Facies (2005) DOI 10.1007/s10347-005-0039-8

ORIGINAL ARTICLE

Markus Geiger · Gunter¨ Schweigert – depositional cycles of the south-western Morondava Basin along the rifted continental margin of

Received: 23 August 2004 / Accepted: 23 November 2005 C Springer-Verlag 2005

Abstract After rifting and final breakup of Gondwana Keywords Madagascar . Gondwana . Post-breakup . along the former East-African-Antarctic Orogen during . T-R cycles the ToarcianÐAalenian, passive margins formed around the Proto-Indian Ocean. Sedimentological and stratigraphic studies in the southern Morondava Basin contribute to an Introduction improved reconstruction of palaeoenvironmental changes during the syn-rift and post-rift margin formation. Depo- From the Late onwards, crustal extension sitional models based on outcrop and literature data in in the centre of Gondwana resulted in the formation of combination with subsurface data sets provide a strati- three sedimentary basins in western Madagascar: Moron- graphic framework of four transgressive-regressive (T-R) dava, Majunga, and Ambilobe (or Diego) basins (Figs. 1 cycles. The incorporation of stratal architecture models de- and 2). The basins were aligned with a zone of weakness, rived from seismic images is essential. After a syn-breakup which developed along the former Pan-African mobile belt T-R cycle (T-R1), the post-breakup succession commenced (Montenat et al. 1996; Pique« et al. 1999). Madagascar was with a Ð Early carbonate platform (T2). situated on the eastern side of this axis of future breakup MiddleÐLate Bathonian (R2) formed when a (Reeves et al. 2002). These basins are characterised by global sea-level fall forced the shoreline to move basin- thick successions of Late Palaeozoic, Mesozoic and Ceno- ward. Incised valleys and palaeokarst known from seismic zoic sediments. lines are typical for forced regression cycles. In the Early From the Late Carboniferous onwards, a of again a widespread transgression occurs (T3). localised pull-apart basins and from the Early During a short regressive phase from the Late Callovian(?) until the more extensive intracontinental rifts to Early (R3), the siliciclastic shoreface deposits formed in the centre of Gondwana (Coffin and Rabinowitz prograded onto the shelf. From the Early Oxfordian on- 1992; Montenat et al. 1996; Schandelmeier et al. 2004). wards a transgressive trend continued (T4). T1ÐT3 can be The PermianÐTriassic interval is often considered the explained as the response to the structural development of Gondwana Breakup strata, which were followed by the breakup rifting but they follow sea-level changes ob- transitional phase (EarlyÐMiddle Jurassic) and the drifting served in other parts of the world. R3 and T4, in contrast, phase from the Callovian onwards (Luger et al. 1994; reflect eustacy. Montenat et al. 1996; Piqueetal.« 1999). Recent studies of tectono-sedimentary architectures from seismic images have shown that the breakup of Gond- wana took place during the late (Geiger M. Geiger () Department of Geosciences, University of Bremen, 2004; Geiger et al. 2004). A widespread transgression rep- PO Box 330440, 28334 Bremen, resents the breakup unconformity and marks the bound- e-mail: [email protected] ary between the ToarcianÐAalenian syn-breakup half- grabens above extensional fault-blocks and the BajocianÐ G. Schweigert Kimmeridgian/ post-breakup strata (Geiger et al. Staatliches Museum fur¬ Naturkunde, Rosenstein 1, 2004). Extensional faulting faded at the breakup unconfor- 70191 , Germany mity, marking the end of rifting, while Madagascar drifted southwards away from along the Davie Ridge Frac- Present address: ture Zone (Malod et al. 1991). M. Geiger Statoil ASA, This paper addresses two issues. Firstly, it aims to review 4035 Stavanger, Norway the lithostratigraphy and biostratigraphy for the southern Fig. 1 A Gondwana reassembly at 200 Ma with present-day con- Gondwana fragments by Reeves et al. (2002) based on an inter- tinents. Initial extension, as response to compression at the north- pretation of ocean-floor topography. The outlines of ern and southern margin of the supercontinent, was localised along crustal fragments are shown in grey and white. Areas of sedimentary the former East African-Antarctic Orogen (EAAO). Compiled from rocks in Karoo-aged basins are shown in dotted texture. Present-day Stollhofen (1999) and Jacobs et al. (1998). B Reconstruction of Madagascar is shown in dashed outline Morondava Basin to overcome the lack of coherent defini- succession varies from 3,000 to 4,000 m in thickness, al- tions of lithological units and the poor biostratigraphic data, though it may reach 11,000 m in thickness in the southern which in the past have been both a communication chal- Morondava Basin (Boast and Nairn 1982). lenge and a source of inconsistent . Therefore, The syn-breakup in particular is documented in the half- 18 sections of BajocianÐKimmeridgian successions in the graben fillings of mudstones (shales and marls) with a few southern Morondava Basin have been measured (Fig. 2)to beds of the Andafia Formation. This formation is study macroscale- and microscale sedimentary patterns and exclusively recognized in the seismic record of the former to characterise depositional environments. Macrofauna-and shelf (Geiger et al. 2004), where it, probably together with microfauna were studied for stratigraphic and palaeoenvi- the , exceeds some hundred metres in ronmental objectives. Secondly, the revised stratigraphic wells (Clark 1996). At outcrop the Andafia Formation can framework of palaeoenvironmental changes forms the base only be found in the Morondava Basin in the to identify transgressive-regressive (T-R) cycles in the region (Fig. 2), where a few metres are exposed at the type southern Morondava Basin and to interpret their origin. locality at Andafia (Besairie and Collignon 1972; Geiger et al. 2004). In the south-western Majunga Basin yield more than 100 m of the correlative Beronono Formation and the Stratigraphy of the Morondava Basin overlying Aalenian Sandstone is exposed. Above the breakup unconformity rests a carbonate plat- Studies on the stratigraphy of the Morondava Basin started form (Bemaraha and Sakaraha formations), followed by a in the 1950s when SPM (SocieteP« etrole« de Madagascar) forced regression sandstones (Ankazoabo and Sakanavaka began a hydrocarbon exploration campaign. Most data were formations) and deeper shelf deposits (Jurassic Duvalia never published but sedimentological and stratigraphic con- Marls). The post-breakup succession rarely exceeds 300 m cepts were summarized by Besairie and Collignon (1972). in thickness at outcrop, but is reported to reach more than The sedimentary fill of the Morondava Basin can be di- 2,000 m in the subsurface (Uhmann 1996). The Jurassic vided according to their position in the Gondwana Breakup record is cut off by two Early transgressive (Geiger et al. 2004). The pre-breakup strata are stratigraph- events in the Valaginian/Hauterivian and Aptian (Besairie ically and lithologically related to the Karoo Mega Se- and Collignon 1972; Luger et al. 1994). quence of continental Africa (SACS 1980; Kreuser 1995). They comprise the Late Carboniferous Ð Early Permian Sakoa Group, Middle Permian Ð Saka- Biostratigraphy of the southern Morondava Basin mena Group, and the Formation (e.g. Hankel 1994; Montenat et al. 1996; Wescott and Diggens Biostratigraphic concepts of the Jurassic in Madagascar are 1997, 1998; Geiger et al. 2004). The entire pre-breakup generally hampered by faunal and floral provincialism due Fig. 2 Three coastal basins—the Morondava, Majunga and Ambilobe basins—extend along the western coast of Madagascar and are filled with Late Palaeozoic, Mesozoic and Cenozoic sediments. In the southern Morondava Basin 18 sections were measured (zoomed sketch map: Persits et al. 2002)

to palaeogeographical isolation. Madagascar’s west coast (1997) provide the biochronological scale for this study. was part of a narrow embayment that extended southward Endemism also plays a major role in the composition of from the southern Tethyan margin onto the Arabian microfossil assemblages. Ostracods (Grekoff 1963; Mette Peninsula and Somalia. Ammonites are of considerable 2004; Mette and Geiger 2004a,b,c), and foraminifers use for biostratigraphic calibration of Jurassic strata in (e.g., Espitalie« and Sigal 1963a,b; Sigal et al. 1970)have Madagascar. Numerous species were identified from the only few East African or Tethyan analogues and usually measured outcrops and were compared with Tethyan do not provide stratigraphic information. Palynological (Cariou and Hantzpergue 1997), western Indian (Spath analyses of Jurassic strata in the Morondava Basin were 1933; Cariou and Krishna 1988) and Madagascan faunas performed by Hankel (1994) and Dina (1996), but they did (Collignon 1953, 1960, 1964a,b, 1967; Collignon et al. not improve the stratigraphic frame significantly. 1959; Besairie and Collignon 1972; Joly 1976). Apart Ammonites used for the biostratigraphy of the sections from the Early Oxfordian when Boreal zonations are are stored at “Staatliches Museum fur¬ Naturkunde Stuttgart, used, the Tethyan zonations of Cariou and Hantzpergue Germany”.

