The intracontinental High and Middle Atlasmountain belts in Morocco intersect to form thesouthern and western margins of the Missourbasin, an intermontane basin formed as a result ofthe uplift and inversion of the Mesozoic Atlas paleo-rifts. These rifts were areas where the crust wasgreatly attenuated and more subject to deformationin response to nearby plate boundary tectonics.Data from observations based on seismic reflectionprofiles and wells over the Missour basin for hydro-carbon exploration and field mapping were used tounderstand the basin evolution, structural styles,and inversion timing of the nearby Atlas Mountains.Hercynian and Mesozoic normal faults were reacti-vated into high-angle reverse and thrust faults inthe Mesozoic during the Jurassic, Early Cretaceous(early Alpine phase), and the Paleogene (late Alpinephase). The reactivation of synrift normal faults ofthe paleo-Atlas rifts inverted previous half grabensinto anticlinal structures, with the axis of the halfgraben centered below the axis of the invertedanticline. The resulting inverted fold geometriesare controlled by the geometries of the extensionalplanar or listric faults.
The Atlas paleorift system is one of the largestrift systems in Africa. Little hydrocarbon explo-ration has occurred within the Atlas Mountains andthe margins of the paleo-Atlas rift system. Inversionof synrift structures can lead to both the destruc-tion and preservation of synrift traps and the cre-ation of new hydrocarbon traps. The study of theeffects of inversion in the Missour basin may lead tothe discovery of footwall subthrust hydrocarbontraps in the Mesozoic sedimentary sequence of theAtlas Mountains.
INTRODUCTION
Mountain belts located along convergent plateboundaries, such as the Andes or the Himalayas,have been and still are the focus of intense geologi-cal and geophysical studies. In contrast, intraconti-nental mountain belts, including the Atlas system inMorocco, lack even an agreed-upon first-order con-ceptual model of their deep structure and activedeformation. Geological evidence suggests suchintraplate belts have significantly contributed tothe evolution of the continental lithosphere sincethe Precambrian.
Rifting during the Triassic and Jurassic waswidespread around the world. The Atlas rift sys-tem of north Afr ica, the North Sea r ift , theAndean rift system of Colombia and Venezuela,and the Palmyride rift of Syria are just a few of theintracontinental rift systems active during theTriassic and Jurassic. Some of these same rift sys-tems were inverted into intracontinental moun-tain belts (i.e., the Atlas Mountains, Palmyridemountains, and the northern Andes). These riftsystems were the focus of sedimentation duringthe synrift and postrift phases of rifting. Riftbasins contain approximately 5% of the world’ssedimentary volume, but they also contain10–29% of the known hydrocarbon reserve base(∼275 billion bbl) (Katz, 1995). This high concen-tration of reserves is partially due to the limitedmigration distance allowed by the geometries ofrift systems.
1459AAPG Bulletin, V. 80, No. 9 (September 1996), P. 1459–1482.
1Manuscript received June 5, 1995; revised manuscript received January16, 1996; final acceptance April 12, 1996.
2Institute for the Study of the Continents and Department of GeologicalSciences, Cornell University, Snee Hall, Ithaca, New York 14853-1504.
3Office National de Recherches et d’Exploitations Petrolieres, B.P. 8030,Rabat, Morocco.
This study would not have been possible without the assistance ofONAREP (Office National de Recherches et d’Exploitations Petrolieres) andthe Geological Survey of Morocco. We greatly appreciate input from ourcolleagues at Cornell, Francisco Gomez, Dogan Seber, Bob Litak, AlexCalvert, William Philpot, and Rick Allmendinger, in the preparation of thismanuscript. We also thank the donors of the Petroleum Research Fund,administered by the American Chemical Society (ACS-PRF 29505-AC2), andto a grant from AASERT(DAAH04-94-G-0361) for support of this research.We thank Richard G. Stanley, Z. R. Beydoun, and John A. Best for theirconstructive reviews of this manuscript. In addition, we would like to thankHarold A. Illich and Geomark Research, Incorporated, for their geochemicalanalysis and help in evaluating potential source rocks in Morocco. INSTOCcontribution 223.
Intracontinental Rifting and Inversion: Missour Basin and Atlas Mountains, Morocco1
Weldon Beauchamp,2 Muawia Barazangi,2 Ahmed Demnati,3 and Mohamed El Alji3
The uplift and inversion of hydrocarbon-bearingrifts can result in the remigration and redistributionof hydrocarbons into structures generated by thereactivation of preexisting faults formed during rift-ing. One must have a good understanding of thegeometry of structures formed by the reactivationof synrift faults, because these structures have thepotential to trap significant amounts of hydrocar-bons. Additionally, one must generate models tounderstand the development of individual invertedstructures, as well as the uplift and inversion ofentire rift systems such as the Atlas Mountains, tobetter understand the potential of unexploredintracontinental rifts and mountain belts aroundthe world. Our research in Morocco is a steptoward understanding and resolving the historyand present architecture of such belts.
GEOLOGIC SETTING
The Mesozoic and Cenozoic geological evolutionof Morocco can be viewed as a response to twomajor geological events: (1) the opening of theNorth Atlantic and the western Tethys in the earlyMesozoic, and (2) the Africa-Europe continental col-lision in the middle Cenozoic (Michard, 1976;Mattauer et al., 1977; Bensaid et al., 1985; Pique etal., 1987; Jacobshagen et al., 1988; Dewey et al.,1989; Westaway, 1990). These two major events
shaped the present architecture of the four majorgeological structures of Morocco: the Rif fold-thrustmountain belt in the north, and the Middle Atlas,the High Atlas, and the Anti-Atlas mountain belts ofcentral Morocco (Figure 1). The Rif belt is funda-mentally different than the Atlas system. The Rif isan asymmetric, Alpine-type, fold-thrust belt withnumerous, well-mapped thrusts and complexnappe structures (Loomis, 1975; Leblanc andOlivier, 1984; Morley, 1987; Doblas and Oyarzun,1989; Ait Brahim and Chotin, 1990; Leblanc, 1990;Miranda et al., 1991), whereas the Atlas system is anintracontinental, largely symmetrical mountain belt.
Regional Tectonics and Rifting
The Atlas system evolved within the stable plat-form of North Africa. Two major events shaped thegeological evolution of the system: early Mesozoicextension and rifting, and Mesozoic–Cenozoic compressional-transpressional phases that resultedin the inversion of the rift systems (Figure 2). TheAtlas system is thus an intracontinental orogene“sandwiched” within the Proterozoic–Paleozoicnorthern African platform, and is fundamentally dif-ferent from orogenes located along convergent/col-lisional plate boundaries. Thrusts, strike-slip faults,and block uplift tectonics characterize the Cenozoicdeformation of the Atlas system (e.g., Schaer and
1460 Intracontinental Rifting and Inversion
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A
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10° 0° 10°
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Algeria
Spain
Libya
France
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Location Map�Atlas Mountains
& Paleorift
�Rif Mountains
Figure 1—Location map of the Atlas mountains andMissour basin of Morocco.The Missour basin isbounded by the MiddleAtlas and High Atlas mountains.
Rodgers, 1987; du Dresnay, 1988; Fraissinet et al.,1988; Jacobshagen et al., 1988; Medina, 1988; Gieseand Jacobshagen, 1992; Jacobshagen, 1992).
The Missour basin and the High and Middle Atlasmountain belts that form its boundaries (Figure 1)are examples of how large stresses can be transmit-ted to intraplate zones of weakness from the colli-sion zones along the nearby plate margins. Highstrain rates created by the thinning of the continen-tal lithosphere resulted in the deformation of thecrust by extension and rifting in the North Africanplate at the end of the Permian and beginning ofthe Triassic (Brede et al., 1992). Sedimentationrates accelerated through the Jurassic as rifting con-tinued with the breakup of Pangea, and the open-ing of the neo-Tethys Ocean and the North Atlantic(Ziegler, 1982). The High Atlas developed into oneof the largest of the rifts, possibly reactivated along
existing weaknesses and faults formed during theHercynian orogeny. The High Atlas rift extends tothe Atlantic margin where it forms a failed rift oraulacogen, and eastward (High Atlas/Saharan rift)across Morocco, Algeria, and Tunisia (Figure 1).The Middle Atlas rift and mountains trend north-east, where they extend beneath the thrustedAlpine Rif allochthonous sedimentary rocks. Theintersection of the Middle and High Atlas rifts/mountains may represent a failed triple junction, ora focus of thermal upwelling.
Beginning in the Late Cretaceous–early Oligocene,dextral movement on the Newfoundland-Gibraltarfault zone increased the eastward drift of the Iberianplate (e.g., Brede et al., 1992). The geometric rela-tionship between the Iberian plate and the Africanplate resulted in compressional stresses that weretransferred to the North African rift systems. The
���Figure 2—Conceptual model forthe development of the Missourbasin and the Atlas Mountains,Morocco. See Figure 1 for location of AA′.
bulk of the intraplate stresses were absorbed by theHigh and Middle Atlas rift systems, resulting inshortening and subsequent inversion. The inver-sion of these rifts led to the reactivation of preexist-ing Mesozoic and Hercynian faults into reverse andthrust faults, with an oblique-slip sense of move-ment. The uplift of the Middle and High Atlas riftsformed the mountains that are now the boundariesof the Missour basin (Figure 2).
The orientation of compressional stresses rela-tive to the orientation of rift bounding faults result-ed in transpressional deformation in both a dextraland sinistral sense (e.g., Giese and Jacobshagen,1992). The sense of movement on the boundingfaults of the High and Middle Atlas mountains var-ied depending upon the direction of plate motionbetween the European and African plates. In theEarly Jurassic spreading began in the central
Atlantic, while the north Atlantic was in a riftingstage. This resulted in an eastward drift of the African plate in relation to the Iberian plate that was to the north of the Newfoundland-Gibraltar transform (Ziegler, 1982). During the LateCretaceous–early Oligocene, the African plate wasmoving clockwise to the east, resulting in transten-sional deformation in the North African rift sys-tems. Plate rotation of the Iberian plate was coun-terclockwise as the Iberian plate moved eastwardalong the Newfoundland-Gibraltar fault/transform.This counterclockwise rotation is evident in theopening of the Bay of Biscay along the northernmargin of the Iberian plate. The African plate wasmoving northward with respect to the Iberian plateduring the late Miocene and Pliocene (Ziegler,1982), with the same counterclockwise rotation,resulting in a rotation of primary stresses from
1462 Intracontinental Rifting and Inversion
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W5°W8°
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South Viking Graben
N57° N59°
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E2°
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Figure 3—Comparison of the sizeand geometries of the Atlas andNorth Sea rift systems. Both riftsystems were active during theJurassic. The Missour basin (A)was a shelf margin much the sameas the Fladen ground spur (B) ofthe North Sea. The Missour basinis bounded by faults that have aright-lateral component of slip to the south in the High Atlas and by faults with a left-lateral component of slip to the west inthe Middle Atlas. The result is theuplift and escape of the Missourbasin to the northeast. Modifiedafter Stewart et al. (1992).
approximately 180 to 120°. Faults bounding the HighAtlas rift yielded a right-lateral transpressive sense ofdeformation during the Late Cretaceous–earlyOligocene (Figure 3). Folds in the central High Atlasgenerally trend east-northeast, at about a 20–30°angle to the High Atlas bounding faults (Studer anddu Dresnay, 1980), further evidence of a right-lateralphase of deformation in the High Atlas Mountains.Later plate motion during the Oligocene to theHolocene has been that of convergence (Betic-Riforogene) with some wrenching, because both plateshave drifted eastward at a similar rate (e.g., Dewey etal., 1989).
For comparison, the High Atlas rift system is sim-ilar in size and geometry to the North Sea rift sys-tem (Figure 3). The actual geometry between indi-vidual rifts varies between the two rift systems.These differences may be related to the rates and
direction of plate convergence between the Iberianplate and the African plate, and the subsequentdeformation. Extension began in the North Sea andthe Atlas r ift systems during the Triassic andJurassic (Figure 3). The Triassic sedimentary rockspenetrated by wells in the Missour basin contain asignificant amount of tholeiitic volcanics interbed-ded with salts (Figure 4). These Triassic basaltsextend over most of the Missour basin, are encoun-tered in several wells, and are clearly identifiable onseismic reflection data.
Rifting in the Atlas continued into the MiddleJurassic when subsidence continued over the riftsystems during the Late Jurassic–Early Cretaceous.Lower Cretaceous sedimentary rocks are not gener-ally preserved in the Atlas Mountains, but may havebeen deposited in the Atlas rift systems during apostrift subsidence phase; if they were deposited,
Figure 4—Stratigraphy ofthe Missour basin basedon well penetrations(OSD-1, RR-1, KSAB-101,KSAB-102, and TT-1; see Figures 5 and 12, and Table 1), seismicstratigraphy, and outcropswithin the basin. Several phases of deformation are related to major unconformitiesin the basin.
record of these sedimentary rocks was removed byuplift and erosion. Subsidence in the Atlas riftbasins probably was coupled with isostatic uplift ofthe adjacent platform margins (Missour basin, Highplateau, and Moroccan meseta). This isostatic upliftof the rift basin margins resulted in a thinning ofthe Upper Jurassic–Lower Cretaceous sedimentaryrocks. The Upper–Middle Jurassic sedimentaryrocks of the Missour basin are deeply eroded alongthe margins of the High and Middle Atlas moun-tains, a result of isostatic uplift and erosion by thebase Cretaceous unconformity. Well data illustratedeep truncation by the base Cretaceous unconfor-mity into the synrift sedimentary rocks from the riftmargins into the paleo-Atlas rift basins. We foundevidence of isostatic uplift of the rift basin marginsduring the Late Jurassic or Early Cretaceous (Figure4). The oldest Cretaceous sedimentary rocksencountered in the Missour basin are Cenomanian(Figure 4), suggesting a subsidence phase in theAtlas rift systems that lasted 40–50 m.y., from theLate Jurassic to the Late Cretaceous. Subsidenceslowed or ended during the Late Cretaceousbecause Cenomanian to Turonian sedimentaryrocks were deposited uniformly across the rift sys-tems and the rift basin margins. Subsequent upliftin the Paleogene, related to the Alpine orogeny,inverted the Atlas rift system and eroded theLower Cretaceous sedimentary rock sequencefrom the present Atlas Mountains. Compressionand transcurrent movements generated by the rela-tive motion of the African and Iberian plates result-ed in stresses being transmitted into the African
plate, the net result being the shortening andinversion of the Moroccan rift systems (e.g., Lavilleand Pique, 1992).
Regional Stratigraphy of the Missour Basinand Atlas Mountains
Five wells have been drilled in the Missour basin,with three that penetrated the Hercynianunconformity and the Permian–Carboniferous clas-tic sedimentary rocks (sandstones, shales, and con-glomerates). The Triassic section overlying theunconformity is recognized by a lower and uppersalt series separated by layers of basalts. The Jurassicsequence consists primarily of marine limestones,dolomites, and shales. The postrift Cretaceous–earlyTertiary sequence is made up of a shallow andmarginal marine sequence of limestones, calcareousshales, dolomites, and interbedded anhydritic shales(Figure 4). A correlation of the wells in the Missourbasin clearly indicates the regional effects of inver-sion (Figure 5). The Jurassic in the Missour basinthickens dramatically toward the old Middle andHigh Atlas rift systems. The thick Jurassic sequencein the center of the rift is composed mostly of shal-low-marine carbonates (Studer and du Dresnay,1980). On what is now the edge of the mountainbelts, Jurassic carbonates exhibit basin-marginfacies, such as reefs, platforms, and intertidal sedi-mentary rocks (du Dresnay, 1971). In the middle ofthe High Atlas Mountains, Jurassic carbonates mea-sure up to 7 km thick (Studer and du Dresnay,
1464 Intracontinental Rifting and Inversion
–3
–2
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0 10 20
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TD 3525 m
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Figure 5—Well correlationbetween wells drilled inthe Missour basin. Inversion can be seen by the dip of the base Cretaceous unconformityin the opposite direction of synrift thickening.
1980). Erosion in the Atlas has removed part of theJurassic section related to inversion. Illite crystallini-ty in preserved sedimentary rocks (Early Jurassic)was used by Brechbuhler et al. (1988) to estimatealmost 6–8 km of synrift and postrift thickness inthe deepest part of the rift. The Triassic sedimen-tary rocks of the High Atlas are 4–4.5 km thick(Beauchamp, 1988). A composite thickness for thesynrift sedimentary rocks in the High Atlas could beas thick as 10–12 km, based on a measured field sec-tions in the High Atlas.
FIELD MAPPING AND ANALYSIS
We collected field data to constrain the interpre-tation and modeling of subsurface data. TheMissour basin and the Atlas Mountains provide ameans to study inversion structures in outcrop, andthe structural characteristics helped us interpretsubsurface structural relationships. Multiple phasesof deformation along the margins of the Missourbasin indicate a complicated tectonic history thatdeveloped from the beginning of the Triassicthrough the early Tertiary. The tectonic history of
the Missour basin has developed in several phasesof extension and compression rather than only onephase of extension in the Triassic and one laterphase of compression in the Oligocene, as hasbeen previously believed.
The primary goal of our field work was to locateexposures of fault zones along seismic reflectionprofiles and tie the two data sets together to pro-vide models for structural styles of inversion. Mostof the exposures in the Missour basin are locatedalong the margins of the basin (Figure 6). The mostimportant exposure within the basin relative to thisstudy is Jebel Missour (Figure 6). This prominenttopographic ridge trends northeast and is com-posed of two large anticlinal structures. Cretaceousand Jurassic sedimentary rocks are exposed in thetwo anticlines. The most important relationship ofthese two folds is that they verge in opposite direc-tions (Figure 7). Aerial photographs used for map-ping in the field illustrate the pronounced topo-graphic expression of these anticlinal structures.The southernmost fold (F1-A, Figure 7) vergesnorthwest, and the northernmost fold (F1-B, Figure7) verges southeast. Both are asymmetric folds withsteeply dipping to vertical limbs along one side of
Beauchamp et al. 1465
High Atlas
Missour Basin
Middle Atlas
High Plateau
Hercynian Igneous Intrusives
Jebel Missour
High Moulouya
South Atlas Fault
Jebe
l Mec
hkak
our
Figure 6—LANDSAT TM (band 5, infrared) mosaic of the Missour basin and parts of the High Atlas and the MiddleAtlas mountains. Field locations where structural data were collected are shown as stars. The eastern margin of theMissour basin is bounded by the northeast-trending Jebel Mechkakour. The location of Jebel Missour is shown tothe east of the High Moulouya. The structures seen in the High Atlas Mountains show polyphase deformation.
the fold. Strike and dip data were collected alongtransects across the structures and plotted to illus-trate the overall geometry of the structures (Figure8). The northernmost fold (F1-B) is plunging to thenortheast, and the southeast limb is steeply dippingto vertical along strike of the fold (Figure 9). Thesouthern anticline (F1-A) has exposures of bothsynthetic and antithetic faults located in the core ofthe fold. Both the synthetic and antithetic faults aresteeply dipping high-angle faults. Fault plane linea-ments indicate a reverse sense of motion on thereactivated synthetic fault (up to the northwest).These faults displace Jurassic rocks and die outupsection in the anticlinal structure. There is a dis-tinct change in dip between the postrift UpperCretaceous sedimentary rocks and the synriftJurassic rocks, and the examined faults do not cutthe base Cretaceous unconformity.
These two structures (F1-A and F1-B) at JebelMissour evolved initially from two opposing planarnormal faults connected by a ramp (Figure 10).These two faults were active during rifting in theTriassic–Late Jurassic. During the postrift phase,
the base Cretaceous unconformity eroded upperJurassic synrift sedimentary rocks from regions of the Atlas r ift system as subsidence began.Cenomanian–Turonian rocks were deposited in theMissour basin, and in the early Tertiary the two halfgrabens were inverted by an oblique compressionalstress, possibly related to the Alpine orogeny.During the late Tertiary–Holocene, sedimentaryrocks were deposited onlapping the existing struc-ture formed by earlier phases of deformation. LateTertiary–Holocene sedimentary rocks are flat lyingand have not been affected by any significant defor-mation after the Oligocene phase of uplift and com-pression in Morocco.
Fault lineaments and slickensides indicate areverse sense of movement on faults measured atJebel Missour. These lineaments overprint earlierdip-slip and oblique-slip lineaments for which asense of movement could not be determined.Previous lineaments indicate earlier phases of defor-mation, and may be associated with normal andtranstensive phases of deformation related to theinitial phases of rifting. The Middle Atlas Mountains
1466 Intracontinental Rifting and Inversion
Figure 7—Aerial photographof the Jebel Missour region(located on Figure 6). Two northeast-trendinganticlinal structures showtwo phases of deformation(F1 and F2). Fold F1-A and F1-B are verging inopposite directions. Jurassic synrift sedimentaryrocks are exposed in thecore of both anticlines separated by the base Cretaceous unconformity.The Tertiary and Quaternarysedimentary rocks are flatlying and onlap the twostructures. Location of Figure 9 is shown near F1-B.
N32°2'
N33°
W4°14'
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W4°10'
0 1 2 3
km
F1-A
F2
F2 F1-B
Jurassic
Cretaceous
Cretaceous
Jura
ssic Tertiary-Quaternary
Figure 9
west of Jebel Missour are bounded by faults thathave recent movement along inverted normal faultsexhibiting a left-lateral sense of shear (Morel et al.,1993). These faults have been active recently, andoffset Quaternary sedimentary rocks and olderNeogene volcanics. Reactivated rift faults in theMiddle Atlas have thrust Triassic sedimentary rocksover Pliocene and younger sedimentary rocks. Thefaults related to structures at Jebel Missour have notbeen active since the early Tertiary, indicating morerecent convergence is being accommodated alonglarger fault systems in the Middle Atlas Mountains.
The exact geometry of the faults at depth atJebel Missour is difficult to obtain. Based on theasymmetrical geometry of the associated folding,the faults may represent reactivated planar faults.The inversion of a planar fault usually results in amore asymmetrical geometry than an invertedlistric fault, which upon inversion forms a more
open symmetrical fault-bend fold (Mitra, 1993).The geometric styles generated by the inversion oflistric and planar normal faults are controlled bythe orientation of the maximum compressivestress. The inversion of a steeply dipping planarfault can accommodate a limited amount of short-ening because of the geometry of the fault. Onceshortening along a planar fault has occurred byuplift of the associated hanging wall, further stressgenerated by compression is accommodated by lat-eral strike-slip movement along the fault. Steeperfaults with dips of approximately 40–60° can bereactivated only if there is a low coefficient of sliding friction (Coward et al., 1991). Because ofthe geometry of planar faults, steeper faults aremore easily reactivated by oblique-slip or strike-slip motion. This type of deformation may haveoccurred along the faults associated with JebelMissour (Figure 7). The apparent second phase of
Fault planes of inverted high angle normal faults (synthetic and antithetic)
Figure 8—Field data collectedfrom Jebel Missour. The northernanticline (F1-B) is verging to the northeast. The southern anticline (F1-A) is verging to thesouthwest. High-angle reversefaults were measured in the coreof the southern anticline (F1-A).These faults are inverted planarnormal faults (synthetic and antithetic). The vergence of thefolds is related to the original dipof the synrift normal faults.
folding affecting the structures is thought to be relat-ed to oblique-slip movement on the planar faultsassociated with the structures. The deformation of
the original half grabens bounded by steeply dip-ping planar faults may have occurred as follows:inversion of the hanging-wall half graben along aplanar normal fault, folding of the postrift UpperCretaceous sedimentary rocks and synrift Jurassicrocks into an asymmetrical fault-propagation fold,refolding of the asymmetrical folds (F2, Figure 7) byoblique-slip movement along the planar fault, andonlap of the structures by the Neogene–Holocenesedimentary rocks. Jebel Missour is an example ofhow planar faults may respond to inversion within aparticular rift system.
We used topographic maps and a GPS (GlobalPositioning System) receiver to find the surface loca-tion of faults identified on seismic lines. Faults iden-tified on seismic profiles that appear to extend tothe surface were commonly covered by Holocenesedimentary rocks. Many of the faults seen on seis-mic ref lection profiles as individual faults werefound to occur as zones of offset at the surface. We
1468 Intracontinental Rifting and Inversion
Figure 9—Photograph ofJebel Missour along thenortheastern plunging F1-B fold. See location ofphoto on Figure 7. Thefold is verging out of thepicture. Steeply dipping to vertical beds can beseen along the flatirons in the right side of thephoto. The fold is anasymmetrical fold formedby the inversion of a planar fault that dips to the northwest. The fault is most likely a fault-propagation fold. The steeply dipping limbis complicated by severalsmaller folds in the foreground.
