Structure and Evolution of the Neogene Guercif Basin

ABSTRACT

The Guercif basin of northern Morocco occu-pies a 50 × 60 km area where the transpressionalMiddle Atlas mountains terminate and abut the Rifthrust belt. Analysis of over 800 km of 2-D (two-dimensional) seismic reflection profiles and eightexploratory wells, in combination with existinggeological data, suggests a late Miocene episode ofextension (4%, or 1.7 km, maximum) and a subse-quent episode of contraction since the end of theMiocene. Most of the late Miocene deposition wasconcentrated in a narrow graben (herein referredto as the Guercif graben), which contrasts with thewider physiographic expression of the basin today.Geohistory analysis indicates that tectonic subsi-dence persisted until the Messinian, and sedimentloading continued to drive subsidence even afterextension stopped. Timing constraints demonstratethe contemporaneity of the Guercif graben andwest-southwest–vergent thrust tectonics of the Rifthrust belt. Similar timing and proximity with theRif, as well as the graben geometry, suggest thatextension in the Guercif basin, in addition to othersmaller extensional basins in the northern MiddleAtlas region adjacent to the Rif, may represent thedistal effects of a broad lateral shear zone boundingthe thrust belt.

1340 AAPG Bulletin, V. 84, No. 9 (September 2000), P. 1340–1364.

©Copyright 2000. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received March 11, 1999; revised manuscript receivedFebruary 23, 2000; final acceptance February 28, 2000.

2Institute for the Study of the Continents, Cornell University, Ithaca, NewYork 14853; e-mail: [email protected]

3Office National de Réchèrche et d’Éxploitations Petrolières, B.P. 8030,Rabat, Morocco.

The development of the manuscript benefited from helpful suggestionsand informal reviews provided by G. Brew, E. Gomez, R. Allmendinger, T. Jordan, A. Calvert, D. Seber, and A. Al-Lazki. We thank M. Dahmani, A. Er-Raji, M. Morabet, M. Zizi, and H. Achnin for their helpful discussions inMorocco. We thank W. Beauchamp, J. McBride, and C. Morley forconstructive reviews. Special thanks to W. Krijgsman for providing anadvanced copy of his article, which contained valuable age constraints. Thisresearch was supported by NSF grant EAR-9627806. INSTOC contributionnumber 253.

Structure and Evolution of the Neogene Guercif Basin atthe Junction of the Middle Atlas Mountains and the Rif Thrust Belt, Morocco1

Francisco Gomez,2 Muawia Barazangi,2 and Ahmed Demnati3

The Neogene Guercif basin is superimposed onthe Mesozoic Middle Atlas rift, which experiencedbasin inversion during the Cenozoic, and seismicreflection interpretations in the southern Guercifbasin depict old Mesozoic rift faults reactivated asreverse faults. Unconformities illustrate that theuplift of the Middle Atlas appears to be primarily alate Cenozoic phenomenon. The Guercif basinoffers a special opportunity for petroleum explo-ration within an aborted rift basin such as theMiddle Atlas. Mesozoic source rocks in the MiddleAtlas may have been sufficiently buried beneathNeogene basin sediments to reach maturity, andthe late Cenozoic timing of contraction can pro-duce suitable structural traps.

INTRODUCTION

The Guercif basin of northern Morocco contains arecord of the interactions between the late Cenozoicintracontinental deformation of the Middle AtlasMountains and the thin-skinned thrust tectonics ofthe Rif Mountains. The Guercif basin appearsabruptly along-strike to northeast of the Middle AtlasMountains (Figure 1). Similar to the Middle Atlas, theNeogene Guercif basin is superimposed on an earlyMesozoic rift basin; however, in contrast to lateCenozoic uplift of the Middle Atlas fold belt (e.g.,Gomez et al., 1998), the Guercif basin experiencedconsiderable subsidence during the late Neogeneand Quaternary. The adjacent Rif thrust belt experi-enced several episodes of shortening (e.g., Morel,1989; Frizon de Lamotte et al., 1991) that coincidetemporally with the Guercif basin’s development.Thus, the Guercif basin may provide a useful exam-ple for understanding the interactions between aninterplate thrust belt (the Rif) and adjacent continen-tal (foreland) deformation (the Middle Atlas). Thelocal tectonic complexity found in the Guercif basinmay also improve the prospects for petroleumexploration in these regions.

The Guercif basin performed a pivotal role in thelate Neogene history of the Mediterranean region.

Prior to the opening of the Strait of Gibraltar, theAtlantic Ocean and Mediterranean Sea were con-nected via the “Rifian corridor” (e.g., Benson et al.,1991). This seaway, which flowed through what isnow the Guercif basin, constricted during theMessinian (7.25–5.2 Ma). Following closure of thisnarrow seaway, the Messinian desiccation event ofthe Mediterranean Sea ensued (this event is alsoknow as the Messinian salinity crisis) (e.g., Hsu etal., 1973). Recent chronological results of Hilgen etal. (1995) constrain this event to occur between5.5 and 5.3 Ma. If the competing tectonic signals ofneighboring mountain chains are distinguishable,then the stratigraphic record in the Guercif basinshould contain a potentially useful record of thetectonic events that were directly responsible forthe isolation of the Mediterranean.

We present new constraints on the structure andNeogene tectonic history of the Guercif basin; ourconstraints are based on the analysis of geophysicaldata in combination with existing geological infor-mation. An extensive database of subsurface infor-mation helps to constrain the basin geometry, tim-ing of events, structural styles, and approximatemagnitudes of deformation and subsidence, andour results permit further consideration of thebasin’s role in the regional tectonic scheme. Wedocument a two-stage basin history since the lateMiocene involving initial development as a narrowgraben followed by deposition synchronous withweak basin inversion tectonics. We interpret theseresults as reflecting the influence on the African

foreland of different tectonic episodes in the Rifmountains.

GEOLOGICAL SETTING

The North African crust bears the scars of repeat-ed deformations imparted since at least the Paleo-zoic; therefore, studying the Cenozoic tectonics ofMorocco involves considering structural elementsinherited from prior events. Key structural elementsinclude the Hercynian fold belt that can be tracedacross the present-day Moroccan Meseta (Figure 1).Subsequently, during the Permian–Triassic break-upof Pangea, the Atlas rift system developed. Thisincluded the initiation of the transtensional MiddleAtlas rift basin in the northeastern part of the pres-ent-day Middle Atlas and Guercif basin (Brede et al.,1992). With the opening of the central AtlanticOcean and the initiation of sea-floor spreading duringthe Early Jurassic, Atlas rifting essentially ceased.After this time, a general tectonic quiescence per-sisted until the late Mesozoic, although minorepisodes of uplift have been interpreted to punctu-ate this time (e.g., Monbaron, 1982; Brede et al.,1992). During the Late Cretaceous, convergencebetween Africa and Eurasia began (Dewey et al.,1989). This convergence ultimately resulted inbuilding the Alpine mountain belts encompassingthe Mediterranean region. Convergent interplateprocesses also reactivated the crustal weaknessesof the Atlas paleorifts, resulting in the Cenozoic

Gomez et al. 1341

Figure 1—Map of northernMorocco depicting thespatial relationship ofthe Guercif basin toother major Cenozoictectonic elements ofMorocco, including theRif and Atlas mountains.Box denotes the studyarea shown in Figures 2and 5. GB = Guercifbasin, FMA = folded Middle Atlas, TMA = tabular Middle Atlas,NMAF = north MiddleAtlas fault, SB = Saissbasin, RB = Rharb basin,MB = Missour basin, TF =Cape Trois Fourches.Inset map shows the spatial extent of theintracontinental Atlassystem and the interplateAlpine mountain chains of the westernMediterranean region.

0 200 kmAtlas ranges

Rif Mountains

Neogene basins

FMA

GB

Casablanca

Rabat

High Atlas

Central Rif

High Plateau

Moroccan Meseta

2˚4˚6˚8˚10˚ W

36˚ N

34˚

32˚

External Rif

NM

AF

Internal Rif

TMA

Taza

Mediterranean(Alboran) Sea

Atlant

ic

Ocean

SB

TF

Figures2 & 5

MB

RB

500 km

Atlas systemAlpine chain

N. Africa

Iberia

Atlas mountain chains. The basin inversion process-es are well illustrated by seismic reflection data inthe Missour basin (Beauchamp et al., 1996) andHigh Atlas regions (Beauchamp et al., 1999).