ToarcianÐAalenian also proposed by ostracods (Mette and Geiger 2004a). A Bathonian age for Sakaraha Formation is nowhere A universal biostratigraphic constraint for the first else confidently proven. Lathuiliere` et al. (2002), who marine ingression in the East African domain is the studied the meadows in this area, identified the appearance of Bouleiceras sp. (Luger et al. 1994; Hallam rhynchonellids Burmirhynchia termierae (Rousselle) and 2001). With the occurrence of Bouleiceras nitescens Baeorhynchia transversa (Cooper) in a horizon close to (Collignon) the Beronono Formation (Geiger et al. 2004) the transgressive base, which suggest an Early Bajocian to in the south-western Majunga Basin is assigned to the early Late Bajocian age. also correspond to faunas homonymous sensu Collignon (Fig. 3). The Boule- from the Late Bajocian (Lathuiliereetal.` 2002). iceras nitescens Zone corresponds to the Early Toarcian Falciferum Zone of the Tethyan realm. The subsequent suc- cession lacks biostratigraphic index taxa but intercalating Ankazoabo and Sakanavaka formations sandstones are commonly attributed as Aalenian Sandstone of suggested Aalenian age (Besairie and Collignon 1972). The stratigraphic age of the boundary between the Bemaraha and Sakaraha formations and the overlying sandstones of the Bathonian Ankazoabo and Sakanavaka BajocianÐBathonian Formation is unknown. The sandstones appear to be limited to the southern part of the Morondava Basin. Besairie and The boundary between the syn-breakup and post-breakup Collignon (1972) correlate the sandstones lithologically, successions approximates to the Aalenian/Bajocian bound- e.g. at Adabomjonga (VII), but biostratigraphic indications ary. AalenianÐBathonian sediments are devoid of strati- are rare. Luger et al. (1994) correlate the Bathonian sand- graphic index . stones in the south with Late Bathonian units in the central basin based on Clydoniceras sp. and Micromphalites Bemaraha and Sakaraha formations hourcqi (Collignon) (see Collignon 1964b). At the south end of the Morondava Basin, Tongobory section (III) gives In the south-western Majunga Basin Besairie and Collignon a BajocianÐBathonian age range due to the occurrence (1972) attributed interbedded and mudstones of Mesoendothyra croatica (Gusiˇ c).« Uhmann (1996) (shales and marls) above the Aalenian Sandstone to the classified those strata by indet. Parachoffatia sp. as Late Bajocian carbonate platform (Bemaraha Formation; Geiger Bathonian. Our new specimens could not be determined et al. 2004). The carbonate platform at the Manambolo and to the species level, but they agree with a Bathonian age. Tsiribihina river gorges, which cut through the Bemaraha South of the Onilahy River, there is sandstone with the plateau (Fig. 2), are likewise assigned to the Early Bajocian ammonites hians (Waagen) and Delecticeras (Besairie and Collignon 1972; Pique« et al. 1999). At the anjohense (Collignon), which indicate a Middle to Late eastern margin of the Bemaraha Plateau the carbonate Bathonian age (Collignon 1964b; Besairie and Collignon succession directly rests on Isalo sandstones. Farther south 1972). at Besabora (along National Road RN35), limestones Another major sandstone succession, the Sakanavaka with the foraminifers Mesoendothyra croatica (Gusiˇ c)« and Formation is described by Besairie and Collignon (1972) Protopeneroplis striata (Weynschenk) support a Middle from a section along the Sakanavaka River, 20 km NNW Jurassic (Bajocian to Early Bathonian) age (Jekhowsky of Ankazoabo village, and by Uhmann (1996) and Dina and Goubin 1964; Pique« et al. 1999). The sandstones and (1996) from the supposed correlative section (XI) along mudstones in the Besabora section are critical for interpre- the Mamakiala River (tributary of the Sakanavaka River). tation. While Besairie and Collignon (1972) and Montenat None of them gives a biostratigraphically specific fauna. et al. (1996) described debris fan deposits, Clark (1996) The base of the sandstone is not exposed. Neverthe- and Geiger et al. (2004) argue for a misconception derived less, Besairie and Collignon classify this sandstone at from poor mapping and correlate the siliciclastic units Sakanavaka to be younger than the Ankazoabo Formation. with the mixed carbonate-siliciclastic facies of the coastal They correlate it with the Formation north of the equivalent to the carbonate platform (Sakaraha Formation). Mangoky River, which itself is considered to overlie the In the southern Morondava Basin the Sakaraha Forma- Ankazoabo and Besabora formations (cf. Boast and Nairn tion rests directly on sandstones of the Isalo Formation. 1982). However, there is no biostratigraphic support for Uhmann (1996) gave a maximum Bajocian age at Sakaraha this lithological correlation. section (VIIIa), and also for Anjeba (Ia) and Andamilany South of the Onilahy River, in the vicinity of Betioky, (Ib) sections. The latter two were classified by Besairie the sandstones and bioclastic conglomerates at Saririaka and Collignon (1972) as Bathonian. A Bathonian age is section (IIa) are lithologically correlated to the Ankazoabo sandstone at Tongobory section. Biostratigraphic markers  are absent. Fig. 3 Ammonite biostratigraphy of the Callovian Ð Early Kim- The age of the top of the sandstone formations can only meridgian sections. Taxonomic comparison with Tethyan (Cariou be estimated indirectly by ammonites in the overlying mud- and Hantzpergue 1997), western Indian (Spath 1933; Cariou and Kr- ishna 1988) and Madagascan faunas (Collignon 1953, 1960, 1964a,b, stones and limestones to predate the Early Callovian (see 1967; Collignon et al. 1959; Besairie and Collignon 1972;Joly1976) below).

Callovian Ð Early Oxfordian Early Oxfordian Ð Early Kimmeridgian

From the Callovian onwards ammonites are useful bios- Reappearing mudstones from the Early Oxfordian tratigraphic tools (Fig. 3). The earliest Callovian is iden- onwards are precisely dated at the Dangovato section tified with cf. apertus (Spath) at the An- (IVa). Proscaphites episcopalis (de Loriol) indicates the tainakanga section (VIa). Together with other specimens of upper Mariae Zone to lower Cordatum Zone (Early the same taxa with wide umbilici, the most likely age ranges Oxfordian). Middle Oxfordian is evidenced by Enayites between the Bullatus and Gracilis Zone (Early Callovian). birmensdorfensis (Moesch; upper Transversarium Zone, At the Antainakanga section the contact to the underlying Luciaeformis Subzone). Late Oxfordian is identified by the Ankazoabo Formation is not exposed. occurrence of an unspecified but morphologically typical Amparambato section (VIb), only 0.5 km NE of An- Dichotomoceras sp. which belongs to the lower Bifurcatus tainakanga, belongs to the Indosphinctes patina Zone (late Zone, and Clambites ex gr. hypselum (Oppel) which is Early Callovian) due to the occurrence of Indosphinctes significant for the lower Bimammatum Zone (Hypselum besavoensis (Collignon). Thus the Antainakanga section Subzone). The occurrence of Benetticeras benettii (Checa) is slightly older than the Amparambato section.The Anka- dates the top of the succession to the Planula Zone (Late zomiheva section (V) contains the boundary to the under- Oxfordian) or slightly younger to the Platynota Zone lying Ankazoabo and stratigraphically correspond to the (Early Kimmeridgian). Antainakanga and Amparambato sections. At Ankazomi- The Beraketa section (IVb) is classified as early Late heva, mudstones can be determined as Early Callovian with Oxfordian (Bifurcatus Zone) based on a poorly preserved Subgrossouvria wanneri (Collignon) of the Indosphinctes Pachyplanulites fasciculata (Collignon). Furthermore, a patina Zone (Early Callovian). Pachyplanulites cf. subevolutus (Waagen) and Pachyplan- The minimum age of the Sakanavaka sandstone was esti- ulites roedereri (Collignon) signifies the upper part of the mated indirectly on the basis of an unspecified fauna from section as late Late Oxfordian (Bimammatum Zone). the overlying Callovian mudstones (Besairie and Collignon The Antsampagna section (IX) contains Perisphinctes 1972). At Mamakiala section (XIII) the occurrence of orientalis (Siemiradzki) of the Bifurcatus Zone (Late Obtusicostites ushas (Spath) confirms a Middle Callovian Oxfordian). Pachyplanulites mahabobokensis (Collignon) age (“ anceps and Hubertoceras mutans Zone” and Dichotomosphinctes germaini (Collignon) indicates sensu Collignon = Tethyan Coronatum Zone). Since no the “Rauracien” sensu Collignon (Latest Oxfordian Ð convincing stratigraphic or lithological concept exists to Early Kimmeridgian). Subdiscosphinctes cf. soucadauxi distinguish the Ankazoabo and Sakanavaka formations, (Collignon) marks the uppermost top of the section it is more credible that they both belong to the same unit. as being slightly younger than the youngest fauna at Thus the Callovian transgression tops both the Ankazoabo Dangovato, possibly correlative to the European Platynota and Sakanavaka formations. Zone (Early ). The Callovian mudstone succession passes into sand- At the Ankilimena section (XI) Perisphinctes orientalis stone, which has been variously reported as Oxfordian (Siemiradzki) and a typical Dichotomoceras sp. indicate the (Besairie and Collignon 1972; Luger et al. 1994; Uhmann lower Bifurcatus Zone (Late Oxfordian). Typical species of 1996), but the stratigraphic position remains unclear. Pachyplanulites sp. and Dichotomosphinctes sp. are possi- It appears that the Oxfordian Sandstone post-dates the bly slightly younger. Athleta Zone (Mamakiala section) and pre-dates the upper The Middle Oxfordian is also identified in the An- Mariae Zone. A further but weak stratigraphic constraint drea section (XII) based on finds of Epimayaites lemoinei for the base of the sandstone comes from Keliambia, 19 km (Spath) and Euaspidoceras beraketensis (Collignon) of the SSE of Ankazoabo, where Besairie and Collignon (1972) D. wartae and P. anar, niveau superieur« Zone of Collignon describe a MiddleÐLate Callovian mudstone and - which correlates with the Tethyan Transversarium Zone. oolitic limestone succession underneath the sandstone. Slightly younger but still Transversarium Zone is the up- Similar Late Callovian mudstones and limestones are also per part of the mudstone succession with Epimayaites cf. known from the slopes above Mamakiala (Besairie and pruvosti (Collignon), Epimayaites aff. axonioides (Spath) Collignon 1972). Consequently, the maximum age of the and Paryphoceras lautus (Spath). Oxfordian Sandstone ranges from latest Callovian to Early Oxfordian. The Ranonda section in the very south of the Mo- Sedimentary environments rondava Basin cannot be unambiguously dated, but the occurrence of a lytoceratide suggests a Callovian Four major sedimentary environments were recognised age. in the Jurassic syn-breakup and post-breakup strata of  the southern Morondava Basin: (1) a ToarcianÐAalenian Fig. 4 Correlation scheme of measured sections of the Sakaraha, dysoxic basin environment, succeeded by marine sand- Ankazoabo, and Sakanavaka formations in the study area illustrates stone, (2) a Bajocian Ð ?Early Bathonian carbonate plat- the stratigraphic difference of the Bajocian platform carbonates and form, (3) MiddleÐLate Bathonian marine sandstones, and the Bathonian sandstones. See Analamanga (X) and Adabomjonga (VII) sections for further exemplary details (4) Callovian Ð Early Kimmeridgian dysoxic basinal

