DISCOVERY OF FOSSIL SILICEOUS CHIMNEYS IN THE ALLOLS GEOTHERMAL SITES, SE AFAR RIFT (DJIBOUTI)

The Republic of Djibouti is located in the Southeastern part of the Afar Rift, where volcanic and tectonic activities occur since 30 my. Numerous fossil and recent hydrothermal activities are identified mostly in the Western part of the country. Most of the hydrothermal edifices encountered in Djibouti rift system are carbonate chimneys (e.g. Lake Asal and Lake Abhé). Recent field investigations in the Allols (Harralol, Haralé Awda and Sakallol) geothermal areas show the presence of fossil siliceous chimney deposits up to 15 m high formed sub-aqueously in the lake water body at a higher water level than observed today. These chimneys are distributed along major faults and consist of breccias and tall spires partly stained by iron-oxides.

The Allols areas are located in the North-Western region of the Asal Rift. The physiographies of the basins are dominated by NW-SE fault scarps which may exceed 500 m in height. These basins are characterized by Quaternary sediments and hydrothermal constructions. In these areas, the hydrothermal calcite edifices, several meters high are located in the vicinity of still active thermal springs, in veneer crowning basaltic escarpment. These deposits are sometimes overlapped by aquatic or aerial basaltic flows which reflect a volcanic activity during and after lacustrine high stand. The distribution of active hydrothermal manifestations show strong control by structural fabric and hydrogeologic setting with most hot springs found at the bottom of the main escarpment. The discovery of fossil siliceous chimneys in a recent-active continental rift system supplies new insights about hydrothermal processes associated with volcanic activity in a spreading context.