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Marine and Petroleum Geology 57 (2014) 561e571

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Marine and Petroleum Geology

journal homepage: www.elsevier .com/locate/marpetgeo

Research paper

The Davie Fracture Zone and adjacent basins in the offshoreMozambique Margin e A new insights for the hydrocarbon potential

Estev~ao Stefane MahanjaneInstitute National Petroleum (INP), Av. Fern~ao Magalhaes N. 34, 2nd Floor, PO Box 4724, Maputo, Mozambique

a r t i c l e i n f o

Article history:Received 21 December 2012Received in revised form8 May 2014Accepted 12 June 2014Available online 5 July 2014

Keywords:BreakupDavie compressional zoneDavie fracture zonePetroleum systemStratigraphySeismic interpretationBasinsTectonic rifting

E-mail addresses: estevao.stefane@inp.gov.mz, stef

http://dx.doi.org/10.1016/j.marpetgeo.2014.06.0150264-8172/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The interpretation of 2-D seismic reflection data provides a modern structural framework includinghydrocarbon potential in the present-day stratigraphic and structural traps of both the Davie FractureZone and the adjacent Nacala and Angoche basins. Possible stratigraphic traps were identified in sub-marine fan and channel depositional environments during Cretaceous to Tertiary times. Structural trapsare mostly defined within compressional structures formed by a variety of fault-related folds and riftgrabens within the Jurassic and Cretaceous successions.

The Nacala and Angoche basins form two depressions separated by the Davie compressional zone. Thiscompressional structure is a prominent interior high running approximately north-south. An event oftranspression and contraction characterizes the main tectonic setting commonly hosting several de-tached compressional structures along the western edge of the transform zone.

Both basins are associated with the Late Jurassic/Early Cretaceous rifting during the opening of theMozambique Channel. The Angoche basin is proposed here to have formed by the earliest stage of break-up in mid-Jurassic time. The basin is bounded landward by the Angoche volcanic zone, a dyke swarmbranch oriented N64degE forming part of the Karoo and Dronning Maud Land magmatism at c. 180 Ma.

Subsequent rifting and break-up led to the drift of East Gondwana southwards along the dextral strike-slip Davie Fracture Zone. At about 150 Ma (Tithonian), East Gondwana appears to have rotated slightlyclockwise about a pivot in the proximity of the Angoche basin leading to extension and rifting in theRovuma basin to the north of the pivot point and compression west of the Davie Fracture Zone to thesouth. Consequently, the eastern boundary of the Angoche basin was compressed developing a typicalgrowth wedge of massive thrust imbrication structures while extensional tectonics created several de-pressions and rift-grabens forming the Nacala and Quirimbas basins.

Basin stratigraphy is interpreted along seismic reflection lines and correlated to the regional strati-graphic information and wells from the Zambezi Delta and Rovuma basins.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Recent gas discoveries in the Rovuma Delta basin in bothstratigraphic and structural play-types (e.g. Law, 2011) call forfurther hydrocarbon exploration towards the south in similarstructures within the Davie Fracture Zone (DFZ, also called DavieRidge or Davie Transform Zone). The offshore Angoche and Nacalabasins are presently attractive for petroleum exploration.

The Davie Fracture Zone forms the continent-ocean transformboundary that crosses the Mozambique Channel between theMozambique and Madagascar continental margins (Fig. 1A). It is a

ane.ac@gmail.com.

prominent 2000 km long lineament bounding the Rovuma andSomali basins in the north-west and the Mozambique andMorondava basins in the south-east (Rabinowitz and Woods,2006). The modern Davie Fracture Zone includes several sea-mounts, namely the St. Lazare, Paisley, Macua and Sakalaves sea-mounts (Fig. 1A). Is it possible that they are part of much morerecent volcanism (East African Rift System, such as the Comoros?).

Despite its prominent nature, the DFZ is relatively poorly stud-ied as part of the frontier area. Structurally, the DFZ developed asshear zone during the southerly movement of the East Gondwanafragment (including Madagascar), starting from initial break-upand lasting until the Early Cretaceous (Coffin and Rabinowitz,1987; Nairm et al., 1991; and Coffin and Rabinowitz, 1992). Dur-ing the active phase, the fracture ridge was in purely strike-slip

Figure 1. A e General map of the northern Mozambique Channel and the navigation map with the 2-D seismic data ship’s track zig-zag lines (red colour) used for this study,including the location of the Figures (dark blue). Tectonic features that predominate in the study region: the Central Davie Fracture Zone (dashed line), the chain of seamounts(Sakalaves, Macua, Paisley and St. Lazare), and the main sedimentary basins: Rovuma, Mozambique, Majunga, Morondava. Green full-circles: location of the oil seeps (ECL and ENH,2000; Maenda and Mpanju, 2003). B e The structural map that resulted from the seismic interpretation is displayed. The foremost geological features are the Davie compression,the Angoche and Nacala basins. Figures 2A_1 and 2C_1 flagged by blue dashed lines depict two portions of Figs. 2A and 2C, respectively. The volcanic zone (dotted line) was mappedalong the continental margin (Raillard, 1990). See text for discussion. C e A simplified stratigraphy chart constructed based on regional correlation and seismic interpretation. (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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movement (Reeves and Mahanjane, 2013), however, the presentstudy indicates that DFZ had experienced other tectonic rolesbefore the relative motion of Madagascar stopped.