mudstones (shales and marls) with a thick intercalated? and sandstone. Besairie and Collignon (1972) attributed Late Callovian Ð Early Oxfordian Sandstone. the regional appearance of the mixed facies to tectonism- induced increase in sediment supply. Siliciclastic succes- sions, such as the Ankazoabo, Sakanavaka, Mandabe and ToarcianÐAalenian rift basins Besabora formations were also included in the mixed fa- cies concept, promoted by stratigraphic uncertainties in At the beginning of the Jurassic marine conditions reached all involved formations. This concept strongly influenced into the East-African- Madagascan domain and partly over- stratigraphic nomenclature by SPM in the early 1950s and stepped onto Karoo rift deposits. Early Toarcian shaly mud- was widely used by SPM field geologists and for producing stones and siltstones with thin shelly limestones, and more geological maps (e.g. Besairie 1969). massive limestones in distal oxygen-depleted basins were Recent studies interpret the Sakaraha Formation as a followed upwards by a prograding shoreface sandstone. coastal plain association that accumulated landwards of the Exposures in the Morondava Basin are only localised in barrier/lagoon complex, which belongs to the Bemaraha the north and their quality and stratigraphic frameworks Formation (Clark 1996; Clark and Ramanampisoa 2002; are poor. Corresponding data, used and discussed in the Geiger et al. 2004). The deposition of the Sakaraha and Be- present paper, are from outcrops in the south-western Ma- maraha formations probably terminates during the EarlyÐ junga Basin (Clark 1996; Geiger et al. 2004). Middle Bathonian and is followed by the Bathonian sand- stones (see below) rather than interfingering with it (Fig. 4). Sakaraha section (VIIIa) and Anteninde section (VIIIb). Bajocian carbonate platform The area around Sakaraha is the for the Ba- jocian Sakaraha Formation (Besairie and Collignon 1972). The Bemaraha Formation of the carbonate platform system Uhmann (1996) and Dina (1996) measured a hill section is only present at the Bemaraha Plateau (Fig. 2), where the (S22◦55.402/E44◦29.983) along the road RN7 to Toliara Sakaraha Formation has been entirely eroded. In the south (Fig. 2). Above the transgressive contact to the underly- the Bemaraha Formation disappears into the subsurface and ing Isalo sandstone, the succession starts with an oolitic- only its coastal facies equivalent, the Sakaraha Formation, grainstone, followed by a thinly-bedded succession of is exposed (Fig. 4). mudstones, sandstones, and limestones. Calcareous ooids and bioclasts occur in some of the sandstones and lime- stones. Only some 100 m further west, Anteninde section Bemaraha Formation (S22◦55.013/E44◦29.008) describes the overlying marls in the westward dipping strata. The Bemaraha Formation is made up predominantly of Uhmann (1996) and Dina (1996) interpreted the Sakaraha massive limestones (carbonate mudstones, pelletoidal- section as an intertidal to shallow subtidal, open lagoonal grainstones or oolitic-grainstones). Typically, they form environment which is the same as the Anteninde section. a lenticular body that runs along the western edge of the Analamalanga section (X). The Bajocian section at Morondava Basin (Clark and Ramanampisoa 2002). The Analamalanga (Fig. 5;S22◦51.738/E44◦32.950) closely limestones vary in thickness from 30 to 1,000 m and resemble those at the type locality at Sakaraha (Geiger are clearly evident on many seismic lines in the northern et al. 2004). A transgressive oolitic-grainstone unconfor- Morondava Basin. Clark (1996) and Geiger et al. (2004) mity overlies the Isalo sandstone followed by mudstones provide a comprehensive description and discussion of intercalating with thinner, partly oolitic and bioclastic lime- subsurface data and some additional outcrops. Thicker stones (packstones and grainstones) and sandstones (Figs. 5 limestone successions found in the Sakaraha Formation and 6aÐd). show the interfingering of both lithofacies associations. Rhynchonellids, bivalves, gastropods, and echinoderms are generally concentrated in the upper and lower parts of the mudstone section. Solitary corals are preserved in one Sakaraha Formation horizon in the central part and possible biohermal structures in the upper part of mudstone unit. Microfossils such as Besairie and Collignon (1972) describe sediments of a the calcareous algae and lenticuline foraminifers occur in mixed carbonate-siliciclastic facies (the so-called Facies limestones in the upper section. A low diverse ostracod Mixte: “mixed facies concept”) in the south-central part assemblage is also present (Mette and Geiger 2004a). of the basin with proposed BajocianÐBathonian ages. The After the basal transgression, a series of high-energy carbonates are similar to those typically found in the lime- shallow sand shoals formed during flooding events that stones of the Bemaraha Formation, whereas the siliciclas- reached far landwards. Subtidal, intertidal, supratidal tics comprise varying admixtures of mudstone, siltstone, marsh, inner lagoonal, and coastal swamp environments  were established (Geiger et al. 2004). The low diversity Fig. 5 Analamanga section (X) illustrates the basal Sakaraha For- of the ostracods (Mette and Geiger 2004a) and brackish mation with the contact to underlying Isalo sandstone, whereas Am- water bivalve faunas (Geiger et al. 2004) supports the parambato section (VIb) reveals the sharp boundary to the overlying interpretation of intertidal and brackish lagoonal or swamp Ankazoabo Formation. For geographical reference see Fig. 1 conditions, whereas the presence of solitary corals and

bioherms, together with rhynchonellids, is more indicative Ankazoabo Formation of subtidal, outer lagoonal conditions. The massive oolite at the top of the section is interpreted as a coastal oolitic At the type locality, in the vicinity of Ankazoabo (Fig. 2), barrier complex that marks a major flooding event that the sandstone succession is interpreted as fluvial and shifted the facies belts far landwards. deltaic environment (Besairie and Collignon 1972). A cor- Anjeba (Ia) and Andamilany (Ib) sections. The Bajocian responding sandstone facies exists west of Sakaraha at Anjeba (S23◦42.090/E44◦20.607) and Andamilany Adabomjonga (VII; Fig. 4), where it directly overlies the (S23◦42.265/E44◦20.395) sections are located west of Sakaraha Formation. The sandstones lack marine indica- Betioky (Fig. 2) and were also previously described by tors, with the exception of a few calcareous, highly bio- Uhmann (1996), where Anjeba represents the upper part turbated horizons. Further south, at Tongobory (III) and of the composed Andamilany section. Geiger et al. (2004) Saririaka (IIa), bioclastics of echinoderms indicate an un- described the Anjeba sandstones, mudstones and partly ambiguous marine environment. Stratigraphic distinctive cross-bedded oolitic and bioclastic limestones. Andami- fossils are not known from either the Ankazoabo or the lany contains bioclastic, peloidal and oolitic limestones Sakaraha area. (mudstones and packstone) and partly conglomeratic, Adabomjonga section (VII). The basal unit of Ad- cross-bedded sandstones. abomjonga section (Fig. 6;S22◦54.901/E44◦28.618)is Microfaunas consist of algae, predominantly Dasy- presumably the top of the Sakaraha Formation and con- cladacea, textulariid and lituolid foraminifers (Fig. 7aÐf), sists of thin interbedded mudstones and sandstones, of and ostracods (Mette and Geiger 2004a). Anjeba is topped which the partly ripple laminated sandstones are fin- by a bivalve meadow. ing upwards into mudstones. Uhmann (1996) described Sedimentological interpretations and faunal assemblages a metre-thick sandstone channel with conglomerates fur- suggest deposition under intertidal to shallow subtidal con- ther down the section but that could not be confirmed. ditions in a protected lagoonal environment. A hardground, The interbedded sandstones and mudstones are capped by topped with glauconite sandstones, at Andamilany infer a a succession of partly dolomitised conglomeratic sand- short episode of shallow subtidal conditions. stones and dolostones. An overlying thick sandstone suc- cession shows several fining upward cycles from con- glomerates to trough cross-bedded and finally laminated Bathonian sandstone sandstone (Fig. 6f, g) with claystone interlayers, capped by a horizon of interlacing burrows (Taenidium isp.). At In the south-central basin Bathonian sandstone succes- the top of the section a red clayey horizon with rhi- sions rest unconformably on the carbonate platform and zoids is interpreted as a palaeosoil recording subaerial appear to be regionally confined. Various formations, e.g. exposure (Fig. 6h). the Ankazoabo and Sakanavaka formations in the south, Despite the absence of biostratigraphic markers, Besairie and Besabora and Mandabe formations in the north, are and Collignon (1972) and Uhmann (1996), attributed the poorly described and often lack reliable stratigraphic con- interbedded mudstones and sandstones, and the dolomites straints. North of the Tsiribihina River, comparable sili- at the base to the Sakaraha Formation with a Bajocian age, ciclastic deposits are unknown. Clark (1996) describes a while the thick sandstone succession is assigned to the sandstone lens with off-lapping clinoforms from seismic Bathonian Ankazoabo Formation. images that are located almost immediately above the Be- Uhmann (1996) interpreted the depositional environment maraha carbonate platform. He attributed them to a set of of the basal part (Sakaraha Formation) as supratidal inner Upper Jurassic sandstones. lagoonal with intertidal channel. Cross-bedding and rip- ple lamination indicates wave-influenced subtidal to inter-  tidal sandy shoreface conditions and the mudstones record Fig. 6 Photos and photomicrographs from the Analamanga (X) and Adabomjonga (VII) sections. Analamanga: a Transgressive bio- protected mudflats. Periodic exposure is evidenced by the clastic oo-grainstone sometimes with quartz grains as ooid nucleus palaeosoil horizon. above Isalo sandstone. b Dolomitized bio-grainstone; zoomed in- Tongobory section (III). The Tongobory section (Fig. 8; set shows dolomite rhomboids at a rim of a recrystallized shell. c S23◦42.265/E44◦20.395) along both shores of the Bioclastic fine sandstone and siltstone; zoomed inset depicts marine Onilahy River (Fig. 2) is of Bathonian age (Besairie and isopachous cementation, followed by a prismatic meteoric phreatic cementation in various generations. d Top of the roadcut section at Collignon 1972; Uhmann 1996). The section comprises Analamanga. e Bioclastic oo-grainstone sometimes with quartz as of bioclastic, partly oolitic sandstones with locally planar ooid nuclei. Adabomjonga: f Wave-modified bedforms in the cut off and trough cross-bedding and bioturbation. Frequently walls at the Antenide River, 200 m upstream from the bridge of intercalated conglomeratic beds with erosive base contain RN7. Cross-bedding infers turbulent flow as in a tidal environment. g Trough cross-bedded sandstone of the Ankazoabo Formation, the carbonate mudstone and sandstone lithoclasts, and bio- box outlines prominent bedding planes within the sandstone. River clasts (Fig. 9b). Small and large wave-length wave ripples cut parallel south to the RN7, see Hammer for scale. h A red clayey and sand wave horizons with crest spacings of up to 40 cm palaeosoil with root marks within the yellow and white, cross-bedded are present (Fig. 9a). Ankazoabo Formation sandstone, cut off walls at the Antenide River, 500 m upstream from the RN7 bridge. See rucksack for scale Ripples crests and the alignment of wood infer EÐW oriented palaeocurrents. Foresets dip predominantly to the east.

