Developing a Coherent Stratigraphic Scheme of the Albertine Graben-East, Africa

Abstract

Abstract:The Albertine Graben is one of the most petroliferous onshore rifts in Africa. It forms the northernmost termination of the western arm of the East African Rift System. Its surface exposures were first studied by Wayland [1] and Pickford et al.[2] among others. Pickford et al. [2] especially developed the basic stratigraphic framework of the graben which was later modified by the government geoscientists and international oil companies using subsurface data. However, the stratigraphic units were not fully and formally described, and have been used informally in different and often confusing ways. The current study therefore aims to solve this challenge by establishing a coherent stratigraphic scheme for the entire graben through an integral study of surface and subsurface data.The study involves precise description of the type and reference sections for various formations both in exposure and wells; and has therefore led to the development of lithostratigraphic columns of different basins in the graben. The approach reveals that the Semliki area, south of Lake Albert, has the most complete sedimentary succession in the graben, spanning the period from middle Miocene (ca 15 Ma) to Recent. It also reveals that platform deposits, which form a small fraction of the thickness of the basinal succession, represent a highly condensed sequence which only saw deposition at times of Lake highstand.

An exceptional sedimentary record of initial rifting on the East African Plateau in the Miocene: Lessons from depositional cyclicity and palaeoenvironmental proxies

“…Hence, there is an uncertainty about the occurrence of the Nyaburogo Formation at the top of the road cut. On the other hand, some change in petrophysical data can be recognised at this boundary (Figure 7) as described from well logs in the area by Lukaye et al (2016). If the topmost part of the succession is classified entirely as Oluka Formation its minimum thickness would be 26 m. However, if the topmost 20 m of the road cut is classified after Lukaye et al (2016) as Nyaburogo Formation, the Oluka Formation shows a thickness of only 6 m. Basically, this model is possible, because significant erosion occurs between the Oluka and Kakara formations.…”

“…In conclusion, the changing lithofacies composition, proportion and stacking of architectural elements and evolution F I G U R E 6 Depositional environments 1-5 from Figure 5 with sketched depositional model, their typical stacking patterns and frequency of lithofacies types (abbreviations explained in Tables 1 and 2) related to the compiled road cut section. At right the revised stratigraphic assignment to formations according to Lukaye et al (2016). Ages are from Simon et al (2017) and references therein.…”

“…The lithostratigraphy of the region has been constrained using the suggestions of Lukaye et al (2016) and Simon et al (2017) with the authors relying on the findings of the national stratigraphic commission of Uganda in outcrops and wells from oil exploration in the vicinity. Figure 6 correlates the row of depositional environments to this lithostratigraphy for the road cut.…”

“…Ages are from Simon et al (2017). Lukaye et al (2016) suggest in their revised stratigraphy that the uppermost part of the outcrop already belongs to the Nyaburogo Formation. A clear indication for that boundary should be a silicified sandstone layer.…”

“…For interpretation see text. Stratigraphic assignment to formations according to Lukaye et al (2016) with ages from Simon et al (2017).…”