International Conference on Geology, Geotechnology and Mineral Resources of Indochina (GEOINDO 2005) 28-30 November 2005, Khon Kaen, Thailand

Sedimentary facies and environments of an unusual thick shell bed in Mae Moh Basin, Lampang Province, Thailand

Wickanet Songtham Bureau of Geological Survey, Department of Mineral Resources

Rama VI Road, Ratchathevee, Bangkok 10400 Thailand

ABSTRACT

A Middle Miocene unusual thick shell bed had ever been spectacularly posed in the southwest margin of the Mae Moh Coal Mine. It was a part of a claystone layer sandwiched by two layers of coal, thin lignite K4 underneath and productive lignite K3 on top. The thickest part was about 12 meters and thinning out toward the east, north, and south of which disappearing into claystone layer. On the western side it was cut by a series of faults that impossible to reconstruct the original distribution but the existing shell bed was widely distributed covering an area not less than 300,000 m2, approximately. The lower boundary was well marked by a thin layer of lignite K4. The shell bed and lignite K4 was, however, intercalated by a layer of claystone that thinning out and disappearing in the vicinity around the thickest shell bed portion that becoming directly contact. The upper boundary was marked by a contact between the shell bed and overlying lignite K3. The shell bed consisted of vivipareous shells genus Bellamya in major and tiny shells of spire snail Bithynia in minor with common turtle plates in some places. The shell bed possessed a succession of interformational layers of mollusk shell that each layer had thickness ranging from 15 to 30 centimeters. Each interformational layer was dominated by upward-coarsening sedimentary structure consisting of various sizes of mollusk shell detritus and completed mollusk shell bodies but some upward-fining sedimentary structures were also presented. The claystone layer between the two layers of coal was deposited by the influx sediments of clay with numbers of shell fragment by a river running from the western mountains. The detritus of mollusk shell were accumulated at the river mouth as a delta and partially redeposited creating a linear terrigenous shoreline where northerly wind generated currents were strong enough to redistribute land-derived detritus of mollusk shell like a shell beach running parallel to the coast. Whereas, the finer particles of clay were farther transported into deep lake and shore far away from the river mouth by some wave actions. This burial shell beach of the Moh Moh Lake had been, unfortunately, vanished by mining activities leaving only a small vulnerable volume of mollusk shell for national conservation by the Department of Mineral Resources. INTRODUCTION Mae Moh Valley situates in Mae Moh District, the Province of Lampang, about 26 kilometers on the east of Lampang City (Fig. 1) regarded as a fault-bounded basin containing Cenozoic sediments. The basin floor is, more or less, a flat area about 300 to 320 meters above mean sea level with a main stream, Nam Mae Moh, and its tributaries flowing from the mountains surrounded to the south joining Nam Mae Chang in Ban Sop Moh of Mae Moh District. Nam Mae Chang further flows to the south in about southwest direction and joining with the Wang River in Ban Sop Chang of Ko Kha District about 20 kilometers south of Lampang City. The basin margin is clearly marked by some high mountain ranges of Lower Triassic limestone of the Lampang Group in the northern and eastern sides and Permo-Triassic volcanic rocks in the western side. In the south of the basin is dominated by rolling hill terrain and some short mountains of Pleistocene basaltic flows with some distinctive forms of ancient volcanic craters like Doi Pha Khok Hin Fu. There are some outcrops of the Triassic Lampang Group in the central area of the basin shifting to the west clearly observable along Mae Kham - Huai Sai Irrigation Canal dividing the basin into two sub-basins, western coal barren sub-basin and eastern coal measured sub-basin. The basin is about 10 kilometers wide in E-W direction and about 22 kilometers long in N-S direction covering an area about 155 squared kilometers.