The partly oolitic shallow water sandstone at the base was trough cross-bedded sandstone with mudstone lenses. recently interpreted as transition from Sakaraha Formation Wave-ripple lamination was observed in sandstones below facies to the Ankazoabo Formation facies, but probably the mudstones. Plant debris are common. Helmintopsis represents more distal facies of the Bathonian Sandstone. isp. was found on bedding planes. Extraformational carbonate mudstone clasts in conglomer- Close to Mamakiala village (S22◦07.651/E44◦24.634) atic beds can be reworked debris of the Bajocian carbon- a larger outcrop exposes laminated to wavy-bedded ate platform. The conglomeratic beds suggest high energy sandstone, fine-grained sandstone with ripple lamination episodes. Coarse, cross-bedded sandstones are interpreted in a few places. Conglomeratic beds contain elongated as upper shoreface deposits. Fining upward at the top of mudstone clasts and cut erosionally into the sandstone the section implies a landward retreating shoreline. succession at several levels. , vertebrate bone Saririaka (IIa). Saririaka section (Fig. 4;S23◦42.125/ debris, and bivalve shell debris are found in the conglom- E044◦18.682) can be lithologically compared with Ton- erates. Uhmann (1996) also identified the bivalve Corbula gobory section (III). Local profile descriptions by Besairie sp. and Collignon (1972) suggest a correlation with the Anka- The strong interfingering of trough cross-bedded sand- zoabo sandstones. The section starts at the base with a thick stones with mudstones is interpreted as distributary chan- ferruginous conglomeratic sandstone, containing coarse nels framing interdistributary bay deposits. Cross-bedding bioclasts and mudstone and sandstone lithoclasts. The over- and coarsening upward cycles reflect a wave dominated lying fining-upward, partly bioturbated sandstone is trough shoreface with recurring shallowing. The conglomeratic cross-bedded. beds are storm-produced deposits. It is uncertain if the The basal bioclastic conglomerate contains bone debris contact to the overlying mudstones is conformable, since it and belemnites and bivalves, the latter indicating marine is poorly exposed. conditions. Fossilised leaf fragments and tree trunks, up to 50 cm in diameter and up to 3 m long, are predominantly WNWÐESE oriented. CallovianÐKimmeridgian shallow shelf basin The basal conglomerate is interpreted as a transgressive conglomerate with regards to the marine fauna. Trough In the Early Callovian, deeper and distal shelf conditions cross-bedding sandstone succession is interpreted as transgressed far landward and formed a thick mudstone suc- wave-dominated upper shoreface. The presence of large cession. Onlapping relationships of corresponding strata tree trunks indicates the proximity of the land. have been described from seismic lines (compare Stoakes and Ramanampisoa 1988 and Geiger et al. 2004), where the mudstones can be seen to successively overstep the Sakanavaka Formation basin-plain, the slope, and finally the platform carbonates (Clark 1996). In accordance with this, the basal mudstones The Sakanavaka Formation has its type locality along the become progressively younger towards the basin margin. Sakanavaka River (20 km NNW of Ankazoabo), where it In the deeper part of the basin, the earliest mudstones consists of sandstones with thin mudstone intercalations are possibly even latest Bathonian in age, according to (Besairie and Collignon 1972). Some horizons are highly well reports (Clark 1996). The mudstones generally reflect fossiliferous with mainly bivalves (Corbula sp.) and petri- deeper, open marine conditions and are interbedded with fied wood. Ripple marks and cross bedding infer deposition sandstones and limestone beds in some places. From the in shallow, agitated water. sections at Dangovato (IVa), Antsampagna (IX) and Ankil- Mamakiala section (XIII). The Mamakiala section imena (XI) sandstone of probable Early Oxfordian age is (Fig. 2;S22◦06.924/E44◦26.487) is the northernmost known. section at the Mamakiala River. Its biostratigraphic range is widely unknown but overlying mudstone contains am- monites of at least Middle Callovian age. Uhmann (1996) Jurassic Duvalia Marl and Dina (1996) described the entire sandstone, siltstone, and mudstone succession, which we found only partly Although the name Duvalia Marl originally applied to exposed. The outcrop patches yielded partly bioturbated, the mudstones (Valanginian Ð Earliest Aptian?) as introduced by Besairie and Collignon (1972),  it became loosely defined thereafter (Clark 1996). Some Fig. 7 Thin section microphotographs showing foraminifer and al- gae assemblage at Anjeba (Ia), Andamilany (Ib) and the Dangovato workers have since included MiddleÐLate Jurassic and (IVa) sections. Bajocian Sakaraha Formation: Foraminifers: a Tex- earliest Cretaceous sediments into the Duvalia Marl tularia sp., Ib. b Valvulina meentzeni Klinger, Ia. c Mesoendothyra demonstrating that the depositional history of these croatica Gusic,ˇ 1969, Ia. d Haurania sp., Ia. e Haplophragmoides sp., sediments is poorly understood. The mudstones are in- Ia. f Valvulina sp., Ia. Algae: g Rivularia sp., Ib. h Cylindroporella terbedded with thin silt-/sandstones, as well as limestones sp., Ib. i Terquemella sp., Ia. j Neomeris sp., Ia. k Heteroporella sp., Ia. l Indet. Dasycladacea,Ia.Oxfordian Duvalia Marl,IVa:Ostra- containing calcareous and iron-oolites and frequently cod: m Cythere. Alga: n indet. Dasycladacea. Foraminifers: o indet. abundant oysters (Gryphaea sp.) to form prominent nodosariid form. p/q Citharina sp. r indet. hyaline rotaliid form. For coquinas (“lumachelles”, see Besairie and Collignon stratigraphic reference see Fig. 8 1972).