Two prominent tectonic elements neighborthe Guercif basin: the Rif thrust belt (part of thewestern Alpine collisional belt) and the MiddleAtlas mountains located in the foreland of the Rifthrust belt. The Rif mountains can be divided intothree structural zones (Figure 1) reflecting differ-ent episodes of tectonic transport (Morel, 1989;Frizon de Lamotte et al., 1991): the internal Rif andflysch zone (early Miocene west vergence), the cen-tral Rif (middle to late Miocene west-southwest ver-gence), and the external Rif (late Miocene–Pliocenesouth vergence). The paleostress history of northernMorocco (e.g., Morel, 1989) documents a progres-sive change in the regional stress field involving a

reorientation of the maximum horizontal compres-sion from northeast-southwest to north-north-west–south-southeast (and thus more consistentwith the relative plate convergence). In the easternexternal Rif, this change occurred during theMessinian, while in the western external Rif, west-southwestward thrusting persisted through thePliocene (Morel, 1989; Morley, 1992).

Although minor uplift of the Middle Atlas may haveoccurred earlier, the main episode of uplift appears tobegin during the Neogene (e.g., Charrière, 1984; Morelet al., 1993). The Middle Atlas can be divided into twoprovinces: the folded Middle Atlas and the weaklydeformed tabular Middle Atlas (Figure 1). The north-east-southwest–striking Middle Atlas system is orientedobliquely within the late Cenozoic continental stressfield of Morocco (north-northwest–south-southeast ori-ented) (e.g., Galindo-Zaldivar et al., 1993), resulting

1342 Neogene Guercif Basin

Figure 2—Map of the Guercif basin (see Figure 1 for location) illustrating the Neogene geology (after Benzaquen,1965) and the database used in this study. More than 800 km of seismic reflection data are shown covering most ofthe Guercif basin. T = Taza, G = Guercif, M = Msoun, JG = Jebel Guillez, JA = Jebel Ahmar. Large star denotes theapproximate region of the surface stratigraphic section described by Krijgsman et al. (1999).

AKL-101

Mesozoic & Paleozoic

Mid Miocene - Tortonian

Messinian

Late Messinian- Pliocene

Quaternary

Neogene volcanic

Quaternary volcanic

seismic reflection line frontal thrust of External Rif

exploratorywell

84GR-05

GR-01

GR-0284GR-09

GR-25

ATM-1

GRF-1

MSD-1

KDH-1

TAF-1X

TAF-1

TAF-2

200 km

T G

External Rif

High PlateauMiddleAtlas

34˚

N34

˚ 30

'

4˚ W 3˚ 30' 3˚

JG

M

JA

in a transpressional system referred to as theMiddle Atlas shear zone. Fault kinematic evidencefrom the central Middle Atlas suggests that obliquedeformation has been partitioned during the lateNeogene as follows. Strike-slip faulting occursalong the north Middle Atlas fault (Figure 1) andwithin the tabular Middle Atlas, and southeast-directed thrusting is observed in the Middle Atlasfold belt (Gomez et al., 1998).

The regional geology of the Guercif basin wasmapped by Benzaquen (1965) (Figure 2), and subse-quent work by Colletta (1977) and Bernini et al.(1992) further defined the Neogene sedimentarygeology. The recent stratigraphic improvements alsobenefited from improved chronological constraintsbased on magnetostratigraphy and biostratigraphy(Krijgsman et al., 1999). The generalized Neogene

stratigraphy of the Guercif basin as described byBernini et al. (1992, 1994a) is illustrated in Figure 3.The base of the stratigraphic succession containscontinental and shallow-marine conglomerates andsandstones (middle Miocene(?)–Tortonian). The suc-cession rapidly changes upward into deeper marinecalcareous mudstones (Tortonian). In the southernGuercif basin near the boundary with the MiddleAtlas, these pelagic sediments are interfingered withclastic lenses interpreted as turbidites. Upward inthe section, these strata change to sandstonesdeposited in shallow-marine and lagoonal environ-ments (Messinian), and, ultimately, strata consist offluvial conglomerates and lacustrine limestone.

Previous geophysical studies illuminated some ofthe Guercif basin’s structural aspects. In a subsur-face study using seismic reflection and well data,

Gomez et al. 1343

Tort

onia

nM

essi

nian

Plio

cene

Pre

-Te

rtia

ry

Lith

olog

y

Age

cross-beddedfluvial sandstone

sandstone

Ostreidae-bearing horizon

calcareous mudstone

muddy sandstone

sandy mudstone

limestone

conglomerate

dolomite

100 m

10.5 Ma

7.2 Ma

5.2 Ma

cont

inen

tal

shal

low

mar

ine

deep

mar

ine

depositionalenvironment

Quat.

Figure 3—Generalized Neogene stratigraphic column of the Guercifbasin. During the earlyTortonian, the depositionalenvironment rapidlychanged from continentalthrough shallow marine todeep marine. Shallowingof the basin also beginsabruptly during theMessinian. Adapted fromBernini et al. (1992, 1994a).

Zizi (1996a, b) showed examples of Neogeneextensional structures; however, Zizi primarilyfocused on the Mesozoic rift basin, and most of thedata used in that study were located outside theactual Neogene depocenter. In another study,Bouguer gravity data were analyzed by Saaidi(1996), who used derivatives of the gravity anoma-lies to map the extent of known and possible saltdiapirs, most of which are located primarily on thesoutheastern side of the Guercif basin. Recentmobilization of some of these diapirs is suggestedfrom field observations documenting locally chaot-ic structures involving upper Miocene and Pliocenestrata (e.g., Colletta, 1977).

Late Miocene–Pliocene shortening directionsdetermined from fault and fracture analyses in theGuercif basin show a general northwest-southeastorientation (e.g., Bernini et al., 1994a, b; Colletta,1977). These directions are similar to observationsfrom the folded Middle Atlas (e.g., Gomez et al.,1996; Morel et al., 1993). Obliquity with respect tothe north-northwest–south-southeast–orientedregional σ1 in northern Morocco suggests a parti-tioning of deformation within the Middle Atlas/Guercif basin system.

Late Cenozoic volcanism of the Guercif basinwas most recently studied by Hernandez andBellon (1985), who demonstrated two suites thatvary with time. Potassic (shoshonitic) volcanismoccurred in the northern Guercif basin at JebelGuillez (Figure 2) from the late Tortonian throughthe middle Pliocene. This was followed during thePliocene and Quaternary by alkali volcanismaround Jebel Guillez and in the southern part of thebasin.

The data used herein illustrate two distinctextensional systems within the Guercif basin: aMesozoic rift system and a smaller Neogene grabenlocalized in the northwestern half of the Guercifbasin. To avoid confusion, we believe it is helpfulto clarify and standardize some nomenclature. Thename “Guercif basin” refers to the present-day,physiographically expressed basin. We refer to theMesozoic rift system (including the part in theGuercif basin) as the “Middle Atlas rift.” The smallerNeogene extensional system is labeled as the“Guercif graben.”

DATA

We have interpreted more than 800 km of 2-D(two-dimensional) seismic reflection and well data(Figure 2) made available for this study by theOffice National de Réchèrche et ÉxploitationPetrolières (ONAREP), the Moroccan nationalpetroleum company. Several seismic reflection sur-veys have focused on the Guercif basin, most

notably those taken during 1977, 1984, and 1985.Data quality varies, particularly for the older sur-veys; the more recent data (1984 and 1985) are 48-fold, whereas older data are 24-fold. Both migratedand unmigrated seismic reflection data have beenused in mapping structures, generating structuralcross sections, and constructing isopach maps.Constraints for seismic reflection interpretationsare provided by surface tie-ins with available geo-logical maps (e.g., Benzaquen, 1965; Bernini et al.,1994b) and from well data.

Several exploratory wells have also been drilledin the Guercif basin (Figure 2), penetrating intoMesozoic and Paleozoic strata. The GRF-1 well pen-etrated nearly 2 km of Neogene and Quaternarystrata, whereas other wells such as TAF-1X, contain-ing less than 100 m of Cenozoic strata, are clearlylocated outside the Neogene basin. These wells pro-vided critical depth and lithological constraints onthe seismic reflection interpretations. Sonic logs ofwells GRF-1, MSD-1, and KDH-1 were digitized andused to construct synthetic seismograms that werecorrelated with seismic reflection profiles. Sonicvelocities were averaged over key stratigraphicintervals to derive interval velocities used in depthconversion of seismic interpretations. Additionalinterval velocities were provided from the process-ing parameters of the seismic reflection data.

Neogene stage boundaries defined by micropale-ontology provide first-order age constraints in thewell logs. Further age constraints are provided bythe new stratigraphy (Bernini et al., 1992), whichcan be tied from the surface using seismic reflec-tion data. Herein, we follow the stage boundariesand chronostratigraphy presented by Krijgsman etal. (1999). In addition to the micropaleontologicalage constraints, the GRF-1 well contains a volcani-clastic layer within the Tortonian section. The vol-canic rocks are basaltic and andesitic in composi-tion and are probably sourced from the shoshoniticvolcanism at Jebel Guillez to the north [4.9–8 Maaccording to Hernandez and Bellon (1985)]. Thebiostratigraphic constraints are consistent with thelower boundary on the age range of the volcanism.Age constraints permit the use of the wells andstratigraphic section for analyzing the subsidencehistory of the basin.

