The Geological and Geomorphological Value of Ha Long Bay

The geological history of Ha Long Bay is a journey spanning more than 500 million years, recorded through clastic and carbonate sedimentary formations dating from the Paleozoic to the Cenozoic eras. These rock layers are often likened to a gigantic “stone book,” documenting the great geological events in Earth’s history.

Paleozoic Sedimentary Formations (from about 500 million years ago)

Ha Long Bay contains numerous sedimentary formations bearing fossil remains, providing important evidence of the development and evolution of life on Earth.

• Ordovician–Silurian Period (488–416 million years ago):
  During this time, the Ha Long area was a deep-sea environment, with corresponding marine sediments deposited.
• Carboniferous–Permian Period (359–251 million years ago):
  • This was a stage when Ha Long Bay became a shallow sea.
  • A remarkable feature is that while many regions of the world experienced hot and humid climates that led to the formation of coal basins, Ha Long Bay, under dry and hot climatic conditions, developed an exceptionally thick limestone layer exceeding 1,200 meters.

Mesozoic Sedimentary Formations

• Triassic Period (251–199 million years ago):
  In contrast to the dry and hot conditions of the Carboniferous period in many regions, the Ha Long area during the Triassic was dominated by humid swamps. Vast forests of cycads and giant ferns were buried over successive generations, forming sedimentary layers that later became the famous coal-bearing strata of present-day Quang Ninh Province.

Cenozoic and Quaternary Sedimentary Formations

• Late Paleogene – Early Neogene (approximately 26–20 million years ago):
  The region continued to experience phases of marine transgression along the coast.
• Quaternary Period (from about 1.8 million years ago to the present):
  Sediments from this period include uplifted marine terraces and ancient river valleys that now lie beneath the floor of Ha Long Bay.
  • Holocene Stage (about 11,500 years ago):
    Sediments from this stage help clarify sea-level fluctuations associated with the development of ancient cultures such as Soi Nhu, Cai Beo, and Ha Long.

Sediments found in caves and ancient marine notches (marine notches or “sea notches”) serve as vivid evidence of geological events during this period.

The present system of limestone islands is primarily the result of chemical erosion of carbonate rocks in an alkaline marine environment, creating spectacular landforms and a rich cave system that developed between approximately 700,000 and 11,000 years ago.

Main Stages of Geological Formation

The geological evolution of Ha Long Bay can be divided into four major tectonic stages:

1. Basement Formation and Sediment Accumulation (500–250 million years ago)

• Caledonian and Hercynian tectonic movements:
  During this stage, the Ha Long region experienced multiple cycles of marine transgression and regression.
• Formation of raw material:
  Continuous accumulation of sedimentary layers (mud, sand), especially the remains of marine organisms (corals, shells), created extremely thick limestone strata (over 1,000 meters). This limestone served as the “material” later sculpted into today’s limestone islands.

2. Mountain Building (Uplift) and Folding Stage (250–65 million years ago)

• Indosinian tectonic movement:
  Strong tectonic collisions uplifted the entire region, raising it above sea level and turning it into land.
• Rock deformation:
  Originally flat limestone layers were folded, faulted, and fractured, forming vertical and horizontal cracks. These fractures acted as pathways for rainwater to infiltrate, initiating chemical dissolution (karstification) that later formed cave systems.

3. Erosion and Planation Stage (65–2 million years ago)

• Tropical climate:
  High temperatures and heavy rainfall caused intense erosion of massive limestone mountains.
• Consequences:
  The landscape gradually evolved from large limestone blocks into clusters of sharp peaks (fengcong) and isolated limestone towers (fenglin). Ancient underground cave systems also began forming within the mountains during this stage.

4. Marine Inundation and Present-Day Landscape Formation (2 million years ago – present)

• Quaternary Period (Pleistocene–Holocene):
  This is the most crucial stage shaping the present “bay” landscape of Ha Long.
• Holocene marine transgression:
  After the last ice age, rising sea levels flooded karst valleys, transforming mountain peaks into the limestone islands scattered across the bay today.
• Wave action:
  Sea waves eroded the bases of islands, forming distinctive marine notches that testify to ongoing sea-level adjustments shaping the bay’s landscape.

Typical Tropical Karst Geomorphology

The geomorphological value of Ha Long Bay is internationally recognized as an outstanding example of mature tropical karst. Its uniqueness lies in the combination of limestone dissolution by rainwater and marine erosion and inundation. Key geomorphological features include:

1. “Fengcong” and “Fenglin” Landscapes

Ha Long Bay is one of the rare places in the world where both classic karst landforms appear together in a marine-influenced environment:

• Fengcong (peak-cluster karst with enclosed depressions):
  • Characteristics: Conical limestone peaks clustered closely together, sharing a common base.
  • Distribution: Common on large islands such as Bo Hon and Dau Be. The depressions between peaks are often flooded by the sea, forming Ang or Tung.
• Fenglin (tower karst):
  • Characteristics: Isolated limestone towers rising steeply from the sea like giant pillars, with nearly vertical cliffs.

2. Ang and Tung: “Lakes Within Islands”

These are unique landforms found only in marine-influenced karst environments:

• Ang:
  Enclosed saltwater lakes located entirely within limestone islands, surrounded by steep cliffs. Water circulates with the sea through underground cave systems (e.g., Ba Ham Lake).
• Tung:
  Sheltered marine lagoons with relatively wide openings to the bay, enclosed by limestone islands, creating calm and clear waters (e.g., Tung Sau).

3. Diverse Cave Systems

Geomorphological processes have created three main types of caves that illustrate geological changes over time:

• Ancient submerged caves
• Ancient karst base-level caves
• Marine notch caves (sea caves)

Together, these features make Ha Long Bay a globally significant geological and geomorphological heritage site, vividly recording the long and complex history of Earth’s evolution.