these assumptions are close to M100lf UoSE Of corrcct. there appears to be a fair degree of symmetry in Ihe CRETACEOUS 08S-9 PlEISTOCENE di ~ tribul io n of transitional c r u~t on either side of the oceanic crust (Fig. 3). :_-~_~-~_=~'~· S!'~_::~w~_~_~__ iii~,~ ~~~:-!.... ;;:?,-~ô~ _ . _ _ An acoustic basemenl seen on the multifold seismic data 6 over most of the cenlral and eastern deep Gulf is interpreted .-~... - . ~--- ~~ ----"""31 to represent the top of the crust or basement. Il has ...... - ' ...... ,.... diHerent charactcristics in diHerent areas. which provides ~"-- . ~ _ -:- ~ - - - • data to better define the di ~ tributi o n of the crust (Fig. J). -~ ~ - " -..., - -_..6 .8 '- 1'<._ - - 12: - 8~O .. SEC (KM/SEC) ' ''''. 1 ~.- ... '" ; ACOUSTIC BASEMEHf 1TOP OCEANIC VOl.C»I1C LAYER 21 Figure 4 PortÎon of mullifold .lti.lrnic lillt 16-1- 101. 15 along OB5-9 (Jtt Fig. 1 for local/(JR). Top III 4.8 kml.lte". faytr C'orrnponds ... ;Ih major mÎddlt C fI'/aCtQ II S rtfltcw"rmC'o l1/orm ÎI)'. AC'olls/ ÎC' bastmtlll OCC'l(' S btl1ttllh tht 4.8 km/ste". layu alld Îs ilrltrpfl'rfd /(J rl'p'l'.ftnl /lit 10P of oC't/mÎC "olC'orrÎC lu)'u 1. Aiternlilively. the irregular acoustic basement in Ihe oceanic part of the deep Gulf could represent the top of li thin salt layer with the relief being local salt pillows. The seismic reftection characleristics are very similar 10 those observed for the IOp of salt domes and pillows in the salt provinces to ' .. ' the south and north. The lack of any weil substantiated salt Figure 3 dome in the luea, however. pl us the absence of a smoolh. Map shu ... ing gtlleralized dislribl.l/;Qn of octmlic "S. IrU/Isil iOllul sub-sal t rd lector seen throughout the SOUl hern salt province c,usl in Ih .. ,..nlml f1 .. lf of Mtxico basilr bastd on stismic rtlltc/ion (d iscussed below and by Buffler t! / lIl .. 1980). s ug gest~ an dala and nlrac/ion da/u. Bars ...· ;Ih dork ô rclt s rtprtSt/lt pllblislrtd interprela tion that salt is absent here. ftfroc/ion da/a. Hta.,y dashtd /int;'1 SO lllhtasltrn Gillf rtprtstnu rnnd ul majo r slruc/llfal bounda,y. Lcng doshtd Unn rtprlStnl ln the area underlain by inferred Iransitional crust north of Irtnd of North Allan/le Iracturt ~o nts (FZ). }V indicalts orta of the Campeche Escarpment (Fig. 3) acoustic basement is }urossic- ..olca nies in tht slIbsurfact OI .fI! lIlhtrn Frorida I)'ing sO llth characterized by a high-amplitUde. relal ively smooth, baSÎn 01 Palt 132 A MODEL FOR THE EARLY EVOLUTION OF THE GULF OF MEXICO BASIN N Fiiture S Portia" of mul/ifold rt;smic Ii", 072·/0 ill rou/heOS/lm Oulf (Stt Fig. 1 for loctJfiQn) sllQ"'i"y rifltd cO lllinlnrol crusl Qr /ransi/ionalouSI conlain/" B rifl btuins. Promi"t,,' unconfor· milits iflll1'p rtftd la fit ISO m y B.P. and 97 m y B.P. brock.tl a Ihick posl-rifl Stqul"cl rtprtstfllilly transition IIp'''ord from shollolO'-lo dttp-mori"t stdimtnls. Nlarby DSDP97 ill projec Itd illlO Ih t stelion, and ENA- TZ is a projl'cled DSDP holl' 10 bl' drilll'd in No'·l'mbl'r-Dtcl'mbl1' 1960, 08S-ll locoll'd "tor Ihe north end of Ihl stetion shows thl ,·t/oeil)' Slructurt 0/ tltl rron si/ionol ousl. PR E-MIDDLE CRET ACEOUS SEDIMENTARY smoO lh refleetor/unconformity intcrpreted to represent the SEQUENCES top of the transitional crust (diseussed ltbove and by Buffl er el al .. 