In addition to borehole data, we have also useda detailed stratigraphic section from the Zobzitregion (see Figure 2 for approximate location) inthe southern Guercif basin (Krijgsman et al.,1999). Relatively high chronological resolutionof upper Miocene and Pliocene strata is providedby the magnetic polarity time scale, much higherthan the paleontological constrains in the welldata. In addit ion, Kr i jgsman et al . (1999)presented paleobathymetry constraints based onmicropaleontology.


STRUCTURE AND TIMING OF THE GUERCIF BASIN

A correlation of well logs demonstrates somegross structural features of the Guercif basin area(Figure 4). The northwest-southeast well correla-tion (Figure 4a) depicts the Neogene depocenter aslocated between the town of Msoun and the TAF-1X well. The isopach maps presented in followingfigures provide a better view of the lateral varia-tions in the thickness of the Neogene basin fill.

In the seismic ref lection data, two classes offaults are distinguished: (1) Mesozoic normal faultsaffecting Triassic and Lower Jurassic strata and (2)Tortonian normal faults that offset the Mesozoic–Cenozoic unconformity. Based on the growth ofcompressional folds and associated onlap relationsabove both types of normal faults, there is evidencefor a younger, post-Tortonian contractional reactiva-tion of these faults (inversion tectonics). Examplesof all of these structures are shown in the followingsections.

These faults and Neogene folds were correlatedbetween seismic lines to construct the structuralmap shown in Figure 5. In general, folds in theGuercif basin trend northeast-southwest, parallel tothe structural grain of the Middle Atlas mountains(Figure 5). The map interpretation depictsTortonian extensional structures primarily concen-trated in the northwestern half of the physiograph-ic basin, suggesting a noteworthy differencebetween the physiographic basin and the Neogenedepocenter. In contrast, faults constituting theMesozoic rift system are mapped farther towardthe southeast.

A broad anticlinorium trends northeast from thenorthern Middle Atlas (Jebel Ahmar) and continuesbeyond Msoun (Figure 5). This structure, hereinreferred to as the Jebel Ahmar–Msoun arch, is oneof the four main anticlinal ridges reported by Colo(1964) to span the length of the Middle Atlas moun-tains. Comparison of wells west and east of theJebel Ahmar–Msoun arch suggests thicker Meso-zoic stratigraphic sections beneath the Guercifbasin to the east (Figure 4a); therefore, the JebelAhmar–Msoun arch appears to approximate thenorthwest boundary of the early Mesozoicdepocenter in the northern Middle Atlas/Guercifbasin region. Farther southwest, the arch trendsinto the north Middle Atlas fault zone (Figure 1),and perhaps the arch represents a structural contin-uation of that fault zone.

The Northern Part of the Basin

Tortonian normal faults in the northwestern areaof the Guercif basin are well expressed in seismic

reflection data (Figure 6). Bright reflectors charac-teristic of the Mesozoic–Neogene unconformitydisplay individual fault throws of up to 600 ms(∼800 m for an interval velocity of ∼2500 m/s).These Tortonian normal faults correlate betweenneighboring seismic lines, suggesting approximate-ly northeast-southwest–striking faults (Figure 5).Lower Tortonian strata thicken toward the footwalland truncate against the fault (Figures 6, 7). Due topoor imaging below the Mesozoic unconformity, itremains unclear whether these Neogene normalfaults are reactivated structures inherited fromearly Mesozoic rifting. Upper Tortonian strata capthe northeast-striking normal faults and show nosigns of synsedimentary fault movement, such asstratigraphic truncation or thickening across thefault (Figure 6). This implies that extension ceasedduring the late Tortonian.

Two anticlinal folds in Figure 6 provide evidenceof post-Tortonian contraction. The northwesternanticline (Figure 7) has developed above a graben-bounding normal fault, suggesting that contraction-al reactivation of this fault has produced the fold.Onlapping strata constrain the initiation of uplift,and unconformities within the Messinian andPliocene strata suggest two pulses of contractionand growth for this particular structure (Figure 7).A contractional episode occurring since theMessinian is also supported by paleostress analyses(e.g., Colletta, 1977). The seismic reflection datashow that significant thicknesses of Messinian andPliocene strata accumulated contemporaneouslywith contraction. The Neogene strata also appearto thin toward the west and onlap the Msoun arch,suggesting geologically recent growth of the arch.

A line drawing interpretation of two seismicreflection profiles (including that shown in Figure6) depicts a graben approximately 20 km in width(Figure 8a). Neogene strata are well imaged, andthe lines tie together near GRF-1. A prominentstructure within the Tortonian graben is the BouMkhareg anticline [following the nomenclature ofColletta (1977)], penetrated by both the GRF-1 andMSD-1 wells (Figure 5). Onlap of early Tortonianstrata onto this structure (Figure 8a) is interpretedto ref lect the rotation of a fault-bounded blockwithin the graben.

The geometries of Neogene strata (seismicreflections) from this composite profile are used todefine depositional sequences. Onlapping andtruncated reflections define stratigraphic sequenceboundaries, and these geometric relationships andtiming constraints were used to construct a chro-nostratigraphic diagram (Wheeler, 1958) depictingtemporal and spatial patterns of deposition acrossthe graben (Figure 8b).

Five sequences are defined. The lower two (N1and N2) are synextensional. The base of N1 (early

Gomez et al. 1345

ATM-1AKL-101

GRF-1

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KDH-1

TAF-1X

TAF-1

TAF-2

T G

200 kmMiddle Atlas

High Plateau

External Rif

M

(b)

(a) ExternalRif

Guercif Basin High PlateauNW SE

N S

sea level

sea level

Msoun

GRF-1

MSD-1

KDH-1

ATM

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0 km 20

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Figure 4—Well correlation diagrams across the Guercif basin with surface topography (see also Figure 2). (a) Thenorthwest-southeast profile depicts the Neogene Guercif basin confined between Msoun and the TAF-1X well. (b)The north-northeast–south-southwest profile suggests younger subsidence in the north than in the south. Boldnumbers on the north-northeast–south-southwest profile indicate interval velocities (km/s) derived from sonic logs.Northeastward thickening of Tortonian strata suggested by the well correlation is, in part, artificial. Well KDH-1 waslocated on a structural high outside of the Neogene depocenter, whereas GRF-1 and MSD-1 were drilled within thedepocenter. Neogene units N1–N3, N4, and N5 are defined based on seismic reflection data. Inset depicts the loca-tions of the well correlation profiles.

Gomez et al. 1347

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NW SE

0 km 1 vertical exaggeration ~1.8

Figure 6—An example of late Miocene (Tortonian) normal faults in the northwestern flank of the Guercif basin asexpressed in seismic reflection line 84GR05 (see Figure 5 for location). Bright reflections characterize the Meso-zoic–Cenozoic unconformity. Because of the short wavelength of the fold growing above the anticline, a shallowlydipping thrust fault is interpreted. The fold appears to develop as a fault propagation fold, and the fault ramp isinterpreted to result from the footwall of the normal fault acting as a buttress. Age constraints are provided by theGRF-1 well. BMA = Bou Mkhareg anticline. Arrows indicate sense of shear on faults; two-headed arrows indicatereactivated fault with the younger sense of movement indicated by the larger arrowhead.

Figure 7—Close-up view of the northwestern normal fault shown in Figure 6 and the anticline developing above.Onlap and truncation (denoted by arrows) define the unconformity that represents the initiation of fold growth.Folding of Pliocene strata also attests to more recent contraction.

Gomez et al. 1349

Fig

ure

8—

Co

mp

osi

te l

ine-

dra

win

g in

terp

reta

tio

n o

f tw

o s

eism

ic r

efle

ctio

n p

rofi

les

and

ch

ron

ost

rati

grap

hic

dia

gram

acr

oss

th

e n

ort

her

n G

uer

cif

bas

in (

see

Fig

ure

5 f

or

loca

tio

n).

(a)

Th

e cr

oss

sec

tio

n d

epic

ts a

gra

ben

at

dep

th, a

pp

rox

imat

ely 2

0 k

m i

n w

idth

, wit

h t

he

pro

min

ent

Bo

u M

kh

areg

anti

clin

e (B

MA

) n

ear

its

cen

ter.

Hea

vy l

ines

den

ote

dep

osi

tio

nal

seq

uen

ce b

ou

nd

arie

s d

efin

ed b

y t

he

geo

met

ries

of

the

stra

ta (

refl

ecto

rs).