This paper contributes to the understanding of this tectonichistory with an interpretation of the Davie compression zone andtwo rift basins, namely the Angoche and Nacala basins (Fig. 1B).These basins are situated in the south-western extremity of thefracture zone, bounded by the DFZ in the east and the Mozambiquecontinental margin in thewest. The interpretation is based on high-quality 2-D reflection seismic profiles transecting the basins andpartially crossing the DFZ (Fig. 1A). Horizon picking was done usingthe regional correlation of the seismic data with stratigraphic re-cords from the Mozambique and Rovuma basins. The aim of thispaper is to give an overview of the tectonic settings in this frontierarea, to provide a description of local stratigraphy, and correlating itwith the petroleum systems known from neighbouring basinsalong the East African Margin.

1.1. Regional geology

The result presented by structural map of Figure 1B contributesto the tectonic framework of the region, giving insight in thebreakup of Gondwana and the development of Davie Fracture Zone.

Paleogeographic reconstruction models from recent studiesusing magnetic data in the Mozambique basin and the conjugateRiiser-Larsen Sea, e.g. K€onig and Jokat (2010) and Leinweber andJokat (2012), provide important new evidence of conjugate M-se-ries magnetic anomalies between Africa and Antarctica that defineswell their relative movements going backwards in time from theyoungest M-series anomaly (about 125 Ma) until about 153 Ma.

Reeves (2009) and Leinweber and Jokat (2012) depict thedevelopment of the AfricaeAntarctica Corridor during the openingbetween Africa and Antarctica. Leinweber and Jokat (2012) alsotentatively date the onset of the sea-floor spreading in theMozambique basin and conjugate Riiser-Larsen Sea using the oldestMagnetic anomaly M25n (154 Ma) in these basins. But the age ofinitial sea-floor spreading could be older than this period because,judging from the location of the anomaly, there is still a significantdistance left of oceanic and stretched continental crust, which hasto be taken in account for timing of the onset stage. Extrapolatingbackwards in time before 154 Ma, Reeves and Mahanjane (2013)obtained a closure at the age of 167.2 Ma (K€onig and Jokat, 2006)just before initial sea-floor spreading started. This age is coherentwith the last period of emplacement of the Angoche dyke swarm atc. 170 Ma (Reeves, 2000).

Besides, M25n is very close to the northernmost end of the NeSFracture zone F (K€onig and Jokat, 2010) created by southerlymovement of Antarctica. Here, the anomaly defines the onset of thebreak-up stage 2 between Africa and Antarctica (Mahanjane, 2012).This break-up was the main stage between West and East Gond-wana leading to initiation of the north-south oriented rifting-drifting phase creating a marginal fracture ridge (DFZ) in the eastand the Lebombo Monocline in the west (Reeves and Mahanjane,2013).

The earlier movement between East and West Gondwana wasessentially dextral strike-slip on the proto-Davie Fracture Zonealong a segment, parallel to the present day coastline of NorthMozambique and SE Tanzania (Reeves and Mahanjane, 2013). Theonset of drift occurred probably in Middle Jurassic (Bajocian-Bathonian, 170e166 Ma?) time (Kreuser, 1995). Subsequently, EastGondwana (consisting of Madagascar-India-Antarctica) separatedfrom West Gondwana (i.e. AfricaeSouth America), moving incre-mentally southward along the DFZ until Early Aptian (c. 121 Ma)(Rabinowitz et al., 1983) or middle Aptian (c.118Ma) (Bassias, 1992;Coffin and Rabinowitz, 1992) when Madagascar came to rest.

Earlier, probably once East Gondwana no longer had direct con-tinentecontinent contact with Africa (Reeves and Mahanjane,2013) it seems likely that the whole assembly east of the DFZ,including Madagascar, rotated slightly clockwise about a point offthe Mozambique coast (~15� South, Fig. 1B), leading to extensionand rifting in the Rovuma basin (offshore MozambiqueeTanzania)and compression west of the DFZ further south. In this regard, theSE corner of Madagascar (e.g. the model of Konig and Jokat, 2010)would lie off the coast between Nacala and Angoche at about 5 Mabefore Jurassic-Cretaceous boundary time (i.e. ~150 Ma). Thus, thecompression could already have started in the Jurassic, later reac-tivated by subsequent Cretaceous tectonism of northern Africa andcurrent rifting of East Africa.