1980). It is lim ited on the north by an inferred basement high (Ladd et al. , 1976: Watkins el al.. 1977 : The UTMSI-GG L seismic d:lla show Ihal the deep Gulf is 1978 ; and Buffler el al.. 1980). underlai n by Il thick sedimcntary section tha!. in general. l) thins 10 the soul h and eaSI IInd pinches out along the base A thick section of salt also underlies the Texas-Louisiana of the Campec he and Florida Escarpmenls and 2) Ihiekens slope and the area jusi west of the northern Florida to Ihe north and west to over 10 km benellth deformed Escarpme nt (Fig. 6). The exact distribution of Ihe salt. continental slopes. Sorne of the details of I.hese sediments however. is unknown. One model suggesis thlll most of the are described in previous UTMSI slUd ies (Ladd el al., 1976 : sal t beneath the slope is allochlhonous and has moved OUI Watkins II al .. 1977: 1978: Buffler lt al.. 1980). A over younger sediments for distances up to several l 00 km widespread strong reflector/unconformity tentatively dated (e.g .. H umphris, 1978). Seismic refl eetion data have pene· as middle Cretaceous (97 my B.I'.) divKies the sedimentary trated only the leadint; edge of the sail al the Sigsbee Scarp section into IWO major seqoences (Buffler el al. , 1980). and show sail thrusI up to 10 10 20 km over young rise E:o;amples are shown on Figures 4 and S. sediments (e .g .. SuHler et al .. 1978). Even though the salt may be allochthonous. it still must have originated from an The prc-middle Crctaeeous sequence represents rocks extensive salt basin somewhere below the shelf-slope deposited during the carly cvolu tionof the basin. It varies in having the same general distribution as shown on Figure 6 . thickness between 2 and 3 km over much of the deep basin T he original basin may be located further landward. T his (Buffler el al.. 1980). The sequence can be generally distributiOn shows u defini te symmetry On either side of the subdivided into syn-rift sediments and post-rift sediments. area of oceflnic crust. where salt is in terpreled to be absent. ln the II rea nonh and west of the Campeche Escarpment T he syn-rih rocks in the area of the southeastern Gulf ovcrl ying transitional crust. the syn-rift sediments consist underlain by transitional crust eonsist of thick sedimentary mainly of a thick salt section. The in ferred age of thi !> salt is seq uences filling basins in the rift topography. T hese rocks Mi ddle to Late Jurassic . and il i5 interpreted to bc approxi lire in terpreted tu represe nt volel1n ics and conline ntl1 l sedi mately equivalent to Ihe Louann Salt beneath the northern ments illlsed mainly on their sett.i ng (Fig. 5). Apparenlly. Gulf coast (Kirkland, Gerhllrd . 1971). The seismic refJection there is no salt in this area, possibly bccause these rift data :tllow us to map the d is tribut ion of this sa lt in this area b:tsins never were connc cted to the seu during the time S'11t fairly accurately (Fig. 6). Il pinches OUI to the south along was being deposited elsewhere in the Gulf. These sequences the base of the Campeche Escarpmcnt against the strong. are eonfined to the nrea of rifted transitional crust and pi nch o ui nort hward against li basement high al the inferred oceanic-transitional crust boundary. These sediments lie below a major uneonformity esli mated by correlation with glObal sea-Ie vcl chatt to be about 150 my B.P .. the postulal cd lime o r formation of oce,lII crust in the central Gulf (Fig. S). A post-rifl sedimenlflry !