Ankazomiheva section (V). The Ankazomiheva section Amparambato section (VIb). At the river cut of Am- (Fig. 8;S22◦58.078/E44◦26.941) consists of Bathonian parambato section (Figs. 2, 10;S23◦00.001/E44◦24.929) Sandstone of the Ankazoabo Formation at the bottom and the river floor is formed by a highly fossiliferous iron- a Lower Callovian mudstone succession at the top. The oolitic limestone and sandstone beds with thin mud- sandstone is partly bioturbated and trough cross-bedded at stone interlayers. The limestones vary between iron-oolitic the bottom of the section and laminated at the top. Within bio-wackestone and sandy bio-wackestone (Fig. 11). Mud- the laminated sandstone shale lenses and small coarser stone with few thin sandstone and limestone beds overlie channels occur. A 1 m thick sandy bioclastic conglomer- the limestone and pass into a thick monotonous mudstone ate (sandy litho-bio-grainstone) transgressively overlies the succession. sandstone and marks the boundary to the overlying Duvalia Within the oolitic limestones at the bottom of the section Marl. The conglomerate is followed by locally fossiliferous an abundant ammonite fauna is preserved. The succeeding mudstones with intercalated cross-bedded sandstones and mudstones contain numerous ostracods (Mette and Geiger limestones (partly lithoclast-bearing, oolitic and bioclastic 2004c) and foraminifers. Foraminifers are exclusively no- grainstones and packstones). dosariid (Geiger 2004). Vertebrate bone debris were found in the Bathonian Ranonda section (IIb). The lithology of the Callovian Ra- Sandstone. The conglomerate contains abundant bioclasts nonda section (Fig. 2; S023◦43.438/E44◦18.328) resem- including solitary corals, wood debris, echinoids, and bles the one at Antainakanga (VIa). Here, chalky siltstones bivalves, but no determinable specimen could be ex- and mudstones with few interbedded sandstones are topped tracted. The mudstones and limestones of the Duvalia by an iron-stained limestone (bioclastic oo-packstone). Marl contain a few Lenticulina sp., macrobenthos and Chalk points to a distal open marine high productive ammonites. carbonate environment. The bioclastic oolite at the top of The thick Ankazoabo sandstone at the bottom of the the succession is interpreted as a shoal during a period of section is interpreted as shoreface deposit. The trough shallowing and agitated water conditions. cross-bedding indicates agitated water, as in the upper Beraketa section (IVb). The MiddleÐLate Oxfordian Be- shoreface. In contrast, the laminated sandstone suggests raketa section (Fig. 2;S22◦54.901/E44◦28.618) along the lower shoreface conditions. The bioclastic conglomerate at Sakondry River yields dark mudstones (shales) at the base the BajocianÐCallovian boundary is interpreted as a trans- which fade to grey towards the top. Lenses enriched in gressive conglomerate and the ammonite fauna in the over- small, white, thin-shelled bivalve fragments disperse within lying mudstones and limestones suggests basinal condi- the dark mudstones. Lenticular calcareous concretions are tions (outer shelf). also present. Red and ochre weathering halos indicate in- Antainakanga section (VIa). The Lower Callovian An- creased iron contents. Several horizons of nodular concre- tainakanga section (Figs. 2, 10;S22◦58.330/E44◦24.247) tions spread within the middle and upper part of the section. starts cross-bedded, well sorted, bioclastic, and carbonate- Concretions contain belemnites and shell and plant de- cemented, bioclastic sandstone which forms a prominent bris. Others show bioturbation. Ammonite findings are scarp and allows a correlation at outcrop with the Ampara- usually in red fissile clay in the core of a dark nodu- mbato section (VIb). Above follows bioturbated chalky lar concretion. The shells were bleached, white in colour mudstone succession with abundant bivalve fossils that and were highly fragile. In the basal section, a few hori- show an oolitic texture in shell cavities. Several coarsen- zons reveal a highly diverse foraminifer fauna dominated ing upward cycles from chalky marlstone to marly siltstone by textulariid and nodosariid forms (Geiger 2004). The succeed until they are topped by a sandstone bed similar to middleÐupper part of the section contains abundant ag- the one at the base of the section. glutinating foraminifers which are dominated by lituolid Ammonites, bivalves and ostracods are common (Mette forms. and Geiger 2004c). The foraminifer assemblage is highly The dark colour of the mudstones infers a high diverse with a few nodosariid and neoflabelline forms preservation of organic material which is typical for (Geiger 2004). oxygen-depleted conditions. Ammonites indicate a fully Ostracods and bivalves reflect normal marine conditions. marine environment. In organic-rich, oxygen-depleted, The bioclastic sandstones at the bottom and at the top of acidic environments calcareous concretions are common. the succession are characteristic for sandbars. Oolites indi- Both interpretations are backed by the foraminifers (see cate agitated shallow water. Chalk as a product of plankton below). indicates open marine distal high productive carbonate en- Dangovato section (IVa). The ?Callovian Ð Early Kim- vironments. meridgian Dangovato section (Fig. 2, 12; S23◦06.895/ E44◦23.254) along the Dangovato River, a tributary of  Fig. 8 Tongobory section (III) shows a typical siliciclastic lower the Sakondry River, starts at the base with shaly mudstone shoreface setting with shallow marine sandstones of the Ankazoabo which contain nodular concretions similar to those of Formation. The section is composed of outcrops from the north and Beraketa section (IVb). Sporadically shelly beds of white the south shore of the Onilahy River. Ankazomiheva (V) section thin valve debris occur. The mudstone coarses upwards illustrates the boundary to the overlying Callovian mudstones of the into a partly cross-bedded and ripple-laminated sandstone, basal Duvalia Marl. For geographical reference see Fig. 2 the Oxfordian Sandstone (Fig. 12a). At the top of the sandstone exists a well-developed hardground which is Fig. 9 Photos and photomicrographs from Tongobory (III) and Landscape view facing westwards on the Ankazomiheva cuesta. The Ankazomiheva (V) sections. Tongobory: a Sand waves with crest scarps and ravines provide a section along the slope. d Coarse bio- spacing exceeding 40 cm, see hammer for scale. b Conglomeratic clastic debris in a litho-bio-grainstone (bioclastic conglomerate). e sandstone with predominantly carbonate mudstones lithoclasts over- Sandy iron-oolitic limestone (bio-packstone) with algal oncoid. lies an erosional surface, see hammer for scale. Ankazomiheva: c overlain by limestone (bio-oo-wackestone). Above succeed (Early Cretaceous) in age (Besairie and Collignon 1972; interbedded mudstones and limestones. The limestones Luger et al. 1994). are often (iron-)oolitic oysters coquinas (Fig. 12b, c). The Several coalified plant debris, leaf imprints, and belem- section terminates with mudstones and a few intercalated nites, rhynchonellids, and bivalves were found in the shaly thin sandy beds when it is erosionally capped by a major mudstone. The ostracod fauna is highly endemic and conglomerate (S23◦06.066/E44◦22.544) which is Aptian stratigraphically and palaeoecologically unspecific (Mette Fig. 10 Lower Callovian Antainakanga (VIa) and Amparambato (VIb) sections illustrate the facies immediately above the Bathonian Sandstone. Compare Ankazomiheva section (V). For legend see Fig. 4

