A Multichannel Seismic Transect of the Somalian Continental Margin
Authors: Coffin Millard F. , Rabinowitz Philip D. 
Published: October 2008

R/V VEMA cruise 3618 multi-channel seismic line 84 combined with DSDP Site 241 drilling results documents important events in the sedimentary history of the southeastern Somaliland-northeastern Kenyan continental margin. Prominent diapers of presumed evaporate origin appear near the landward end of the profile. These evaporates were deposited in Early to Middle Jurassic time during the rift and early drift stages of the separation between Madagascar and Africa. A zone of chaotic reflectors, primarily confined to the Upper Cretaceous/Paleocene section, occurs down slope and seaward of the diapers and terminates abruptly. This zone is interpreted to be a massive sediment slide, whose seismic reflection character results from intense deformation. The slide may have been initiated by halo kinesis upslope and/or earthquake activity associated with the East African rifting. Beneath the lower continental rise seaward of the gravity slide is a large buried channel of Miocene age. This channel is likely the result of rapid sea level fluctuations and turbidity current activity initiated by East African rifting. A strong reflector is observed at ? 8 s depth near the seaward edge of the sediment slide. This reflector, which continues to the seaward end of the line and immediately overlies basement is thought to represent lime stones of Middle to Late Jurassic age.

Introduction

From November, 1980 through January, 1981, R/V VEMA cruises 3618 and 3619 were devoted to studying the East African continental margin and evolution of the western Somali Basin. Multi- and single-channel seismic reflection, son buoy reflection/refraction, gravity, magnetic and echo sounding measurements were recorded during these cruises. We will show here prominent features that we observe on one of the multi-channel seismic lines (line 84, location in Fig. 1) that demonstrate important processes affecting the passive continental margin of southeastern Somalia and northeastern Kenya. These features include diapers, a sediment slide and a deep-sea channel system (see Fig. 2). We are able to constrain the ages of the above processes by seismic correlation with a deep sea drill hole (DSDP hole 241, Leg 25)1.

The VEMA multi-channel data were recorded at 4 ms intervals by a Texas Instruments DFS IV digital acquisition system from a 1200 m long 12-channel Seismic Engineering streamer. Two synchronized Bolt Associates 466-cu. in. air guns, fired at 15 to 20 second intervals, provided the sound source. SSQ 41, 42, and 57 son buoys were deployed frequently along the lines to obtain velocity information.

Completed processing of line 84 off East Africa includes demultiplexing, common-depth-point gather, l2-channel velocity analysis, normal move-out correction, 12-fold stacking, band-pass filtering, time variable gain, disconsolation before and after stacking and, in places, wave-equation migration 2 Velocity analyses were performed using the semblance technique of Tanner and Koehler3 Input velocities for the move-out correction were also based on son buoy refraction information, especially for the deeper 1ayers.

Seismic Stratigraphic Correlations

Deep Sea Drilling Project Site 241, drilled in a water depth of 4054 meters, is located on the continental rise off northeastern Kenya-southeastern Somalia1