Locality of the basin on military topographic map covers parts of map sheet 4945 I (Ban Tha Si), 4945 II (Amphoe Long), 4945 III (Amphoe Mae Tha) and 4945 IV (Changwat Lampang). The deposits in the eastern sub-basin are Tertiary semiconsolidated to consolidated and Quaternary unconsolidated sediments over 1,000 meters in thickness these include the potential coal measures. The coal has been exploited for lignite since 1955 for power plants by the Electricity Generating Authority of Thailand (EGAT). The lignite is now produced 17 million tons a year supplying 8 power plants with electricity generating capacity over 2,400 megawatts.

Fig. 1 Map of northern Thailand showing important cities and locality of Mae Moh Basin.

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The Tertiary sediments in the Mae Moh Basin have been named as the Mae Moh Group comprising 3 formations including Huai King, Na Khaem and Huai Luang Formations in ascending succession (Fig, 2). The Huai King Formation is fining upward transgressive sequence from conglomerate in the bottom to sand, silty sand, and clay with a thin layer of coal, lignite S, on the uppermost portion indicating the depositional environments had changed from a fluvial, lacustrine and ended up with swamp through times. The Na Khaem Formation is alternations between a series of claystone and lignite layers indicating lake and swamp environments alternately. The coal seams comprise lignite R, lignite Q, lignite K, and lignite J in ascending. There are various kinds of fossils discovered from this formation including ancestral forms of vertebrate of rhinoceros, elephant and otter with some forms of fish and turtle and various taxa of mollusk like Brotia, Paludina, Planorbidae, Bellamya, Bithynia, and Melanoides, with many indeterminant taxa of both bivalve and gastropod. The Huai Luang Formation appears to be a red bed formation comprising claystone, silty claystone, sandstone, conglomerate indicating fluvial environments. There is a local lens of dark greenish grey claystone with a series of thin coal layers, lignite I, and a thin shell bed of Margarya at the middle portion of the formation. Abundant gypsum crystals, selenites, occurring in the lignite Q, lignite K, lignite J and in the red bed Huai Luang Formation had been crystallized in different styles of environment (Silaratana, 2005).

Fig. 2 Schematic stratigraphic succession of the Mae Moh

Group and its subdivision units.

The lignite K zone is further physically divided into four coal sub-zones including lignite K4, lignite K3, lignite K2, and lignite K1 in ascending with Middle Miocene in age (Benammi et al, 2002). Between the lignite K4 and lignite K3 is widely intercalated by a claystone layer with about 15 meters in thickness. An unusual thick shell bed (Fig. 3) was a part of this claystone layer on the southwestern margin of the coal mine. It was reported by the EGAT in 2003 due to its spectacular huge shell bed body, with about 12 meters in thickness regarding as the thickest shell bed of Thailand ever reported. The Department of Mineral Resources (DMR) had moved forwards to protecting the shell bed site by submitting proposal to the cabinet-council for conservation and developing the shell bed site as an open museum and tourism

attraction. The cabinet-council had adopted the DMR’s proposal by allocating a specific shell bed area of 68,800 m2 on February 17, 2004 of which considerable as a national heritage. However, according to some technical problems in mining plan, the EGAT had interrupted the DMR’s conservation plan of the 68,800 m2 area due to the rock strata including the shell bed were dipped into eastward direction with some critical dip angles disturbing open pit mining plan particularly the slope stability design in the nearby eastern area of the shell bed. Another proposal was later resubmitted to the cabinet-council to conserve only 28,800 m2 plus other extra non-shell bed area totally 83,200 m2 and was adopted by the cabinet-council on December 21, 2004. Accordingly, main part of the shell bed site had been, unfortunately, bit by bit trimmed out to keep slope stability in the mining plan. During the shell bed was successively removed, some significant geological sections had been cropped out to updating geological information involving the shell bed site describing in this paper.

Fig. 3 Schematic stratigraphic succession of the Mae Moh

Group and its subdivision units.

