Petrography and U/Th dating of travertine deposits in Bongongo (Cameroon volcanic line): Implication for paleoclimate, hydrothermalism and tectonism

Abstract

The studied travertine deposits have been analysed using sedimentology, petrography and geochronology to evaluate their resurgence variation, Neotectonic reconstruction, hydrothermal/paleohydrology, and tectonic influence for their precipitation. These deposits display a dome structure with milimetric layers containing some dicotyledonous leaf prints mainly of angiospermic plants class and animal traces. Traces of gastropods fossil and imprinted plant leaves are seen scattered all over the samples. Microscopically, the travertine reveal laminae characterising a long spike-like dendritic millimetric crystals. The spike-like structure composed of crystals calcite enclosing putative microbial filaments which are longitudinal within some places, micrite partially filling inter-crystalline voids. The petrographic characteristics of the travertine corroborated with isotopic analyses, reveals processes such temperature variation, degassing variation, fluctuations of the physico-chemical fluid and dissolution as controlling factors to the formation of the different carbonates textures. Four units representing different sample points discloses ages of 19.839 ± 4.8 ka, 50.3645 ± 9.4 ka b2k, 2.00 ± 1.8ka b2k and 5.77 ± 1.7ka b2k. Conclusions drew from the study suggest that, (1) a forest vegetation type testifying to a humid climate prevailed at the time of the travertine precipitation, (2) there were repeated reactivation of fault systems as well as episodic high hydraulic head in the aquifers underlined by U-series dating that showthe presence of different spring systems generations in the travertine deposits (3) three phases of travertinization were disclose: first phase (Upper Pleistocene, ca 50 ka b2k), marked by significant and regular rainfall leading precipitation of massive travertine deposits, second phase (Between 50 ka b2k and 19 ka b2k), marked periodicity of rainfall forming laminated travertine, third phase (beginning of the Holocene between 5.77 ± 1.7ka b2k and 2.00 ± 1.8ka b2k), marked by obstruction of the main canal of circulation inducing a possible change of exsurgence forming new deposits recorded at the base of the dome (4) the travertine precipitation was influence by hydrothermal system and recent extensional tectonic activities owing to the presence of saline springs.