On

lap

of

stra

ta i

n t

he

bas

al (

syn

exte

nsi

on

al)

seq

uen

ce a

re i

nte

rpre

ted

as

refl

ecti

ng e

xte

nsi

on

al b

lock

ro

tati

on

. (b

) T

he

chro

no

stra

tigra

ph

ic d

iagra

mil

lust

rate

s a

wid

enin

g d

epo

siti

on

al p

atte

rn t

hro

ugh

th

e la

te M

ioce

ne

(To

rto

nia

n a

nd

Mes

sin

ian

) an

d s

ub

seq

uen

t co

nfi

nem

ent

du

rin

g th

e P

lio

cen

e.T

hes

e d

epo

siti

on

al s

equ

ence

s co

rres

po

nd

wit

h e

pis

od

es o

f ex

ten

sio

n (

To

rto

nia

n)

and

su

bse

qu

ent

con

trac

tio

n (

Mes

sin

ian

–Pli

oce

ne)

.

nond

epos

ition

(hi

atus

)

eros

iona

l lac

una

Seq

uenc

e N

3 (T

orto

nian

- e

arly

Mes

sini

an)

Seq

uenc

e N

5 (la

te M

essi

nian

- P

lioce

ne)

Seq

uenc

e N

4 (m

iddl

e M

essi

nian

)S

eque

nces

N1&

N2

(Tor

toni

an)

Sei

smic

Lin

e G

R-0

1S

eism

ic L

ine

84G

R-0

5

NW

SE

2 M

a 5 10

Mid

Mio

cene

Torto

nian

Mes

sinia

nPlio

cene

Qua

tern

ary

a) b)

BM

AG

RF

-1

0 km

24

0.0

1.0

2.0

TWTT (sec.)

010

2030

dist

ance

(km

)

N4

N1

N2

N3

N5

Tortonian or older) onlaps the Mesozoic basement,as well as the footwalls of the Tortonian normalfaults. Local unconformities above the fold andwithin N1 and N2 probably represent either differ-ent pulses of extension and uplift, or possiblyshort-term sea level fluctuations that could havereduced the detrital f lux to the interior of thegraben. Azdimousa and Bourgois (1993) document-ed several short-term sea level fluctuations in theRifian corridor during the Tortonian at Cape TroisFourches (Figure 1).

Graben formation ceased in the late Tortonian,and sequence N3 was deposited. This sequence ischaracterized by parallel ref lectors showing noeffects of normal faulting, and it represents a tec-tonically quiet period up to the earliest Messinian.Deposition continued filling space remaining fromthe previous extensional episode.

The brief period of tectonic quiescence repre-sented by N3 was succeeded by contraction docu-mented in two sequences beginning in the middleMessinian. N4 is distinguished from N3 by localstratigraphic truncation of N3 and onlap of N4above the Tortonian normal faults. This contractionmarks the closing of this part of the South Rifiancorridor, and the final closure (e.g., Morel, 1989)corresponds with the unconformity at the top ofsequence N4. As Krijgsman et al. (1999) reported,the post-N4 unconformity in the Guercif basin pre-dates the isolation of the Mediterranean by morethan 0.5 m.y. Thus, perhaps another part of theRifian corridor remained open after the Guercifbasin closed.

N5 was deposited during another contractionalpulse, distinguished from the previous sequence by abroad, regionally mappable unconformity truncatingN4 ref lections. Thinning of latest Messinian–Pliocene–Quaternary reflectors above the crest ofthe Bou Mkhareg anticline attests to growth of thisfold during this time. The geometry of the N5 reflec-tors on the north side of the anticline is interpretedas offlap rather than truncation. The GRF-1 well logdemonstrates that this sequence comprises continen-tal conglomerates and lacustrine sediments.

One possible scenario for the structural develop-ment of the graben is depicted schematically inFigure 9. Extension and block rotation resulted inonlap of the Bou Mkhareg anticline during theTortonian. This was followed by contraction andweak basin inversion. The uplift and warping of theJebel Ahmar–Msoun arch was superimposed onthis deformation as well.

The Southern Part of the Basin

Seismic reflection data in the southern Guercifbasin also depict Mesozoic normal faults that have

contractionally reactivated, such as that beneaththe structural high on which the KDH-1 well wasdrilled [the Bled Marhrane anticline of Colletta(1977)] (Figure 10). The broad anticline in thisexample grew above a blind listric fault. The wedgeof Triassic and Lower Jurassic strata that rapidlythickens toward the west attests to the listric geom-etry (i.e., extensional growth faulting). The base ofthe Mesozoic synrift strata is denoted by verystrong reflections characteristic of the Hercynianunconformity in northern Morocco (Beauchamp etal., 1996). The existence of a fault cutting theMesozoic strata was also interpreted from dipmeterdata in the KDH-1 well by GECO (1986). In planview, these southern structures align with struc-tures in the northern Middle Atlas, and this inter-pretation suggests that the northern Middle Atlasmay involve the reactivation of synrift faults.

The unconformity between middle Miocene(?)strata and Mesozoic strata suggests a pre-Tortonianinitiation of growth for both the Bled Marhraneanticline and the Haloua-Richa anticlinorium to theeast (Figure 10). If these structures are representa-tive of the northern Middle Atlas, these relationssuggest that the initiation of the main contractionof the Middle Atlas occurred prior to or during themiddle Miocene. Onlap onto the Bled Marhraneanticline persisted through the Tortonian, whereasTortonian strata above the Haloua-Richa structureare subparallel with the Mesozoic unconformity.Folded and subparallel Miocene–Pliocene reflec-tions suggest a late Neogene–Quaternary pulse ofcontraction and uplift of the Bled Marhrane anti-cline; furthermore, Neogene strata approaching theHaloua-Richa anticlinorium depict a significantwestward tilting, suggesting renewed growth ofthat fold.

The southern boundary between the Guercifbasin and the northern Middle Atlas is also bound-ed by normal faults striking approximately east-west to east-southeast–west-southwest, and thesestructures are evident in the northeast-south-west–oriented seismic line (Figure 11). Althoughtiming constraints are not as precise as they are forexamples in the northern Guercif basin, the appar-ent draping of Tortonian strata across the faults sug-gests synsedimentary fault movement, and possiblymore recently. In fact, longitudinal profiles ofstream terraces suggest continued uplift of theMiddle Atlas with respect to the Guercif basin dur-ing the Quaternary (Rampnoux et al., 1979).

Regional Tectonic Picture

The structural relations described above illus-trate significant along-strike variations in the struc-ture of the Guercif basin/Middle Atlas region.


These variations are also illustrated by two regionalcross sections through the northern and southernparts, respectively, of the Guercif basin (Figure 12).Depth-converted seismic reflection interpretations(including those shown in previous figures) pro-vide the basis for these two regional cross sections.These interpretations were depth converted bymultiplying the thickness in time by the intervalvelocities (see the data section).

The northern profile (Figure 12a) depicts theNeogene graben located on the northwestern side ofthe profile. Other minor Tortonian extensional struc-tures are located farther east, but this graben repre-sents most of the Neogene extension, as well as thelocus of subsequent contraction. Assuming Tortonianextension was oriented perpendicular to the trend ofthe Guercif graben, the minimum magnitude of

Tortonian extension can be estimated by summingthe heaves of the Tortonian normal faults using thedisplacement of the Mesozoic unconformity. Thisexercise suggests 1.5–1.7 km of northwest-southeasthorizontal extension for the Guercif graben. Farthertoward the southeast along this profile, an earlyMesozoic normal fault evident in seismic reflectiondata shows signs of contractional reactivation priorto deposition of late Miocene sediments.

The southern profile (Figure 12b) shows aNeogene half graben located in the northwest.Horizontal shortening is accommodated by old riftfaults that have reactivated as reverse faults; fur-thermore, Triassic salt locally was mobilized alongsome of these reverse faults. According to ourinterpretation, the Haloua-Richa anticlinoriummarks the southeastern edge of the Mesozoic rift,

Gomez et al. 1351

Figure 9—A conceptual andschematic illustration ofthe structural evolution ofthe Guercif graben. Tortonian extension produced block rotationand a gentle uplift of theBou Mkhareg anticline(BMA). The growth of thisstructure resulted in anonlapping unconformity atthe base of the Tortoniansequence. Subsequent contraction resulted in thegrowth of two anticlines.

dip too steep for con-tractional reactivation,

new fault

more moderate dip permits contractional reactivation

growth of anticlines above propagating faults

rotation

Extension (Tortonian)

Prior to Extension

Contraction (Messinian)

future Tortonian normal faults

NW SE

not to scale

BMA

Guercif Grabenonlap


Syn Atlas Rift(Triassic-Lower Jurassic)

Post Atlas Rift (Upper Jurassic)

0 km 1 2 3

0.0

1.0

3.0

2.0

TW

TT

(s)

NW SE

Middle Miocene?

