Audiomagnetotelluric soundings on the basement-sedimentary transition zone around the eastern margin of the Douala Basin in Cameroon

Abstract

Audiomagnetotelluric (AMT) soundings were made on two profiles traversing a metamorphic basement-sedimentary transition zone on the eastern margin of the Douala basin in southern Cameroon. The sounding curves and pseudosections of both profiles suggest the existence of two blocks of different lithologies. Also the interpretation of the AMT data reveals a distinct electrical conductivity discontinuity on both profiles for each of the two orientations of the telluric line. This discontinuity marks the metamorphic basement-sedimentary contact. On one profile, some stations correlate well with each other, but not with one of them which presents singular characteristics. There is evidence of a conducting layer in the entire sedimentary section (5 Ohm.m resistivity) of thickness about 100 metres and the variability of its depth can help us to evaluate the dip of the sedimentary deposits. A close examination of the transition zone suggests vertical tectonic movements of several hundred metres. Using the base of the supposed basal sandstone layer as a reference horizon, we have estimated: the dip of the sedimentary syncline to be 5%; the top of sandstone layer to have a dip of 3%; the thickness of the basal sandstone to vary between 500 to 850 m and calculated the total thickness of the sedimentary deposits to be about 1100 m. The high electrical conductivity in the sedimentary section demands further soundings in the lower frequency range (10 −2 to 10 Hz) required in deep MT exploration work.