Moreover, the present position of Madagascar may not beidentical to that at the end of the first period of activity of the DFZ,and possibly during the reactivation of the DFZ as part of East Af-rican Rift system activity. Likewise, a change of spreading directionalong DFZ has occurred in the Kimmeridgian (?). The DFZ beforeand after this change is expected to be different, also regarding itslocation as indicated by the several transfer faults along its length.

From a sedimentary point of view, deposition started fromMiddle Jurassic (Bajocian) in the modern DFZ. An optimistic inter-pretation suggests dominantly lacustrine deposition along the DFZ,over a Proto-oceanic rift on both the northern and southern ex-tremities of the fracture zone before themain strike-slipmovementstarted. This is supported by facies of the Makarawe Formation(typically shallow-water lagoonal and reefal sediments) correlatedwith the adjacent Tanzanian areas (Salman and Abdula, 1995).

TheOxfordian-Aptian successionsoverlayunconformably themid-Jurassic deposits (Fig. 1C). They are related to marine transgression ofPaleo-Tethys between Africa and Madagascar in a time span of157e118 Ma (Salman and Abdula, 1995). Predominately shelf carbon-ates andbasinal sedimentsweredeposited in a restrictedenvironmentwithin local depressional basins in the East African margin.

2. Database

The principal data for this study include 1300 km of 2-Dreflection seismic profiles consisting of 8 dip zigzag lines (Fig. 1A)covering an area of approximately 73,000 km2. This dataset is asubset of a considerably larger database provided by the NationalPetroleum Institute of Mozambique as part of ongoing explorationprojects. Seismic data were collected by Western Geophysical in2000 (spec surveyWG-00). An array of airguns provided the sourcefor the data collected in water depths between 100 and 3000 m. Alllines are time-migrated seismic data, which were interpreted bothin paper-record form and by using Seismic Micro-TechnologyKINGDOM™ software. The part of the seismic lines used in thispaper only covers the western flank of the DFZ, except for thenavigation of Figure 3B (Fig. 1A).

The principle of seismic sequence stratigraphy analysisdescribed by Catuneanu et al. (2009, 2010, and 2011) was applied tointegrate the approach for a detailed reconstruction of the basin-fillhistory, the prediction of rock types, and depositional environment,thereby indirectly delineating reflection packages in space andtime.

3. Results and discussion

3.1. Regional stratigraphic correlation

A compilation of stratigraphic information from the northernMozambique and Rovuma basins [Salman and Abdula (1995), ECL&ENH (2000)] (Fig.1A) wasmade and integrated with seismic ties fora better correlation of the regional stratigraphic records. The result

Figure 2. A e In the upper panel is shown an uninterpreted 2D reflection seismic section (migrated). The profile crosses several features described in this paper: The Daviecompressional zone and the Angoche basin. In the lower panel, the seismo-stratigraphic interpretation is displayed as proposed in this study. Stratigraphic evolution is described inthe text. The Mid-Tertiary extensional tectonic (EARS) is shown by active young faults over the Davie compression. Jurassic and Aptian-Albian source rocks are expected fromshallow-marine shale, see text for discussion. A_1 e Enlargement of the south-eastern portion of A presenting an interpretation as proposed in this study. Details of geotectonicstructures and some sediment stratigraphy are shown. The prominent features are thrust imbrications of growth wedge structures underlying the Davie compression. Fault-relatedfold were identified within the thrust imbrications and form structural traps. Onlap terminations are the main characteristic of the sedimentary deposition in the Angoche basinover the Jurassic unconformity. See Fig. 2A for the abbreviations, and text for discussion. B e In the upper panel is shown an uninterpreted migrated 2-D seismic section. Theseismo-stratigraphic interpretation (the lower panel) displays prominent thrust faults and fault-bend folds observed within the Davie compressional zone. The continentalbasement is a very steeply dipping reflector that defines a pre-kinematic margin. The slope and basin floor fans are shaped by mounded configurations predominate in the K-Units.See text for discussion. C e An uninterpreted 2-D reflection seismic section is shown (upper panel). The profile transects the Davie compressional zone in a distance of ~35 km long,

E.S. Mahanjane / Marine and Petroleum Geology 57 (2014) 561e571564

Figure 2. (continued).