Tortonian

Haloua Richaanticlinorium

Messinian

Bled Marhraneanticline

KDH-1(projected ~5 km)

84GR-09

out-of-syncline fault

Reactivated Mesozoicnormal fault


Mesozoic

Tortonian

0 km 1 2 3

0.0

1.0

3.0

2.0

TW

TT

(

s)

SW NEMiddle Atlas Guercif Basinextensional draping of strata

diffractionat fault plane


Figure 11—The boundary between the folded Middle Atlas and the Guercif basin as shown by a northeast-south-west–oriented seismic reflection profile (see Figure 5 for location). These data depicting east-west–striking normalfaults lower the level of the Mesozoic “basement” toward the northeast. This seismic reflection profile is unmigrat-ed, and we interpret diffractions to denote the fault locations. Draping and onlapping of Tortonian strata across thefault attest to syntectonic deposition.

Figure 10—Seismic reflection profile 84GR09 across the Bled Marhrane anticline (BNA) in the southern Guercifbasin (see Figure 5 for location) depicting a Mesozoic normal fault reactivated as a propagating reverse fault duringthe Neogene. Thickening of middle Miocene(?) strata toward the southeast of the fold suggests a pre-Tortonian initi-ation of growth. The reflections southeast of the Bled Marhrane anticline demonstrate a general northwestward tilt,suggesting late growth of the Haloua-Richa anticlinorium. Age constraints are provided by the KDH-1 well (tied inwith other seismic lines) and surface outcrop (Bernini et al., 1994b; Benzaquen, 1965).

Gomez et al. 1353

Fig

ure

12—

Reg

ion

al c

ross

sec

tio

ns

acro

ss t

he

Gu

erci

f b

asin

bas

ed o

n s

eism

ic r

efle

ctio

n p

rofi

les,

wel

l d

ata,

an

d p

ub

lish

ed g

eolo

gic

info

rmat

ion

(se

eFig

ure

5 f

or

loca

tio

n).

(a)

Th

e n

ort

her

n p

rofi

le d

epic

ts m

ost

of

the

mid

dle

Neo

gen

e ex

ten

sio

n a

nd

su

bse

qu

ent

con

trac

tio

n o

ccu

rrin

g o

n t

he

no

rth

-w

est

sid

e o

f th

e p

rofi

le,

pri

mar

ily

in

th

e vi

cin

ity

of

the

Gu

erci

f gr

aben

. (b

) T

he

sou

ther

n p

rofi

le a

lso

ill

ust

rate

s T

ort

on

ian

ex

ten

sio

nal

str

uct

ure

sre

stri

cted

to

th

e n

ort

hw

este

rn p

art

of

the

cro

ss s

ecti

on

. A

sm

all

Mes

ozo

ic g

rab

en i

s in

terp

rete

d b

enea

th t

he

Hal

ou

a-R

ich

a an

ticl

ino

riu

m (

HR

A)

toco

nta

in t

hic

k T

rias

sic

evap

ori

tes

that

cro

p o

ut

sou

th o

f th

e cr

oss

sec

tio

n.

To

rto

nia

n e

xte

nsi

on

est

imat

es o

f 1

.5–1

.7 k

m i

n t

he

no

rth

an

d 0

.5 k

m i

nth

e so

uth

are

bas

ed o

n t

he

hea

ves

of

the

Mes

ozo

ic–N

eoge

ne

un

con

form

ity a

cro

ss l

ate

Mio

cen

e n

orm

al f

ault

s. B

elo

w 5

km

dep

th,

stru

ctu

res

in b

oth

cro

ss s

ecti

on

s ar

e u

nco

nst

rain

ed.

010

2030

4050

60

10

0

100

reac

tivat

ed fa

ult,

larg

er a

rrow

hea

d de

note

s re

cent

sen

se o

f mov

emen

t

dist

ance

(km

)

BM

A

?

KD

H-1

(pro

ject

ed)

depth (km)

84G

R09

GR

25H

igh

Pla

teau

?

HR

A

no v

ertic

al e

xagg

erat

ion

BN

A

GR

F-1

1005 55

10050

1020

3040

5060

70

no v

ertic

al e

xagg

erat

ion

NW

SE

depth (km)

84G

R05

GR

01G

R02

Mso

un A

rch

Hig

h P

late

au

Pal

eozo

ic

Tria

ssic

& L

ower

Ju

rass

ic "

syn

Atla

s rif

t"

Mid

dle

Jura

ssic

"pos

t Atla

s rif

t"

Mid

dle

Mio

cene

-

early

Mes

sini

an (

N1-

N3)

Mes

sini

an (

N4)

Mes

sini

an -

Plio

cene

(N5)

(a)

No

rth

ern

Pro

file

(b)

So

uth

ern

Pro

file

salt

salt?

and to the east is the undeformed High plateau. Weinterpret the southeastern concentration of possi-ble salt diapirs (e.g., beneath the Haloua-Richa anti-clinorium) (Saaidi, 1996) and the northwest tilt ofstrata as suggestive of a northwest-dipping masterfault inherited from Mesozoic extensional tecton-ics. Summing the fault heaves across the normalfaults suggests about 0.5 km Tortonian extension inthe southern Guercif basin.

These two profiles, in addition to the structuralpattern shown in Figure 5, depict the Mesozoic riftsystem as broadening toward the northeast. Thenorth Middle Atlas fault (which may be traced intothe Jebel Ahmar–Msoun arch) corresponds withthe northwestern margin of the early Mesozoic riftbasin (e.g., Fedan, 1989; Charrière et al., 1994).Relative to this northwestern boundary, the widthof the northern Middle Atlas rift is approximately45–50 km, but the Mesozoic extension spanned60–65 km in the southern Guercif basin and 85–90km in the northern Guercif basin. Thus, the transi-tion from the Middle Atlas to the Guercif basin cor-responds, to a first order, with a broadening of theearly Mesozoic rift system. As described by Morley(1995), localized extension in rift systems frequent-ly splays into smaller faults as the rift terminatesinto a zone of diffuse extension.

Isopach maps help define the geometry of theNeogene basin. Because the N2-N3 (Tortonian)sequence boundary was not easily discernible onmany of the seismic reflection lines studied, theunits N1–N3 were considered together for con-structing an isopach map of “synextensional” fill(Figure 13a). This isopach map of the preservedTortonian and earliest Messinian strata illustrates theNeogene depocenter as relatively long and narrow,approximately 20 × 40 km, further emphasizing thedistinction between the Neogene depocenter andthe broader present-day physiographic expressionof the Guercif basin. In addition to the long trough,there is a small area containing up to 1 km of pre-Messinian strata located southeast of the BledMarhrane anticline (Figure 13a). This local accumu-lation, which may predate late Miocene extensionaltectonics, appears to be related to the initial upliftand contraction of the anticline rather than deposi-tion onto the hanging wall of a normal fault.

Considerable Messinian and Pliocene deposition(sequences N4 and N5, respectively) is contempo-raneous with growth of contractional structures(e.g., Figure 6), and this appears to fill in accom-modation space remaining from the “graben”episode (Figure 13b). Locally, the isopach for thispostextensional period exceeds 1 km. The post-Tortonian depocenter is wider than during the pre-ceding stage, but it still displays major axis orient-ed northeast-southwest. The similar depocenterorientations may suggest that Messinian–Pliocene

deposition filled in space remaining from theextensional episode. There also appears to be anortheastward shift in the location of the depocen-ter, which may reflect the propagation of uplift ofthe Middle Atlas fold belt after graben extensionceased.

Timing

According to Krijgsman et al. (1999), marinedeposition in the southern Guercif basin began atapproximately 8 Ma. This is considerably later thanthat suggested by the well logs in the northernGuercif basin. The 8 Ma level in GRF-1 lies abovemore than 500 m of Neogene marine strata; there-fore, barring an exceptionally high rate of sedimen-tation, marine deposition in the northern Guercifbasin probably initiated earlier.

Geohistory analysis was performed following themethod described by Allen and Allen (1990). Thisapproach quantifies subsidence rates and identifiesepisodes of tectonic subsidence and uplift. Thisexercise was performed for two wells within theNeogene depocenter (GRF-1 and MSD-1), as well asthe surface stratigraphic column from the southernGuercif basin published by Krijgsman et al. (1999).Both of the wells are located above the same struc-ture, but lie about 5 km apart. Because these wellsare located on a structural high, they do not neces-sarily record the maximum subsidence; however,they are sensitive to the localized reactivation anduplift of the structure. Furthermore, as previouslyshown, contraction was contemporaneous withsignificant basin subsidence. Other wells were notused because they contained limited or no Neo-gene stratigraphic record.

Decompaction was accomplished by assuming adepth-dependent, exponentially decreasing porosi-ty function:

(1)

where φ is the porosity at depth y, φ0 is the surfaceporosity, and c is the coefficient describing the rateof decrease in porosity (Allen and Allen, 1990). φ0and c are dependent upon the lithology, and coeffi-cients provided by Allen and Allen (1990) andMakhous et al. (1997) for similar lithologies wereused here. The stratigraphic intervals and physicalproperties used are listed for each stratigraphic sec-tion in Table 1.