Fig. 11 Silty iron-oolitic bio-wackestone with concentric iron-ooids and bladed calcite rhombus (indicates fresh water mixing). Stratigraphic position as indicated in Amparambato section, Fig. 10 2004; Mette and Geiger 2004b). Shaly mudstones below the Oxfordian Sandstone contain a wide spectrum of agglutinating foraminifers (Geiger 2004). The assemblage is dominated by lituolid forms with accessory textulariid and saccamminid foraminifers. In the interbedding mudstoneÐlimestone succession above the Oxfordian Sandstone, agglutinating forms are successively replaced by calcareous forms (Geiger 2004). In Kimmeridgian samples agglutinating forms reoccur but calcareous forms remain predominant. Calcareous concretions, microbenthos (see below) and the black colour of the shaly mudstones below the Oxfordian Sandstone reflect dysoxic basinal conditions. The coarsening upward cycle into the Oxfordian Sand- stone displays prograding shoreface conditions. The Oxfordian mudstones, and oolitic coquinas and limestones represent distal shallow-water conditions (see below). The comparably thin Oxfordian interval suggests condensation and emphasizes shallow water with erosion and restricted accommodation space. Antsampagna section (IX). The Antsampagna section (Fig. 2;S22◦52.470/E44◦24.950) is a ground section across the road RN7 from Sakaraha to Toliara, 8 km ENE of Mahaboboka village and 16 km W of Sakaraha village. The section covers the Upper Oxfordian Ð Lower Kim- meridgian. The section starts 100 m north of the road at the upper bank of the Fiherenana River with cross-bedded red weathering sandstone succession. Clasts are aligned to the bedding planes and bioturbation is present at several levels. This sandstone succession is topped by an erosional surface which is succeeded by a grey mudstone succession of 4 m thickness. At top the mudstone grades into 10 m white sandstone with lamination at the bottom, bioturbated, cross-bedding in the middle part and a massive appearance at the top. About 20 cm of bioclastic coarse-grained sand- stone overlies the succession erosionally. This bioclastic sandstone is considered to constitute the inclining surface on which RN7 is built. A few metres south of RN7, 15 cm of pisoidal mudstone is probably overlying the sandstone. The overlying 22 m succession contains mainly mudstones with a few dm-thick iron-oolitic limestones (iron-oolitic bio-packstones). At the top of this succession several cm- thick limestones (pel-packstone) are intercalating with the mudstone. Petrified wood debris were discovered in the sandstone at the bottom and in the limestone at the top of the sec- tion. A few marine bivalves, belemnites and ammonites are present in sandstones and limestones. Several samples re- vealed foraminifers which are dominated by similar lituolid forms than in the Beraketa (IVb) and Dangovato (IVa) sec- tions (Geiger 2004). Epistomina sp. was identified in the  Fig. 12 Dangovato section (IVa) is the stratigraphically most ex- tensive section covering the Lower Oxfordian Ð Kimmeridgian in- terval. The top is cut off transgressively by the Aptian conglomerate (Besairie and Collignon 1972). It is the only section to contain the complete succession of the Oxfordian Sandstone. Letters in black circles refer to photographs in Fig. 12. Letters in grey circles refer to photographs in Fig. 7 mudstone interlayer between the sandstones at the bottom. and Uhmann (1996) measured the same section, but they Discobotellina sp. occurs in one horizon at the very bottom describe a conglomerate that tops the section at about the of the thick mudstone succession further up. Pisoidal lime- central part. This conglomerate was found to represent a stone at the top of the sandstone is an emersion horizon river bed, which cuts into the Jurassic section. which is followed by a transgressional mudstone succes- However, the section extends further upstream until a thick sion with open marine conditions but episodes of shallow- cross-bedded sandstone truncates the mudstones. Besairie ing (oolites). and Collignon (1972) described OxfordianÐKimmeridgian Ankilimena section (XI). At the MiddleÐUpper Oxfordian and Cretaceous sandstones at Manama, which both can be Ankilimena section (Fig. 2;S22◦45.417/E44◦25.096)a correlative. cross-bedded sandstone forms the base of the section. The The limestones and in places also the mudstones con- sandstone grades into a mudstone succession which is over- tain a macrofauna of ammonites, bivalves, belemnites and lain by a 2 m thick coquina (iron-oolitic bio-packstone) of plant debris. Foraminifer assemblages are characterised by disarticulated oyster shells and other reworked inner ner- a frequent change between lituolid and calcareous assem- itic components. Above the coquina mudstone, bioclastic blages (Geiger 2004) with only a few mixed assemblages. sandstone and recurring iron-oolitic limestone are succeed- Stratigraphically and palaeoecological insignificant ostra- ing. Overlying mudstones contain thin gypsum interlayers. cods are described by Mette and Geiger (2004b). An iron-oolitic limestone (bio-packstone) forms a morpho- The mudstones represent an outer shelf environment logical step at the scarp. The steep slope is apparently (see above). Episodic shallowing is documented by the of monotonous mudstone which sometimes contains bio- oolitic limestones. Besairie and Collignon (1972) de- clasts. At the top chalky limestone forms the roof of the scribed the sandstone at the top of the section as a fluvial scarp. Trough and planar cross-bedded sandstone channels sandstone. with bioclastic horizons bound to bedding planes and wave ripples are spread within the chalk. Ammonites, rhynchonellids, bivalves, belemnites, echin- Oxfordian sandstone oderms and wood debris are found in and close to the lime- stone beds throughout the succession, which elsewhere is Several of previously described sections contain a part of devoid of macrofossils. But some lituolid foraminifers were or are interbedded with a more or less prominent sandstone found in the mudstones but nodosariid forms dominate the succession of probably Early Callovian age (see above). assemblages associated with the limestones (Geiger 2004), The yellowÐpink sandstones are generally well sorted but together with ostracod assemblages (Mette and Geiger contain conglomeratic beds. Cross-bedding is a common 2004b). feature and ripple lamination indicates shallow-water con- The sandstone at the base is fining upwards into the ditions. Common plant debris records a proximal position mudstone, representing a transition from lower shoreface to the land. to deeper shelf deposits. The monotypic character of the Besairie and Collignon (1972) included several Late macrofossil assemblage within the thick oyster coquina Jurassic sandstones in the southern Morondava Basin to suggests a period of shallowing. This is supported by the what he called “Argovian”. Clark (1996) correlated those oolitic texture of the coquina which infers agitated shallow- sandstones with the Sakanavaka and Ankazoabo forma- water conditions. Similar shallowing events are represented tions which are considered to have a Bathonian age. With by the iron-oolitic limestones in the succeeding limestones. the term “Argovian,” as a subdivision of the Lower Oxfor- Chalk production at the top of section infers high productive dian, Besairie and Collignon (1972) indicated the supposed conditions in distal shallow water. Embedded sandstone age, but stratigraphic indicators are missing. The stratigra- channels are of intertidal origin during shallowing. phy of this study suggests that the age of the sandstones at Andrea section (XII). The Middle Oxfordian mud- Dangovato (IVa), Ankilimena (XI) and Antsampagna (IX) stone succession at Andrea section (Fig. 2;S22◦25.695/ ranges in the Late Callovian Ð Early Oxfordian interval E44◦23.459) is comparable to the Dangovato (IVa), Ankil- and thus correlate with the “Argovian”. They are herein imena (XI), Antsampagna (IX) and Beraketa (IVb) sec- addressed as Oxfordian Sandstone. tions. The LowerÐMiddle Oxfordian Sandstone known A second sandstone unit above the Oxfordian mudstones, from these correlative sections, is not directly recognised to such as it is present in the Andrea section, appears as a underlie the section but approximately 2 km NNE at Am- localised phenomenon. There are three possible explana- pasy, in the Andrea River-bed, 4 m cross-bedded sandstone tions: Firstly, the sandstone is not Jurassic, but Cretaceous is exposed. Outcrop correlation to the Andrea section litho- in age (probably accounts to Andrea section). South of the logical comparison provides an argument for classifying it Onilahy River, Cretaceous sandstones occur, which have a as underlying Andrea section. similar lithology and the same order of thickness. Secondly, The entire 45 m succession comprises mudstone with the Aptian transgression cut off corresponding strata in the only a few iron-oolitic limestone (bio-packstones) and very south. The oldest preserved age of Early Kimmerid- sandstone beds. Spherical concretions align in several gian, however, infers a Kimmeridgian age for the sand- horizons. In a few places the mudstone contains common stone. Thirdly, regional tectonism formed localised facies shell fragments. Iron-ooids were also recognised in contrasts. concretionary horizons in the mudstone. Dina (1996) Iron-ooid formation, ocean floor spreading tion to pin down events to a biostratigraphic zone must be and sequence stratigraphy dismissed. Hallam (2001) outlined that such a generalisa- tion necessarily means that the applicability of sedimentary From the Callovian onwards the presence of frequent iron- patterns for interpreting global sea-level changes has to be oolites points to changes in sea water chemistry and thus the reduced to second-order cycles though he conceded the in- depositional environment. Sturesson et al. (2000) propose fluence of eustacy on thirdÐorder transgressive-regressive that iron ooids form in a shallow turbid sea directly next to cycles (Embry 1993). volcanism. In contrast, Yoshida et al. (1998) suggest rather The first transgressive-regressive cycle (T-R1) in the Mo- a non-marine or brackish environment to promote iron-ooid rondava Basin covers the syn-breakup phase with the trans- formation. Oxyhydroxides of iron and aluminium, silicon gressive Toarcian shales and marls, followed by the regres- oxide and calcium carbonate are the main components of sive Aalenian Sandstone (Fig. 13). The Bajocian Ð Early and modern iron-ooids (Sturesson et al. 2000). Fluid Bathonian marginal carbonate platform sequence which in- plumes from modern mid-ocean-ridge vents are found to cludes the Bemaraha and Sakaraha formations overlies the carry increased contents of iron, silicon and various ox- breakup unconformity and forms T2. It is followed above ides, hydroxides, sulphates and sulphides (Hekinian et al. by the MiddleÐLate Bathonian siliciclastic sequence (R2) 1980; Edmond et al. 1982). This argues for a connection with the Sakanavaka, Ankazoabo, Besabora and Mandabe of the appearance of iron-ooids and ocean-ridge volcanism formations. T3 covers the Early Callovian Ð Early Oxfor- from the Callovian onwards and consequently establishes a dian interval with a deepÐshallow shelf sequence of the connection to the onset of sea-floor spreading at minimum Jurassic Duvalia Marl. T3 is overlain by the siliciclastic Early Callovian. This passive dating of sea-floor spreading, sequence R3 of the Oxfordian Sandstone. Sequence T4 however, enhances the usually cited (e.g., de Wit 2003) old- represents an Early Oxfordian Ð Kimmeridgian/Tithonian est oceanic crust measured by magnetic sea-floor anomaly deep-shallow shelf environment and is also assigned to the M25 from the Oxfordian/Kimmeridgian boundary (Coffin Jurassic Duvalia Marl. The upper part of T4 is diachroni- and Rabinowitz 1992; calibration by Palfy« et al. 2000). cally truncated by several Cretaceous transgressions (Luger Iron-oolites are often discussed to be of sequence strati- et al. 1994). graphic relevance (Jervey 1988; Loutit et al. 1988; Kidwell 1989; van Wagoner et al. 1990). Macquaker et al. (1996) conclude that iron-oolites form in shallow conditions where ToarcianÐAalenian cycle (T-R1) sediment supply is low and detrital iron is available. This applies to sequence boundaries (e.g. partly Dangovato IVa), The syn-breakup succession of the Andafia and Beronono major flooding surfaces or maximum flooding surfaces (e.g. formations and the Aalenian Sandstone as described by Amparambato VIb) and thus limits the unambiguous prac- Geiger et al. (2004) illustrate a transgression and a maxi- ticability. mum flooding (Luger et al. 1994) during the Early Toarcian. In western the Oxfordian Dhosa Oolite Member of The successions are characterised by dark shaly mudstones highly condensed oolites is associated with hardgrounds, with several thin fossiliferous limestone beds. The shales reworked concretions, intraformational conglomerates, are rich in organic material and are interpreted to be de- shell lags, iron crusts and iron oncoids (Fursich¬ et al. 1992). posited in oxygen-depleted conditions (Clark 1996; Geiger The top of the member is interpreted as a maximum flood- et al. 2004). Toarcian anoxia has been explained by sev- ing surface of a relative sea-level highstand. This highstand eral global (e.g. Bailey et al. 2003) and regional models is a result of the combined Callovian transgression and (e.g. Rohl¬ et al. 2001). The anoxia coincides with a wide- thermal subsidence after the Gondwana Breakup. spread transgression in the Falciferum Zone (Hallam 2001) With regards to its sequence stratigraphic relevance, the which is well known from (Hallam 1981; Jacquin Oxfordian oolites of Madagascar mark a transgressive event et al. 1998), (Legarreta and Uliana 1996), above the Oxfordian Sandstones and the low siliciclastic Siberia (Zakharov et al. 1998), Middle East (Hallam 1981), contamination argues for the deposition in a distal shelf and northern Pakistan (Fatmi 1972). position. The interaction of thermal subsidence and eustacy Shallowing and regression in the Aalenian is recorded by is unclear. Major regional tectonism has not been proved the overlying Aalenian Sandstone which are poorly con- by thermochronological studies during this time (Emmel strained by biostratigraphic markers. The shales grade suc- et al. 2004). cessively into the 25 m thick cross-bedded sandstone. In the Morondava Basin a transgressive surface indicates a sudden change as a result of basin-wide flooding reach- Transgressive-regressive cycles ing far onto the rift margins, whereas the transition into the overlying post-rift carbonate platform happens more or less The study and intercontinental correlation of sedimentary successively at the outcrops in the south-western Majunga sequences during the Jurassic is generally hampered by Basin (compare Besairie and Collignon 1972; Geiger et al. the lack of global biostratigraphic correlation schemes. 2004). Seismic images from the Morondava Basin (Geiger In Madagascar the poor outcrop conditions and the lack et al. 2004) show the T-R1 strata with syn-rift reflection of biostratigraphic completeness require a considerable divergence in half-graben structures (Fig. 14) and infer amount of generalisation. This also means that the destina- the coincidental syn-breakup rifting. Such a syn-breakup Fig. 13 a Dangovato river gorge cuts through the Oxfordian Sand- the ooids. c Top surface of an iron-oolitic coquina; the white shells stone. b Thin section microphotograph of an iron-oolitic coquina in are predominantly oysters; see lens cap for scale (7 cm diameter) the Lower Oxfordian; the inset illustrates the concentric character of

