PRELIMINARY LITHOLOGICAL AND PALYNOLOGICAL INVESTIGATION OF MESOZOIC-CENOZOIC STRATA IN THE GORGE OF THE NILE, ETHIOPIA

The Gorge of Nile in Ethiopia exposes layers of rock laid down over hundreds of millions of years of earth history. The Triassic sandstones of Gohasion Formation lie below the younger Triassic and Jurassic limestone and gypsum layers in the Gohasion and Antalo formations. The sandstones and limestones are sandwiched between Precambrian granites and 30 million-year old flood basalts. Samples from the Gohasion, Antalo and organic-rich chert interbedded with the flood basalts were studied for their palynological contents. Palynology has been integrated with lithologic data to make preliminary interpretations about depositional conditions. Transmitted light microscopy was used to identify and count palynomorphs and dispersed organic matter. Palynomorph recovery is poor in most samples, but one organic-rich gypsum sample is very productive. Common spore taxa recovered are Converrucosisporites sp., Matonisporites sp., and Concavissimisporites variverrucatus, while pollen grains are rare. Identified organic components include fungal remains, pollen, spores, structured phytoclasts (wood, cuticles), degraded phytoclasts, comminuted phytoclasts, opaque debris and amorphous organic matter (AOM). Organic components were point counted (300 specimens) and analyzed using principal components analysis and cluster analysis to recognize four palynofacies assemblages. Palynofacies assemblage A correlates with the flood basalt interbedded sediments and is exceptionally rich in structured phytoclasts (>=50%). Degraded phytoclasts and the highest percentage of AOM define assemblage B. Assemblage C correlates with the Antalo limestones. Palynofacies assemblage D is characterized by an elevated percentage of AOM and comminuted phytoclasts but fewer structured phytoclasts; it correlates with the siliciclastic sediments of the Gohasion Formation. Although marine palynomorphs appear to be absent, abundant phytoclasts and moderate amounts of AOM coupled with lithological data suggest marine depositional conditions with high input of terrestrial material into the basin. Kerogen data also yield information on source rock potential in the study area when analyzed on the AOM-Phytoclast-Palynomorph ternary plot. Sixteen out of the 21 samples plot in the gas-prone, phytoclast-rich type 3 kerogen field.