Geological History of Caves and Conservation Values of the World Nature Heritage Phong Nha-Ke Bang National Park

Abstract

The long development history and geological diversity of the Phong Nha-Ke Bang area is shown by the rich and diverse lithological composition of the stratigraphic units ranging in age from Devonian to Present day: granite, terrigenous rock, thin-bedded limestone, and massive limestone. The geomorphology of the national park includes limestone mountain terrain alternating with terrigenous rock mountain terrain surrounding the karst valley system. The formation of the cave is an endogenous-exogenous geological process that occurs according to the cycle of global sea level change. The tectonic activity is the mother that gives birth to the caves of different heights and ages. The higher the cave, the older it is, and the oldest cave is 32 million year old-Khe Ry cave. The four fault systems of Northeast - Southwest (NE-SW), Northwest - Southeast (NW-SE), West - East (WE), and North - South (NS) create four cave systems that are deep faults with strong destructive intensity. Exogenous geological activities in the cave have created a uniquely beautiful landscape including three simultaneous processes: i) Chemical washing of driftwood,  mechanical abrasion, polishing of the cave walls, and ceiling caused by travertine floods pouring into it from outside the cave; ii) Precipitation forming stalactites from the ceiling of the cave and precipitation forming stalagmites from the floor of the cave slowly over millions of years; and iii) Travertine sedimentation on the cave floor resulting in many unique sedimentary bodies, specifically filling the rough terrain of limestone blocks, covering the ancient stalactites falling from the cave ceiling, forming spiral mushroom island in the Son Doong cave, and creating gem pebbles located in fan-shaped sunken cells of the cave.