Khao Sok Geology, Part 1: A Natural History

Posted on: August 21st, 2013 by bodhi

Morning mist in Khao Sok

Khao Sok’s relevant geologic history begins approximately 345 million years ago, during the Carboniferous period.  At this time, the Shan Thai craton (a craton is an extremely old chunk of land that has been relatively stable) was actively eroding.  Ancient mountains of the craton’s interior were slowly transferring their mass, via rivers carrying off gravels, sands, silts and mud, onto the continental shelf of the craton.  This was similar to the modern day process of major river deltas (like the Mississippi or the Nile) extending out into the sea from the land.  The sediments would accumulate on the edge of the shelf, just above a deep ocean basin.  Every so often, the sediment pile would catastrophically collapse into the basin, sometimes triggered by earthquakes, to form chaotic deposits known as turbidites.

 

Over the ensuing 65 million years, the deep basin slowly filled up with sediments from the Shin Thai craton’s eroded mountains, becoming a shallow sea. These sediments became mudstones, siltstones and sandstones known as the Kaeng Krachan Formation. The craton was at tropical latitudes during this time (and has been ever since, an important factor in Khao Sok’s unique environment).  The combination of a now shallow sea with warm tropical waters created favorable conditions for corals and other calcareous (meaning derived from calcium carbonate) marine organisms. Interestingly, calcium carbonate (CaCO3) only attains solid form in warm shallow waters, due to its unique property of becoming more soluble with decreasing water temperature.  The ocean gets colder as it gets deeper, and therefore CaCO3 tends to dissolve in deeper water.  There are other factors at play as well, but this is a good simple explanation of the phenomena.

Khao Sok Geology formations

The next 55 million years (from approximately 280 million years ago to 225 million years ago) witnessed the creation of a vast coral reef that stretchedthroughout Southeast Asia.  In fact, all the famous karsts of this region, from China’s Wulong Karst to Vietnam’s Hua Long Bay to Gunung Mulu in Borneo, are derived from this ancient coral reef.  In peninsular Thailand, this reef became the source for the Ratburi Limestone, of which the karsts of Khao Sok are composed.

Eventually, the reef building period came to an end, and the accumulated deposits of calcium carbonate began to be buried with sediments from nearby land masses.  Seawater from the ocean above infiltrated some parts of the deposit, leading to a process known as dolomitization, where calcium is replaced by magnesium.  Over time, the overlying mass of sediment became heavy enough that the carbonate underwent lithification, meaning that it turned into solid rock, in this case limestone.

Around 136 million years ago, during the Cretaceous period, the Indian Ocean plate dove underneath the Shan Thai craton.  The subduction of the oceanic crust brought the crust deep down towards the mantle, allowing a complex melting process to occur.  The resulting magma was of a granitic composition, and gradually migrated up towards the surface.  Eventually, these granitic intrusions came into contact with the sediments (Kaeng Krachan Formation) beneath the limestone.  This produced contact metamorphism, creating minerals such as tin and tungsten in the metamorphosed contact zones.  Like the magnesium created by dolomitization, these minerals became economically important in the modern era and have created the impetus for exploitation.

Karsts, a prominent feature of Khao Sok GeologyThe next major event in Khao Sok geology, which continues today, was the earth-changing collision of the Indian sub-continent with Eurasia, beginning around 55 million years ago.  The collision of these tectonic plates has altered and affected land forms far away from the immediate contact area. Both plates are composed of continental crust, which is relatively buoyant, as opposed to oceanic crust, which is relatively dense.  This means that subduction cannot occur; neither plate will subduct underneath the other.  The result is a terrific collision of land masses, and resulting deformation of landforms near and far.

The Himalaya mountain range, and the Tibetan Plateau behind it, are the obvious results of this collision.  But Southeast Asia was dramatically affected as well.  As the Indian plate drove into the underside of Eurasia, Thailand and its neighbors were pushed to the southeast, rotated clockwise and compressed.  The compressional forces folded ductile, flexible rock units, whereas it faulted brittle rock units.  This can be seen today in smoothly folded sedimentary units contrasting with sharply faulted limestone.  The compression also uplifted the Shan Thai craton, slowly exhuming its many layers.  Uplift of layerered sediments allowed erosion, much like a deli meat slicer pushes the roast beef into the slicing blade. The resultant processes of the Indian collision created the morphology, or physical form, of Khao Sok geology we see today.

Modern geologic processes continue unabated.  The Indian plate continues to push into Eurasia, so compression, rotation, and movement continue in Southeast Asia.  The tortured reactions of subsurface rock to deformation, whether by folding, faulting or metamorphosing, continue to be revealed as these rocks rise to the surface. Erosion is always acting, inexorably whittling down any exposed land forms.    The karst landscape reacts in a unique manner to these forces.  This will be explored in more depth in “Khao Sok Natural History – Karsts”.

Cheow Lan Lake, Khao Sok National Park

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