The stratigraphic sections were progressivelydecompacted for the time period represented byeach stratigraphic package to generate an estimateof total subsidence. Unconformities were con-strained by the age of the strata above and below,

φ φ= −0

cy


Gomez et al. 1355

Tab

le 1

. In

pu

t D

ata

for

Geo

his

tory

An

alysi

s*

Dep

thA

ge T

op

ρc

WD

∆SL

Inte

rval

(m

)Li

tho

logy

(Ma)

Seq

uen

ce(k

g/m

3 )φ 0

(1/k

m)

Sou

rce

(m)

(m)

GR

F-1

117

79–1

937

Co

ngl

om

erat

e, s

and

sto

ne

10.0

±1.

00N

126

500.

560.

0002

7A

llen

an

d A

llen

(19

90)

100

±50

752

1413

–177

9M

arl

8.0

±1.

00N

227

200.

635

0.00

051

Mak

ho

us

et a

l. (1

997)

250

±50

653

950–

1413

Mar

l7.

2 ±

0.00

N3

2720

0.63

50.

0005

1M

akh

ou

s et

al.

(199

7)45

0 ±

5055

462

8–95

0M

arl

7.0

±0.

25N

327

200.

635

0.00

051

Mak

ho

us

et a

l. (1

997)

200

±50

505

414–

628

Mu

dst

on

e6.

8 ±

0.10

N4

2680

0.56

0.00

039

Mak

ho

us

et a

l. (1

997)

25 ±

1045

619

8–41

4Sa

nd

sto

ne,

limes

ton

e6.

5 ±

0.10

N4

2650

0.42

90.

0002

7A

llen

an

d A

llen

(19

90)

5 ±

535

719

8–19

8U

nco

nfo

rmit

y6.

3 ±

0.10

n/a

n/a

n/a

–200

±20

030

80–

198

San

dst

on

e,cg

.4.

0 ±

1.00

N5

2650

0.49

0.00

027

Alle

n a

nd

Alle

n (

1990

)–4

00 ±

250

20M

SD-1

112

23–1

247

Co

ngl

om

erat

e, s

and

sto

ne

10.0

±1.

00N

126

500.

560.

0002

7A

llen

an

d A

llen

(19

90)

100

±50

752

1100

–122

3M

arl

8.0

±1.

00N

227

200.

635

0.00

051

Mak

ho

us

et a

l. (1

997)

250

±50

653

820–

1100

Mar

l7.

2 ±

0.00

N3

2720

0.63

50.

0005

1M

akh

ou

s et

al.

(199

7)45

0 ±

5055

460

0–82

0M

arl

7.0

±0.

25N

327

200.

635

0.00

051

Mak

ho

us

et a

l. (1

997)

200

±50

505

420–

600

Mu

dst

on

e6.

8 ±

0.10

N4

2680

0.56

0.00

039

Alle

n a

nd

Alle

n (

1990

)25

±10

456

282–

420

San

dst

on

e,lim

esto

ne

6.5

±0.

10N

426

500.

429

0.00

027

Mak

ho

us

et a

l. (1

997)

5 ±

535

728

2–28

2U

nco

nfo

rmit

y6.

3 ±

0.10

n/a

n/a

n/a

–200

±25

030

80–

282

San

dst

on

e,cg

.4.

0 ±

2.00

N5

2650

0.49

0.00

027

Alle

n a

nd

Alle

n (

1990

)–4

00 ±

250

20Z

ob

zit

117

60–1

875

San

dst

on

e, m

ud

sto

ne

7.85

±0.

05N

226

800.

560.

0003

9A

llen

an

d A

llen

(19

90)

210

±50

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ater

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200

400

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BM

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approximate trace ofthe N3-N4 contact

34˚

N34

˚ 30

'

4˚ W 3˚ 30' 3˚

G

M

(a) Tortonian-early Messinian ("graben" episode)

ATM-1AKL-101

GRF-1

MSD-1

KDH-1

TAF-1X

TAF-1

TAF-2

0

200400

600

800

1000

1200

0 200

0 020

0

400

grow

th o

f Mso

un "A

rch"

BNA

BM

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200 km

34˚

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˚ 30

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M

(b) late Messinian - Quaternary ("intermontane" episode)

ATM-1AKL-101

GRF-1

MSD-1

KDH-1

TAF-1X

TAF-1

TAF-2

Figure 13—Isopachmaps (thicknesses inmeters) of preservedNeogene strata in theGuercif basin basedon seismic reflectionand well data. Onlyfaults active duringeach time period(based on the structural interpretationin Figure 5) areshown. (a) The pre-Messinianisopachs show thesynextensional and subsequent tectonically quiescent period.The narrow depocenter is confined to thenorthwestern part of the basin. Stratigraphic thicknesses thintoward the Rif thrustbelt in the northwest.(b) Deposition contemporaneouswith the two contractionalepisodes since theMessinian fills in thenorthwestern part ofthe basin. Despitethe contraction,deposition locallyexceeds 1200 m in thickness. Abbreviations as inFigure 2.

Gomez et al. 1357

and no effort was made to account for eroded stra-ta at truncational unconformities. To estimate thetectonic subsidence, the total (decompacted) subsi-dence was subsequently corrected for sediment

loading, water depth, and long-term sea level varia-tions (Haq et al., 1987) assuming local compensa-tion (Airy isostasy) following the method ofSteckler and Watts (1978). Paleobathymetry was

0

400

800

1200

1600

2000

24681012

0

400

800

1200

1600

2000

24681012

0

400

800

1200

1600

2000

24681012

Age (Ma)D

epth

(m

)D

epth

(m

)D

epth

(m

)

N1 N3N2 N4 N5graben intermontane

MiddleMiocene Tortonian Messinian Pliocene

decompactedundecompactedsediment and waterload correctedtectonic subsidence


GRF-1


MSD1


Zobzit

Figure 14—Geohistoryplots based on Neogenestrata from the Guercifbasin. Two of the plots arebased on well logs fromthe northern Guercifbasin (GRF-1 and MSD-1),and the third is based onthe composite stratigraphicsection for the Zobzitregion of the southernGuercif basin presentedby Krijgsman et al. (1999).The locations of thesedata are depicted in Figure 2. All three plotsillustrate rapid tectonicsubsidence during theTortonian, correspondingwith extensional tectonics.The lack of tectonic subsidence from theMessinian onward suggests that subsidencewas driven by sedimentloading. This change intectonic subsidence corresponds with thetransition from the Guercif graben to theintermontane Guercifbasin. The input data andphysical properties arelisted in Table 1. Thedepth errors primarilyreflect the uncertainty inpaleobathymetry.

estimated based on facies and paleontology ofwells and exposed stratigraphic sections (Colletta,1977; Bernini et al., 1992; Krijgsman et al., 1999),and error estimates were assigned to these waterdepths (Table 1). Constraints are poor for paleoalti-tude estimates (negative water depth in Table 1),and we have assumed these values to be betweensea level and the present elevation.

The geohistory plots in Figure 14 show consis-tent results for all three sections. Most notably, com-parison of the total (decompacted) subsidence withthe corrected “tectonic” subsidence demonstrates aprofound change in basin subsidence patterns inthe late Tortonian. The major component of subsi-dence on all three plots prior to approximately 7.2Ma is tectonic subsidence. From the previous analy-sis of stratal geometries, this time period corre-sponds with extensional faulting and the activity ofthe Tortonian graben. Significant tectonic subsi-dence of the Bou Mkhareg anticline (on which wellsGRF-1 and MSD-1 were drilled) is consistent withthe earlier interpretation that the anticline did notgrow significantly during the extensional period.

On all three plots, total subsidence remains sig-nificant after approximately 7 Ma, despite very lit-tle or no tectonic subsidence. In fact, these threegeohistory plots depict a general tectonic uplift; there-fore, most of the subsidence since the Messinian prob-ably resulted from topographic uplift of adjacent areas,which raised the base level, and sediment loading. Thebroad, similar patterns of the syngraben and post-graben isopachs (Figure 13) suggest thatMessinian–Quaternary deposition has filled in theremaining space. In addition, rapid infilling andsubsidence may have been facilitated by increasedsedimentation rates resulting from the growth ofnew, proximal source areas in the external Rifthrust belt. The shallowing of the marine basin thusresulted from infilling of the Tortonian graben, aswell as regional uplift.

