Morondava Basin Research Articles

Sedimentation in an intracratonic extensional basin: the Karoo of the Central Morondava Basin, Madagascar

AbstractThe late Carboniferous to Triassic Karoo Supergroup of Madagascar is a sequence of predominantly continental clastic sediments deposited during a long period of regional crustal extension. In the Morondava Basin of western Madagascar the lower two divisions of the Karoo sediments – the Sakoa and the Sakamena – are deposits of fluvial and lacustrine sedimentation systems supplied from the Precambrian metamorphic basement terrain to the east. East–west crustal extension produced a series of graben and half-graben structures after the Sakoa period which were reactivated after the Sakamena. The position and orientation of these half graben, which were marginal to a larger rift system to the west, were partly controlled by the steep NNE–SSW mylonitic fabric in the metamorphic basement. Palaeocurrents in the braided river deposits of the Sakoa and Lower Sakamena indicate flow to the southwest and west in both sequences. The rivers followed a regional palaeoslope to the southwest/west and were apparently not significantly influenced by the local structural trends which were oriented perpendicular to this slope. The absence of local structural control is attributed to extensive erosional events which followed each tectonic episode and preceded the onset of further sedimentation which took place on an essentially peneplained surface. The tectonic episodes brought about changes in base level which caused this part of the basin to fluctuate between erosion and deposition.

Le jurassique supérieur et le Crétacé inférieurd'Ankilizato (bassin de Morandava, Madagascar)

Upper Jurassic and Lower Cretaceous from Beritsa, near Ankilizato, are an important landmark between well known cross sections of Northern (Antsalova) and Southern (Sikily) parts of the Morondava Basin. New ammonites and belemnites fauna point out an Upper Jurassic to Upper Valanginian age for this section. A paleogeographical scheme shows the dynamical structure of «Ankilizato gulf at that time.

Microbial alteration of 17α(H)-hopanes in Madagascar asphalts: removal of C-10 methyl group and ring opening

The detailed investigation of the saturated hydrocarbon fractions of two biodegraded asphalts from the Morondava Basin, Madagascar, and the comparison with a related non-biodegraded asphaltic oil revealed new information on the compositional alteration of reservoired hydrocarbons due to biodegradation. A nuclear-demethylated 17α(H)-hopane was isolated from one of the asphalts and the preferred structure is suggested to be 25-nor-17α(H)-hopane. In addition to a homologous series of 25-nor-17α(H)-hopanes and 25-normoretanes a series of tetracyclic triterpanes was detected. These are thought to be derived from 17α(H)-hopanes by ring C opening. Steranes in the biodegraded asphalts were found to be altered but not completely removed.

Triassic palynology of the Carnarvon Basin, Western Australia

Palynological investigations have been carried out on 281 productive samples from Triassic sediments intersected by 14 oil exploration wells in the northern part of the Carnarvon Basin, Western Australia. Ninety-three species of spores and pollen grains were identified, ten of which are described as new. Staurosaccites (type species: S. quadrifidus Dolby n. sp.) is proposed as a new form genus. Five assemblage zones have been recognised in the Triassic succession, viz.: 1. V. Minutosaccus crenulatus Zone (Carnian—?Norian) 2. IV. Samaropollenites speciosus Zone (Carnian) 3. III. Staurosaccites quadrifidus Zone (Anisian—Carnian) 4. II. Tigrisporites playfordii Zone (Smithian—Anisian) 5. I. Kraeuselisporites saeptatus Zone (Griesbachian—Smithian) Assemblages from zones III–V differ notably from any described from strata of comparable age in eastern Australia. They resemble in important ways the microflora of the Isalo I succession in the northern part of the Morondava Basin, Malagasy. Well-characterized taxa that occur in the Upper Triassic of Western Europe are also found in the Carnavvon Basin. These include Ovalipollis, Aulisporites, Enzonalasporites, Camerosporites, Infernopollenites and Decussatisporites. Middle and Late Triassic plant microfossil assemblages that contain a mixed association of Gondwana and European forms have been called the Onslow Microflora. It is believed to represent temperate rain-forest formations, established along continental margins, at palaeolatitudes between about 30°S and 35°S. The distinctive European elements listed have not so far been recorded from Triassic assemblages from eastern and southern Australia. These are less diverse and almost always dominated by Falcisporites. Assemblages of this type are called here the Ipswich Microflora and may have derived from Triassic plant communities characteristic of higher latitudes. The crude phytogeographic pattern indicated by the distribution of these two types of plant microfossil assemblage is consistent with the model of Triassic geography recently presented by Smith, Briden and Drewry.

Subsurface Palynology in Madagascar: Permian, Triassic, and Jurassic of Morondava Basin: ABSTRACT

Palynological investigations are summarized for several hundred cores and cuttings samples from the Permian, Triassic, and Jurassic found in about 30 wells drilled by the Societe des Petroles de Madagascar in the Morondava Basin (southwest of Madagascar). The nature of the microflora, which includes about 300 forms of organic microfossils (numerous spores and End_Page 269------------------------------ gymnospermic pollen grains, a few Hystrichospheres and Dinoflagellates) is presented. Evolutionary trends of the associations and lateral variations and environmental influences are discussed. Choice of stratigraphical characteristics and zonation of the series, which have an aggregate thickness of several thousand meters, are indicated. Correlation is made between wells at distances extending to as much as 600 kilometers and across various deposits of continental, brackish, and marine facies. End_of_Article - Last_Page 270------------