42. EVOLUTION OF THE MOROCCAN OCEANIC BASIN AND ADJACENT CONTINENTAL MARGIN— A SYNTHESIS Yves Lancelot, Département de Géologie Dynamique, Université Pierre et Marie Curie, Paris, France, and Geological Research Division, Scripps Institution of Oceanography, La Jolla, California. and Edward L. Winterer, Geological Research Division, Scripps Institution of Oceanography, La Jolla, California ABSTRACT Although drilling in the Moroccan Basin failed to reach strata older than uppermost Jurassic, regional geology and geophysics combined with drilling results provide the basis for a reconstruction of the evolution of this basin and the adjacent margin since the Triassic. Rifting began around the Triassic/Jurassic boundary, and drifting probably started a little later, probably during the late Liassic (Domerian-Toarcian). Evaporites accumulated on the rifting margin prior to drifting, and possibly in the deepening adjacent oceanic basin during the late Liassic-earliest Dogger. Accumulation of thick turbidites in the basin characterized the Middle Juras- sic-Neocomian interval, with a probable interruption during the ear- ly Late Jurassic "Atlantic" transgression at the time of widespread carbonate accumulation on all margins around the central Atlantic. The Upper Cretaceous through Cenozoic record reflects the predom- inant influences of oceanic circulation on sedimentation during that time. A major hiatus omits most of the Late Cretaceous, and the suc- ceeding Cenozoic shows alternations of erosion and pelagic deposi- tion. INTRODUCTION In Morocco itself, the western basins that border the Atlantic are the Essaouira Basin, and more to the east Direct access to older strata lying close to the foot of the Western High Atlas (Figure 3). Sometimes the two the Moroccan margin continues to elude all drilling at- are grouped under the name of Southwestern Morocco tempts. The deepest level reached during Leg 50, at Site Basin (Société Chérifienne des Pétroles, 1966). The 416, lies at a sub-bottom depth of 1624 meters, within westernmost part of that basin, south of Essaouira, is uppermost Jurassic strata (Figure 1). also called the Haha Basin (Choubert and Faure-Muret, Without direct examination of rock samples, we must 1962). This area was intensively studied for several infer the early evolution of the Moroccan Basin from years, and the results were included in a set of remark- geophysical data on the oceanic basin and from the able syntheses by Choubert and Faure-Muret (1962), the geological record of the various sedimentary basins on Société Chérifienne des Pétroles (1966), and later by the adjacent margin. This approach is valid for this area Faure-Muret and Choubert (1971) and Ager (1974). because of the detailed published results of years of Choubert and Faure-Muret (1962) clearly demonstrated geologic work in Morocco, where exploratory wells and that one cannot fully understand the record from these outcrops of Mesozoic strata are abundant. For the deep basins nor from the Central Atlantic itself without also basin itself, we had several good-quality multichannel considering the record from the Atlas (Middle Atlas and seismic-reflection profiles — including parts of two Central High Atlas). commercial lines that could not be released for publica- For areas south of Morocco, we use for comparison tion in this volume — to help complete the picture and the record from the Aaiun-Tarfaya Basin (Querol, 1966; tentatively correlate deep-water units with geological Martinis and Visintin, 1966; Arthur et al., 1979; von events recognized on land. Rad and Arthur, 1979) and from the Senegal Basin In assembling this synthesis, we have relied heavily (Castelain, 1965). on the record from sedimentary basins of western Mo- Naturally, the record from the opposite side of the rocco. We also tried to integrate data from various Atlantic, especially for the Jurassic and Early Cre- other basins surrounding the central Atlantic (Figure 2), taceous, is especially relevant, and several comparisons and even from the North Atlantic (north of the Gibral- between the American and African margins have been tar-Azores and Newfoundland fracture zones). Our ef- attempted (see, for example, Bhat et al., 1975; Rona, fort was greatly facilitated by the abundance and quality 1970). The Canadian margin, off Nova Scotia and of syntheses that have been published on these various beneath the Grand Banks, has been studied in detail areas. through numerous commercial wells, which have pro- 801 oo •< ~ <~~ B ~F~~l 3 sS- I 1 SITE 415 (Lat. 31 01.7'N; Long, 11 39. TW) SITES 416 and 370 (Lat. 32 50.2'N; Long. 10 48.TW) J* MMII"A^*J,''"""'/ C ' \\^ 'lf/f) I' °^ i LithO'°9V A9e " DMh|| LithologY A9β " m Pleist0 e 1 "~\^"^>',i ) „-' METEOR 3902 —/«^0 j l l^ / • / ^ -I^li* Nannofossil marl " R^C""-] Naπnofossil marl ^ V O V~'y ^-' PR0FILE I £•y I i y••S• f' -~-^- a"d Plioce"e ^i^ a"d Late >H Nannofossil oze 4000m .