Upper Cretaceous to Neogene Palynology of the Rawat Basin, White Nile State, Sudan

The palynology of the Upper Cretaceous to Neogene non-marine succession in the Rawat Basin, White Nile State Sudan was investigated. An attempt was made to determine the relative age, and depositional environment of the studied interval. Based on the stratigraphic distribution of selected pollen and spores from two exploration wells, seven informal palynozones from the Campanian to the Neogene are proposed. The zones, in stratigraphically ascending order, are as follows: Assemblage Zone I, Campanian (Gelhak Formation); Assemblage Zone II, Maastrichtian (Melut Formation); Assemblage Zone III, Eocene (Yabus Formation); Assemblage Zone IV, Oligocene (Adar Formation); Assemblage Zone V, Oligocene/Miocene (Jimidi Formation); Assemblage Zone VI, Miocene (Miadol Formation); Assemblage Zone VII, Miocene/Pliocene (Daga and Agor formations). The ages are based on stratigraphic ranges of marker species in contemporaneous basins in Africa and South America and a series of assemblage species such as Ladakhipollenites lehmanii, Triorites sp (Zone I), Proteacidites sigalii (Zone II), Proxapertites operculatus (Zone III), Deltoidospora cf. africana (Zone IV), Magnastriatites howardii (Zone V), Cyathidites minor (Zone VI), Verrucatosporites usmensis (Zone VII). The paleo-environment of deposition were determined to range from fluvial to lacustrine environments.


Introduction
The Rawat Basin is one of the rift basins in the interior of Sudan that formed after breakup of Gondwana during the late Jurassic-Early Cretaceous and the opening of the South Atlantic.
The basin lies in the northern part of White Nile Rift system, around 350 km south of Khartoum City between latitudes 11.6 -12.8 N and longitudes 31.6 -32.9 E. It is located around 400 km to the southeast of the CASZ and represents the northern extension of the Melut Basin and shares global sedimentary and architectural features. The basin is up to 175 km long and 50 km wide and locally contains up to 4000 m of Upper Cretaceous to Neogene sediments.
Due to the lack of surface exposures in the Rawat Basin, oil exploration has provided much of the information on the basin history.
Based on seismic and log interpretations eight formations have been identified for the Rawat Basin, which are analogous to Northern Melut Basin. The formations from the bottom to top include: Gelhak, Melut, Yabus, Adar, Jimidi, Miadol, Daga and Agor.
Palynology is the most important biostratigraphic tool for studying the nonmarine successions of the Rawat and adjacent basins in southern Sudan.
Two oil fields were discovered in the Rawat basin but were not developed. The integration of the evolution of the sedimentary environments with time; timing is to be deduced from the palyanological evolution will help in understanding the basin characterization and reservoir properties and help in assessing and adopting suitable production technology.
The main objective of this study is to establish a pollen/spore zonation for the Rawat Basin, based on the identification of qualitative and quantitative changes of palynofloras in the sections studied. The successions in several deep wells in the Rawat Basin consist of sandstones, clay stones, and siltstones deposited with fluvio-lacustrine environments.

Material and Methods
A total of 25 ditch-cutting samples from two deep wells (M-1 and M-2) located in the Rawat Basin together with their corresponding log data (gamma ray, SP and resistivity logs) form the data base for the present investigation. The two wells penetrate whole the basin and combined together to give a complete dating for all formations in the Rawat Basin.
Preparation of palynological slides was carried out in the Petroleum Laboratory Research and Study (PLRS) in Khartoum, Sudan.
One slide per sample was comprehensively scanned and the abundance of pollen grains, spores, and other palynomorph types was determined. Counts were made along the traverse lines and the positions of the specimens were noted. Coloured microphotographs were made using transmitted light microscope Leica DM LB equipped with a digital camera Leica, type DFC 280, serial No. 022611304. Wellpreserved specimens were chosen for image processing and presented in the Plates 1-5.

Palynostratigraphy
The Rawat Basin palynofloras are generally characterized by wellpreserved miospores. Most of the Paleogene and Neogene samples yielded rich and diverse palynomorphs. Due to an extensive sand-rich interval at the Gelhak Formation and at the mid of the Melut Formation,  Many of the palynomorphs in this study are assignable to pollen species previously described from Africa and South America.

Palynostratigraphic results and biozones:
The Rawat Basin palynoflora is characterized by fairly well-preserved miospores of terrestrial origin. The palynofloral composition is dominated mainly by pteridophyte-bryophyte spores e.g. Concavissimisporites, Cyathidites, Deltoidospora, Laevigatosporites and Zlivisporis. Angiosperm pollen and gymnosperm pollen grains are rare and less diverse include among others: Proxapertites, Proteacidites, Triorites, Praedapollis are rare and less diverse.
Based on the stratigraphic distribution of the guide forms in relevant phytogeographic provinces seven palynofloral assemblage zones for the interval spanning the Late Cretaceous to Neogene have been proposed for the Rawat Basin. This zonation is correlated with previous schemes established in Sudan and West Africa. These are described and discussed below. Discussion: Ladakhipollenites lehmanii reported from the Maastrichtian of Somalia [1] and Campanian of Melut Basin [2]. Butina andreevi was reported from the Campanian-Maastrichtian of the Sudan [3] and from the Maastrichtian of Somalia. Triotites sp reported from Campanian to Maastrichtian of Sudan, Foveotriletes margaritae together with B. andreevi were reported from Campanian-Maastrichtian of Nigeria [4,5]. Longapertites microfoveolatus was originally described as Monocolpopollenites sp from the Campanian of Senegal and Senonian of Ivory Coast. It was later reported from the Paleocene and Late Campanian-Early Maastrichtian strata in Nigeria. The species was also recorded from the Maastrichtian of Somalia. Late Maastrichtian-Paleocene strata of central Sudan and together with Cycadopites ovatus have been reported from Campanian of Melut Basin. The combined stratigraphic ranges of the aforementioned taxa suggest a Campanian age for the Assemblage Zone I.