character was inferred by Kreuser (1995) for the possibly? ern part of the basin (Clark 1996) which probably caused HettangianÐToarcian prograding deltaic Ngerengere beds the quick facies changes within the Bajocian in the south in . The Ngerengere beds lithologically correlate (Analamanga X). with the Aalenian Sandstone. In the sequence stratigraphic Similar to Madagascar, the East African margin was framework of Somalia the lower Hamanlei Depositional widely covered with carbonate-dominated marine deposits Sequence and the Meregh Fromation (Bosellini 1992)is of the Lugoba Formation (Kapilima 1984), partly of the comparable with T-R1. The presumed more basinal Meregh Mtumbi and Kidugallo formations of Tanzania (Kreuser Formation derived from the first marine ingression is con- 1995), the Baidoa Member of the Baidao Formation of sidered to be (Bosellini 1992). Lithologi- southern Somalia (Kassim et al. 2002) and the Hamanlei of cally, the Meregh Formation resembles the Andafia and north-eastern Somalia (Bosellini 1992). The transgressive Beronono mudstones in Madagascar. In the global context character can often be seen by the overstepping architec- the regression with the Aalenian Sandstone deposition is ture onto basement or older sedimentary strata (Mbede concordant to a regressive event in Europe (Jacquin et al. 1991). In Tanzania the major transgression is sometimes 1998) while elsewhere in the world (Argentina: Legarreta dated to have started during the Middle Aalenian with Ruvu and Uliana 1996; Himalayas: Li and Grant-Mackie 1993) Formation (Kapilima 1984) which can be correlated with evidence is rather poor (Hallam 2001). Kidugallo Formation of Kreuser (1995). A similar basin- ward younging of the transgressive surface is possible in the south-western Majunga Basin where the transition from Early Bajocian Ð Early Bathonian transgressive Aalenian Sandstone into more distal mudstones and lime- cycle (T2) stones predates the Bajocian (Besairie and Collignon 1972; Geiger et al. 2004). With a global transgression in the Early Bajocian (Hallam Comparable transgressive events in the Early Bajo- 2001) marine conditions were introduced to the entire cian are known from Europe (Hallam 1988), the Andes basin and the margins were flooded far inland. The trans- (Legarreta and Uliana 1996) and possibly from the Hi- gressive surface unconformably overlies sandstones which malayas (Westermann and Callomon 1988; Gradstein et al. belong to the Isalo Formation (Analamanga section X and 1991). A Late Bajocian transgression which has been sug- Sakaraha section VIIIa). Du Toit et al. (1997) described gested from northern Europe (Jacquin and de Graciansky this surface as the MAD 6 seismic marker and put it at the 1998), (Surlyk 1991) and Argentina (Legar- Lower/ boundary but Geiger et al. (2004) reta and Uliana 1996) has not been clearly recognised in showed that it represents a hiatus ranging almost through- Madagascar. However, changes in the dinoflagellate frac- out the Early Jurassic till Toarcian times. The transgressive tion from the Upper Bajocian is interpreted by Dina (1996) surface coincides with the breakup unconformity and marks as a regression which is possibly part of the an Upper Ba- the onset of post-breakup deposition (Geiger et al. 2004). jocian T-R cycle. The overlying carbonate succession shows several T-R cycles of higher order at the Bemaraha plateau in the north-