>equence rallging in age from LaIe Jurassic 10 middle Cretaceous (1S0 to 97 my B. P.) occurs Ihroughout the deep Gulf basin. In the areas or transitiomll crusi seismic stratigraphie analysis suggesls Ihe sequence probably represents a ttansition upward trom shaJlow-to deep-m:trine deposi ts, while in the area or ocellOic crust the sequence probably is totally decp marine. North of the Campeche Escarpmenl , the lower part of the sequence is deformed by early s1I It flowage and is interpreted to be Ihe southern Gulf equivalent of the shall ow-waler Upper J uras sic Smac kover Formation and associatcd rocks beneath the .,. ... northern Gulf cuast (B urner et CIl .. 1980). The upper .,,"" undcformed part of this .~ eq uence is interpreled to represent Figure 6 the deep-wntcr equivalent of the massive Lower Cretaceous MUII sI1l> ... ill}/ illferreJ Jil'lrillll/iQII Q/ sull (s Ii/l/lkJ) ill Ille dU/I cO\rbo nllte banks that farm the adjacent Banco de Climpeche cell/ml Gllif (Jf Mexico b(uill. (Buffl er el al.. 1980). 133 R T. BUFFlER e/ a/. The po:.t-rift :.ediments in the SOUlheastern Gulf Iying cooling and contraction of a newly (ormed ocean crust. Thi ~ between the inferred 150 my and 97 my unconformities also provides a logical mechanism :.imilar to Ihal proposed for may represenl an upward transition from rclatively :.hallow the subsidence of Ihe Atlantic margin (Sleekler. Wam. marine 10 deep-marine deposits (Fig. 5). The gently progrr. 1978: Walls, Sieckler. 1979). ding sequences are interpreted to be shallow shelf-slopc A ~chemalic cro:.s·section across the soulhern margin of the deposits. which are overlai n by a hummocky sequence basin from Yucatan tO the central Gulf is used 10 iIIustrate intcrprelcd 10 bc deep-sea fan deposits (Fig. 5). Again. Ihe furlher Ihe subsidence history (Fig. 7). The diagmm is very upper part of the :.equence prob1lbly represents the generalized and i:. ooioed un four vertical ~ection:.. See deep-water equivalenl of the Lower Cretaceous carbonate lions A and B are controlled ~trictly by seismic dal .. : C i~ margins forming the adjacent Florida and Campeche controlled in pan by DSDP 94 and seismie data ; and D i ~ Escarpments. This old carbonate bank margin evidently conlrolled by a weil (Yucatan 4) on the Yucatan Peninsula. rimmed the cntire GuI( bilsin during Lower Cretaceous time Seclion A lies on inferred oceanic crust. If ail the sedime nts (Neocomian-Ccnomani:.n) (dotted line on Fig. J). are stripped off Ihe crust at A using methods outlined by The post-rift sediments in the ccnlml Gulf overlying oceanic Steckler and Wall~ (1 97!!), the crust rebound:. N A o 5 s_n'''E (Yuc.4) 8AHCO <10 CAIoI P[CH( o p. ,. " '""'" 1 p. (;O'Itltlt"tAL 0 PL[.nOC.E"E tllUst • " 1 ,• " .. LUE ~ • ,·• p. J ,• 1T : " o • ~ •• .. = ,.,. ~ 1 .1 \I.E." 30 :1 -_. ~ ()Ç[A ,,!C .- • tllUST. . . f' OWITLE ' l. ._ !}""" <\, Fil/ure 7 Sc/.ellla/ie eross-sec/IOII acro,U .~ (/U,lIem nU/fgin (Jf Ille dc~p Gulf blm'" (He Fig. 1 /Uf lun/,lu" u/ Hdill"~}' A ' ffpft'l'ftl/S f eSllmuioll 0/ wp of IX·t'all C"I$I a/ur Slrippillg en tiTe sediment CO/Unlll. A " fl'pfeselll.f illferred dep/II 0/ ocea/lie NI/II ju.1I IIfler /ofllwlioll (1 lpproXllllUldy /JI) AI}' H.P.), CfU.!f lIilS .mh .~ided Il WIIII (J/ 7.4 km (3.4 km hy udim'''t IIJl/dill/,! and 4.0km b)' ('1j()Ung v/ I/.r Ii,hl)~phefr). 