Diary of a flood basalt: A stratigraphic tour of two sections within the Oligocene Ethiopian Traps

Thursday, 17 December 2015
Poster Hall (Moscone South)
Tyrone O Rooney1, Bodaniel L Bradley1, Susan R Krans2, John W Kappelman3, Gezahegn Yirgu4 and Dereje Ayalew4, (1)Michigan State University, East Lansing, MI, United States, (2)Michigan State University, Geological Sciences, East Lansing, MI, United States, (3)University of Texas at Austin, Anthropology, Austin, TX, United States, (4)Addis Ababa University, School of Earth Sciences, Addis Ababa, Ethiopia
Flood basalts are the most significant magmatic events on the planet, influencing our environment by modifying the lithosphere and atmosphere. Our understanding of flood basalt processes comes almost entirely from within the rock record – typically a vast pile of basaltic lavas, hiatuses, and sedimentary horizons. Stratigraphic continuity is thus a critical characteristic to constrain the processes associated with the formation of flood basalt provinces. Oceanic flood basalts are difficult to access in situ, leaving continental equivalents our primary target. The Oligocene Ethiopian Traps is among the youngest and most intact flood basalt provinces, making this a premier region to examine flood basalt stratigraphy. Leveraging recent road building, we have constructed a flow by flow stratigraphy for two ~1500m independent sections of the Low-Ti region of the Ethiopian traps, separated by ~70 km. These sections lie towards the edge of the modern exposure of the Ethiopian flood basalts, however the remarkable parallelism of flows and the lack of evidence of pinching suggests that the province was significantly more aerially extensive towards the west. For the most continuous ~1200m section, median flow thickness is about ~15m in the lower ~400m and upper ~400m of the flood basalt sequence, but flows thin significantly to a median of 4m in the central portion of this sequence. The petrography shows distinctive patterns through the pile: largely aphyric and clinopyroxene/olivine phyric flows are more common towards the base of the sequence and transition to largely plagioclase phyric basalts. At the top of the flood basalt pile a further transition from plagioclase phyric to largely aphyric flows is observed. Paleosols become more frequent and are thicker towards the top of the sequence. These observations point to a gradual temporal shift in the differentiation depths and flux of magmas entering the Ethiopian lithosphere.