N ew Concepts on the D evelopm ent of East Brazilian M arginal Basins by H ung K . Chang, R enato 0 . K ow sm ann and A ntonio M .F. de Figueiredo O ne obvious focus in recent years for B razilian geology has able im provem ents in the acquisition of data (especially been the m arginal basins of the eastern part of the country, seism ic) along the m arginal basins. T he other is the devel- w hich now produce 70 ％of Brazil's oil. Indeed this region is opm ent of conceptual m odels for the therm o-m echanical now regarded as a classic A tlantic-type m argin. The behaviour of sedim entary basins and the recognition o f authors review here recent seism ic data and basin m odels, global variations in M esozoic-C enozoic sea levels. w h ich e n ab le a rec o nstru ction o f th e J urassic-C re ta ce o us stratigraphy and its relationship to the evolution of the The general Jurassic to C retaceous stratigraphy of the A tlantic O cean. (E d.) eastern Brazil offshore basins (Fig. 2) can be represented by five m egasequences: continental, transitional evaporitic, In t ro d u c tio n shallow carbonate platform , m arine transgressive and m arine regressive (Ponte et aL , 1978). These m ega- The continental m arg in of Eastern B razil com prises six rift- sequences are linked to the continental breakup of the initiated basins (Sergipe-A lagoas, B ahia Sul, Espirito Santo, Pangea and to the evolution of the A tlantic O cean. C am pos, Santos and P elotas), w hich occupy the coastal plain, continental shelf and slope o f the w estern portion of T he C ontinental M egasequence the Sou th A tlantic O cean (F ig. 1). Since the late 1960s, Three synrift sequences com prise the continental m ega- w hen offshore exploration for hydrocarbons began, a la rge sequence, based on their characteristic facies associations am ount of geological and geophysical data has been obtained (Fig. 3) and structural styles, and follow ing a schem e used o n th e s e b a s in s. by Figueiredo (1981). The first sequence (Syn-R ift I) is T w o m ajor developm ents have helped to increase our know l- hardly affected by faulting, w hereas the other tw o (Syn-R ift edge of the origin and developm ent of these basins in recent II and 111) are subject to intense faulting. years (see Ponte and A sm us, 1978; O jeda, 1982, and A sm us Syn二丝望_. During the latest Jurassic (Dom Jogo or Volgian and Baisch, 1983, for general review s). O ne is the consider- Stage, see F igure 2, tw o areas becam e separate as a con- sequence of the initial process of G ondw analand breakup. ｝ ‘三经二 ＿ To the north of the E spirito Santo B asin, a huge depression 8 0 ' 三至至呈至呈全丝呈兰 16 0 * ；石． form ed, know n as the "A frican-Brazilian D epression" (Ponte et al., 197 1). This w as rapidly filled w ith a com plex package of arid alluvial fans and by coarse-g rained fluvial deposits 0 - w ith subordinate evaporites, representing local playa一ake environm ents. E olian deposits are also com m on in this sequence. To the south data are lacking, but the sequence here is thought to be com posed of the sam e sedim entary sequence present to the north or volcanics, or both, as in the low er C ongo R iver B asin of Zaire (R . Palagi, personal com m unicatio n). S v n-rift 11: T h e