The main aims of this paper are to describe sedimentary facies of the shell bed including geometry, lithology, sedimentary structure, paleocurrent and paleontology to explaining sedimentary environments of the shell concentration in which processes of sedimentation producing the spectacular huge shell bed body. A real earth historic episode of the shell bed is no more presented in the field except contexts and some pictures in this paper with, unfortunately, no more good scientific references of the real outcrops due to only a negligible volume of the shell bed has been conserved. SEDIMENTARY FACIES During my first visiting the thick shell bed in August 2003, it exposed in the southwest margin of the coal pit covering an area over 80,000 m2. The shell bed dipped into eastward direction with about 20 degree dip angle observable the underlying lignite K4 and overlying lignite K3. First thickness estimation of the shell bed was about 12 meters in the middle part of the shell bed. The shell bed was cut as a wide trench for installing conveyor belt separating the shell bed into two outcrops, western outcrop and eastern outcrop. The western outcrop covered an area about 68,800 m2 with fault boundary in the western margin. Meanwhile, the eastern outcrop covered an area about less than 16,000 m2 due to large area of the shell bed in the eastern side had been mined

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that large volume of shell bed had been removed. The real dimension of the shell bed is, therefore, impossible to reconstruct but regardable as an exceptional geological outstanding.

The shell bed was considered as an elongate distribution lying in N-S trend as a part of claystone layer of which sandwiched between two layers of lignite, lignite K4 underneath and lignite K3 on top. The north-south cross section showed a lens shape with about 1,000 meters long that the thickest part was at about the middle portion with more or less 12 meters in thickness (Fig. 4). In the far north, the shell bed edge was a sharp wedge and thickening toward south. The northernmost part of the shell bed connected with claystone layer that was overlain by the lignite K3. Beneath the shell bed was underlain by claystone that thinning toward south disappearing at the thickest shell bed vicinity forming a wedge shape in N-S cross section. Further south via the thickest shell bed portion, the shell bed and claystone between the lignite K4 and lignite K3 became rapid thinning to be about 2.5 meters and into about 0.8 meters thick in such less than 200 meters in distance. However, when moving further south about 200 meters, the shell bed was much thickened but immeasurable due to the shell bed was being removed in the mining. Continuing to the south about 200 meters, the shell bed contained turtle plates in common and changing into non-fossilifereous claystone layer that still sandwiched by the two layers of coal.

Fig. 4 N-S geological cross section showing distribution of

the thick shell bed between two the coal layers.

In the E-W cross section via about the thickest shell bed part, the shell bed was dipped into opposite directions controlled by a N-S strike fault with steeply dip into west direction. The shell bed on the western side of the fault dipped into west direction, on the other hand the shell bed on the eastern side of the fault dipped into east direction. Unfortunately, east-west cross section shows incompleteness of shell bed due to western margin was cut by a set of faults and eastern margin had been mined by the EGAT. Nevertheless, from the EGAT’s information, the shell bed extended from west to east about more than 300 meters in distance.

The lignite K4 was under the shell bed with sharp contact (Fig. 5) in the thickest portion of the shell bed but between the lignite K4 and shell bed somewhere else was intercalated by a layer of claystone from a very thin layer to gradual increase in thickness into the northward and southward directions. The claystone was recognized into two types depending on shell fragment concentration. The claystone in the lower part was rare in shell fragments meanwhile in the upper part was common to abundant in shell fragments. The demarcation of the two types of the claystone was dully observable by a very thin layer of carbonaceous claystone with little darker in color from the nearby greenish grey claystone (Fig. 6). The shell bed was thinned towards north and south and finally disappeared being claystone instead and then widely extending laterally.

Fig. 5 Lower boundary of the shell bed showing sharp contact between lignite K4 and shell bed in the thickest portion of the shell bed.

Fig. 6 Lower boundary of the shell bed in northern part

showing claystone layer intercalated between lignite K4 and the shell bed The upper part of the shell bed was overlain by lignite

K3 layer with abundant shells of Planorbidae in some places but somewhere showed gradational contact varying from shell concentration into lignite K3. There was an extensive thin coal layer, about 5-20 centimeters in thickness, in the upper part of the shell bed located about 20 centimeters under the upper boundary of the shell bed (Fig. 7). This thin coal layer extends covering all over the shell bed nearly parallel to the lignite K3 layer. In the southern part, the formation between lignite K4 and lignite K3 was divided by a thin layer of coal into two parts, lower part and upper part. The lower part was characterized by grade bedding dominated by shells of mollusk in the bottom and graded to be finer shell debris and ended up with claystone on top characterizing upward-fining grade bedding with thickness about 1 meter. The upper part in contrast, the lower portion was claystone and graded up to be dominated by shells of mollusk characterizing upward-coarsening grade bedding. The upper boundary was unknown due to the outcrop was partially removed.

