Control of cave levels in Kanaan, Kassarat and Jeita karst systems (Central Mount Lebanon, Lebanon)

摘要

Mount Lebanon is a mountainous Mediterranean karst ridge, consisting of high plateaus reaching 3,088 m altitude and entrenched by deep valleys. Fluvial drainage systems run on a steep gradient and reach the Mediterranean Sea coast in less than 30 km distance. In Central Mount Lebanon, the downstream parts of Antelias and Kalb valleys host the Kanaan cave (160 m), the Kassarat cave (4.6 km) and the Jeita cave (10.05 km), whose subterranean rivers are connected to the karst springs located close to the contact between the Cretaceous aquidude and the Jurassic karst aquifer. The area thus comprises a dammed Jurassic aquifer in which all the studied cave levels developed at certain altitudes. A geomorphological study performed by speleological and geomorphological surveys and completed by analyses of cave sediments allowed the formulation of a first evolutionary framework of the karst system. Several features suggest a long-term and multi-stage evolution of the karst in both Antelias and Kalb valleys. The horizontal caves develop between 60 and 160 m a.s.l. and constitute water table caves in a steeply dipping structure (Mount Lebanon's western fold). These cave levels are part of an ancient system that developed near the water table controlled by the position of the aquiclude and correspond to the base level of a past fluvial network. An important phase of base level cave development is documented in the relict passages above 90 m a.s.l. Speleogenetic features between 90 and 160 m developed in a phreatic environment where paragenesis or "per ascensum erosion" processes occurred during base level rise. Epiphreatic features were identified at 60-70 m in Kassarat and Jeita caves. Typical morphological features such as horizontal solution passages and solution notches suggest a phase of stillstand of the base-level. The evolution of the water table level from a paragenetic rise above 90 m to a continuous lowering below 90 m and with a temporary stillstand phase at 70-60 m, is related to the fluvial base-level evolution. Preliminar chronological constraints for the evolution of the karst system are discussed based on U/Th dating of speleothems and cave levels (90-160 and 60-70 m) and their relationship with the altitude of the low-gradient surfaces. The cave levels between 90 and 160 m are related to a water table rise that caused an alluviation process at the river's mouth probably during the Pliocene transgression phase of the Mediterranean Sea (PHS). Abandoning of the upper cave levels (90-160 and 60-70 m) is the result of a continuous lowering of the water table related to fluvial incision. The lowering of the water table level below 90 m is mainly related to the fluvial incision of the acquiclude in a high geodynamic context (Mount Lebanon uplift) during the Pleistocene.