Quaternary travertine deposits,AI-Fatha area, Iraq

Abstract

Travertine crusts of Al-Fat'ha area, Iraq, are situated in a mountainous and tectonically-active area overlying a major subsurface fault. The deposits display morphological and petrographic features resembling those described from North America. Calcite spar crystals, comprising the bulk of these accumulations, are elongate and columnar resulting from aggrading neomorphism. Blue-green algae fila-ments obtained from these crusts clearly belong to two separate genera (Phormidium and Schizothrix). However, the most striking feature of these travertines is the presence of microtube-rich calcite crystals primarily occurring in the dark layers. The precursers of these microtubes are bacterial rods approxi-mately 0.3 !-Imin diameter. The coexistence of bacteria and the blue-green algae Schizothrix in the dark layers, strongly indicate that the latter must have adapted to living in sulfide-rich water. Travertine crusts, therefore, are not restricted to forming in normal non sulfide-rich waters, as has been assumed, but can form in harsh (sulfide-rich water) environments as well. In addition, the precipitation of either dark or light crusts could have been the result of the changing H2S content in water.