E.S. Mahanjane / Marine and Petroleum Geology 57 (2014) 561e571 565

is a simplified local stratigraphic chart shown by Figure 1C. Thefollowing unconformities/tops were selected to describe the mainstratigraphic packages in the Angoche and Nacala basins:

i. Top Cretaceous/Base Tertiary Unconformity (66.0 Ma) corre-sponds to an erosional surface formed in associationwith thelate highstandmajor regression during the late Drift phase inLate Cretaceous time.

ii. Top Turonian (89.8 Ma) is correlated from both Rovuma andMozambique stratigraphies as the top of the first cycle ofsedimentation associated with the development of the EastAfrica continental margin. A lowstand tract system domi-nated the sedimentation of clastics during the short-liveduplift linked to the cease of the strike-slip motion betweenAfrica and Madagascar.

iii. Top Albian Unconformity (100.5 Ma) was formed by the mainregression in the Neocomian/Aptian and resulted in non-deposition or erosion in the Mid-Cretaceous, associatedwith the end of Early Drift phase.

iv. Upper Jurassic Unconformity (145.5 Ma) is presented in theRovuma basin stratigraphy (Salman and Abdula, 1995) as apronounced angular unconformity linked to the Late Jurassicmarine section. The unconformity transgressed onto theAfrican platform with the deposition of extensive platformcarbonates, later replaced by prograding Upper Jurassic/Lower Cretaceous marine clastics.

the Angoche basin and partially the Nacala basin. The lower panel displays the seismo-stratthe detached thrust fold-belts. Cretaceous and Tertiary plays are mainly stratigraphic traps dthe central portion of Fig. 2C presenting an interpretation as proposed in this study. The illusmoderate to high amplitude are imaged as potential Jurassic structural plays within the Dduring the rifting of Nacala basin. Active young faults rejuvenate the Mesozoic and Cenodiscussion.

v. Drift onset Middle Jurassic Unconformity (Bajocian-Bathonian,170e166 Ma) depict the initiation of movement on the DavieRidge transform, which inmarked by spreading in the Somalibasin (ECL and ENH, 2000 report).

vi. Top acoustic basement depicts the top of basement underly-ing the sedimentary successions.

3.2. Seismic mapping and stratigraphy (Figs. 2 and 3)

General appearances in the seismic profiles are described asfollows:

(i) The acoustic basement is mapped on both sides of the DFZ(Figs. 2C, 3A and 3B). On the western side (Angoche basin),the surface of the acoustic basement is a very steeply dippingreflector with no evidence of tectonism preserved (Fig. 2Aand B). In the eastern DFZ, mapping was done with help ofthe profiles crossing the fracture zone (Figs. 2C, 3A, B). Thetop of the inferred acoustic basement is overall characterizedby strong positive impedance contrast bounding well-stratified reflectors above and hummocky patterns below.The tracing of this reflector from thewesternmargin towardsthe east is difficult along the pre-kinematic margin due toamalgamation with the Davie compressional structures (e.g.Figs. 2B, C), while, in the Nacala basin, this reflector is

igraphic interpretation including the proposed plays. Jurassic Plays are defined withinefined by slope fan and onlap pinch-outs. See text for discussion. C_1 e Enlargement oftration shows details of the morphology of the Davie compressional zone. Reflectors ofavie compressional zone. Half-graben morphologies dissect basement along the DFZzoic successions as part of the Mid-Tertiary extensional tectonic (EARS). See text for

Figure 2. (continued).

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identified below the Jurassic succession underneath the rift-grabens. Commonly, the basement is dissected by a series ofNNW and NNE faults forming classical grabens (Figs. 3A, B).

The composition of the material below the acoustic basement ishard to evaluate by using only the data at hand. However, the

Angoche margin is a steeply dipping basement. The length of thesteep-slope shape narrows down northwards until a few tens ofmetres in the area close to the Nacala basin. In map view, thisinterpretation ties pretty well with the Angoche volcanic zone(Raillard, 1990) more recently described as the Angoche dykeswarm (Reeves and Mahanjane, 2013). It is therefore correlated to

Figure 3. A e The upper panel is uninterpreted migrated 2-D reflection seismic section selected to exemplify the main geological setting along the Nacala basin. In the lower panelthe profile shows the intensive extensional tectonics that led to the formation of the Nacala basin. Predominate, are synthetic and antithetic listric faults dissecting the entirestratigraphy including the basement along the western DFZ. Prolongation of the Davie compression is speculated by similar morphology present in the south. See text for discussion.B e The upper panel is an uninterpreted migrated 2-D reflection seismic section transecting the DFZ at the location of the Paisley seamount. The seismo-stratigraphic interpretation(lower panel) displays the tectonic setting similar as described in A. The extensional deformation (graben system) predominates in the west edge, while the Indian Ocean basintypifies a normal tectonic subsidence (east side) with no subsequent deformation. See text for discussion.

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the northern limit of the Karoo magmatism (basalt flows) that tookplace during the final phase of Gondwana stability between 205and 175 Ma (Salman and Abdula, 1995).

(ii) Jurassic successions are mapped in the basins. The presenceor absence of Karoo sediments in the offshore DFZ cannot be

ascertained from this seismic dataset. Besides, evaluating theextension of these sediments from the Karoo, Selous(Tanzania), Duruma (Kenya), and Morondava (Salman andAbdula, 1995; Catuneanu et al., 2005) basins into the DFZarea leads to the conclusion that the DFZ is a later event,formed after Karoo magmatism. Thus, Jurassic sediments are

E.S. Mahanjane / Marine and Petroleum Geology 57 (2014) 561e571568

expected to be exclusively post-Gondwana disruptiondeposits.

