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The Shungura and Usno Formations

The Shungura and underlying Usno Formations provide the most continuous record of Plio-Pleistocene sedimentation in the Turkana Basin. Spanning from about 4.2 to about 1.1 MY, these strata provide a Rosetta Stone representing thousands of superimposed landscapes punctuated by 18 dated volcanic ashes. The local magnetic polarity sequence has been correlated to the world-wide magnetic polarity time scale, providing additional independent time control. Based on interpolation, the age of most beds and fossils can be deduced to within a few tens of thousands of years.

The Shungura strata are documented by 34 sometimes overlapping columnar sections (de Heinzelin and Haesaerts, 1983, figures 6-50, p. 34-127). Many schematic compilations of these sections have been published. Kidane et al., (2007, 2014) compiled 632 m of strata ranging from the base up to the middle of member H and published a second-generation paleomagnetic data set. I have used their thickness patterns and completed the column with measurements from de Heinzelin and Haesaerts, 1983 and paleomagnetic data from Brown, Shuey and Croes, 1978. Close inspection of the original sections and the various compilations (including those of Kidane et al. 2007, 2014) reveal minor discrepancies of individual unit thickness. These variations are a function of on-the-ground differences where the same interval is measured several places (de Heinzelin and Haesaerts, 1983) where thickness variations can be on the order of 2-4 meters. The overall compiled package, comprised of hundreds of fluvial cycles, is considered to be representative and reproducable. Sedimentation rates are slower in the Kalam area (about 70%) than in the type Shungura, due to the more axial position of the thicker type area.

Stratigraphic sections in the Koobi Fora and Nachukui formations, strata from Ethiopia at Konso and Hadar, and from deep marine sediments in the Gulf of Aden and Indian Ocean may be referenced to this diagram.

References:

Brown, F.H., Shuey, R.T., and Croes, M.K., 1978, Magnetostratigraphy of the Shungura and Usno formations, southwestern Ethiopia: New data and comprehensive re-analysis; Geophysical Journal of the Royal Astronomical Society, v.54, p. 519-538.  

de Heinzelin, J., editor, 1983, The Omo Group; Tervuren: Musee Royal de l’Afrique Central, Annales, Serie in 80, Sciences Geologiques, v. 85, 365 pp. This two volume publication includes geological maps and is cited as the source of the term Omo Group.

de Heinzelin, J., and Haesaerts, P., 1983, The Mursi Formation, in: de Heinzelin, J., ed., The Omo Group, Musee Royal de l’Afrique Central, Tervuren; annals serie 8, Sciences Geologiques, v. 85, p. 141-147.

Kidane, T., Otofuji, Y-I., Brown, F.H., Takemoto, K., and Eshete, G., 2007, Two normal plaeomagnetic polarity intervals in the lower Matuyama Chron recorded in the Shungura Formation (Omo Valley, southwest Ethiopia); Earth and Planetary Science Letters, v. 262, p. 240-256. 

Kidane, T., Brown, F.H., and Kidney, C., 2014, Magnetostratigraphy of the fossil-rich Shungura Formation, southwest Ethiopia; Journal of African Earth Sciences, v. 97, p. 207-223.

Bob Raynolds, August 2022

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