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Figure 10—Schematic model depicting the structuralevolution of Jebel Missour. (A) Synrift faults result in thedeposition of sedimentary rocks in an asymmetricalhalf graben. The two normal faults are dipping oppositeone another connected by a ramp or transfer zone. (B)The postrift phase begins the deposition of Cenomanianand Turonian age sedimentary rocks unconformablyabove the Jurassic. (C) Uplift and inversion reactivateopposing planar faults that both form oppositely verg-ing fault-propagation folds. The inversion of (A) hasresulted in approximately 20% shortening.
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were able to record dips of several rocks in the hang-ing wall and footwall of faults that define the overallgeometry of the surface structure (Figure 11).Displacement in fault zones generally occurs along asystem of related small faults in the zone of deforma-tion. Slip, illustrated by kinematic indicators alongbedding planes in the footwall and hanging wall offaults, gave a sense of shear across fault zones.
Many of the fault zones along the southern marginof the Missour basin indicate oblique-slip movementassociated with the most recent phase of deforma-tion. The sense of slip across all of the fault zonesmeasured in the field along the southern margin ofthe Missour basin was either right-lateral oblique-slip or right-lateral strike-slip movement. The age ofthis right-lateral deformation is thought to be asso-ciated with an early Tertiary or older inversionphase because younger sedimentary rocks werenot deformed. Lineaments and slickensides thatrecord right-lateral oblique and strike-slip sense ofmovement cut across several (up to three) earlierlineations that record previous phases of move-ment on the same surface.
An important conclusion based on the study ofthe Missour basin is the concept that the entirebasin may have been uplifted and inverted as awhole (Figure 2). Most of the deformation in thebasin has occurred along the margins of the basin.Extensional features that are not near the flanks ofthe Atlas Mountains normally do not indicate evi-dence of reactivation. Reactivation and inversion ofextensional faults may have occurred in the interiorof the basin, but these faults still illustrate a “net”extension. This observation leads to the conclusion
that shortening has been accommodated by previ-ously existing synrift faults within the paleo-Atlasrift and current-day Atlas Mountains. The density offaults observed in the field indicates a direct corre-lation between the degree of inversion and preex-isting synrift faults. This relationship may be truefor the magnitude of the shortening and inversion(relative to the size), and the amount of throw andextent of synrift faults in the Missour basin andAtlas Mountains. These observations also may havea correlation with the present topography in thebasin (Figure 12). Topography is generally relatedto the density of faults and structure in the Missourbasin, as can be seen by Jebel Missour (Figure 7).
Left-lateral oblique and strike-slip offset in theQuaternary and Neogene recorded in the MiddleAtlas (Morel et al., 1993), combined with right-lateral movement along the southern margins ofthe Missour basin, would indicate the overall rela-tive movement of the Missour basin is to the north-east (Figure 3A). The concept of the Missour basinhaving been uplifted and inverted as a whole maybe similar to the concept of “escape” tectonics pro-posed by Sengor et al. (1984). The basin may havebeen uplifted and inverted by the culmination ofseveral phases of deformation. The region was pre-viously part of the shelf margin or shoulder of theAtlas rift system, and has since been uplifted andtranslated to the northeast.
The basin is bounded to the east by Jebel Mech-kakour (Figures 6, 12). The large-scale geometry ofthe basin as defined by the High Atlas to the south,the Middle Atlas/High Moulouya to the west, andJebel Mechkakour to the east is rhombohedral. The
Beauchamp et al. 1469
Figure 11—Fault-propagationfold that is verging to thenorth near the southernmargin of the Missourbasin. Onlap of growth strata can be seen fromnorth to south across thestructure.
North South
0 10m
High Plateau east of the basin (Figure 1) also ischaracterized by a more obvious rhombohedralshape. These shapes are inherent in transtension-al pull -apart basins associated with r ift ing(Morley, 1995). The High plateau basin is distinct-ly different from the Missour basin because of athick (>1000 m) sequence of salt penetrated bywells in the High plateau basin that is not presentin the Missour basin. The anticlinal feature east ofJebel Mechkakour (Figure 6) was partially formedby the movement of Triassic salt. The distinct dif-ferences between the Missour basin and the HighPlateau basin have been present since the earlysynrift phase in Morocco. The High Plateau basinwas most likely an extensional pull-apart basinduring the rift phase, and the Missour basin wastopographically higher relative to the currentHigh Plateau. The present Missour basin has beenuplifted, inverted, and translated to the northeastby a style of escape tectonics along boundariesformed during rifting.
GEOLOGICAL AND GEOPHYSICAL ANALYSIS
A total of 3400 km of seismic ref lection datahave been acquired in the Missour basin and wereused in this study. These surveys were acquiredbetween 1974 and 1986, resulting in a collection ofseismic reflection data with various qualities andprocessing parameters. The digital poststack dataof several lines were obtained from ONAREP andwere used for further processing, migration, anddepth conversion for more accurate analysis andmodeling. Velocity data in the form of time-depthcurves and synthetic seismograms were used to tiewells in the basin to the seismic ref lection data(Table 1).
The combination of surface and subsurface datain the Missour basin constrains the geometry offaults recognized at the surface into the subsurface.Seismic lines were migrated and depth convertedprior to modeling, balancing, or interpreting theseismic data. Seismic lines used for modeling in this
1470 Intracontinental Rifting and Inversion
Figure 12—Map showing digitaltopography of the Missour basinand the adjacent High and Middle Atlas mountains; seismicreflection profiles, well data, andfield locations are shown. A totalof 3400 km of seismic lines were used to study the tectonicevolution of the Missour basinand Atlas Mountains. The seismiclines in white are those used inthis paper; both black and whitelines were used in the study.
N
HighAt las
W3°
W3° 30'
N33°
N33°30'
N32°30'
Missour
Basin
South Atlas Fault
Jebel Mechkakour
Seismic lineField LocationWell Location
0 km 50
MiddleAtlas
MR
07
MR
10
PKM09
MT9KB11KB2
KB8
MR22
MR17
MR20
W4°
RR-1
KSAB-101
KSAB-102
TT-1
OSD-1
High Plateau
study were reprocessed by applying an FX decon-volution and coherency filters, migrating the datausing a Stolt FK migration, and then depth convert-ing the line using interval velocities. This process isnormally bypassed due to time constraints or thelack of digital seismic data. The migration and con-version of key seismic lines in the Missour basinproved to yield important interpretations and struc-tural models in the basin.
Two important regional stratigraphic horizonswere clearly identifiable on most seismic data in thebasin: (1) the base Cretaceous unconformity, and(2) the Hercynian unconformity (Figure 4). Thesehorizons form the stratigraphic boundaries for thepostrift and synrift sedimentary sequences mappedthroughout the basin. The base Cretaceous uncon-formity is usually identifiable on dip lines as anangular unconformity. The Hercynian unconformityis characterized by an angular unconformitybetween the Paleozoic and the synrift sedimentarysequence. The Hercynian unconformity can also berecognized as the base of the highly ref lectiveTriassic sedimentary rocks composed of salts, anhy-drites, volcanics, dolomites, and clastics (Figure 13).Reflections from within Paleozoic strata were evi-dent on many of the seismic sections in the basin.On several seismic sections, strong reflections werepresent to 7 s two-way traveltime. Paleozoic struc-tures are present on several lines in the basin. Wellsdrilled in the Missour basin did not penetrate deeply
enough into the Paleozoic section to allow us tocorrelate the units within the Paleozoic. The onlystructures that could be defined in the Paleozoicwere at the structural level of the Hercynian uncon-formity.
