Les réseaux souterrains de la région de Lébamba (Gabori), 1987, Peyrot B. , Massala J.

CONTRIBUTION TO THE STUDY OF KARSTIC SYSTEMS OF THE LEBAMBA REGION (GABON) - The Lebamba region is situated in the extreme southern part of Gabon close to the Republic of Congo. Geologically, it can be included within the Niari-Nyanga synclinorium, where the Precambrian schists and limestones show many karstic features and particularly a remarkable subterranean network, as to yet only partially surveyed. The dolomitic limestone of Lebamba, although of little importance, contains many cave systems: Lebamba, Bongolo, Mbenaltembe. A severe tectonic mesh together with fine micro-stratifications have opened the way to biochemical corrosion which, after observations and analysis, appears to result from a climate wetter than at present. The dimensions of the underground passages appear disproportionate, relatively to the low aggressivity of the present environment, thus illustrating the importance of paleoclimatic factors on the morphogenesis of central african karsts.

Sedimentary manganese metallogenesis in response to the evolution of the Earth system, 2006, Roy Supriya,

The concentration of manganese in solution and its precipitation in inorganic systems are primarily redox-controlled, guided by several Earth processes most of which were tectonically induced. The Early Archean atmosphere-hydrosphere system was extremely O2-deficient. Thus, the very high mantle heat flux producing superplumes, severe outgassing and high-temperature hydrothermal activity introduced substantial Mn2 in anoxic oceans but prevented its precipitation. During the Late Archean, centered at ca. 2.75[no-break space]Ga, the introduction of Photosystem II and decrease of the oxygen sinks led to a limited buildup of surface O2-content locally, initiating modest deposition of manganese in shallow basin-margin oxygenated niches (e.g., deposits in India and Brazil). Rapid burial of organic matter, decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the Archean-Proterozoic transition. Concurrently, a massive drawdown of atmospheric CO2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused Paleoproterozoic glaciations (2.45-2.22[no-break space]Ga). The spectacular sedimentary manganese deposits (at ca. 2.4[no-break space]Ga) of Transvaal Supergroup, South Africa, were formed by oxidation of hydrothermally derived Mn2 transferred from a stratified ocean to the continental shelf by transgression. Episodes of increased burial rate of organic matter during ca. 2.4 and 2.06[no-break space]Ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere. Black shale-hosted Mn carbonate deposits in the Birimian sequence (ca. 2.3-2.0[no-break space]Ga), West Africa, its equivalents in South America and those in the Francevillian sequence (ca. 2.2-2.1[no-break space]Ga), Gabon are correlatable to this period. Tectonically forced doming-up, attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric CO2 drawdown causing glaciation on the Neoproterozoic Rodinia supercontinent. Tectonic rifting and mantle outgassing led to deglaciation. Dissolved Mn2 and Fe2 concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation. During the Sturtian glacial-interglacial event (ca. 750-700[no-break space]Ma), interstratified Mn oxide and BIF deposits of Damara sequence, Namibia, was formed. The Varangian ([identical to] Marinoan; ca. 600[no-break space]Ma) cryogenic event produced Mn oxide and BIF deposits at Urucum, Jacadigo Group, Brazil. The Datangpo interglacial sequence, South China (Liantuo-Nantuo [identical to] Varangian event) contains black shale-hosted Mn carbonate deposits. The Early Paleozoic witnessed several glacioeustatic sea level changes producing small Mn carbonate deposits of Tiantaishan (Early Cambrian) and Taojiang (Mid-Ordovician) in black shale sequences, China, and the major Mn oxide-carbonate deposits of Karadzhal-type, Central Kazakhstan (Late Devonian). The Mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans. During the Early Jurassic OAE, organic-rich sediments host many Mn carbonate deposits in Europe (e.g., Urkut, Hungary) in black shale sequences. The Late Jurassic giant Mn Carbonate deposit at Molango, Mexico, was also genetically related to sea level change. Mn carbonates were always derived from Mn oxyhydroxides during early diagenesis. Large Mn oxide deposits of Cretaceous age at Groote Eylandt, Australia and Imini-Tasdremt, Morocco, were also formed during transgression-regression in greenhouse climate. The Early Oligocene giant Mn oxide-carbonate deposit of Chiatura (Georgia) and Nikopol (Ukraine) were developed in a similar situation. Thereafter, manganese sedimentation was entirely shifted to the deep seafloor and since ca. 15[no-break space]Ma B.P. was climatically controlled (glaciation-deglaciation) assisted by oxygenated polar bottom currents (AABW, NADW). The changes in climate and the sea level were mainly tectonically forced

THE UNDERGROUND KARST OF THE NEOPROTEROZOC SERIES OF NIARI-NYANGA (CONGO AND GABON). A KARSTOGENESIS CONTROLLED BY ENVIRONMENTAL CHANGES, 2012, Peyrot, Bernard

The area of Niari-Nyanga, divided between Congo-Brazzaville and Gabon, corresponds to a Neoproterozoic synclinal whose schisto-limestones and dolomitic layers shelter many caves as well as vast underground karst systems that are hardly known. In a sub-equatorial climate characterized by sparse rainfall with very variable intensity, a dry season of five months and a savanna environment, the endokarst presents a vast array of forms with sometimes large, sometimes small dimensions. The caves are mostly horizontal and oriented along tectonic lines. Old fossil perched caves contrast with epiphreatic caves and drowned inaccessible systems. In current bioclimatic conditions, corrosion seems not very effective and not in equilibrium with some vast morphologies. The stacked levels, the presence of fossil speleothems and detritic material suggest a polyphase genesis in link with important paleoclimatic changes, where humid and dry periods alternate. Recent age dating with 14C on stalagmites show that the speleothemes are holocene and grew during the important rainfalls of this time, before drying up at the general chlimate change 3000 years before present. Thus, the endokarst of Niari-Nyanga as well as its neighbours is an archive of large importance.