Age estimation of the deposition of the cretaceous Upper Sandstones and a later remagnetization event, from Hirna area, SE Ethiopia

Outcrops of the Cretaceous Upper sandstone formation some 375 km to the East of Addis Ababa on the motor Highway to Harar was paleomagnetically investigated. About seventy core samples were collected at various stratigraphic levels from 250–300 meters thick sedimentary formation. After standard sample preparations in the laboratory the resulting specimens were subjected to routine paleomagnetic demagnetization protocol. In the first steps of demagnetizations process the recent and viscous magnetizations were removed by heating until a temperature of level of 300 °C. Further demagnetization of the samples resulted in the isolation of the final magnetization with stable line segments that is directed towards the origin, which is interpreted as Characteristic Remanent Magnetization (ChRM). Rock – magnetic experiments have identified goethite (αFeOOH), hematite (αFe2O3), detritial hematite, and magnetite as the magnetic mineral phases carrying the remanence. The ChRM identified resulted in an average value of (Ds = 0.5°, Is = −0.7°, α95 = 4.3°, N = 34) for the red sandstones while an average value of (Ds = 335.8°, Is = −31.8°, α95 = 4.7°, N = 14) for the limestone intercalations. The former ChRM in the red sandstone is determined to be secondary while the latter ChRM is known to be primary. Comparison of these directional results and their pole equivalents with the African plate Apparent Polar Wander Path curve established by Besse and Courtillot (2003) give ages of between 115–130 Million years for the limestone intercalation and ages of 30 million years for red sandstone unit. These are interpreted respectively as estimates of the age of deposition and a later remagnetization respectively.

► The age of the Upper Sandstone unit in the Ethiopian Mesozoic sequence is problematic.
► Paleomagnetism of this unit with a limestone intercalation resulted in age determinations.
► Two paleomagnetic directions were identified.
► The first one found within the red sandstones is a remagnetization.
► The second and primary magnetization is dated 115–130 Ma.