Styles of Faulting
Low-angle thrusts in the Atlas Mountains havebeen documented on geological maps and in previ-ous field studies. These thrusts have been interpret-ed previously on published cross sections as low-angle faults that detach at the top of the synriftTriassic salts. The results of our study indicate thatthese low-angle thrusts may be related to the reacti-vation of synrift listric faults that detach well belowthe synrift Triassic sedimentary rocks. Many faultsin the Atlas are steep to vertical, thus making it dif-ficult to recognize the direction of dip of the fault.The footwall and hanging wall of the fault also aredifficult to recognize. Although it is common forextensional faults related to Mesozoic rifting to beinverted upon compression in the Missour basinand the Atlas Mountains, it is difficult in many casesto document the dip of the fault plane and thesense of movement on the faults using only seismicreflection data.
We found it common for large-scale (>5 km, mapview) faults to dip both toward the paleorift basinsand out of the basins. This relationship is commonin many rift systems (Rosendahl et al., 1986). Whenthe relationship of extensional and dual fault polari-ty is applied to an inverted rift system, the resultsare that thrusts and reverse faults verge both intoand away from the paleorift basins.
Sedimentary relationships on seismic data thatillustrate extension and active sedimentation dur-ing rifting are important to recognize. Otherwise,determining whether a fault is a reverse fault,thrust fault, or a reactivated synrift fault is difficult.Seismic line 85KB11 (Figure 14), for example,shows a well-defined high-angle planar fault thatdips to the northwest toward the Middle AtlasMountains. This fault is not exposed at the surface,but the highly reflective package of the Triassicsedimentary rocks above the Hercynian unconfor-mity indicates the sense of throw and the amountof offset. The Cretaceous is not present west of thefault on line 85KB11. The sense of throw on thefault is reverse, but it is difficult to determine if thefault is a reactivated normal fault or a fault that wasnewly formed as a reverse fault.
Another consideration when interpreting reacti-vated faults is the uplift of the hanging-wall halfgraben above what has been previously referred to as the null point (Williams et al., 1989). A faultmay have been a normal fault during rifting, with
Beauchamp et al. 1471
Table 1. Wells Drilled in the Missour Basin
Total Fm. atCoordinates Depth Total& Elevation Company Year (m) Depth
Well TT-1 S.C.P.* 1954 2813 TriassicX = 611.916.7Y = 252.593.2 EL = 1212.6 m
Well RR-1 S.C.P. 1965 2794 TriassicX = 704.372.2Y = 271.197.5EL = 1633 m
Well KSAB-101 Phillips 1983 1831 TriassicX = 632.666Y = 249.989EL = 1395.5 m
Well KSAB-102 Phillips 1983 2188 JurassicX = 608.889Y = 230.609EL = 1395.5 m
Well OSD-1 ONAREP** 1986 3525 Carbonif.X = 648.824Y = 288.959EL = 839 m
*Societe Cherifiene des Petroles.**Office National de Recherches et d’Exploitations Petrolieres.
thickening of synrift sediments into the active exten-sional fault. Upon inversion, the hanging wall isuplifted until the prerift unconformity (Hercynian) isuplifted above the prerift unconformity in the foot-wall. The null point, as referred to by Williams et al.(1989), would occur when there is no stratigraphicdisplacement of the prerift unconformity across afault. This relationship can result in interpretationsthat assume deformation by extensional tectonics,when a significant amount of compression normalto the fault may have occurred. Identifying inversionon faults is important because inversion may affecthydrocarbon migration and trapping in either a posi-tive or negative manner. For example, on seismicline 85KB11 the null point would occur when thebase Triassic–Hercynian unconformity (Figure 14,AA′) is juxtaposed across the fault that is dipping to the west.
An example of a reactivated fault that has beenuplifted above the null point can be seen on seismicline 86MR17 (Figure 15). This line has been migrat-ed and depth converted to help position and restorethe fault geometry. There has been an apparentuplift of the base Cretaceous unconformity to thenorth of the fault by at least 2 km. North of the fault,reflections in the Jurassic can be seen truncatingbeneath the base Cretaceous unconformity. The
highly reflective Triassic basalts can be seen clearlyabove the Hercynian unconformity. The reactiva-tion of this fault may have occurred along a previ-ous synrift fault or by reactivation of a prerift fault.Steeply dipping planar faults allow for a limitedamount of shortening normal to the fault plane.Faults such as the fault on line 86MR17 reactivateby initial slip along the fault plane, inverting thehanging wall, and forming a fault-propagation–stylefold above the synrift fault (below the baseCretaceous unconformity). Further shorteningacross the fault zone was accommodated by the gen-eration of a fault-propagation fold (verging to thesouth). This fold was later cut by the breakthroughof the fault upward along the forelimb of the fold.The syncline to the south of the fault on line86MR17 (Figure 15) is the conjugate fold related tothe initial fault-propagation fold. If the principalcompressive stress was oriented at an angle obliqueto the fault plane, oblique or strike-slip deformationwould be expected to accommodate further strainacross the fault (Coward et al., 1991). This style ofdeformation is similar to the style of deformationthat was mapped at Jebel Missour (Figure 7).
Another more subtle criterion that indicatesinversion is seen on seismic line PKM-09 (Figure16). This seismic line ties well KSAB-102 drilled by
1472 Intracontinental Rifting and Inversion
Figure 13—Major unconformitiesthat illustrate tectonic phases(Hercynian, base Cretaceous, and Turonian?) are seen on seismic line MR-7. The Hercynianunconformity has been “flattened” by the inversion of the stratigraphic section. The synrift and postrift sectionshave been rotated upwards reversing the original sense of regional dip (location on Figures 12, 17).
0.0
1.0
0.0
1.0
Tw
o-W
ay T
rave
ltim
e (s
)
NWMiddle Atlas
Uplift and Onlap of Turonian
Base Cretaceous Unconformity
High AtlasSE
Jurassic
Triassic
Cenomanian
Hercynian Unconformity 0 1km
(Postrift)
(Synrift)
(Prerift)
Line MR-7
Phillips in 1983 that reached total depth in theJurassic. Line PKM-09 shows a thickening of theJurassic from east to west across the section, from644 to 826 ms two-way traveltime. This thickeningoccurred during rifting, and the sedimentary rocksthicken toward a reactivated normal fault to thewest. The inversion has resulted in an anticlinalstructure (in the subsurface and surface) wherethere was previously a half graben.
A common feature of rifting is syndepositionalgrowth strata or a progressive unconformity asso-ciated with an extensional half graben. Majorunconformities, such as the base Cretaceous,commonly appear to have been flattened due touplift and inversion. These unconformities origi-nally dipped basinward as a result of postrift subsi-dence (Figure 17). Regional seismic lines thatextend across the basin were tied to wells drilledin the basin using synthetic seismograms andtime-depth curves, and major stratigraphic bound-aries were mapped throughout the basin. Theseinterpreted stratigraphic boundaries were thendigitized and redisplayed to produce the inter-preted cross sections in Figure 17. One important
feature is the lack of faulting in the Missour basinas a whole, with the overall deformation havingoccurred along the margins of the basin both dur-ing the extensional synrift phase and in laterinversion phases of deformation (Figure 17). Acommon inversion characteristic recognized onthe seismic data in the basin is dip reversal rela-tive to the direction of sedimentary thickening(Figure 13).
Evidence of a previously unrecognized phase ofuplift (Turonian) or a change in sea level can beseen on seismic line MR-7 (Figure 13) based ongrowth strata. The lack of Lower Cretaceous(Berriasian–Albian) sedimentary rocks above thebase Cretaceous unconformity indicates a missing40 m.y. of sedimentary rocks in the Missour basin(Figure 13). The lack of deposition during this timemay be related to isostatic uplift of the Atlas riftshoulders or margins. The basin was positioned onsuch a shoulder or margin during the onset of thepostrift subsidence phase. This isostatic uplift ofthe rift margins (Missour basin, High Plateau, andMoroccan meseta, Figure 1) during the return ofthe rift to thermodynamic equilibrium would haveresulted in the deposition of Lower Cretaceous sed-imentary rocks into the subsiding Atlas rift basin.The Lower Cretaceous sedimentary sequencewould not necessarily have been represented onthe shoulders of the Atlas postrift subsidence basin(Missour basin). Some Lower Cretaceous sedimen-tary rocks (Albian) are preserved in the High AtlasMountains to the south of the Missour basin. Theinversion of the postrift Atlas basin would haveresulted in the uplift and erosion of the LowerCretaceous sedimentary rocks from most of theAtlas Mountains.