134 A MOOEL FOR THE EARLY EVOLUTION OF THE GULF OF MEXICO BASIN massive carbonale bank mllrgins around the perip hery of southwesl (Pacifie Ocean), all owed deposition of great the Gulf in Late Jurassic·Early Cretaceous ti me (Fig. 3). Ih ic knesses of evaporiles in some o f the basins as the y Most of the subsidence due to sedime nt loading probably continued to subside. pan ie ularly in Ihe cenual Gulf. took pl l\Ce since 97 my B.P., as Ihis is the lime represented Deposilion of the evaporites probably occurred in relatively by the thickcst sedimentary section in the deep mlsin shallow-waler environments. Some arells such as the rift ed (Fig. 7). southeastern Gulf did not have early access to the sea. Here The schematic d iagram (Fig. 7) also suggests that the cnti re thick. nonmarine syn-rift sediments continued 10 f iIJ the rift Banco de Campeche subsided somewhat uniformly. while bllsins. the lIrea of IransitionaJ crust and oceanic crust north of the The drifl and subsidence I,/rase began during the carly LaIe bank subsided much more rapidly. The blln k margin at the Jurassic (about 150 my B. P.) when a major rift opened in the Campeche Escarpmenl see ms to be controlJ ed by some sort central G ulf and new ocean crust began to form as the rift of major hi nge zone in the basement surface. A sim ilar widened . II is not known how long the period of ocean crust hinge zone has becn observed along the Atlantie margin formation or seafloor spreading lasted. Il probably was (Watts, Stcckler, 1979: Austin el al., 1980) and may fairl y brief and on the order of several millions o f years. represent the boundary between contine ntal and a thinner sÎ nee the amounl of separation is only severa] hundred transitional crust. kilometers. This period is interpreted to correspond appro ximately wilh Ihe major unconformity (150 my) in the;: southeastern Gulf separating syn-rift from post-rift sedi ments (Fig. 5). The openi ng separated the salt basins o n DISCUSSION. A MO DEL FOR T HE EARLY EVOL U cilhcr sitle of the newly-formed ocean crust. T ION OF T H E GU LF O F MEXICO BASIN The symmetry of crust and sll11 (Fig. 3 and 6) ~ugges l that Interpretation of new UTMSI-GG L seismic refJeetion lmd the Gulf opened in a NW-SE d irection. The most logical direction would he along flow Hnes parai leI to Ihe North refraction data liiong with ail previous data allow us 10 suggest a model or scenario for Ihe early geologie evolution Atlantic fracture zones (FZ-Fig. 3). SimiJar trends have o f the Gulf of Mexico lxlsin. The model Îs b..1Siclllly similar been mapped inlo the eaSlern Gulf as mllgnetic li neati ons to earlier proposed models , which suggests that new ocean (Klitgord , US Geological Survey. pers. comm.). This trend crust fo rmed in the Gulf at the same time (Triassic-Jurassic) is also parall el to the major structural boundary mappcd in and by Ihe same mechanisms involved in the evolution of the southeastern Gulf (Fig. 3) and other structural trends in Ihe early North Atlantic margin as North American separa the deep Gulf. Reconstruction of the basin along Ihese lines ted from Arrica-South America (e.g., Humphris. 1978 : 111l0WS for a fairlY good fit between the salt basins. A sligh tly Dickinson. Coney. 1980 : Salvador, 1980: Walper, 1980). beller fi t is obta ined. however, by reconstrucling the Gulf Our model, however. is supported by new seismic refrac along a more N-S d irection. Perhaps the re were several tion and reflection data , which constr;lill the mode!. Major periods of movement in different directions. No magnelic feal ures incl ude : 1) Ihe seismic characler and symmetrical lineatio ns or st ripes have yet been indentified in Ihc central distribution of inferred oceanic vs. transitionnl crust ; 2) the G ulf to indicate direction of opening. distribution of salt o n either side of oceanic crust ; and 3) A logical plate boundary al Ihe time of opening of Ihe Gulf the inferred subsidence history of the bnsin. These dala arc woul d he across norlhern Mexico, through the ccntral Gulf. presented here or sorne h;we been presented earlier (Buffler and then out through the Straits of Florida north o f Cuba. el al .. 