formations In 2000,
UNESCO's World Heritage Committee inscribed Hạ Long Bay in the World Heritage List according to its outstanding examples representing major stages of the Earth's history and its original limestone karstic geomorphologic features. Hạ Long Bay and its adjacent areas consist of a part of the Sino-Vietnamese composite terrane having its development history from pre-Cambrian up to present day. During Phanerozoic, terrigenous,
volcanogenic and
cherty-carbonate sediments containing in abundance
graptolites, bivalves,
brachiopods, fishes,
foraminiferans, corals,
radiolarias, and flora, separated from one from another by 10 stratigraphic gaps, but the boundary between
Devonian and
Carboniferous has been considered as continuous. The limestone karstic geomorphology of the bay has developed since the
Miocene, especially the cone-shaped hills (fengcong), or isolated high limestone
karst towers (fenglin) with many remnants of old phreatic caves, old karstic foot caves, and marine notch caves forming magnificent limestone karst landforms unique to the world. The
Quaternary geology was developed through 5 cycles with the intercalation of marine and continental environments. The present Hạ Long Bay, in fact, appeared after the
Middle Holocene maximum transgression, leaving ultimate zone of lateral undercutting in the limestone cliffs bearing many shells of
oysters, having the 14C age as 2280 to >40,000 y. BP. Geological resources are abundant, including
anthracite, petroleum,
lignite, phosphate,
oil shale, limestone and cement additives,
kaolin, silica sand,
dolomite, quartzite of exogenous origin,
antimony, and
mercury of hydrothermal origin. Additionally, there is surface water, groundwater and thermal mineral water on the shore of the Hạ Long – Bái Tử Long Bays, as well as other environmental resources. In terms of marine geology, this area is recorded as an especially coastal sedimentary environment. In the alkaline seawater environment, the chemical
denudation process of
calcium carbonate proceeds rapidly, creating wide, strangely shaped marine notches. The bottom surface sediments are various from clay mud to sand, however, silty mud and clay mud dominate in distribution. Especially, the carbonate materials originated from organisms make up 60 to 65% sedimentary content. The surface sediments of coral reefs are mainly sand and pebbles, of which the carbonate materials account for more than 90%. The intertidal zone sediments are various, from clay mud to sand and gravel, depending on distinguished sedimentary environments such as mangrove marshes, tidal flats, beaches etc. At the small beaches, the sand sediments may be dominated quartz or carbonate materials. The sediment layers of the intertidal zone, the upper sea bed with a plain surface conserving ancient rivers, the systems of caves and their sediments, traces of ancient marine action forming distinctive notches, beaches and marine terraces, and mangrove swamps are important evidence of geological events and processes taking place during the Quaternary Period.
Karst geomorphology value Due to a simultaneous combination of ideal factors such as thick, pale, grey, and strong limestone layers, which are formed by fine-grained materials; hot and moist climate and slow tectonic process as a whole; Hạ Long Bay has had a complete karst evolution for 20 million years. There are many types of
karst topography in the bay, such as
karst field. Hạ Long Bay is a mature karst landscape developed during a warm, wet, tropical climate. The sequence of stages in the evolution of a karst landscape over a period of 20 million years requires a combination of several distinct elements including a massive thickness of limestone, a hot wet climate and slow overall tectonic up lift. The process of karst formation is divided into five stages, the second of which is the formation of the distinctive do line karst. This is followed by the development of fengcong karst, which can be seen in the groups of hills on Bo Hon and Dau Be Inland. These cones with sloping sides average 100m in height with the tallest exceeding 200m. Fenglin karst is characterized by steep separate towers. The hundreds of rocky islands that form the beautiful and famous landscape of the Bay are the individual towers of a classic Fenglin landscape where the intervening plains have been submerged by the sea. Most towers reach a height of between 50 and 100m with a height to width ratio of about 6. The karst dolines were flooded by the sea, becoming the abundance of lakes that lie within the limestone islands. For example, Dau Be island at the mouth of the Bay has six enclosed lakes including those of the Ba Ham lakes lying within its fengcong karst. The Bay contains examples of the landscape elements of fengcong, fenglin and karst plain. These are not separate evolutionary stages but the result of natural non-uniform processes in the denudation of a large mass of limestone. Marine erosion created the notches which in some places have been enlarged into caves. The marine notch is a feature of limestone coastline but, in Hạ Long Bay, it has created the mature landscape. Within Hạ Long Bay, the main accessible caves are the older passages that survive from the time when the karst was evolving through its various stages of fengcong and fenglin. Three main types of caves can be recognized in the limestone islands (Waltham, T. 1998): • Remnants of old
