Palynological and palaeoenvironmental interpretation of the Late Cretaceous to Tertiary strata of the Melut Basin (southeast Sudan)

Abstract

m ABSTRACT A Late Cretaceous to Tertiary non-marine sequence of the Melut Rifi Basin in the southeastem Sudan was palynologically investigated. The recovered palynomorphs are overwhelmingly of terrestrial origin; few specimens of brackish water dinoflagellates and scolecodonts were encountered within the Lower Miocene and Oligocene/Miocene assemblages. The recovered palynomorphs are generally well preserved and fairly diversified. Based on the stratigraphic distribution of ninety-five selected spores and pollen species, compiled from three wells in the northem Melut Basin, eight palynozones covering the Campanian through the Neogene have been proposed. The identified zones are designated as Assemblage Zone I through VIII in stratigraphically ascending order as follows: Assemblage Zone I (Campanian), Assemblage Zone II (Campanian/Maastrichtian), Assemblage Zone III (Maastrichtian), Assemblage Zone IV (Mid-Late Paleocene), Assemblage Zone V (Early-Mid Eocene), Assemblage Zone VI (Oligocene/Miocene), Assemblage Zone VII (Early Miocene) and Assemblage Zone VIII (Late Miocene/ Pliocene). Out of more than 150 palynomorphs identified in the present investigation, only 25% of the species are Late Cretaceous elements, the remaining are Cenozoic material. In the Cenozoic assemblage, angiosperm pollen are most diverse, but pteridophytic spores and freshwater algae are most abundant. The angiosperm pollen are represented, among others, by Striatopollis, Bambacacidites, Retitricolpites, Retitrescolpites, Psilarriporites, Peregrinipollis, Pachydermites and Graminidires. The fem spores are represented by genera such as Deltoidospora, Cyathidires, Polypodiaceoisporites, Pteridacidiles, and Verrucatosporites, which represent the most abundant component of the Paleogene/Neogene palynomorph assemblages. Late Cretaceous angiosperm pollen are represented by Fuveotricolpites cf. giganleus, Cristaecolpites echinaceus, Monocolpopolleniles spheroidires, Auriculiidiles reticulatus, Syncolporites schrankii, as well as several pollen attributed to the Palmae and Proteaceae. Aquatic fern spores such as Gabonisporis vigourouxii and Ariadnaesporites spinosus and the hepatic spore Zlivisporis blanensis are present in the Campanian-Maastrichtian assemblages but disappear from the record in the Cenozoic assemblages.iv The coexistence of abundant structured organic palynodebris with the spores of aquatic ferns of the families Salviniaceae and Marsiliaceae observed within the Late Cretaceous sediments suggests swampy conditions within a predominantly fluvial setting. Frequent occurrence of pollen grains assigned to the Palmae group indicates the prevalence of warm, humid conditions during the Late Cretaceous, although seasonal aridity might be suggested due to the presence of Ephedripites spp. recorded from the Campanian and Campanian/Maastrichtian of well AY-3. Late Paleogene and Neogene assemblages are characterized by abundant amorphous organic matter, freshwater algae, appreciable amounts of fem spores and Gramineae pollen. Deposition in aquatic habitats such as fluvial plain and coastal swamps under warm-humid conditions is inferred. The occurrence of Gramineae pollen points towards the development of grassy areas under generally dry climate with marked rainy seasons. A presumably short marine invasion might have taken place during the Oligocene-Miocene, inferred from the rare occurrence of brackish water dinoflagellates and scolecodonts within the assemblage.