Sequence stratigraphy, petrography, and geochronology of the Chilga rift basin sediments, northwest Ethiopia

This dissertation presents results of an integrated approach including geochronology, paleomagnetism, petrography, x-ray diffraction analysis, depositional environment and sequence stratigraphic interpretation of Oligoceneage continental rift sediments. It provides a convincing analysis of the sedimentary and structural processes involved in the evolution of an Oligocene rift basin and offers inferences for both modern and ancient rift basins. The Chilga rift basin is located in northwestern Ethiopia, between N 12º 25' to 12º 39' latitude and E 37º 03' to 37º 11' longitude. It is a continental rift basin with isolated outcrops of sedimentary and volcanic basin fill separated by faults related to rift processes. Sediments filling the Chilga basin are rich in volcanic ashes, lignite beds, silty sandstones, silty claystones, and vertebrate and floral fossil rich sandstones. Continental rift basins, such as the Chilga basin that contain outcrops which can be correlated with marker beds and that can be dated with a suitable dating method provide a unique opportunity in understanding the tectonic and stratigraphic evolution of rift basins. Correlation and dating of ash beds allows the tectonic and stratigraphic evolution of the Chilga basin to be determined. Radioisotopic and paleomagnetic age dating techniques were used to constrain the age of the basin filling sediments and the underlying basaltic rocks. Results indicate that the Chilga sediments were deposited between 27 – 28.5 Ma. To interpret the depositional environment, the fine-grained Chilga sediments were analyzed using petrographic and x-ray diffraction analysis. Results of the analyses indicate that the Chilga sediments are composed of framework grains of mainly unaltered plagioclase, potassium feldspars, and volcanic rock fragments. The matrix is commonly enriched in kaolinite and illitemontmorillonite and the cement is mainly composed of authigenic siderite and some iron oxide coatings. Interpretations of these characteristics suggest that the Chilga sediments were deposited in alluvial-lacustrine environment with a nearby fine-grained sediment source and indicate a reducing diagenetic environment. A sequence stratigraphic interpretation of the Chilga basin sediments is inferred from the vertical and lateral stacking pattern of stratigraphic units. Key units that were used for the sequence stratigraphic interpretation of the Chilga basin were siderite-cemented silty claystones, which indicate a reducing condition and an increase in lake water depth, and channel incisions and lignite beds, which indicate lake shallowing. The Chilga sedimentary basin records three 4th order sedimentary cycles formed by the rise and fall of the Chilga Lake throughout the course of basin evolution. The identification of normal faults that cut the oldest Chilga sediments, tectonic growth faults, and fault sets that cut the entire Chilga sediment succession indicate that the Chilga basin was tectonically active both during and after sediment deposition. Regional correlation of the isolated Chilga outcrops using ash beds combined with structural interpretations indicate that the Chilga basin started with two sub-basins bounded by a paleo-high. Later, regional tectonic subsidence eliminated the marginal paleo-highs and caused the enlargement of the Chilga basin. Later stages of uplifting, faulting, and erosion have resulted in the formation of the existing, isolated Chilga outcrops.