17 Journal of the Geological Society of Thailand No. 1, 17-22 , 2009
Evidence of Mollusk Shell Deposit, Middle Holocene Marine Regression, Wat Sai Thai, Muang, Krabi, Thailand Wickanet Songtham and Lertsin Raksaskulwong Department of Mineral Resources, Rama VI Road, Ratchathewi, Bangkok 10400, Thailand Email: firstname.lastname@example.org, email@example.com
Abstract A mollusk shell deposit at Wat Sai Thai (a Buddhist temple) has been discovered comprising several taxa of Crassostrea sp., Arca granulosa, Trochus niloticus, Strombus cf. S. canarium, and Nerita lineata and some other indeterminant species. The whole assemblage consists of marine mollusk shells occurring under the floor of a small limestone cave in the Wat Sai Thai area where people dug down to about 0.5 meters in depth for construction purpose. The mollusk shells are densely aggregated with loosely sandy clay sediment. The locality together with the nearby general topographic landforms and previous studies on Holocene sea-level history suggest that the mollusk shell deposit may be the fauna accumulated at the period of sea-level peak around Middle Holocene time, likewise the locality is inferred to be at about 4 meters above present mean sea level. The evidence is regarded as the remnant of sea-level rise prior to regression and eventually standing at the present level. The shell deposit as archaeological evidence is also discussed and debated. Keywords: marine mollusk shells, Holocene, sea-level changes.
Introduction The study area is in Wat Sai Thai (a Buddhist temple), Tambon Sai Thai, Muang, Krabi province, southern peninsula of Thailand. It is about 7 kilometers in distance on the west of Krabi town along highway 4034 and is located on the RTSD topographic map number 4725-II (Krabi sheet). Its coordination is 8º 04′ 75″ N and 98º 52′ 25″ E at about less than 20 meters above mean sea level. The mollusk shells site is on the north and a little bit shifting to the northwest margin of Khlong Chi Lat wetland. The wetland covers an area about 20 square kilometers mainly composed of mangrove forests with a main river, Khlong Chi Lat, and its tributaries flowing out to the Krabi Bay (Andaman Sea). The general landscape covering the study area and major area of Krabi province is characterized by steep, limestone head cliffs along its shoreline and by limestone karst towers both offshore from the headlands and inland along its alluvial plains. The coastal karst towers rise directly out of the shallow water or emerge from mangrove-fringed tidal flats whereas the inland karst towers are surrounded by Quaternary alluvial and colluvial deposits. The tower karst in Krabi is developed in massive Permian limestone and dolomitic limestone. These folded carbonate beds strike NE-SW and dip gently to moderately to the NW and SE (Stephen Harper). The northern-western-southwestern areas off the Khlong Chi Lat wetland are dominated by high Permian limestone mountains of both isolated peaks and group of peaks with a common rocky base
extending until the seashore on the southwest side forming high limestone cliff shores intercalated with some various sizes of gentle-slope shores where sand beaches have been developed such as Nopparat Thara Bay, Nang Bay, Ton Sai Bay, Rilay Bay (West), Phra Nang Bay, Rilay Bay (East), and Nam Mao Bay. South of the wetland is demarcated by a sand barrier in form of a slender narrow cape with about 100 – 500 meters wide and about 4 – 5 kilometers long namely “Laem Pho” as well as name of a village. Along this sand barrier crops out Tertiary rock sequences along the Andaman coast with shell beds known as the famous “75 million years old shell cemetery” site for tourism. A shell bed densely contains viviparous snails (Margarya sp.), Malaysian trumpet snails (Melanoides tuberculata), and some kinds of bivalve. There is a report on palynological study including various kinds of pollen and spores of angiosperms, gymnosperms, ferns, and algae. Freshwater fern spores, Striatriletes susannae, and algae Pediastrum spp. and Botryococcus braunii strongly suggest that the deposition of the shell beds occurred in freshwater environment under tropical rain forests in the Early Miocene (Songtham and Watanasak, 1999). On the east of Khlong Chi Lat wetland is formed by undulating terrains in form of cape from the north mainland to the south protruding into the Andaman Sea. The cape is sandwiched by Khlong Chi Lat wetland on the west and Krabi wetland on the east. The cape is about 1.5 – 3 kilometers wide and 5 – 6 kilometers long where Krabi city is situated (Fig. 1).
Wickanet Songtham, Lertsin Raksaskulwong
Fig. 1 Map showing location of Wat Sai Thai (study area) in the northwest side of Khlong Chi Lat wetland (dark grey = inland, pale grey = wetland, white = water surface)
Evidence of Mollusk Shell Deposit due to Middle Holocene Marine Regression, Wat Sai Thai, Muang, Krabi, Southern Thailand
Locality and Mollusk Shell Deposit The area of Wat Sai Thai where the shell deposit has been found is on the north to northwest margin of the Khlong Chi Lat wetland. The temple is well known and well marked by a large reclining Buddha statue with about 16 meters long making people call it another name as Wat Phra Non Yai (large reclining Buddha statue temple). The shell deposit is in a small limestone cave at the base of Khao Khong Khang (Khao means mountain) with about 137 meters high and not quite far from the Khlong Chi Lat wetland in distance. The cave has been used as a holy place by Buddhists since the temple was built about 50 years ago and later floored by a concrete lining. In May 2009, the concrete slab was broken up by digging a small shallow soil pit for construction where the mollusk shell deposit of various species was exposed (Fig. 2). It was once believed that the shell deposit at Wat Sai Thai cave was caused by human activities and it was thus regarded as an archaeological site. The about 10 centimeter thick concrete slab in the cave is in contact with the underneath shell deposit. Therefore, the shell deposit would have been observed by people at that time before the concrete lining was built. The concrete floor level in the cave is approximately one meter higher than the area outside the cave and an 11-step concrete stairs for walking up to the cave was built. Most shell deposit occurs in mixing aggregation among species of marine shells with very rare fragmented crabâ€™s chelips. The shells are abundant consisting of Crassostrea sp. and Arca granulosa, common Nerita lineata, Trochus niloticus and Strombus cf. S. canarium, and other indeterminant taxa of both gastropods and bivalves. The bivalve Arca granulosa are still commonly preserved as pairs of valves as their original natural positions. However Trochus niloticus is somewhat corroded but the color and pattern on the shell surface are still clearly visible. The assemblage of the shell is densely aggregated in dark grey to black sandy clay soil (Fig. 3).
Results The present topographic features of the area coupled with the occurrence of marine shells as well asnumerous previous studies concerning the Quaternary sea-level changes enable us to explain that during the last glacial period the sea level declined down to about 120 meters compared to the present level (Goudie, 1992). When the glacial period came to the end, more or less 10, 000 years BP, the sea level rapidly and continuously rose up to about four meters above present level at about Middle Holocene (Sinsakul, 1992). In this study, ground survey and age determination are unavailable. However, it is believed that the shell deposit at Wat Sai Thai is
probably fitted to the sea-level peak at about Middle Holocene time. Therefore the height of four meters has been regarded as the shell deposit elevation above mean sea level, and Middle Holocene as the age of the event previously described by Sinsakul (1992) in this study. During the sea-level peak at about Middle Holocene, the sea level stood at about 4 meters above the present level. At that time, the Khlong Chi Lat wetland as well as Laem Pho was fully flooded by sea water forming a bay. The shoreline was dominated by mangrove forest strips with some limestone headlands. Wat Sai Thai area including its limestone karst tower, Khao Khong Khang, situated along the shoreline and was affected by some wave actions and storms creating a cave into the base of the limestone karst tower. The cave is perhaps created by such long term wave actions in accordance with weak planes of the folded limestone cliff that dipped accordingly to the wave direction. These characteristics are comparable well to the modern limestone headlands along the shoreline of Krabi province. There are not any archaeological artifacts found in the cave and its vicinities to be convinced that the shell deposit was resulted from human activities. Some bivalve shells show pairs of valves that are still firmly enclosed together as in their natural positions. Additionally, there is no reason to elucidate that the ancient humans ate the mollusks and disposed the shells into the cave, â€œWhy did they have to do like that?â€? After the Middle Holocene onwards, the sea level has continuously regressed with some minor fluctuations and has finally been standing at the present position. The marine regression has left a remnant of wetland beyond namely Khlong Chi Lat wetland that dominated by mangroves. At present, Khlong Chi Lat wetland shows some features which depict that it was used to be a bay where Wat Sai Thai area was along the shoreline.