3.2.1. Middle Jurassic successionsSalman and Abdula (1995) discussed the mid-Jurassic marine

deposits in the Rovuma basin being equivalent to the marine sed-iments of the Makarawe Formation, Mtumbei Limestone, andKizimbani Shale known in Tanzania. The major question is how farthese sediments could extend further south along the DFZ.

More than one kilometre of Middle-Jurassic succession ismapped in this study as post-break-up deposits, and correlatedwith the Makarawe shale (Bajocian-Bathonian) (Fig. 1C). The top ismarked by the ‘drift onset unconformity’ pronouncing the initia-tion of spreading in the Mozambique basin. It is distinctively tracedin the south and central area of the Angoche basin topping reflec-tion patterns of lower impedance contrast, gradually thinningseawards (e.g. Fig 2A, B). The ‘drift onset unconformity’ commonlyparallels the steepness of the basement in the west, terminating byonlap onto the acoustic basement in the slope area. As with theacoustic basement, tracing of this unit becomes difficult towardsthe east underneath the Davie compressional zone (Fig. 2A, B and C).

3.2.2. Late Jurassic successionsThe entire Jurassic succession is only faintly expressed

morphologically and completely dissected by several imbricatedthrust-and-fold-belts in the eastern edge of the Angoche basin(Figs. 2A, B, C). Prominent internal reflectors delineate at least threedistinctive post-rift units in the Angoche basin (J-Unit I, II, III). Theyare well-imaged within the Davie compression zone, characterizinga typical growth wedge fromwest (~1 km) to east (~3.5 km). J-UnitII pitches-out against J-Unit I in the west edge of the compressionalzone (e.g. Fig. 2A). Conversely, in the Nacala basin, the Jurassicsuccession is a syn-rift deposit with uniform internal configura-tions forming a single unit (J-Unit N):

J-Unit I Themaximum thickness of the Jurassic basal sequence ismeasured in the Angoche basin as ~1.3 km. The sequence iscommonly thicker in the eastern direction and pinches out towardsthe west on the slope of the Angoche margin (Figs. 2B, C). Althoughthe seismic grid is sparse, this interpretation seems to be consistentwithin the Angoche basin (Fig. 2A, B, C). Mahanjane et al.(submitted for publication) are discussing the basin evolution ofthe northern Mozambique basin. The burial history shows a rela-tively rapid subsidence shortly after break-up started. A maximumof ~3 km of Bajocian-Valanginian sediments was already buriedbetween Late Jurassic and Early Cretaceous contributing for ther-mal maturation at the earliest stage of the basin evolution.

J-Unit II is only mapped within the Davie compressional zone,dissected by an imbricated thrust-and-fold-belt. This unit consistsof a wedge growing towards the east, formed mainly by fault-propagation folds. In contrast, the basal detachment shallows tothe west, where the fault-propagation folds turn into fault-bendfolds (Fig. 2A). Towards the west, the horizon gradually merges(pinches out) with the top J-Unit I at approximately shot point 3000(Fig. 2A). The reflectors (wavelet frequency) are well-imaged layersexpressing strong negative impedance contrasts, displaying anopposite polarity of the water bottom (see Fig. 2C). However,additional data analysis is essential for accurate prediction ofhydrocarbons.

J-Unit III The top is mapped as a regional unconformity toppingthe Jurassic successions from the compressional zone in the eastuntil onlapping onto the acoustic basement in the Angoche margin(Figs 2A, B, C). The geometry of the seismic characters changestowards the north, particularly on top of the Davie compressionalzone, showing significant loss of the impedance contrast but

maintaining the continuity of reflectors (Fig. 2C). The sedimentarywedge is stratigraphically correlated to marine shale deposits fromthe adjacent Rovuma basin associated to transgression onto theAfrican platform at the Late Jurassic (Salman and Abdula, 1995).

J-Unit N is the Late Jurassic succession mapped in the Nacalabasin as a single unit approximately 1e2 km thick. The sedimentarywedge in this area shows seismic reflectors increasing impedancecontrast from moderate to high amplitude within the rift-grabens.The geometry of these reflectors is commonly fragmented by syn-thetic and antithetic faults. Their shape suggests widespread syn-rift deposition of a homoclinal wedge of drift deposits, whichthickens drastically to the east towards the DFZ (e.g. Fig. 3B).

