M. Feseha1, G. Kocurek1, J.W. Kappelman2, P. Copeland3
(1) University of Texas at Austin, Austin, TX
(2) University of Texas, Austin, TX
(3) University of Houston, Houston, TX
ABSTRACT: Sequence Stratigraphy, Petrography and Geochronology of the Oligocene Chilga Rift Basin Sediments, Northwest Ethiopia
The Oligocene Chilga sedimentary basin, with an approximate areal extent of 12 km east/west by 23 km north/south, is located in the northwestern part of Ethiopia between 12°25' - 12°39' latitude and 37°03' - 37°11' longitude. Our fieldwork in the fall of 1998 and spring of 1999 has documented stratigraphic exposures, some of them up to 120m thick, in the Chilga basin. The most common lithologic types in this area include extensive but thin beds of lignite, indurated volcanic ashes organic-rich silty sands, and thick, homogeneous, laterally traceable silty clays.
The presence of lignite beds in association with volcanic ash marker beds in the Chilga basin is particularly important for correlating stratigraphy in the region. Sedimentary rock samples were collected for petrographic and x-ray diffraction (XRD) analysis. Sediment and basaltic samples were collected for paleomagnetic and radioisotopic age dating.
Petrographic and XRD analysis results indicate that plagioclase feldspars, potassium feldspars, and volcanic rock fragments are the dominant framework grains in the Chilga sedimentary succession. Siderite is the most dominant authigenic cement. The presence of siderite cement and stratigraphic relationships suggest that Chilga sediments were deposited in alluvial-lacustrine environment.
Radioisotopic age dating of the basal volcanic rocks in Chilga yielded an age of 32.4 +/- 1.6 Ma. Similarly, volcanic tuff samples from the Chilga basin sediments have been dated at 27.36 +/- 0.11 Ma. Paleomagnetic constraints indicate that sediments in the Chilga basin were deposited between 27 - 28 Ma.
The measured and described Chilga sections have been correlated with in the basin. Using the stratigraphic relationships between the silty clays, lignites and volcanic ashes, the Chilga basin is divided into five sedimentary packages each bounded by volcanic ashes. Sequence stratigraphic interpretations suggest that climate and tectonics more than sediment supply were the major controlling factors during the Chilga basin evolution.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado