GEOTHAI’07 International Conference on Geology of Thailand: Towards Sustainable Development and Sufficiency Economy
MIDDLE HOLOCENE PEAT AND MOLLUSK SHELLS FROM ONGKHARAK AREA, NAKHON NAYOK, CENTRAL THAILAND: EVIDENCE OF IN SITU DEPOSITS DURING A MARINE TRANSGRESSION PERIOD Wickanet Songtham 1, Poramita Phanwong 2, and Tosak Seelanan 2 1 Department of Mineral Resources, Bangkok 10400 Thailand 2 Department of Botany, Faculty of Science, Chulalongkorn University, Bangkok 10330 Thailand ABSTRACT About seven meter-thick Bangkok Clay in Ongkharak area, Nakhon Nayok, Central Thailand had been studied. Clay samples were collected for palynology. Peat and mollusk shells were recovered for carbon fourteen dating. The Bangkok Clay distinctly yielded mangrove pollen, peat, and abundant marine mollusk shells. The clay sediment dating back from 7,600 to 5,000 years B.P. is prominently considered as marine clay deposited during a marine transgression period. At the beginning of the marine invasion, 7,600 to 7,000 years B.P., abundant peat was accumulated in littoral zone under mangrove forest. The area was continuously flooded by seawater and was subsequently in subtidal zone at about 5,000 years B.P. indicated by a mollusk shell layer containing abundant Cyrtopleura sp. cf. C. costata, the species today occurs near the low tide boundary of subtidal zone as burrower. The shell species from the Bangkok Clay is strongly regarded as in situ deposit on the basis of its occurrence that is suitable for age determination. The carbonized woods are reasonably regarded as in situ deposit as well. The carbonized woods and shells are evidences of a marine transgression period during Holocene. Key words: mollusk, central Thailand, transgression
INTRODUCTION
STUDY AREA
Precise carbon fourteen dating for Holocene geological formation needs suitable materials, peat or marine shell, to representing their original deposition. The basic requirement is to make sure that the material used is not reworked sample, likewise, the in situ materials is needed. The Holocene Bangkok Clay in the lower central plain of Thailand is one that contains abundant materials of peat, carbonized woods, and marine mollusk shells of which are possible to date. However, these materials are hard to proof that either they are in situ or ex situ deposit. The Bangkok Clay has been proven as estuarine deposits that reworked woods and shells were possibly taken place in general. Some carbonized wood and mollusk shell occurrences from Lad Chang soil pit, Ongkharak, Nakhon Nayok province (Fig. 1), are needed to prove for this application. Common carbonized woods at the lowermost portion of the Bangkok Clay play significant role for precise age determination. A species of shell from a thin shell layer at the middle portion of the Bangkok Clay shows a systematic depositional pattern enabling to believe as an in situ deposit. These led us to collect the carbonized woods and shell specimens for carbon fourteen dating and reconstructing the ancient shorelines. This paper aims to describe and record some special characteristics of materials used for age determination and their occurrences.
The study area is a large soil pit producing dumped soil for construction site. It is 380 meters wide, 1050 meters long, and 27 meters deep. The soil layers in the pit are consistently considered as Quaternary deposits. The lower portion exposed about 20 meters thick is stiff clay of Pleistocene sediments covered with Holocene Bangkok Clay containing abundant marine animal remains with thickness about 7 meters. The youngest sediment is a thin layer of clay without marine animal remains regarding as a recent fluvial deposit. The pit is located in Ban Lad Chang, Tambon Chumpon, Ongkharak district, Nakhon Nayok province. It is on the area between Khlong Sib Si and Khlong Sib Ha on the southern area of Khlong Rang Sit. It is located on latitude 14° 00' 40" N longitude 100° 55' 30" E of the Royal Thai Survey Department map sheet 5137 II, scale 1: 50,000 (Nong Sua sheet). The area is on the lower central plain of about 150 kilometers wide in east-west direction and about 190 kilometers long in north-south direction bounded by mountain ranges to the west, north, and east with the Gulf of Thailand in the south. The plain is where four main rivers run through from north to south flowing out to the gulf including Maeklong, Tajeen, Chao Phaya, and Bang Pakong rivers arranging from west to east respectively. The plain is under tropical monsoon climate alternated between wet and dry period in a year rounded. The plain is usually flooded during heavy rains with flooding sediment accumulation from year to year over
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thousands of years. Large cities settled in the plain include Bangkok, Pathumthani, Nontaburi, Samut Prakan, Samut Sakhon, Samut Songkarm, Nakhon Pathom, Supanburi,Ratburi, Ayuthaya, Angthong, Singburi,
Chasoengsao, and with some parts of Petchaburi, Kanchanaburi, Prachinburi, Nakhon Nayok, and Chonburi provinces.
Figure 1 Map of lower central plain of Thailand showing four main rivers, Mae Klong, Tachin, Chao Phaya, and Bangprakong rivers; province localities, and study area.
MATERIALS AND METHODS Materials used in this study consist of clay samples, peat, and mollusk shells. The clay is soft and black in general collecting along northern wall of the Lad Chang soil pit, Chumpol sub-district, in Ongkharak district of Nakhon Nayok province. The Bangkok Clay is about 7.6 meters thick and the samples were collected from the base to the top of the horizon with about 10 centimeters intervals. Each station of the sample was precisely located by Theodolite and using ½ inch-size PVC pipe to collecting the clay. The samples were put in plastic bags and marking labels. The clay samples are being used for palynological extraction. Carbonized woods from the Bangkok Clay are located at the contact between the lowermost part of the Bangkok Clay and the underlying sedimentary layer (Fig. 2). They are black, brittle as characteristics as lignite and annual rings are well observable. They exposes as the basal parts of small tree trunks in upright position with roots deeply submerged down into the soil. Three samples were collected and wrapped with aluminum foil for carbon fourteen dating. In the middle portion of the Bangkok Clay contains a thin mollusk shell layer about 15 centimeters thick. The shell layer is at 4.428 meters under present mean sea level
Figure 2 Stratigraphic log from a pit wall of Lad Chang soil pit showing the Bangkok Clay where mollusk shell and carbonized wood were collected for carbon fourteen dating.
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5,000 years B.P. as the result of the age determination of the shell. Carbonized woods together with abundant peat densely occur in the lowermost part of the Bangkok Clay. The woods sparsely occur from place to place with some significant interesting. They are characterized by tree trunk shape with annual rings clearly visible. The tree trunks pose in vertical standing with roots deeply submerged downward in the soil. The woods are said to be in situ deposit that have stood at the original living tree positions perhaps in the mangrove forest at the time of 7,600 to 7,000 years B.P. These carbonized woods are, thus, considered as good materials for carbon fourteen dating. The area was first affected by the marine incursion at about 7,600 to 7,000 years B.P. At the time, the area was a shore where abundant crabs living along. Many crab burrows were in the Pleistocene stiff clay and at the same time abundant peat was also being accumulated probably under the early stage of the mangrove environment. The Holocene peat deposit took place on the Pleistocene stiff clay shore marking unconformity boundary with more clay deposition onward forming the Holocene Bangkok Clay as seen today. The area had continuously flooded by seawater forming subtidal environment at about 5,000 year B.P. where abundant mollusks living in especially the Cyrtopleura sp. cf. C. costata. The time span between 7,600 to 5,000 years B.P. is, thus, considered as a marine transgression time span.
whereas the ground surface is about 2.2 meters above the mean sea level. There are diversified species of mollusks but Cyrtopleura sp. cf. C. costata is significantly interesting. The shell is characterized by an oval to elongate shape with narrowing towards the posterior end. Valves have umbones at about Âź position of the shell length from the anterior end. Outside surfaces of the valves are patterned with radial riblets, which are sharp and angular in the anterior, and broader posteriorly. Inside surface of the valves, behind the umbones, is a thick, winged plate that lies transverse to the plane of the valves. Each mollusk shell body separately occurs from each other about 5 to 30 centimeters separation widely distributed all over the study area. The shell body always occurs as a pair of valves as its original nature and almost always orientated its long axis in upright position to the shell layer with anterior side pointed downwardly and posterior side pointed upwardly (Fig. 3). Many samples were collected but one of them was chosen and wrapped by aluminum foil for carbon fourteen dating.
Table 1 Results of carbon fourteen dating.
DISCUSSION AND CONCLUSION
Figure 3 Occurrence of mollusk shells Cyrtopleura sp. cf. C. costata at the middle part of the Bangkok Clay showing orientation of the shell valves of in situ burrowing deposits.
The occurrence of Cyrtopleura sp. cf. C. costata plays systematic depositional pattern with high degree of confidence in saying that the shell is in situ deposit. The 5,000 years B.P. age of the shell clearly represents the age of deposition. The carbonized woods seem to be in situ deposits but not highly confident by some features of depositional styles. However, the result 7,600 to 7,000 years B.P. in age determination is reasonable enough by mean of older than the shell deposit. Therefore, the ages from the carbonized woods are relatively acceptable in term of stratigraphic sense but absolute acceptance is not yet available until more detail investigation is confirmed.
RESULTS The mollusk shell, Cyrtopleura sp. cf. C. costata, seems to be much interested as in situ deposit as suitable for carbon fourteen dating. Each shell body is oval elongated shape and still kept into pair valves as its original nature with anterior part pointed downwardly and the posterior part pointed upwardly, the long axis of the valves orientated in vertical position. This manner is interpreted that the shell body has been preserved in its own burrow since it was still alive. As the present Cyrtopleura costata, it is burrower living on sandy mud near lower tide in subtidal zone. It has survived in the burrow since juvenile stage and never come out the burrow until die. Thus, the shells Cyrtopleura sp. cf. C. costata from the study area are said to be in situ deposit and can be used to represent its original stratigraphic level. The shells and the embedded sediment were, therefore, accumulated at about
ACKNOWLEDGMENTS The authors sincerely thank Mr. Santi Sricham and Mr. Montree Kaenthong from the Department of Mineral Resources who kindly provided a ground level survey of the study area. We thank the Department of Mineral Resources in providing financial and logistical supports throughout the field works. 179