(iii) Cretaceous sediments unconformably overlie the Jurassicsuccessions in both basins with seismic reflectors of lowamplitude on-lapping in both west and east directions. Theseismic patterns in the basins suggest basin-fill predomi-nated by a thick wedge sequence of layered sediments, sub-horizontal, overlying the Jurassic successions. The depositionstyle suggests facies with favourable juxtaposition of sourceand reservoir rocks. The Cretaceous successions are sub-divided into three units (Lower, Middle and Upper Creta-ceous, Fig. 1C):

K-Unit I This Albian unconformity seems to be widespread inAfrica. It seems to be linked to the separation of South America andAfrica in the Equatorial Atlantic being achieved - but this question isbeyond the scope of the present paper. Sediments of the K-Unit Ireach maximum thickness in the Angoche basin depocentre.Approximately 1.5 km of sediments is recorded at the location ofFigure 2A. This thickness decreases gradually to the west and east,onlapping onto the slope of the Angoche margin, and the Daviecompression zone. The seismic characteristics of the lowermost partconsist of an interplay of moderate to high amplitude reflectionsexpressing contrasts between lithologies of different rock proper-ties. The geometry of the reflection patterns changes gradually tothe east, where low amplitude impedance contrasts predominate(uppermost part). Sediments of this unit show good correlationwith the stratigraphy of the Rovuma (Pemba Formation Equiv.) andMozambique basins (Domo Shale Equiv) (see Fig. 1C). A descriptionof the Pemba Formation and Domo Series equivalent facies hasbeen provided by Key et al. (2008) and Nairn et al. (1991), respec-tively. The Domo Shale is regarded as potential source rocks knownonly in subsurface, consisting of dark grey to black, thinly bedded,marly shale (Nairn et al., 1991).

Towards the Nacala basin, tracing of K-unit I indicates exten-sional tectonics dissected by several listric faults (Figs. 3A, B). Themaximum thickness in this basin is ~700 m. The K-unit I is alsocorrelated on the eastern side of the DFZ described by seismic re-flectors with more parallel configurations than the western side,suggesting gradual transition to open marine conditions;

K-Unit II This is a ~750 m thick section of a stacked and morecontinuous sediments showing distinctive characteristic relief ofthe reflectors with moderate to high amplitudes widespread in thecentral part of Angoche basin. These characteristics indicate a dif-ference in seismic response compared with the lower and the up-per units. A suitable correlation of the K-Unit II is made towards thenorth in the Nacala basin within the rift-grabens. However, in theAngoche basin, the principal morphologies show a configuration ofmounded patterns, suggesting slope fan or basin floor fan deposits(Figs. 2A, B). The top reflector terminates onlapping onto the Daviecompressional zone indicating possible stratigraphic trapping.Deposition during Turonian time was influenced by marineregression caused by the short-lived uplift after cessation of thestrike-slip motion along the DFZ. Hence, typical lowstand clastic

E.S. Mahanjane / Marine and Petroleum Geology 57 (2014) 561e571 569

sediments are expected to be the litho-stratigraphic component ofthe Domo sandstone equivalent, enclosed between shales (Salmanand Abdula (1995);

K-Unit III is topped by the Cretaceous-Tertiary boundary pro-nounced by a regional unconformity which was formed during theLate Cretaceous erosion associated with major, late highstandregression during the late drift phase. At this period, the develop-ment of East Africa as a passive margin is proposed typified bywidespread marine transgression at the end of the Cretaceous(Salman and Abdula, 1995). The impedance contrast observed inthe reflection seismic data is similar to that of K-Unit I. The seismiccharacters vary with depth and exhibit lateral changes in the wholebasin. Towards the central Angoche basin, reflection patterns withlow to moderate amplitude are restricted to the lowermost part. Inthe uppermost part, typical mound-like reflectors predominate,suggesting slope and/or submarine floor fan deposits (Fig. 2B).Similar facies have been described in the Mozambique basin withfavourable interplay of sand and shale forming gas reservoirs andseal rocks of the Lower Grudja Formation.

(iv) The Cenozoic successions are manifested by predominatelysouth-to-north downlapping terminations onto the BaseTertiary unconformity. In Figure 2A, these terminations arebarely visible on the seismic display, typifying a highstandsystem tract that developed through lateral accretion ofmigrating point bars. They indicate ~800 m thickness ofprograding wedge from southwest to northeast, suggesting apredominance of sediment-feeding systems during the mid-Tertiary. Droz and Mougenot (1987) discussed the SerpaPinto Valley and Zambezi Valley systems in the northernMozambique basin, linking them to the creation and erosionof the East African Rift System in the Oligocene. Accordingly,the Serra Pinto Valley was the main path towards the north,filling the depression along the DFZ (Angoche basin) withmassive depositional sequences. At a regional scale, reflectorpatterns indicate a prograding wedge from NW of the BeiraHigh, developing typical contourite deposits (Reichert et al.,2008).

On top of the prograding wedge, one observes a ~1000 ms TWTthis series of reflectors showing predominate mounds, slumpingrollover and channel-fill deposits well-imaged in the south(Fig. 2A). Prominent channel cuts, with well-defined top and base,occur at shallow depths. These channels are well outlined by theseismic amplitude section (Figs. 2A, B and 3B). In the Nacala basin,the reflection patterns have similar internal arrangement to theAngoche basin but are dissected by several listric faults (Figs. 3A, B).