1980). This would in volve movemcnl of Mexico Hnd Yucatan away lJiscussion of the ellrly geologic evo!ulion of Ihe basin i ~ from North America as one block. Transverse motion divided into two main periods ; a rift plU/se and a drift (II/d woul d he taken up lIcross norlhern Mcxico by an inferred subside/ICe phase. Each phase is discussed in more detail megashear (Fig. 3). Such li fellture has been postulated by below. several authors (e.g .. Pilger. 1976: Dic kinson. Coney, The rift phase consists of a long period (Triassic through 1980). particularly 10 explain carly (Triassic-Jurassic) rota' Mitldle Jurassic) of widcsprcad dorning. oplift. rifting. and tions or northern Mexico independenl of North Ameri ca as fillîng o f basins wilh volcanics and Ilonmarine sediments. evidenced by recenl paleomagnetic data (Gose el al .. 1980 ). Evidence for this ocçurs ail along the margins of the North An alternatÎve plate boundary would be south across Ihe Atlantic, the northern Gulf coast, Hnd in Mexico and lsthrnus of Tehuantepec, which would altow movement of Guatemala. T he J urassie volcanics in the subs urfaee of Yucal;ln so ulh as a block indepcndenl of the rest of Mexico. soulhern Florida (JV in Fig. 3) may havc formed during the After seafloor spreading in the Gulf flborled. possibly due to latter part of Ihis period. T he se ismic data in the Gulf pla ie reorganization. the basin began to subside ;I S Ihe ind icate Ihat similar rifting liiso occurred benearth parts of lithosphere cooled and contracted . Evidence for subsidence the deep sO Ulhern Gulf. as evidenced by tillcd blot:ks fmd is supported by in terpreling the sedimentary hi story of Ihe rift bllsins filled wilh synrift sediments. This evidently was Jeep basi n from the seismic data. Early deformatÎon of sail pllrt of Il period of great crustal sIret ching, altenuation. and and overlying sediments by gravit y flowflge was probably formation of thinneJ "transiti onal" crust along the entire triggered by subsidence of the basin. Shallow-to deep-willer Atlantic-Gui! margin accompanying the early separation of sediments were deposited along Ihc margins of Ihe b,lsin in North America from Arrica-South America. The mecha areas ovcrlain by subsiding transiriomtl crus!. T he upper ni sm in the mantlc for forming th is trllnsition,ll crust is not sediments were the deep-water equi vale nts of the mllssÎve weil understooJ but probabl y involves SUlne combinalion of cHrbonale plalforms Ihat built up on the adjacent. more subcrustal erosioll, injection of igneous material. and alle stable conti nental blacks. The Lower Cretaceous carbonate nuation of the upper crus!. mllrgi ns were conlroll ed. in part. by a tectonic hinge zone During the latter p;lft of Ihe rift phase, the Gulf ;Irea separaling Ihe continenlal blocks from the Ihinner transitio scparated into ind ividual subsiding basin s. Early Iransgres nal crust. A re latively thin deep-water section was deposi tcd sion of the sea in the Midd le Jurassie. probably from the in the newly- formcd OCClln bllsin. 