inc eD tio n o f the m a in rift Dh ase . in th e - 2 0 ' h a riy L;re ta c eo us tii io aa b e rra- a ra tu o r iN e oc o m ia n stag e s), 2 0 、 、 S A N T O S generated a series of a rapidly subsiding half-grabens along the entire m argin. T o the no rth, a series of deep and strat- ified lakes form ed and filled w ith dark-coloured organic-rich shales and by turbidites associated w ith fluvial-deltaic cla stic s. Su bo rd ina te os trac od al ca lca ren ite s a re rela ted to episodic flooding. T o the south, how ever, volcanism w as very active (Fig. 3). AIS 匆久 T he volcanic-sedim entary sequence is best studied in the 洲尹 C a m pos Basin, w here it is com posed o f basalt fl ow s, vol- caniclastics and sedim entary rocks w ith ages ranging from 120 to 130 M a (M izusaki, 1986; Ponte and A sm us, 1978). B07 60- ＼ M izusaki recognizes areas w ith subaqueous, laeustrine and 明． subaereal volcanism , the latter characterized by explosive episodes. Fi gure 1 ： Location of the East Brazil marginal b as in s . S . M ar : S er r a do M a r co a s ta l ra n ge . Later, lakes becam e progressively shallow er and volcanic SE/A l: Sergipe众lagoas Basin . ESP . SANTO: Espirito events less m arked. Shallow lakes to the north w ere slow ly S a n t o B a s i n . filled by very extensive, cyclic fluvial-deltaic clastics con- 194 E PISO D ES, Vol. 11, N o. 3, Septem ber 1988 0 W S A N 下 O S E IW C A M 户 O S E lW S E R G IP E / A L A G O A S E － H O LO C E N E / P L E IS T O ‘ 跳参是笼厂呈多牙扭 ＾ P L IO C E N E 比 舀签5 ＜ 之z 二 于任 万三于三 毛夕 M IO C E N E 瑟砂毛 手 卜－ 比 O L IG O C E N E 田 1 E O C E N E P A L EO C E N E S 户笠瘩蓉昌 仁毛 艺 礼 n M A A S T R IC H T IA N 二 三子二三二气笼二习 卜痊 竺廷 从 o 氮 口奏窦t7孟备之二二 - sm 山 C A M P A N IA N 0 艺竺 互或竺梦 艺己 竺 芡多婆获 三 } q 哎 卜三口 任二 鬓 －L S A N T O N IA N 乏忿弓芍吞燕K 创 匕定后 二毛只目 于三I uJ ?-I是笔分叹 O 公 卜玄于舀＝＝ ＝洲 户万二三 匡 C O N IA C IA N 桩三笼 决三 刹 b }丈d 口城丈二 石乙饭乃甘口 0 T U R O N IA N 到 阵 n． C E N O M A N IA N ．恻翔日纽戴｛围 〔三二污 S 3 石 n A L B IA N ．O O 几 -f 国 石。巴弋 0 哎 A L A G O A S J 曰 J 1 、 护、护、子、J 、 子、目产、2 、护、韶、廿 、2 、子 、矛 鬓鬓爵翼德 山 J IQ U IN 止 0 } 墓幸 B U R A C IC A 矛 葺舀霭 =q_ 远理全霆 0－ 气， ． 一 1 夕 v v V v v V v v v v V v v v v v v v v v v V v v v V v v V v V 心 ．。 二 一， A R A T U / R IO D A S E R R A V v V v V v v V v v v v v v v V v V V v v v v v v V V V v v v 菩月生毛 二户吧少竺当冲任丛笼仪 夕 V/ V V V V V V V V V V V V D O M J O A O 盘 ‘二士J二三二三二二云二之匕二二 ？ 盗气 二少甲二 IVV：篮攀署举署 P 十 ＋ ＋ ＋ 画 ＋ 十 十 十 ＋ ＋ ＋ ＋ ＋ ＋ I ＋ 十 ＋ 十 ＋ 图B A S E M E N T ｝毛是冬｝S H A L E JJ_J -i｝H A L IT E P"- __ C A LC IL U T IT E 盈V O L C A N IC 匡口 S A N D S T O N E 既盈 A N H Y D R IT E 匝因 M A R L C O Q U IN A 嚎澎：1C O N G L O M E R A T E 卜气月 C A L C A R E N IT E ｝烤忿｝T U R B ID IT IC ,SA N D ST O N E F ig ur e 2 : S t ra ti g rap h ic c h ar t o f th r ee r ep r es en 一 tative margin al basins of E a s t B r a z i l . A r r o w s m a r k T he Transitional E vaporitic M egasequence t r a n s i t i o n b e t w e e n 付a r i n e T ran sg re s si ve an d M a r in e This m egasequence, deposited during the A lagoas (A ptian) R eg re s si ve m e ga se qu en c e s . Stage, is com posed of sedim ents typical of the transition be tw ee n c on tin en t an d m a rin e e n viron m e nts. It w a s d e- trolled 勿 rapid base-level changes in response to clim atic posited over a w idespread post-rift unconform ity (Fig.