TECTONIC SETTING AND ORIGIN OF THEGUERCIF BASIN

Our results demonstrate that the main Neogenedepocenter in the Guercif basin is confined to anarrow region that experienced subsidence con-trolled by normal faults during the Tortonian. TheTortonian isopach map demonstrates the localizednature of the basin with its long axis orientednortheast-southwest. Small magnitudes of exten-sion, along with the localized nature of the basin,imply that the Tortonian graben is not a crustal-scale rift similar to, for example, the MesozoicMiddle Atlas rift.

Previous interpretations of the tectonic contextof the Neogene Guercif basin represent a variety of

scenarios: (1) a pull-apart basin in an east-west–striking dextral shear zone (Colletta, 1977), (2)flexure in the foreland of the Rif with secondaryextensional subsidence (GECO, 1986; Zizi, 1996a),and (3) structural interference between the trans-pressional Middle Atlas shear zone and the Rifthrust belt (Boccaletti et al., 1990; Bernini et al.,1999). Subsurface and surface information show noevidence of a throughgoing east-west–striking faultsystem as required by the first possibility; further-more, the main structural trend of the Guercifbasin is oriented northeast-southwest.

The proposed flexural origin is based on a report-ed regional thickening of the Tertiary isopach (Zizi,1996a). To the contrary, isopach maps presentedhere (using a considerable amount of data in thewestern part of the basin) suggest a general thin-ning toward the Rif thrust belt in the northwest. Asmentioned, most of Zizi’s data were outside of theNeogene depocenter. A more typical flexural fore-land basin of the Rif is probably represented by theRharb basin to the west which, unlike the Guercifbasin, displays significant stratigraphic thickeningtoward the thrust belt (Flinch, 1996).

Structural interference between the Middle Atlasand Rif thrust belt as suggested by Boccaletti et al.(1990) and Bernini et al. (1999) seems most appeal-ing owing to the two adjacent tectonic elements:the Middle Atlas mountains and Rif thrust belt;however, the original suggestion proposed thatextension in the Guercif basin resulted from a flex-ural overprinting of the Middle Atlas shear zoneresulting from crustal loading in the Rif, and thisdoes not readily explain the late Miocene changefrom basin extension to basin contraction docu-mented in our study. Another implication of thishypothesis is that extension should correspondonly with the northern part of the Middle Atlasshear zone; however, Tortonian extension wasmore widespread, and other, smaller examples canbe found farther west in the tabular Middle Atlasand Saiss basin (Figure 1) (e.g., Charrière, 1984,1990). This zone of minor extension occurs adja-cent to and contemporaneous with the west-south-west–vergent thrust system of the central Rif.

Tortonian extension in northern Morocco is con-temporaneous with west-southwest–vergent thrust-ing in the central Rif, and extension in the Guercifbasin and northern Middle Atlas ceased at the sametime that west-southwest thrusting in the centralRif stopped (Figure 15). The coincident timing andproximity of tectonic events in the Guercif basinand the Rif suggest an intimate relationshipbetween Guercif basin extension and west-south-west tectonic transport in the central Rif. The gen-eral northeast-southwest orientation of theTortonian graben illustrated here and the paleo-stress directions reported by others (e.g., Morel,


Gomez et al. 1359

1989; Galindo-Zaldivar et al., 1993) are consistentwith west-southwest transport of thrust sheets andthe associated sinistral shear along laterally bound-ing faults in the Rif, such as the Nekor fault (Figure16). We therefore suggest that extension in thenorthern Middle Atlas region may be a distant man-ifestation of the sinistral shear zone bounding thecentral Rif thrust system. Extension did not occurin the northern Middle Atlas region during the pre-vious stage of west-southwest–vergent thrustingbecause this prior episode occurred farther away inthe internal zone of the Rif (see Figure 1). TheGuercif graben is the most prominent example ofthis extensional system and may be a result of thepreexisting crustal weakness inherited from theMiddle Atlas rift system.

Coincident with the initiation of south-vergentthrusting in the external Rif thrust belt, extensionin the Guercif basin stopped and was succeeded bycontraction in the Middle Atlas/Guercif basin(Figure 16). Despite this regional contraction anduplift, the Guercif basin itself experienced furthersubsidence as an intermontane basin driven by sed-iment loading. Minor contraction also occurred in

the Guercif basin, and the weak surface expressionof these Late Neogene contractional structuresdemonstrates that depositional processes outpacedtectonic processes during the latest episode of thebasin’s history. Also during this time, the Msounarch began to grow, presumably due to transpres-sion along the northern part of the north MiddleAtlas fault zone.

The mechanism for the kinematic change in theRif thrust belt remains unclear. Although differentplate models may disagree about early Tertiarymovements of the African and Eurasian plates, theyall agree that since the Tortonian, the relativemotion has been an approximately northwest-southeast convergence between the two plates(e.g., Dercourt et al., 1986; Dewey et al., 1989;Srivastava et al., 1990). Because the relative platemotions have not varied over the time periodrecorded in the Guercif basin, plate tectonics can-not explain the significant kinematic changes in theRif and the Guercif basin during this time period.Many workers have suggested that west-south-west–vergent thrusting in the Rif is directly relatedto extension and exhumation of the Alboran Sea

Figure 15—Diagram summarizing the tectonic events in northern Morocco. Of particular interest is the general pro-gression of tectonic activity in the Rif from the internal to the external zone and the contemporaneity of Guercifbasin extension and west-vergent thrusting in the external Rif. This summary is compiled from several sources,including Frizon de Lamotte et al. (1991), Morel (1989), Comas et al. (1999), and Charrière (1990).

Middle Atlas

alkalivolcanism

Guercif Basin

potassicvolcanism

cont

ract

ion

exte

nsio

n

exte

nsio

n

alkalivolcanism

ExternalRif

CentralRif

W T

hrus

ting

WSWthrusting

S to SSE vergent

thrusting

InternalRif

Alboran

calc-alkalinevolcanism

folding andstrike-slip faults

5

10

15

Ma AgeM

id M

ioce

neLa

te M

ioce

neP

lioce

ne

Quat.

L.

Mid

E.

Mes

s.To

rton

ian

Ser

rava

lian

Lang

hian

cont

ract

ion

cont

ract

ion

Exh

umat

ion

of

Alb

oran

Dom

ain

potassicvolcanism

NW

-SE

to N

-S c

ontr

actio

n,su

bsid

ence

(th

erm

al?)

northern central

North South

?

App

roxi

mat

e M

otio

n of

Eur

asia

with

res

pect

to A

fric

a


T

R

G

Tortonian (~8 Ma)

T

R

G

Late Messinian (5.5 Ma)

Present coast linepaleostress orientation(schematically shown)

thrust transport directionemergent

marine R = RabatG = GuercifT = Tanger

Messinian

5˚ W40˚N

35˚

0˚

Tortonian

5˚ W40˚N

35˚

0˚(a)

(b)

NF

alkalic volcanism

shoshonitic volcanism

paleostressobservation

interpolated stresstrajectory (σ1)

Africa-Eurasiarelative platemotion (Dewey et al., 1989)

paleostressobservation

interpolated stresstrajectory (σ1)

Africa-Eurasiarelative platemotion (Dewey et al., 1989)

basin, as a result of either extensional collapse anddelamination of a thickened lithosphere (e.g., Plattand Vissers, 1989; Seber et al., 1996; Platt et al.,1998; Comas et al., 1999; Calvert et al., 2000) orretreating subduction due to collision of an irregu-lar continental margin (e.g., Royden, 1993; Sengor,1993; Morley, 1993; Lonergan and White, 1997). Asdepicted in Figure 15, west-southwest tectonictransport in the central Rif persisted beyond thecessation of Alboran extension. Either mechanismmust sufficiently explain the continued west-south-westward thrusting and its associated results, suchas the extension in northern Morocco.

Evidence of recent extensional faulting duringthe late Pliocene and Quaternary has been report-ed in the southern Guercif basin along the JebelAhmar–Msoun arch (Bernini et al., 1994a, b).Quaternary extensional faulting appears to be rela-tively minor and confined to the arch. Relativeuplift of the Middle Atlas with respect to theGuercif basin appears to have continued into theQuaternary, as suggested by the long profiles of flu-vial terrace surfaces (e.g., Rampnoux et al., 1979).Northeast-southwest extension is compatible withthe generally northwest-southeast incrementalshortening directions obtained by Bernini et al.(1994b) and Colletta (1977) from microtectonicanalyses. We suggest that this relatively minorPliocene–Quaternary extension may represent foldaxis extension or possibly local wrenching due tothe northern Middle Atlas fault (NMAF, Figure 1),but not renewed activity of the Guercif graben.Northeast-southwest extension of the northernMiddle Atlas may reflect the lateral extrusion of thelower crust within the “transpressional” MiddleAtlas shear zone [“vertical strain partitioning” ofGomez et al. (1998)]. Gomez et al. (1998) suggest-ed that this may be required to reconcile estimatesof crustal thickening and horizontal shorteningwithin the folded Middle Atlas. According to thishypothesis, lateral extrusion of the lower crustwithin the Middle Atlas shear zone may explain anapparently thin crust beneath the Middle Atlasdespite Cenozoic contraction. As the lower crust

extrudes parallel to the orogen, it imparts a basalshear on the upper crust resulting in orogen-paral-lel extension at the end of the system, i.e., theGuercif basin region.