Fault Restorations
Both planar and listric faults were formed in theextensional phase of rifting of the Atlas rift systems.Listric faults interpreted in the Missour basin shal-low to a detachment in the Paleozoic. One suchexample is seen on seismic line MR22 and trendsnorthwest along the margin of the Middle AtlasMountains (Figures 12, 18). Thickening of the syn-rift sequence is clearly evident from the southeastto the northwest into a listric normal fault that dipsto the southeast. This line was migrated, depth con-verted, and plotted at a one-to-one scale for model-ing and balancing of the section. Line MR22 wascorrelated to a cross line that was then tied to wellOSD-1 using a time-depth curve that is north of line MR22 (Figure 12). The base Cretaceous andHercynian unconformities diverge from southeastto northwest, and the Hercynian unconformityhas been uplifted so that it is near horizontal. This
Beauchamp et al. 1473
0.0
1.0
2.0
3.0
Line 85KB11NW SE
Base Jurassic
Base Triassic
Base Triassic
Base Jurassic
A'
A
Carboniferous
Carboniferous
Base Cretaceous
0 km 1
Tw
o-W
ay T
rave
ltim
e (s
)
Figure 14—Seismic profile 85KB11 located near thesoutheast margin of the Middle Atlas Mountains (loca-tion on Figure 12). The null point is the point at whichAA′ is adjacent across the fault. Note the uplift of the Tri-assic unconformity. Erosion by the base Cretaceousunconformity has removed any indication of whetherthe fault was a reactivated synrift fault or a newlyformed reverse fault.
seismic line is important, as it illustrates what isbelieved to be a common inversion structuralstyle in the Missour basin and the AtlasMountains. Listric normal faults as illustrated online MR22 are easier to reactivate than steeperplanar faults due to a lower friction on the faultplane, particularly when the maximum compres-sive stress is horizontal. The listric fault on lineMR22 was initially inverted by slip along thelower part of the fault, because the hanging-wallhalf graben was rotated up the fault plane. As thedip of the fault plane steepened, it became easierto generate a new fault that cut through the foot-wall as a lower angle thrust fault than to continuethe inversion of the hanging wall up the steepersection of the original synrift normal fault (Figure19A). This new shortcut fault formed a ramp-flatgeometry similar to that generated in a fault-bendfold style of deformation associated with purelycompressional tectonics. Continued shorteningacross the fault zone developed a fault-bend foldover the ramp in the footwall. The generated foldis an open fold that is slightly asymmetrical.Further shortening across the fault zone results inthe initiation of a breakthrough fault along theforelimb of the fault-bend fold, and continuedshortening is accommodated by uplift along the
steeper reverse fault (Figure 19B). This break-through by reverse faulting occurs instead of fur-ther shortening along the ramp by the fault-bendfold. Reconstruction of line MR22 to the baseCretaceous unconformity (Figure 19C) illustratesthe geometry of the half graben prior to inversion.Stratigraphic relationships in the half graben east ofthe listric fault indicate there may have been aphase of uplift prior to the Cretaceous, becauseerosion and folding are apparent in the synrift sedi-mentary rocks. The synrift geometry may also rep-resent the fold geometry associated with the synriftlistric fault.
Using relationships from the restoration of lineMR22 to a postrift preinversion phase, we can esti-mate the amount of shortening across the faultzone. This technique has been proven to be effec-tive for estimating β as defined by McKenzie (1978)for normal faults across tilted fault blocks (LePichon and Sibuet, 1982):
where φ = angle between listric fault and preriftunconformity and ψ = angle between preriftunconformity and a synrift stratigraphic horizon;
β φ φ ψ= −( )sin sin
1474 Intracontinental Rifting and Inversion
0
–1
–2
–3
1
0 1 2 3km
Dep
th (
km)
Base Cretaceous
Jurassic Synrift
Synrift
Base Cretaceous
Hercynian Unconformity
Triassic Basalts
Prerift
Postrift
NorthMissour BasinLine 86MR17High AtlasSouth
Postrift
Figure 15—Seismic profile 86MR17 shows the uplift of the Hercynian unconformity above the null point. There hasbeen an apparent uplift of 2 km on the Hercynian unconformity. The amount of shortening normal to the fault islimited due to the geometry of the fault, and subsequent strain is accommodated by strike-slip or oblique-slip move-ment (location on Figure 12).
The amount of stretching obtained for this halfgraben (2.15) in the Missour basin is the ratio of thecrust before and after extension as defined byMcKenzie (1978). Values of β for the North Sea(Figure 3) based on seismic ref lection data areapproximately 1.4 (Barr, 1987). Stretching values(β) for the Rhine graben are 1.15, the Armoricanmargin is 1.53, and the Afar rift system is 3.0 (LePichon and Sibuet, 1982).
Similarly, the subsequent amount of shorteningacross the fault zone can be calculated using thedistance between the pin line and loose line of therestoration of line MR22 (Figure 19). The distancebetween the pin line and loose line prior to restora-tion is 17 km, and the distance before shortening is20 km. The resulting amount of shortening acrossline MR22 is approximately 15%.
High-angle normal faults such as interpreted online MR17 (Figure 15) will accommodate muchless shortening with stress oriented normal to thefault plane. Steep faults in the Missour basin areaccommodating strain created by horizontal com-pression by strike-slip or oblique-slip deformation.A significant amount of strain may be accommo-dated by the strike-slip and oblique-slip movementon high-angle normal faults, but the amount of strain and shortening across these faults is difficult to quantify due to movement in and out of the plane of the seismic section. The combina-tion of both high-angle and listric normal faults in the Missour basin and Atlas Mountains results in a complex tectonic history after inversion.Horizontal stress applied to the synrift fault sys-tems of the Atlas probably resulted in deformation
by oblique-slip reverse movement, as well as fault-bend folding, and a thin-skinned style of deforma-tion resulting from inversion.
INVERSION TECTONICS
Structural inversion related to intracontinentalrifting occurs when extensional rift faults reversetheir sense of motion during subsequent episodesof compressional tectonics. Features generated byextension, such as half grabens, are uplifted to formpositive anticlinal structures.
It is important to validate that a structure is actu-ally an inversion feature or a newly generated com-pressional structure, because reactivated rift faultshave prospective stratigraphic relationships. Acommon feature of r ifting is syndepositionalgrowth strata or a progressive unconformity associ-ated with an extensional half graben. Reactivationof the growth fault by later compression normal tothe fault will result in a thicker synrift section inthe hanging-wall anticline than in the footwall syn-cline. Inversion is sometimes difficult to recognizewhen uplift of the hanging wall has occurred to thepoint where erosion has removed any indication ofthe original thickening associated with the halfgraben.
The reactivation of normal synrift faults invertsprevious half grabens into anticlinal structures,with the axis of the half graben centered below theaxis of the inversion anticline. Anticlinal structuresin the Missour basin and Atlas Mountains frequentlyrepresent an inverted half graben. Because of thisrelationship, hanging-wall anticlines formed by theinversion of synrift half grabens will have a thicker
Beauchamp et al. 1475
1.0
2.0
West EastKSAB-102
Line PKM-09
10 km
Carboniferous
700 ms
644 ms
770 ms
826 ms Jurassic
Cretaceous
TD 2188 m
Figure 16—Seismic profilePKM-09. Inversion can bedemonstrated by the upliftof a previous half graben.Thickening from the rightto left across the sectioncan be seen from a reflector in the synriftsequence to a reflector at the top of the Triassicbasalts (location on Figure 12).
synrift section in the hanging wall than in the foot-wall even though the hanging wall may be abovethe footwall. The resulting inverted fold geometryis controlled by the geometry of the extensionalfault (planar or listric) and the depth of detachment(e.g., Mitra, 1993). Reactivated listric faults normal-ly form inversion anticlines that exhibit fault-bendfold geometry, and allow for greater shorteningthan planar faults. This style of inversion and short-ening may have contributed to the creation of thehigh elevations (>4000 m) in the High AtlasMountains. Inversion along listric faults can gener-ate a compressional fault-bend fold that is moreopen or symmetrical than a fault-propagation foldgenerated by the reactivation of a planar fault(Mitra, 1993). Folds generated by reactivated nor-mal faults are commonly associated with faultbreakthrough along the forelimb of the anticlines,as well as footwall thrusts.