MiddleÐLate Bathonian regressive cycle (R2) Legarreta and Uliana (1996) recognized a widespread dis- continuity at the base of a Bathonian regressive succession Regarding the accuracy of the Bathonian stratigraphy of in central Argentina and identified shelfal exposure and the study area, massive sandstones related to a regressional incision. They interpret the features as a forced regression, event are presumed to be of an Early/MiddleÐLate Batho- which exactly reflects the situation in Madagascar. Also nian age. The sandstones were usually included together the Tethyan signal illustrates a regressive cycle (Jacquin with carbonate platform to a BajocianÐBathonian mixed et al. 1998). Nevertheless, Hallam (2001) believes that a facies sequence (Besairie and Collignon 1972; Luger et al. Bathonian regression is globally anomalous and clearly 1994; Montenat et al. 1996). marks a regional and not an eustatic event. A key locality is at Adabomjonga (VII), where Bathonian sandstones of the Ankazoabo Formation erosionally overly a dolomite succession that is assigned to the Sakaraha Callovian transgressive cycle (T3) Formation of the underlying carbonate platform. Besairie and Collignon (1972) and Uhmann (1996) suggest a Lower The coarse bioclastic, sandy limestone at the base of the to Middle Bathonian age for the unconformity. It sharply mudstone succession (Bullatus Zone: Early Callovian age) separates the Bajocian carbonate platform deposits from marks the beginning of the transgression T3, while the the overlying Bathonian sandstones. Such a sharp regres- transgressive surface below it represents the sequence sive contact points to a forced regression. In the north of the boundary (Fig. 14). Nevertheless, Tongobory section (III) basin in a loosely defined Bathonian interval Stoakes and shows deepening at the top of the MiddleÐLate Bathonian Ramanampisoa (1988) described also a gently undulating sandstone succession prior to the major transgression. surface at the top of the carbonate platform sequence with a Epsitimona sp. in mudstones at Antsampagna (IX) are number of broad valleys cut into it. This relief was formed typical for sudden floodings (Gordon 1970). Iron-oolitic during a widespread exposure of the platform when karsti- coquinas immediate at the base of the Early Callovian fication and river incision at the top of the Bemaraha lime- mudstones (Amparambato, VIb) point to initial shallow stone took place (Pierce and Yeaman 1986; Stoakes and conditions. Proximochorate and for the first-time chorate Ramanampisoa 1988). Stratigraphic correlation ranges dinocysts in the succeeding mudstones document deepen- from the predating deposition of the Bemaraha and ing of the water (Dina 1996). Chalky mudstones, such as Sakaraha formations to a postdating sandstone succession those at Antainakanga (VIa) and Ranonda (IIb), represent that is considered to be unspecified Bathonian. The a distal, highly marine environment. Abundant ammonites combined occurrence of a sharp contact between a emphasize the open marine character. Further up in the regressive and a transgressive facies, the exposure of wide stratigraphically uncertain Late Callovian to early Early areas of the shelf, and incised valleys is a strong argument Oxfordian at Dangovato (IVa) just below the Oxfordian for forced regression (Posamentier and Vail 1988;van Sandstone, mudstones with an apparently high content of Wagoner et al. 1990). The Bathonian sandstones infer a organic material are classified at outcrop as black shales. proximal, shallow-water environment (e.g. Tongobory III Oxygen-depleted deeper water conditions point to an and Ankazomiheva V). Pisoidal limestones at the top of episode of maximum flooding. the sandstone at Antsampagna (IX) are interpreted as cal- The lithological contrast between the Bathonian sand- careous crusts which were produced during soil-forming stones and the overlying Callovian mudstones above is emersion episodes. clearly determinable on seismic images and is widely However, the role of the Bathonian sandstones in the known as sequence boundary. It probably corresponds to southern basin is still not completely understood. Localised seismic marker MAD 7 recognised by du Toit et al. (1997) descriptions of Bathonian sandstones, e.g. Ankazoabo between the Middle and Upper Jurassic. and Mandabe formations (Besairie and Collignon 1972; In a regional context, major transgressions due to facies Luger et al. 1990) could be further indicators for incised changes to more basinal mudstones during the Early valley fills. However, the extension of the sandstones is Callovian are interpreted from the Ano«ole« Formation from still unknown. south-western Somalia (Kassim et al. 2002), from the The Bathonian regression of Madagascar represents a Pindiro Shales of Tanzania (Kreuser 1995), from the shales strong correlation to signals in other parts of the world. In of the Malivundo and Magindu formations in Tanzania Somali the stratigraphic correlative Goloda Member of the (Kapilima 1984) and from the in western Baidoa Formation is rather indicative of a transgressive India (Biswas 1980;Fursich¬ et al. 1991). Transgression phase during the Late Bathonian Ð Early Callovian and deepening from the Early Callovian onwards and the (Kassim et al. 2002). Widespread reef formation in Tanza- deposition of organic-rich shales is a global phenomenon nia infers that the transgression died out (Kapilima 1984). (Hallam 2001). It is locally suggested that this transgressive For more global comparison, Riccardi (1983) reports event already started in the Late Bathonian (Greenland: a general regression in western Argentina and Chile. Surlyk 1991; Europe: Jacquin et al. 1998). A twofold stacking of transgression surfaces in the Upper Bathonian  and Lower Callovian and the occurrence of limestones Fig. 14 Model of the sedimentary environments during the syn- breakup and post-breakup T-R cycles of the Morondava Basin. Data (Amparambato VIb) and sandstones (Antainakanga VIa basis was extended as indicated and Ankazomiheva V) correlate with the southern Moron- dava Basin (Fig. 14). However, it is in contrast to a sharp ments the progressive shallowing from deeper shelf to sili- regression at the Bathonian/Callovian boundary in the ciclastic shoreface environment. The age is uncertain but is Andes (Legarreta and Uliana 1996) and other parts of the probably between Athleta and Mariae Zones (Dangovato world (Hallam 1988). In Europe (Jacquin et al. 1998) and and Mamakiala, XIII sections). This sandstone unit proba- Greenland (Surlyk 1991) the major sea-level rise was bly is also the base at Antsampagna (IX) and Ankilimena during the Middle Callovian. Iron-oolites at the base (XI). At the top of the sandstone succession at Dangovato of the transgression in Europe (Jacquin et al. 1998), a hardground forms the sequence boundary where biotur- the Himalayas (Li and Grant-Mackie 1993), Pakistan bation reaches 20 cm into the top surface. (Fatmi 1972) and India (Gaetani and Garzanti 1991) In East Africa a coeval regression has not been clearly are in correspondence with the study area and appear to described. In the Uegit Formation of Somalia, Kassim be a world-wide characteristic. The transgressive facies et al. (2002) found a regression during Late Oxfor- development in Madagascar follows a more or less global dian Ð Tithonian times but stratigraphic determinations trend that is also manifested in the Tethyan cycle pattern are poor. Further north, Bosellini (1992) combines mud- (Jacquin et al. 1998). stones with oolitic coquinas of the Uarandab Formation and oolitic and sandy limestones and mudstones of the Late Callovian? Ð Early Oxfordian regressive Gabredarre Formation (Late Oxfordian Ð Kimmeridgian) cycle (R3) to the Uarandab Sequence. At the base of the Uarandab Sequence a transgressive surface is determined as Late Nodule-bearing mudstones pass successively upwards into Callovian in age (Bosellini 1992). Late Callovian fine- the Oxfordian Sandstone at Dangovato (IVa). This docu- grained siliciclastic sediments in north-western India sug- gest persistent flooding with hemicycles (Fursich¬ et al. Oxfordian times (Bosellini 1992). Highly condensed iron- 1991), although the initiation of the transgressive cycle is oolites of the Dhosa Oolite Member (Cordatum Zone) in uncertain. north-western India (Fursich¬ et al. 1992) closely resemble A minor regressive event during Early Oxfordian times those of Madagascar in facies and age. The Late Oxfordian is known from the Tethyan realm (Jacquin et al. 1998)but Ð Tithonian regression of south-western Somalia (Kassim has not been recognised on a global scale (Hallam 2001). et al. 2002) and a regression within the KimmeridgianÐ Tithonian Gabredarre Formation (Bosellini 1992) is a con- trast to the prevailing transgressive development in the stud- Early Oxfordian Ð Kimmeridgian transgressive ied successions. cycle (T4) The Upper Callovian Ð Lower Oxfordian deposits appear The Early Oxfordian transgression is stratigraphically to be intensively condensed in several parts of the world well determined at Dangovato section (IVa) with a reliable (Norris and Hallam 1995) including the Andes, where the ammonite age of Early Oxfordian (lower Cordatum hiatus can be pinched down to the late Athleta to early Zone). Shaly mudstones overlie a hardground at the top Cordatum Zone interval (Legarreta and Uliana 1996). This of the Oxfordian Sandstone. The same situation has been more or less coincides with a possible age of the Oxfordian observed at Ankilimena (XI) and Antsampagna (IX). The Sandstone and the succeeding mudstones with iron-oolitic succeeding sequence, e.g. at Dangovato (IVa), is rich in coquinas. Compared to the Madagascan sea-level change, iron-oolitic coquinas, marls and limestones and the thin transgression in the Tethys and other parts of the world are biostratigraphic intervals indicate condensation. Despite only known from the Late Oxfordian (Jacquin et al. 1998; unsatisfying stratigraphic control, other sections further Hallam 2001). north, like Antsampagna (IX) and Andrea (XII), show increasingly thicker mudstone successions with sparsely interbedded thin limestones containing iron-ooids. Iron- Conclusions and discussions oolites argue for shallow agitated-water conditions in the surrounding areas. Thicker stratigraphic intervals could The rifted passive margin along western Madagascar be the result of a basinward increase of accommodation experienced several intensive palaeoenvironmental space and a decrease in wave-induced erosion. T4 is con- changes. These changes reflect T-R cycles which all can cordant with an increased diversity of dinoflagellates and be related to cycles in other parts of the world. This acritarchs in the Oxfordian assemblages indicating marine “global” correspondence hampers interpretations of the deep-water environment (Dina 1996). The Kimmeridgian influence of regional tectonism on depositional systems at Dangovato and Antsampagna is devoid of iron-ooids and suggests two scenarios: (1) Tectonic control on the but the mudstones become siltier and indicate increased depositional environment was too small in comparison to sediment supply in distal lower shoreface. Calcareous eustacy to have imprinted in the sedimentary patterns, or foraminifers, partly with thin tests, indicate normal oxy- (2) tectonism coincides with eustacy. genation. These are a precursor of a regression. In some During the syn-breakup event the region was flooded for places Kimmeridgian and Tithonian palynomorphs argue the first time in the Jurassic and led to the deposition of for a shallowing of the water (Dina 1996). In contrast, dark shales. Together with shoreface sandstones of the so- localities to the north and towards the basin margin show called Aalenian Sandstone this T-R1 cycle coincides with a reverse trend, partly with exclusively terrestrial floras the global sealevel. Possibly slow subsidence and attenu- at the base. In the Upper Jurassic also glauconite, which ated tectonism produced relatively thin syn-breakup strata regenerates in entirely marine shallow shelf conditions with a pronounced marine character. The absence of promi- (Chamley 1989), becomes more frequent (Uhmann nent rift shoulder uplift explains denudation rates beyond 1996). thermochronological resolution (Emmel et al. 2004) and The East African biostratigraphy and lithostratigraphy suggests a shallow rift system similar to Galicia and an- becomes more inaccurate during the Upper Jurassic proba- cient Adria (Manatschal and Bernoulli 1999; Wilson et al. bly due to poor outcrop quality. In Tanzania mainly shales 2001). and evaporites of the Pindiro Shales are found in the coastal The T2 reached far onto the rifted margin and formed the basins (Kreuser 1995) and shales and sandstones of the carbonate platform. A clear distinction whether this trans- Malivundo Formation (Kapilima 1984) further inland. In gression responds to the global Early Bajocian sea-level Somalia, dark shaly mudstones with oolitic coquinas in rise alone or is amplified by expected thermal subsidence parts of the Late Callovian Ð Late Oxfordian Uarandab For- of the rift margin is unclear. mation and the Oxfordian part of the Gabredarre Formation The Bathonian R2 has an apparent correspondence with indicate condensation prior to a maximum flooding during a global eustacy signal. A basinward migration of the sili- ciclastic shoreline due to increased sediment influx as re-  Fig. 15 Composed lithostratigraphy and interpreted succession of sponse to uplift/denudation cannot be determined by ther- local T-R cycles compared to Tethyan T-R Tethyan T-R cycles (after mochronological methods (Emmel et al. 2004). Jacquin et al. 1998). Facies distribution is mainly based on the present More critical is the T3. An apparent start of ocean floor work with supplementary data from Besairie and Collignon (1972), production and from the Callovian onwards, could have Clark (1996) and Montenat et al. (1996) induced a flexurally downwrapping of the margin by the heavier ocean floor. On the other hand, post-rift thermal Collignon M (1964b) Le Bathonien marin a Madagascar; limite subsidence possibly promoted the transgression. However, superieure,« rapports et correlations.« In: Colloque du Jurassique a strong correlation of T3 with global sea-level changes (Luxembourg, 1962), pp 913Ð919 Collignon M (1967) Sur quelques ammonites remarquables, nou- argues for a major role of eustacy, which at least superim- velles ou peu connues du Jurassique de Madagascar. In: Comptes poses an additional mechanism. Rendus de la Semaine geologique« (Madagascar 1966), pp 21Ð The Oxfordian Sandstone follows a regressive event (R3) 26 that is recognizable in other parts of the world. Poor out- Collignon M, Rebilly G, Roch E (1959) Le Lias et le Jurassique moyen de la region« de Kandreho (Madagascar). Bull Soc Geol« crop conditions of the CallovianÐOxfordian boundary in- Fr 1:132Ð136 terval constrict further models. The following T3 reflects a De Wit MJ (2003) Madagascar; heads it’s a continent, tails it’s an global event and furthermore demonstrates the relation of island. Ann Rev Earth Planet Sci 31:213Ð248 the depositional sequences to global eustacy. Dina A (1996) Geologie und jurassische Palynologie des sudlichen¬ Morondava Beckens, Madagascar. PhD thesis, TU Berlin, Germany, 130 pp Acknowledgements The authors are indebted to A. Dina and T. Du Toit SR, Kidston AG, Slind OL (1997) The hydrocarbon potential Razakamanana (University of Toliara) for logistic support and as- of the East Africa Continental Margin. Alconsult Intern (On sistance during fieldwork. We would like to thank D. N. 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