3.3. Tectonic setting as seen from seismic

Figs. 2A, B, C and Figure 3A, B show seismic sections crossing theDFZ and adjacent basins. The most prominent features on thewestern side of the DFZ are displayed here. The fault geometrygives the impression of a unique structural fracture; but twodifferent tectonic regimes are identified, characterized bycompressional and extensional tectonics, or a combination of both:(1) Compressional tectonics (that was previously not observed inthis region) is presented by the most prominent feature, the Daviecompression structure (Fig. 2 A, B, C). It runs generally north-southbending to the NW in the location of Figure 2C (see Fig. 1B). Thestructure displays a ridge-like morphology migrating from the SEand progressively losing its character towards the northwest in thelocation of Figure 3A, B (see also Fig. 1B). The character of the Daviecompression structure suggests a tectonic signature distinguishedby transpression and contraction events. This setting is hosted by

several detached compressional structures (like fold-belts) withprominent trailing thrust imbrications of massive growth wedgestructures underlying the Davie compression zone. They are sub-parallel to the DFZ and progressively dip seawards (Figs. 2A, B, C).The compressional structures overlie a pre-kinematic (?) passivemargin sequence developed during the earlier rifting and driftingstages in the mid-Upper Jurassic time. On the western side of theDFZ, tectonic subsidence occurred forming the Angoche basin. Theshape of the basement in the western margin is interpreted as atypical volcanically active rift. It is controlled by limited brittleextension of the upper crust. If this interpretation is correct, theseaward dipping reflectors proposed by Reichert et al. (2008) inadditional mapping of the volcanic zone by Raillard (1990) and theAngoche dyke swarms by Reeves and Mahanjane (2013) along theAngoche continental margin support the interpretation of thevolcanic origin of this margin; (2) The extensional regime charac-terized by asymmetric half-grabens to symmetric grabens pre-dominates from the central to the northern region. In general, listricnormal faults bounding both sides are observed along the entireNacala basin (Figs. 3A, B). The area affected by the faulting widensto the northern basin forming a V shape, reaching a width of 50 kmin the area close to Nacala city (Fig. 1B). The detachment faultscommonly follow the continental margin. They propagate seawardsto end in the DFZ (Fig. 3A) and change morphology gradually,controlling faults dipping and displacing eastwards. They become afull-graben system towards the north where the Nacala basin be-comes wider (Fig. 1B). The graben system is expected to extendfurther north to correlate with similar tectonism overprints inter-preted in the Rovuma basin (Mahanjane and Franke, 2014). Riftingalong the western side of the DFZ formed a series of tilt-blocks inhalf-grabens in the transitional zone (at the location of Figs. 2B, C);and (3) Both the imbricate wedge structures and extensional rift-grabens form a link between extensional tectonics in the northand deformation, with compressional tectonics, in the south.Extensional tectonics is manifested in the Nacala basin andcompressional tectonics occurs along the Davie compression zone.

In general, the detachment surfaces of the growth wedges arelocated at depths between 7000 ms and 8000 ms (TWT) dippingtowards the east. The lateral extension varies from 10 km in thenorth to 20 km in the south (Fig. 1B at the location of Figs. 2A, C).Towards the north, the compressional tectonics structure is grad-ually replaced by extensional half-graben to graben morphology.The lateral extension of the graben varies from 10 km in the southto 40 km in the north (Fig. 1B). It is possible that extensional tec-tonics followed the cessation of transpression. The DFZ involved theMesozoic compressional and extensional events that were rejuve-nated by theMid-Tertiary extensional tectonics (East African RiftingSystem).

Common normal faults stretching out and cutting the Cenozoicsuccessions to the surface were consistently mapped in the seismicsections, favourably located on the top and along the western edgeof the Davie compression zone (Figs. 2A, B, and C). The interpretationbased on their geometry indicates the onset of the reactivation ofDFZ from the Oligocene onwards. It is considered to be a southerlyextension associated with the modern East African Rifting System.In some places, the faulting appears to indicate an active tectonicregime, supported by the occurrence of seismicity along the DFZ atthe present time (Coffin and Rabinowitz, 1987; Yang and Chen,2008; U.S. Geological Survey).

4. Hydrocarbon potential

The critical aspects for hydrocarbon potential plays in the DFZfrontier area and adjacent basins are predictable from the structuralmapping and stratigraphy of the DFZ, and fairways are identified for

Figure 4. Events charts for the potential petroleum system in the Davie Fracture Zone and adjacent basins. Please note, no maturity modelling was carried out in this study, thepetroleum system chart is modified from Bird (1994), Peters et al. (2005), Mubarak and Mariam et al. (2009). The chart indicates the presence of elements of the Petroleum systemdepicted from the stratigraphy of Fig. 1C. Age ranges of petroleum system are shown in coloured boxes. Generation and critical moments need to be estimated by maturity studies.Keys: Gen/Mig/Accum-generation/migration/accumulation; BUeBreakup, CSecondensate sections, MFSemaximum flooding surface. (For interpretation of the references to colourin this figure legend, the reader is referred to the web version of this article.)