Half grabens formed during rifting are dividedinto a synrift and postrift sequence separated by an
unconformity (base Cretaceous in the Missourbasin) (Figure 13). Sedimentary rocks associatedwith the postrift sequence normally have lowerdips, usually associated with regional subsidenceinto the paleorift basins. The synrift sequence,however, is associated with steeper dips related tothe original hanging-wall fold shape and strati-graphic growth. The dips of sedimentary rocks inthe inverted anticlinal structure are steeper in thecore of the anticline than along the f lanks of theinverted structure. Planar faults that do not develophanging-wall anticlines frequently preserve the rela-tionships of steeper dipping synrift sedimentaryrocks after they are inverted into fault-propagationfolds. The structure at Jebel Missour exhibits thisrelationship, where f lat-lying postinversionNeogene sedimentary rocks onlap more steeplydipping Upper Cretaceous rocks that, in turn, over-lie Jurassic rocks exhibiting yet even steeper dips.
Listric faults that generate hanging-wall rolloverfolds during extension must be “unfolded” during
1476 Intracontinental Rifting and Inversion
0 . 0
0 . 4
0 . 8
1 . 2
1 . 6
KB11
J u r a s s i c /
Cenomanian
Turonian-Cenozoic
Carboni ferous
TW
T (
s)
0 . 0
0 . 4
0 . 8
1 . 2
1 . 6
TW
T (
s)
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TW
T (
s)
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1 . 6
TW
T (
s)
J u r a s s i c - T r i a s s i c
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Turonian-Cenozoic
Carboni ferous
Turonian-Cenozoic
Cenomanian
J u r a s s i c - T r i a s s i c
Carboni ferous
Turonian-Cenozoic
Cenomanian
J u r a s s i c
Carboni ferous
Missour Basin
Missour Basin
T r i a s s i c
Middle Atlas
Middle Atlas
Missour BasinMiddle Atlas High Atlas
Missour BasinMiddle Atlas
MR-70 5km
0 10km
0 10km
0 10km
KB-2
KB-8
OSD-1
NW
NW
NW
NW
SE
SE
SE
SE
Figure 13
F i g u r e 1 4
T r i a s s i c
Figure 17—Interpretations ofregional seismic profiles acrossthe Missour basin. Note the thickening of the synriftsequences (Triassic–Jurassic) outof the Missour basin into thepaleo-Atlas rifts (Middle and High Atlas mountains). Regionalinversion effects can be seen bythe uplift of the stratigraphic section near the basin margins.The base Cretaceous unconformityin some cases may have removedthe sense of synrift thickening into the rift (KB-2). The locations of Figures 13 and 14 are shown. TWT = two-way traveltime.
inversion before the hanging wall can be refoldedinto an inversion anticline (Mitra, 1993). Therestoration of the inverted listric fault on seismicline MR22 (Figures 18, 19) shows that there aresynrift reflectors indicating dips that may be relat-ed to an earlier extensional hanging-wall rolloverfold. An inverted listric normal fault is difficult torestore to the exact geometry present prior toinversion, possibly due to slip out of the plane ofthe section.
Inverted structures are important explorationobjectives because extensional half grabens may contain source rocks and reservoir rocks.Hydrocarbons generated during rifting are gener-ally trapped in the synrift sequence updip alongthe crests of footwall anticlines associated withextensional half grabens, such as in the North Sea.The inversion of faults associated with footwallanticlines results in the uplift of the hanging-wallgraben above and sometimes over the top of theoriginal footwall anticline (Figure 19). The resultof this inversion is the creation of a hanging-wallcompressional fault-bend fold in combinationwith a subthrust footwall anticline/ramp. Thisrelationship produced by inversion tectonics cre-ates the opportunity for stacked structural traps,both in the original footwall anticline and in theinverted hanging-wall fold. This type of potentialtrap could be important with significant shorten-ing and inversion. Large amounts of horizontalshortening may place Triassic sedimentary rockscontaining salts and evaporites above Jurassic
source and reservoir rocks in the footwall, provid-ing an effective seal. This seal formed by thethrusting of the Triassic sequence could help tomaintain traps formed during r ifting in theJurassic and help to form a new trap that can col-lect hydrocarbons that are remigrated by subse-quent inversion.
HYDROCARBON POTENTIAL
The Missour intermontane basin was formed bythe uplift and inversion of the margins of the Atlasrift system. The stratigraphic relationships of theMissour basin are such that the distribution of syn-rift sedimentary rocks (Triassic–Jurassic) of sourcerock facies, as in most rift basins, would not beexpected to be along the margins of the rift. Mostsource rocks related to the synrift phase of theAtlas rift system would have been deposited inwhat is now the High and Middle Atlas mountains(Atlas paleorift). For this reason, the most prospec-tive areas for hydrocarbon traps in the basin arealong the margins of the High Atlas and MiddleAtlas mountains. These regions are importantexploration fairways because hydrocarbons gener-ated during the synrift and postrift phases of theAtlas r ift system would have migrated updiptoward the basin margins. Hydrocarbons trappedin the original rift structures may have remigratedtoward the margins of the Atlas Mountains uponthe inversion of the rift system. Two potential
Beauchamp et al. 1477
1.0
0
–1.0
–2.0
–3.0
–4.0
–5.0
Dep
th (
km)
NW SELine MR22
Synrift
Postrift
PostriftPostrift
Synrift
Synrift
Missour BasinMiddle Atlas Mountains
Figure 18—Seismic line MR22 was migrated and depth converted to enable the modeling of the reactivated listricnormal fault seen on the seismic profile. Thickening of the synrift sedimentary rocks is evident from the southeastto the northwest into a listric normal fault that dips to the southeast.
hydrocarbon systems are present in the Missourbasin and Atlas Mountains: (1) the Paleozoic–Triassic system sourced from the Paleozoic andsealed by Triassic salts and basalts with potentialTriassic sandstone reservoirs, and (2) the post-Triassic system with potential sources and reser-voirs in the Jurassic and Cretaceous. The structuralmodels developed for the inversion of synrift nor-mal faults in the Missour basin and High AtlasMountains are favorable for trapping hydrocarbonsin the following three scenarios.
(1) The inversion of a listric normal fault mayposition Jurassic source rocks in the footwall of afault-bend fold-style structure, creating the possi-bility of moving immature synrift source rocks intothe oil window in the footwall of a thrust. Triassicevaporites in some cases may be thrust oversource and reservoir rocks in the footwall, creatingan effective top seal.
(2) Inversion creates the possibility of preserv-ing hydrocarbons trapped during rifting. Asobserved along the margins of the Atlas Mountains,not all synrift faults are reactivated. Some exten-sional structures that trapped hydrocarbons duringrifting may be preserved beneath thrust faults origi-nating from reactivated listric faults (footwall short-cut faults).
(3) Paleozoic source rocks (Carboniferous–Silurian–Devonian) are more prospective in theMissour basin and the margins/shoulder areas of thepaleo-Atlas rift systems. Paleozoic source rocks in theAtlas rift probably would have been buried too
deeply to have any remaining source rock potential.A lower geothermal gradient and burial depth in theMissour basin and other rift margins may yield poten-tial traps sourced by Paleozoic source rocks.Hydrocarbons sourced from the Paleozoic may betrapped in Triassic sandstones and sealed by overly-ing and interbedded salts, anhydrites, and basalts ofthe Triassic.