--- / IA ,„' / gV •/ \ y y ^Z^-IT÷ Naπnofossil ooze ^__ V\~^^ ' ° fTI ,/ -> i£i I '/ >ESSAOUIRA» 100- HErE Late 100- • ~ar• LH f //" / ß.^m.Jy , ^< s^π p!S.^π Middie g r /•' //B / f/"^ / >" _jC-—-~r '~' '"'~ Altemati"9 "annotossil P ^d5- § <! F ^p<<~ / ^ ^ ^Efi N I ("•• ljL-r~•—'^ Z^y^T::• jfjπT^π chalk and marl Midd|e Φ •^,.u.^\\ g ^ ^'' ""--r~•-^> 415^&N X> jEfv^vfe^&^SEi**- • 200- ^j^ I —brown— 200-E^I-^ brown Z II ,' / i ‰ ; S Lλ A/>-liZilrt<rxO^ -C Alternating nannofoss.l -LV^• Turbidites: W ^•' ^ 0«^ / £ ,^•-CA., VTNi----^T^S^^ ^ü^ chalk and Siliceous- Early .^7; sandstone, ?0 -^° / // A^O% TSSUTH^^Φ . •ü F= debris-bearing marl ^•J-= diatom-rich nannofossil I W / ,, / J•/ $j /sùUSS>^ . 300- ___^ 1 300- Φ^b "-"•"-«** ^ ' ^^' p oS^^^ / ^ ir-\i^ with ?H^ f ' l/\7J — <^^ ^r Λ ^'^•'~' — Paleocene ^_^>E^. to Late Eocene r I r-^s CANARY ISLANDS ,«/ //T ^ ^,-- j/Φ^•P * — ~~~~~ Mar|S,one <i "i^r^^lS > / y / //TARFAYA/ / ^ 5 •™^U-K ^•^V^V~L~N* red 7θθ 7θθ 1 *'^JF/ l II I* "~^ - • |H%!=one, - — Bathymetric contour Q Leg 50 Sites Φ Other DSDP Sites I | g^j^ calcarenite | | | Pji^jjiyiy 1300- Sr4r4r; ^jj)y^ Valanginian 1400- fjjTt^πy Jgπ Turbidites: 1500- ^^ ?™m drhrù÷ wi<" hard Berriasian Figure 1. Location of Sites 415 and 416 and stratigraphic summary. MOROCCAN BASIN AND MARGIN W. GRAND BANKS BASIN (AVALON UPLIFT) BALTIMORE TROUGH SCOT I AN SHELF BASIN WESTERN HIGH ATLAS AAIUN-TARFAYA BASIN Figure 2. Index map showing locations of various sedimentary basins of the At- lantic margin, cited in the text, and locations of DSDP sites that have reached Jurassic sediments in deep oceanic basins. vided the basis for excellent syntheses (Mclver, 1972; the top of the basaltic layer 2. Since we really have no Jansa and Wade, 1975). clear idea of what one ought to see in seismic profiles at Finally, in the North Atlantic, the Jurassic sequence the transition between oceanic and continental crust in the Portugal Basin provides interesting additional da- along passive margins, the possibility remains that such ta on the early history of the Central Atlantic (Faugères a transition would remain undetected on these profiles. and Mouterde, 1979; Mouterde and Ruget, 1975). Both the depth and the acoustic character of the base- ment might not exhibit major differences on either side REGIONAL SETTING OF SITES 415 AND 416 of the boundary, or of any sort of transition zone. The The two sites drilled in the Moroccan Basin during boundary sometimes has been placed at the seaward DSDP Leg 50 are close to the base of the continental edge of the diapir field that characterizes the continental slope and about 20 to 30 km seaward of the edge of a slope area, east of Sites 415 and 416. To the north, field of diapiric structures (presumably evaporites) that however, the diapiric structures clearly extend into the extends from the shelf down to the slope and deep basin deeper parts of the basin, all the way to the Seine areas (Figure 1). Both sites are well within the Jurassic Abyssal Plain (Beck and Lehner, 1974; Renz et al., magnetic quiet zone, about 300 km landward from the 1975; Uchupi et al., 1977). On the basis of information boundary. presently available, there is no reason not to believe that The location of the boundary between oceanic and some of the diapiric structures overlie oceanic crust. continental crust in that area remains unknown, and If one accepts the hypothesis that the crust in the area none of the data presently available uniquely solve the of Sites 415 and 416 is indeed oceanic, then its age still problem. Seismic profiles running eastward from the remains difficult to estimate directly. As will be dis- area of the quiet-zone boundary, where the crust is sure- cussed below, the geological record from the various ly oceanic, do not show any change in the character of basins surrounding the North and Central Atlantic sug- the acoustic basement, all the way to the area of Sites gests that oceanic crust appeared in the basin for the 415 and 416, and even beyond, until it disappears to the first time around 180 m,y. ago. An age estimate of west beneath the diapiric evaporite field. The depth of about 170 to 175 m.y. for the basement beneath Sites the acoustic basement, computed from stacking velocity 415 and 416 would be compatible with a spreading rate information based on multifold seismic profiles with of slightly less than 2 cm/yr for the period between the correction for isostatic load, is approximately 5860 early Liassic and the beginning of the Oxfordian. meters, a value quite comparable to the theoretical The acoustic stratigraphy in the basin has been deter- depth of 5320 meters computed from the classical cool- mined from multichannel seismic profiles from Geo- ing subsidence curves for oceanic crust.