Discussions and Recommendations Much more extensive area of investigation is needed particularly along the ancient shoreline at such an episode of time. Ancient sea caves should occur at some ancient headlands that probably well correlated among them. Such as a nearby location of shell deposit in a foot cave at the base of a limestone headland at Ao Nang (Nang Bay), about 8 kilometers SW of Wat Sai Thai area. Harper (2005) reported that this deposit lies immediately above the modern high tide level and has been protected from erosion by its location in a foot cave and by rock fall debris in front of the foot cave. The shells yielded radiocarbon ages of 1425 and 1410 AD., the time that there is no evidence of a high sea level stand. This shell bed deposit is perhaps the result of a tsunami event or an extreme storm event that occurred around 1400 AD or perhaps neither because there is not an accurate
Wickanet Songtham, Lertsin Raksaskulwong
Fig.2 Shell assemblage beneath man-made concrete floor, Wat Sai Thai limestone cave showing shell aggregation in dark grey sandy clay soil
21 Evidence of Mollusk Shell Deposit due to Middle Holocene Marine Regression, Wat Sai Thai, Muang, Krabi, Southern Thailand
Fig. 3 Some selected taxa of mollusk shells from Wat Sai Thai limestone cave: 1 – Crassostrea sp., 16.5x12.5 cm.; 2 – Arca granulosa, 3.7x4.6 cm.; 3 - Nerita lineata, 2.8x2.0 cm.; 4 – Strombus cf. S. canarium, 4.8x3.0 cm.; 5 – Trochus niloticus, 5.5x8.0 cm.
22 Wickanet Songtham, Lertsin Raksaskulwong
historical record for this part of Thailand for this time period. Harper added that radiocarbon of the shell deposit suggests a recurrence interval of about 500 to 600 years for a large magnitude tsunami event in this region of Southeast Asia. The above is just an example to demonstrate that investigating in an extensive area and determining the ancient cave levels as well as radiocarbon ages of the shell deposits are needed for further study. However, it must be kept in mind that the caves and shell deposits could equal or differ in age but the caves could, in fact, not be younger than the shell deposits. Tectonic factors are also taken into account since it plays important role in the changes of earth surface levels. The level of the cave floor compared to the present mean sea level needs to be measured. However, the height of four meters is still used as the sea-level difference between the time the shell bed accumulated and the modern sea level. Even though the sea-level difference might not be exactly four meters, the different values may be regarded as the result of tectonic mechanisms after the shell had deposited. These complicated tectonic frameworks of the region must be taken into account for more detailed investigation and researches.
Acknowledgements We sincerely thank Kesanee Sapchokanan from Office of Natural Resources and Environment, Krabi
province who guided us to the study area and provided a lot of useful information helping us to well run this research. We thank the Department of Mineral Resources for all facilities and supports.
References Goudie, A., 1992 ,Environmental change; contemporary problems in geography: Oxford Univ. Press, 3rd ed, 329 p. Harper, S.B., 2005, Bedded shell deposit at Ao Nang, Krabi Province, southern Thailand; a record of a prehistoric Tsunami event or extreme storm event or neither: Geol. Soc. America, Abstracts with Programs, Salt Lake City, UT, Ann. Meeting October 16-19, 2005, v.3, no.7, p.75. Sinsakul, S., 1992, Evidence of Quaternary sea level changes in the coastal areas of Thailand a review: Jour. of SE Asian Earth Sci., v.7, no.1, p.23-37. Songtham, W. and Watanasak, M., 1999, Palynology, age and paleoenvironment of Krabi Basin, southern Thailand in B. Rattanasathien and S. Rieb, (eds ), Proceeding of the international symposium on shallow Tethys (ST) 5: Geol. Dept., Chiang Mai Univ., Chiang Mai, p.426-439.