phreatic caves • Old karstic foot caves • Marine notch caves The first group consists of old
phreatic caves which include Sung Sot, Tam Cung, Lau Dai, Thien Cung, Dau Go, Hoang Long, Thien Long. Nowadays, these caves lie at various heights. Sung Sot cave is on Bo Hon Island. From its truncated entrance chambers on a ledge high on the cliff, a passage of more than 10m high and wide descends to the south. Tam Cung is a large phreatic fissure cave that developed in the bedding planes of the limestone dividing the fissure cave into three chambers. Lau Dai is a cave with a complex of passages extending over 300 meters on the south side of Con Ngua Island. Thien Cung and Dau Go are remnants of the same old cave system. They both survive in the northern part of Dau Go Island at between 20 and 50m above sea level. Thien Cung has one large chamber more than 100m long, blocked at its ends and almost subdivided into smaller chambers by a massive wall of stalactites and stalagmites. Dau Go is a single large tunnel descending along a major set of fractures to a massive choke. The second group of caves is the old karstic foot caves which include Trinh Lu, Bo Nau, Tien Ong and Trong caves. Foot caves are a ubiquitous feature of karst landscapes which have reached a stage of widespread lateral undercutting at base level. They may extend back into maze caves of stream caves draining from larger cave systems within the limestone. They are distinguished by the main elements of their passages being close to the horizontal and are commonly related to denuded or accumulated terraces at the old base levels. Trinh Nu, which is one of the larger foot caves in Hạ Long Bay with its ceiling at about 12m above sea level and about 80m in length, was developed in multiple stages. Bo Nau, a horizontal cave containing old stalactite deposits, cuts across the 25o dip of the bedding plane. The third group is the marine notch caves which're a special feature of the karst of Hạ Long Bay. The dissolution process of sea water acting on the limestone and erosion by wave action creates notches at the base of the cliffs. In advantageous conditions, dissolution of the limestone allows the cliff notches to be steadily deepened and extended into caves. Many of these at sea level extend right through the limestone hills into drowned dolinas which are now tidal lakes. A distinguishing feature of marine notch caves is an absolutely smooth and horizontal ceiling cut through the limestone. Some marine notch caves had not been not formed at present sea level, but old sea levels related to sea level changes in Holocene transgression, event to Pleistocene sea levels. Some of them preserved the development of old karstic foot caves in mainland environments or preserved the remnants of older phreatic caves. One of the most unusual features of Hạ Long Bay is the Bo Ham lake group of hidden lakes and their connecting tunnel – notch caves in Dau Be Island. From the island's perimeter cliff a cave, 10m wide at water level and curving so that it is almost completely dark, extends about 150m to Lake 1. Luon Cave is on Bo Hon Island and extends 50m meters to an enclosed tidal lake. It has a massive stalactite hanging 2m down and truncated at the modern tidal level. It has passed through many stages in its formation. The karst landscape of Hạ Long Bay is of international significance and of fundamental importance to the science of geomorphology. The fenglin tower karst, which is the type present in much of Hạ Long Bay, is the most extreme form of limestone landscape development. If these karst landscapes are broadly compared in terms of their height, steepness and number of their limestone towers, Hạ Long Bay is probably second in the entire world only to
Yangshuo, in China. However, Hạ Long Bay has also been invaded by the sea so that the geomorphology of its limestone islands are, at least in part, the consequence of marine erosion. The marine invasion distinguishes Hạ Long Bay and makes it unique in the world. There are other areas of submerged karst towers which were invaded by the sea, but none is as extensive as Hạ Long Bay.
Timeline of geologic evolution Some of the most remarkable geological events in Hạ Long Bay's history have occurred in the last 1,000 years, include the advance of the sea, the raising of the bay area, strong erosion that has formed coral, and, pure blue and heavily salted water. This process of erosion by seawater has deeply engraved the stone, contributing to its fantastic beauty. Present-day Hạ Long Bay is the result of this long process of geological evolution that has been influenced by so many factors. Due to all these factors, tourists visiting Hạ Long Bay are not only treated to one of the natural wonders of the world, but also to a precious geological museum that has been naturally preserved in the open air for the last 300 million years. ==Ecology==