Source rock samples collected in the field in theMiddle Atlas Mountains west of the Missour basinindicate favorable source rock parameters (Table 2).Upper Pliensbachian source rocks yield total organ-ic carbon values between 1.66 and 3.87%. The Tmax(Rock-Eval) values of 421–437°C for these upperPliensbachian marls (type II) are approximatelyequivalent to vitrinite reflectance values of 0.5–0.6(Miles, 1989). These data indicate that the upperPliensbachian (Dommerian) source rocks are tooimmature for oil generation. These source rocksmay be similar to source rocks of the same agefound in the Paris basin and other parts of central andsouthwestern Europe (Hallam, 1987). Additionalpotential may be present in Pliensbachian–Bajocianreefs developed along the margins of the basin, suchas those found at Jebel Mechkakour and sourced byrocks of the same age (Figures 6, 12) (El Alji andOuazzaba, 1995). Maastrichtian source rocks yieldhigh total organic values (∼18%) and have excellentpotential for generating hydrocarbons when theyoccur in the footwall of a subthrust-style structure.These Maastrichtian source rocks may be related toother known source rocks deposited in the Late
1478 Intracontinental Rifting and Inversion
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1
0
–1
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–4
Dep
th (
km)
Pin Line
*Footwall Unrestored
Loose Line
0k m1 2
17 km
18 km
20 km
Late Cretaceous Restoration
Paleogene Restoration
Present
�
Jurassic
Jurassic
Jurassic
Cenomanian
Turonian
Turonian
Cenomanian
Cenomanian
Turonian
Jurassic
Jurassic
HercynianUnconformity
Jurassic
Carboniferous
Carboniferous
Carboniferous
Base CretaceousUnconformity
NW SE
(A)
(B)
(C)
Triassic
Triassic
Triassic
Line MR 22
Ψ = 12°Φ = 2 2°β = s i n Φ / s i n (Φ − Ψ) , β = 2 .15 = 1 / β = .4 6
3 k m / 2 0 k m = 15 % s h o r t e n i n g
Figure 19—Restoration ofmigrated and depth converted seismic reflectionline MR22. Reactivation ofthe synrift listric fault occursuntil the fault steepens, andthe synrift fault is bypassed.Shortening is then accommodated by a thrustthat cuts the footwall at alower angle (C). A fault-bendfold forms over the newfootwall ramp (B), and islater faulted along the forelimb by the reactivationof the original synrift normal fault (A). Shorteningthen occurs in the hangingwall along the high-anglereverse fault. Shortening ofthe original half graben isapproximately 15%. The thinning factor β isapproximately 0.46 that isthe inverse of McKenzie’s(1978) stretching factor β.
Cretaceous during a transgressive phase; thesedeposits were widespread organic-carbon–richsediments in Morocco (Schlanger et al., 1987).Shales and similar deposits grade into laterallyequivalent phosphorites of Late Cretaceous toearly Tertiary age laid down on the southern mar-gin of Tethys, stretching from North Africa into theMiddle East (Hallam, 1987). The OSD-1 well drilledin the western Missour basin near the Middle AtlasMountains (Figure 12) shows favorable sourcerock characteristics (type III) in Westphalian andNamurian marls and shales (Table 2). The geo-chemical parameters for the upper Pliensbachian,Maastrichtian, and Paleozoic source rocks, com-bined with the aforementioned trapping mecha-nisms, create exploration potential for hydrocar-bon accumulations along the margins and withinthe Atlas Mountains.
CONCLUSIONS
The integration of surface geological mapping,seismic ref lection data, well data, and remotesensing has given a better understanding of thetectonic processes, timing, and structural stylesresulting from the various deformational phasesforming the Atlas Mountains and associatedbasins.
The Missour basin was a stable shelf marginseparating the High and Middle Atlas rift systems.Regional shortening across the region during theLate Cretaceous and Tertiary resulted in the upliftof the entire Missour basin region contemporane-ously with the uplift/inversion of the paleo-Atlasrifts. Shortening across the region has occurredmainly along the margins and the interior of thepaleo-Atlas rift systems. Most of the shorteningacross these rift systems was along preexisting
faults formed during Mesozoic rifting or the previ-ous Hercynian orogeny. The geometries of struc-tures generated by inversion are controlled by thetype of extensional faults formed during riftingand the orientation of the maximum compressivestresses relative to these faults (Figure 20). Thereare a variety of fault types (planar and listric),fault polarities, and fault distributions presentwithin the Missour basin and the adjacent AtlasMountains.
Several phases of deformation resulted in theshortening and inversion of the paleo-Atlas rift sys-tem and the Missour intermontane basin: upliftrelated to the Hercynian orogeny, an uplift phase inthe Middle Jurassic, uplift in the Early Cretaceousrelated to subsidence, a major uplift/inversionphase in the Early Tertiary (Paleogene), recentdeformation in the Neogene–Quaternary (left-lateral oblique slip) along the Middle Atlas-Missourbasin margins, and right-lateral oblique-slip move-ment along the High Atlas–Missour basin margin(Figure 3A). The Missour basin may have beenuplifted/inverted by the culmination of severalphases of deformation. The region was previouslypart of the shelf margin or shoulder of the Atlas riftsystem and has been uplifted and translated to thenortheast.
The combination of both high-angle and listricnormal faults in the Missour basin and AtlasMountains results in a complex tectonic history fol-lowing inversion. Horizontal stress applied to thesynrift fault systems of the Atlas has resulted indeformation by oblique-slip reverse movement, aswell as fault-bend fold, thin-skinned style of defor-mation resulting from inversion. Applying structuralinversion models to observed structures in the AtlasMountains may present new exploration opportuni-ties that have not previously been attempted as anexploration strategy in Morocco.
Beauchamp et al. 1479
Table 2. Geochemical Data from the Middle Atlas Mountains and Well OSD-1*
Depth TmaxLatitude Longitude (m) TOC S1 S2 S3 (°C) HI OI Age
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1480 Intracontinental Rifting and Inversion
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Base Cretaceous map view
Base Cretaceous map view
Extensional half-graben planar fault
synrift phase
Inversion of extensional half-graben along
planar fault
Postrift phase
Asymmetrical-overturned folding, oblique-slip
deformation
synrift
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synrift
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prerift
postrift
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Inversion of
listric normal fault by a footwall
shortcut
Breakthrough of fault-bend fold
by high-angle reverse fault
Extensional half-graben listric fault
Inversion of Listric Normal Faults
Inversion of Planar Normal Faults
Figure 20—Faults associated with the Atlas rift systemoccur as both listric and planar normal faults. Fault-bend folds are frequently formed by the reactivation oflistric faults, when compression is applied normal tothe fault plane. Fault-propagation folds can form whencompression is normal to a fault plane of reactivatedplanar faults. Listric faults reactivate by dip-slip move-ment when subjected to normal compression, whereasplanar faults commonly reactivate by oblique-slipmovement.
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1482 Intracontinental Rifting and Inversion
Weldon Beauchamp
Weldon Beauchamp received aB.A. degree in geology from NewEngland College, New Hampshire,and an M.S. degree in geology fromOklahoma State University. Heworked for Sun Exploration andProduction Company in OklahomaCity, Oklahoma, and Dallas, Texas,as a geologist in the mid-continentregion, prior to joining Sun Inter-national Exploration and ProductionCompany in Dallas, Texas, and London, England. Heworked for Sun as a new venture exploration geologist inthe North Sea, Africa, and the Middle East regions.Currently, he is working on his doctorate in geophysics atCornell University, Institute for the Study of theContinents. Recent interests are in the Atlas mountains ofMorocco and the tectonics of North Africa.
Muawia Barazangi
Muawia Barazangi is a senior sci-entist and faculty member in theDepartment of Geological Sciencesand the Institute for the Study ofthe Continents (INSTOC), CornellUniversity. He is the associatedirector of INSTOC, and leader andcoordinator of the Middle East andNorth Africa Project at CornellUniversity. Academic backgroundincludes a B.S. degree in physicsand geology from Damascus University (Syria), an M.S.degree in geophysics from the University of Minnesota,and a Ph.D. in seismology from Columbia University,Lamont-Doherty Earth Observatory (New York).Professional experience includes global tectonics, tec-tonics of the Middle East and North Africa, structure ofthe continental lithosphere, and structure of intraconti-nental mountain belts.
Ahmed Demnati
Ahmed Demnati is currentlyheading an exploration division ofthe Moroccan national oil com-pany ONAREP (Office National de Recherche et d’ExploitationPetrolieres), where he is acting aschief geophysicist. Prior to joiningONAREP he was responsible for thegeophysical and sedimentary basindivision of the Moroccan GeologicalSurvey. He holds an M.S. degree ingeophysics from the Bergakademie Clausthal-Z, and aPh.D. from the University of Hamburg in Germany.
Mohamed El Alji
Mohamed El Alji received apetroleum exploration engineeringdegree from ENIM (Ecole Nationalede l’Industrie Minerale) in 1987.Since this date he has been work-ing for ONAREP (Office Nationalde Recherche et d’ExploitationPetrolieres) as a petroleum geolo-gist. He participates in the studiesof different Moroccan basins andhis interests are the Missour basinand the Prerif region.
