Depositional Sequences of Madagascar during the breakup of Gondwana

Markus Geiger, Jochen Kuss

DFG Project KU 642/18

Crustal extension and eventual separation of the eastern and western part of the Gondwana supercontinent left imprints in the Jurassic sedimentary record in the Morondava Basin of western Madagascar. The sedimentary successions were studied with an interdisciplinary approach, based on interpretations of sedimentary textures (outcrop) as well as microfacies interpretations (outcrop and thin sections), stratigraphic studies (ammonites), palaeoecological interpretations (macro- and microfossil), interpretations of the stratal architectures (seismic images), provenance studies and resedimentation studies (detrital apatite fission track analyses). Sedimentological and stratigraphical analyses elucidate local and regional palaeo-environmental perturbations and relative sea-level changes. Seismic images allow the nature and temporal order of tectonic events to be interpreted. The combination of sedimentological and seismic concepts results in a tectono-sedimentary model of the breakup of Gondwana.
During the Late Carboniferous-Late Triassic extensional basins formed all across Gondwana. In contrast to earlier concepts, in which Gondwana breakup was seen as a multiphase, long-lasting process during Late Carboniferous-Late Triassic times, the present study infers that the breakup episode can be constrained to the Toarcian-Aalenian interval (late Early Jurassic).
Seismic images from the northern Morondava Basin illustrate that the Late Carboniferous-Late Triassic Karoo succession is involved in breakup faulting with fault block rotation and therefore is by definition a pre-breakup strata. The typical syn-breakup strata are formed by the Andafia Formation (Toarcian) and the Aalenian Sandstones, when they form divergent strata of shales, limestones, and sandstones, deposited in half-grabens above tilted blocks. Observations from outcrop also suggest a basinward repeating, eastward-tilting of the strata and an eastward pinching out of the Andafia Formation. The Andafia Formation in the Morondava Basin correlates with strata of the Beronono Formation in the Majunga Basin. There, the oldest syn-breakup deposits in Madagascar are classified to the Bouleiceras nitescens Zone (Early Toarcian). A major unconformity, which is overlain by Early Bajocian sediments, marks the end of this early syn-breakup phase, and is followed by post-breakup deposits of the drift phase. The post-rift phase is characterised by the initial formation of a coastal carbonate platform (Bajocian), which extends along the entire continental margin followed by a siliciclastic ramp system (Bathonian) and subsequently by basinal shales and intercalating shallow water limestones (Callovian-Kimmeridgian).

During breakup rifting and drifting four major transgressive events, each followed by a regression, were recognised to mainly follow eustatic sea-level changes and to be only minor affected by regional tectonism:
- Early Toarcian
- Early Bajocian
- Early Callovian
- Early Oxfordian
The first transgressive-regressive (T-R) cycle coincided with the Toarcian syn-breakup rifting. Apart from this three T-R cycles are documented in the passive margin succession. An Early Bajocian transgression overstepped the rift flank of the breakup eastwards far inland and covered Toarcian-Aalenian syn-breakup rift strata and extensive parts of the pre-breakup Karoo strata. At the same time a coastal carbonate platform attached to the newly formed continental margin. Previous studies have failed to recognize the basin-wide extent of the carbonate platform (mudstones, limestones, and oolites Bemaraha Formation), because only the coastal equivalents (mudstones, limestones, siltstones, and sandstones of the Sakaraha Formation) are exposed in the southern Morondava Basin.

A major regressive event in the Early-Middle Bathonian resulted in a partial exposure of basinal strata in the northern part of the Morondava Basin (Stoakes and Ramanampisoa, 1988). There palaeokarstification and incised valleys on top of the carbonate platform are known from seismic lines in the north. At the same time shallow water siliciclastics were deposited in the south to form the sandstone-dominated Ankazoabo and Sakanavaka formations. The combination of basin exposure, incised valleys, and localised siliciclastic wedges are interpreted as typical forced regression features.
A widespread transgressive unconformity introduces deeper marine conditions again in the Early Callovian (Bullatus Zone) with mudstones, siltstones, limestones, and a few iron-oolites at the base. Deeper basinal conditions prevail, with the exception of prominent shallow water sandstones which post-date the Athleta Zone (Late Callovian) and predate the Plicatilis Zone (Middle Oxfordian). This “Oxfordian sandstone” documents a short regression forcing the siliciclastic shoreline to prograde into the basin. The base of the youngest Jurassic transgression of the southern Morondava Basin (Plicatilis Zone) is characterised by the occurrence of iron-oolitic limestones. Stratigraphically upwards mudstones and shales are intercalated with bioclastic and iron-oolitic limestones.
The three post-rift T-R cycles are basically comparable with the second-order cycles of the Tethys. Biostratigraphic bias of the Ankazoabo Formation and of the “Oxfordian sandstone” hampers a precise comparison. However, the Jurassic sea-level curves of both the Morondava Basin and of the Tethys show strong similarities and suggest that the eustatic signal is superimposed on the local sea-level curve.
The depositional concepts derived from studies at outcrop and subsurface are complemented by the compilation of facies maps and isopach maps for the Morondava Basin. Facies maps suggest a basinwide occurrence of the Bajocian carbonate platform, whereas in contrast a pure siliciclastic environment is present during the Oxfordian. Isopach maps of Bajocian-Bathonian and Callovian-Tithonian intervals outline a strong difference in sediment thickness in the southern part of the basin compared to the north. Both time intervals imply two depocentres in the south-central part of the basin, whereas in the north strata are thinner for a large area.

Palaeo-current indicators from the south Morondava Basin suggest a general westward sediment transportation and a N-S trending shoreline during the Middle and Late Jurassic. During the Bajocian-Bathonian a northward directed transport is inferred at the very southern end of the Morondava Basin, where local uplift is probable. During Oxfordian times, at the northern edge of the southern central part of the basin, southward-directed palaeo-currents may indicate an elevated basin margin. Evidence for slightly uplifted marginal areas can also be found at the southern basin margin by the comparison of stratigraphic ages of Bajocian-Callovian sediments with apatite fission track (AFT) ages of detrital apatite grains. The stratigraphic order of detrital AFT ages can only has been formed by a stratigraphic reversal, possibly due to reworking of former Karoo deposits. The erosion of Karoo sediments may refer to uplift of the margin of the breakup basin, which is lined by Karoo deposits.
Considerations of biogeographical vicariance based on new ostracod data indicate high endemism in Madagascar compared to East Africa and other Jurassic findings. Spreading ridge formation and dysoxic basinal conditions in the Proto-Indian Ocean formed a possible migration barrier between Madagascar and the African coast.
Reappraisal of published regional ocean floor ages and consideration of Middle Jurassic volcanism in the region suggests that the maximum age of the Proto-Indian Ocean floor is approximately 180 Ma. This age corresponds to the Toarcian time, which is considered to be the syn-breakup period.
Finally, the new perceptions of basin structure and stratal architecture, as well as their development during the Jurassic can be compared to numerical models of basin formation (Burov and Cloetingh, 1997; Cloetingh et al., 1997; van Balen et al., 1995). Such comparisons indicate that during the Jurassic the Morondava Basin of Madagascar was part of a non-volcanic, fast spreading, narrow, passive rift which rift shoulder experienced no considerable uplift.

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