135 A. T. BUFfLER el al. The carly (pre-middle Crctaeeous) history of the basin Gase W. A. , Seoll G. R. , S"lIrtz O. K., 1980. The GIU/rega/io ll of ended wit h the formation or a major unconformity, which is MesOOlllerica; pultolll/lgnerk el"idencc. Procudillgs of G .!"}""'po· characterized by tru ncation of beds below and onlap of beds !iÎUIlt. The origill of Ihe Gulf of Mexico. ediled br R. H . Pilger. Louisiana StMe University. Baton Rouge. SI-54 . above. The unconformity is correlated with .1 middle lIumphris C. C., 1978. Sull mo,·emem ou (·ollli/lelilai slupe. /lor Cretaceous (97 my) drop in sea level. which may have Iltem Gu/f of Mexico. Pramework facies und oil/rappi,tg rlwraell! caused bottom curTents 10 scour the seafloor. This uncon rislics of Iht upptr CO/ttint/l/al "wrgill. AAPG Studies in Geology formity represents a basinwide change in sedimentation No. 7. cdiled by A. H. Bouma. G. T . Moore. J. M. Coleman. Am. incJuding the drowning of the outer middle Cretaceous A ssoc. Petr. Geol .. Tulsa. Okla .. 69-85. cilrbonate bank marg;ns. Ibrahim A. K., Car)".., J ., Lalharn G., Buffler K. T., in press. CruSlal struclure in the Gulf of Mexico from OBS refraclion amI multichan nel reflcction data. Am. Al·SOC. Petr. Gtol. BII/I. Kirkland D. W .. Gerhllrd J . E .. 1971. Jurassic sali. central Gulf of Acknowledgemems Mexico. and ilS tcmporill rdillion ot circum-Gulf e vaporites. Am. Assac Petr. Geol. Hull .. 55, 680-6K6. ·We aeknowledge the assistance and cooperation of Cecil Ladd J. W., 1976. Relative mOlion of SouIII America wilh respect and Ida Green plus the captain and crew of the UT/MSI R/V to Norlh Amerie:l :lnd Caribbean tectonics. Geul. Suc Am. 8ull. . lda Gree/!. We also appreciate the assistance of Mr. John S. 87. 969-976. Kunselman and his stafr for their help in proeessing much of Ladd J. W., Buffler R. T. , Walklns J . S., Wnrul J . L .• Carran:r.a the data. Financial support for cOllecting. processing. and A. , 1976. Deep seismic reflec[ion results from the Gulf of Mexico. Geolog)'. ". 365-368. interpretation of the data was provided by the National Parsons B., Sclaier J. G .• 1977. An amlly~is of Ihe variation of Science Foundalion and m.my petroleum companies. This ocean floor bathymetry and heat flow willl age, J. Geophys. Res .. support is greally lIppreciated. Ms. Leone Sames assisted 82, 802-825. with typing the manuscript. We also than!.. James A. Austin. Pllger R. H., 1978. A closed Gulf of Mexico. pre-Atlantic Ocean Jr., for reviewing the manuscript. plaie reconstruclion und the eilrly rift lIistory of Ihe Gulf and Nonll This paper is The University of Texas Marine Science Allanlic. Trmls. Gulf Coast A$Soe. GeaI. Suc .. 211 , 38S-]93. Institute Contribution No. 406, Galveston Gcophysics Sah·ador A. , 1980. Lme Tria5sie·Jurassie p(lleogeograplty UIIU the origi" (If the Culf of Mexico (abSlr.). Proceedings af a symposium. L'lboratory. The ",if/in of lite Gulf of Mexico. edited by R. H . l'ilger . Louisiana Slaie Universit y. Baton ROUlle. 101. Sieckler M . S., WaUs A. B., 1978. Subsid..,nce of Ihe Atlantic-type conlinenlal milrllin off New York . Earl!! Platte/. Sci. Leu .. 40, 1-13. REFERENCES Swolls H. S., 1%7. Scismic refraction slUdies in Ille soulhweslern Gulf of Mexico. Mas/er's Tltesi)·. Texu) A /lIld M Vni,·.. College Antain.., J., E ... IRIl. J., 1963. Seismic refraction measuremenlS on Ille Stalion. Texas. margins of tlle Gulf of Mexico. J. 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