Temporal changes in volcanic activity in theGuercif basin do not coincide with the tectonicchanges documented here. Shoshonitic volcanism[8–5 Ma, according to Hernandez and Bellon(1985)] initiated during the activity of the Guercifgraben, but this magmatic activity persisted wellinto the contractional phase; furthermore, sho-shonitic volcanism appears to be limited to JebelGuillez, which is located well away from the locusof Neogene extension. Alkali volcanism [5–2 Ma,according to Hernandez and Bellon (1985)] is dis-tributed about the northern and southern extentsof the Guercif basin. Petrologically, these lavas aresimilar to alkali basalt extruded in the Middle Atlas.The volcanic activity in the Guercif basin probablyrepresents some other external process superim-posed on the Neogene basin.

IMPLICATIONS FOR HYDROCARBONEXPLORATION

The timing and structural styles presented hereprovide important constraints for an improvedassessment of hydrocarbon potential in the Guercifbasin. Although the four exploratory wells drilledin the Guercif basin encountered no hydrocarbons,we believe that the Guercif basin still has potentialfor modest production. There is ample evidencesuggesting good potential for hydrocarbon explo-ration in this part of Morocco. Known source rocks(Figure 4) are found in the Mesozoic strata of thenorthern Middle Atlas and the Paleozoic strata ofthe High plateau/Missour basin (e.g., Beauchamp etal., 1996; Zizi, 1996b), including Middle Car-boniferous (Namurian) shales in the Missour basinshowing up to 11% total organic carbon (TOC)and Lower Jurassic (Pliensbachian) calcareousmudstones with 4% TOC (Beauchamp et al. ,1996). The regional distributions of these sources

Gomez et al. 1361

Figure 16—Two-stage history of late Cenozoic tectonics in northern Morocco. (a) During the Tortonian, extensionin the Guercif basin, as well as other parts of the northern tabular Middle Atlas and Moroccan Meseta, resulted froma broad sinistral shear zone bounding the west-vergent thrust sheet of the central Rif. The west-southwest–vergentthrusting of the Rif at this time contrasts with the approximate northwest-southeast convergence between Africaand Eurasia (Dewey et al., 1989). (b) During the Messinian, the Betic-Rif stress field changes to become more consis-tent with relative plate motions in the western Mediterranean. This resulted in kinematic changes in the Rif andGuercif basin. Thrusting in the external Rif has a southward vergence, and the Guercif basin contracts along withthe rest of the Middle Atlas shear zone. Contraction closes the marine seaway connecting the Atlantic Ocean andthe proto-Mediterranean Sea, resulting in the Messinian desiccation of the Mediterranean region (e.g., Hsu et al.,1973). Paleogeography is modified after Morel (1989). Inset maps depict the paleostress (adapted from Galindo-Zal-divar et al., 1993), including individual measurements (short black lines) and the interpolated stress trajectories(dashed lines), along with the relative motion of the African and Eurasian plates (fat arrows) (Dewey et al., 1989).

include areas adjacent to the Guercif basin, so itseems likely that they may be found in the Guercifbasin. Reservoirs in the Guercif basin includeTriassic clastics, Lower Jurassic reef complexes,Middle Jurassic sandstones, and lower Tortonianclastics (GECO, 1986). These reservoirs each havecorresponding seals provided of Triassic evapor-ites, Middle Jurassic mudstones, and Tortonianmudstones (GECO, 1986).

Our constraints suggest alternative explorationstrategies in the Guercif basin involving the failedand inverted Mesozoic rift and the Neogene graben.These are particularly significant if possible hydro-carbon maturation and migration followed Neogeneburial. For example, one scenario involves matura-tion of Lower Jurassic source rocks located in theMesozoic half graben where they would be suffi-ciently buried. These could potentially migrate intoTriassic clastics in the footwall or into LowerJurassic reef complexes or Middle Jurassic sand-stones of the hanging wall. These could be sealedby Triassic evaporites or Middle Jurassic mudstones.As shown by the Bled Marhrane anticline (Figure10) in the southern Guercif basin, middle Miocenefolding created structural traps prior to Neogeneextension and burial.

Another play concept could involve maturationof the Mesozoic source rocks with hydrocarbonmigration into Tortonian clastics, such as the lowerTortonian strata northwest of the Bou Mkhareganticline in Figures 6 and 12a. The subsequent tec-tonic inversion, although small in magnitude, hasfolded and tilted these strata to create a gentle anti-clinal trap.

In summary, the Guercif basin contains richsource rocks documented in the Middle Atlasmountains that have been buried deeper than inthe Middle Atlas. These conditions may provide abetter opportunity for hydrocarbon maturation.Although we have constrained the timing and basinarchitecture, other information is still needed for aproper assessment of hydrocarbon potential in theGuercif basin. For example, migration histories ofhydrocarbons need to be studied and comparedwith the timing of events documented here; fur-thermore, an improved understanding of theNeogene depositional systems can provide addi-tional constraints on the basin development.Detailed sedimentological studies of provenanceand paleocurrents would yield crucial informationon the sediment inputs of the competing mountainsystems (i.e., the Middle Atlas vs. the Rif).

CONCLUSIONS

Results constraining the structure and timing ofthe Guercif basin demonstrate a two-stage basin

history that appears to reflect the influence of theRif thrust belt upon the foreland uplift of theMiddle Atlas. Through its own development, theGuercif basin documents a dramatic change in thekinematics of the Rif thrust belt; however, itappears that the Middle Atlas was affected by theRif only when elements in the Rif thrust belt wereproximal to the foreland uplift of the Middle Atlas.In the case presented here, very oblique transportin the thrust belt caused localized extension in thenorthern Middle Atlas, as well as other parts ofnorthern Morocco.

The southern Guercif basin also provides con-straints on the timing of deformation in the north-ern Middle Atlas system. In addition to lateMiocene contraction demonstrated in seismicreflection data, an episode of pre-Tortonian foldingis also documented. Because contractional defor-mation predates the Late Neogene basin develop-ment, the structures may serve as traps for poten-tial hydrocarbons maturing due to burial beneaththe Neogene basin fill.

From the perspective of geologic history, thestory of the Guercif basin is interesting because theinitiation of the intermontane stage heralds theimpending isolation of the proto-Mediterranean Seaat the end of the Miocene. Regional uplift of theGuercif basin, in conjunction with thrusting in theexternal Rif, certainly helped seal the fate of theMediterranean Sea; however, the terrestrial emer-gence of the Guercif basin appears to predate theMessinian salinity crisis by 0.5–1.0 m.y. It thereforeseems probable that the emergence of the Guercifbasin may reflect the closure of a significant part ofthe Rifian corridor, but the narrow marine seawaywas not completely closed until later.

The Guercif basin is another example of localcomplexities in continental tectonics, particularlywhen the nearby plate boundary processes them-selves are complicated; however, it is this samecomplexity that may improve the prospects forpetroleum exploration in these very areas.

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Francisco Gomez

Francisco Gomez is a postdoctoral researcher in theDepartment of Geological Sciences and the Institute forthe Study of the Continents (INSTOC), Cornell University.His research in North Africa has focused on regionalCenozoic tectonics of the Atlas Mountains and adjacentregions. He holds a B.S. degree in geology from theCalifornia Institute of Technology and a Ph.D. in tectonicsfrom Cornell University.

Muawia Barazangi

Muawia Barazangi is a professor in the Department ofGeological Sciences at Cornell University. He also servesas the associate director of the Institute for the Study ofthe Continents (INSTOC) and is the leader andcoordinator of the Middle East and North Africa Project atCornell University. His academic background includes aB.S. degree in physics and geology from DamascusUniversity (Syria), an M.S. degree in geophysics from theUniversity of Minnesota, and a Ph.D. in seismology fromColumbia University, Lamont-Doherty Earth Observatory(New York). Professional experience includes globaltectonics, tectonics of the Middle East and North Africa,structure of the continental lithosphere, and structure ofintracontinental mountain belts.

Ahmed Demnati

Ahmed Demnati is currently an independentconsultant based in Morocco. Before retiring in 1999 fromthe Moroccan national oil company, ONAREP (OfficeNational de Recherche et d’Exploitation Petrolieres), heserved as chief geophysicist and headed an explorationdivision. He holds an M.S. degree in geophysics from theBergakademie Clausthal-Z and a Ph.D. from the Universityof Hamburg in Germany.

ABOUT THE AUTHORS

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Structure and Evolution of the Neogene Guercif Basin