E.S. Mahanjane / Marine and Petroleum Geology 57 (2014) 561e571570

future exploration. A summary of the petroleum system (e.g.Magoon and Dow, 1994; Peters et al., 2005) is presented by eventchart of Figure 4. For the evaluation of aworking petroleum system,it is taken into account that effective organic-rich source rocksshould be present and sufficiently buried as well as thermallymature (Peters et al., 2007).

Regrettably, the source rocks of the Mozambique continentalmargin basins are poorly understood, since no well drilled thus farhas penetrated the possible Jurassic and/or Early Cretaceous shaleand shelf carbonate deposits. Nonetheless, an effective source rockshould be present within the DFZ and deep-water areas judged bythe reported massive gas discovered in the Rovuma basin. Inaddition, there are active oil and gas seeps in the Rovuma basin andAngoche margin (Figs. 1A). Additional information indicates Middleand Late Jurassic source rocks in the Mandawa and Ruvu basins,which are capable to generate oil and gas. For instance, Bajociansource rocks from Makarawe-1 well contain ~1.98e2.36% TOC andthe Late Jurassic Nondwa shales in Mandawa basin exhibit0.65e8.7% TOC (Maenda and Mpanju, 2003). Likewise, the Jurassic/Lower Cretaceous marine marls from the Morondava basin containup to ~5% TOC (Matchette-Downs, 2006).

Geochemical data measured from the oil shows found in Man-dawa basin indicate excellent hydrogen index up to 1000 mgHC/TOC, vitrinite reflectance (R0) between 0.5 and 0.9% (Maenda andMpanju, 2003). The maturity window (R0) above “fits” well intothe maturity model discussed by Mahanjane et al. (submitted forpublication). They suggest sufficient maturity of the possibleJurassic source rock to generate hydrocarbons at the present-dayburial depth in the Angoche basin.

In general, both geometrically robust structural and strati-graphic play types occur along the DFZ and adjacent basins. Trap-ping developed from Late Jurassic to Cenozoic time preservingconfigurations similar to those pooling hydrocarbons in theRovuma basin. The Jurassic structural plays are predominatelydefined within the compressional structures and are formed by avariety of fault-related folds and fault-propagation folds geometriesinterpreted within the Davie compression zone (Figs. 2A, B, C) whilstthe stratigraphic trap plays are defined within Cretaceous andTertiary successions by pinch-out geometries of submarine fans(Figs. 2A, B, C).

5. Summary and conclusions

The results of this study present structural elements of twobasins, the Angoche in the south and the Nacala in the north,separated by a compressional zone. The Angoche basin was formedby thermal subsidence during the earliest stage of the Gondwanabreak-up in mid-Jurassic time. The evidence of the timing is theregional scale Karoo magmatism widespread along the westernflank of the Angoche basin. A basement with steep dipping surfacebounds the northern extent of the Angoche dyke swarms dim-out

in the Nacala basin and appears to be overprinted by the Daviecompressional structure. This structure is well-imaged in theeastern flank of the Angoche basin, deforming completely theJurassic succession along its extent. The compressional zone ismore prominent in the south. It shows progressive loss of charactertowards the northwest, being replaced by rifting of the Nacalabasin. This basin is formed by series of NNW and NNE grabensystems developed further north in the Rovuma basin.

The petroleum systems identified in structural mapping includethe following:

� Effective structural and stratigraphic traps are present due togood sealing structures, e.g. fault-related folds, and presence ofnumerous pinch-outs with anomalous amplitude reflectors. Thestratigraphic play is identified from Jurassic to Tertiary timewhereas the structural plays are defined in the Jurassic andCretaceous intervals.

� Geochemical data from neighbouring basins and the recentmassive gas discoveries in the Rovuma basin confirm the exis-tence of working source rocks along the DFZ. The possiblesource rocks are depicted within the stratigraphy of Jurassic,Cretaceous, and PalaeoceneeEocene (?) times.

Acknowledgements

This paper has received support from the management of theInstituto Nacional de Petr�oleo (INP) including the data. I would liketo thank BGR and Dr. Christian Reichert for the logistical supportthey provided. The author alsowishes to express his gratitude to Dr.Axel Ehrhardt, Dr. Norbert Ott, Martin Block, Dr. Walter Roest, theAssociate Editor Dr. AllardW. Martinius as well as the reviewers Dr.Colin Reeves and Dr. Ian Kane whose helpful remarks and com-ments on the paper resulted in significant improvements of thefinal version. Finally, I would like to thank Laura da ProsperidadeMahanjane and Anna Hacker for helping in the final edition of thispaper.

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