Fig. 7 Upper boundary of the shell bed showing an extensive

thin coal layer at about 20 centimeters under the shell bed-lignite K3 boundary.

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The shell bed entirely consisted of shell interformational layers alternating over 30 layers with thickness ranging from 15 to 30 centimeters each. Each thin layer of shell was clearly defined by sharp contacts between fine fragmented shells and non-fragmented shells. Each shell layer was, therefore, characterized by grade bedding of both coarsening upward (Fig. 8) and fining upward but coarsening upward was dominant. However, each shell layer probably contained both types of those sedimentary structures showing lateral facies changes. Fig. 8 Thick shell bed showing interformational shell layers

possessing upward-coarsening sedimentary feature. Sedimentary variation of the shell bed from north to south was subject to degree of clay matrix between the mollusk shell fragments. The shell bed in the northern part mostly contained shell fragments without any matrix of clay with gradual increase of clay to the south. This characteristic indicated that the paleocurrents were from north to south. SEDIMENTARY ENVIRONMENTS The Na Khaem Formation is a series of fine grain clastic sediments of greenish grey to dark grey claystone and lignite. This series of deposits is of low energy condition of lake and peat swamps. Abundant and diversified fossils of freshwater fauna and flora including mollusks and alga Pediastrum without any traces of marine evidence strongly suggest that the depositions occurred in freshwater environments. Mollusks from the formation are good markers in stratigraphic references and paleoecological preferences. Mollusk shells of Brotia spp., Bellamya, Bithynia, Paludina, Melanoides, and some indeterminant taxa are in exclusive assemblages in claystone layers. Planorbidae is always found in lignite seams including lignite Q, lignite K3, and lignite K2 and completely absented from any claystone layers. This indicates that the Planorbidae inhabited in swamp environment with dense vegetation rather than lake. Bellamya shell is, however, in wide ranges of molluscan assemblages and in different stratigraphic positions. Some horizons of mollusk in underburden claystone consist of Bellamya, Paludina, Brotia spp and other indeterminant taxa in claystone layers. In the lower portion of overburden claystone contains another Bellamya species in association with Melanoides cf. M. tuberculata. These Bellamya-associated claystone layers strongly suggest that the Bellamya snails inhabited on the mud floor of lake. The Bellamya-dominated unusual thick shell bed with negligible

clay matrix is said to be ex situ deposit by water current transportation. Widely distribution of two sedimentary cycles of which underlying and overlying the thick shell bed are of regressive sequences. The first cycle starts with lake sediments of interburden greenish gray claystone and changes into shallower sediments with more enrichment of organic matters onward and finally ended up with lignite K4 layer. The second cycle is the same manner of the former starting with claystone layer sandwiched between the lignite K4 and lignite K3 and ended up with the lignite K3 layer. However, in the claystone layer of the second cycle contains a local elongate shell bed situated in the southwest margin of the eastern sub-basin. The claystone layers were accumulated by clay-sized particles transported from rivers into lake forming a delta. On the other hand, peat (lignite) layers were formed where there was no influx of terrigenous detritus. The shell bed in the claystone layer between the two lignite seams had been formed by redeposition of Bellamya and Bithynia shells (Fig. 9) that transported by northerly wind generated currents from the river mouth. The wind generated currents carried both clay particles and various sizes of shell fragments and non-fragmented shells from the river mouth down to the south forming a shell beach parallel to the coast of the lake. The sedimentary variations from north to south were function to the northerly paleocurrents.

Fig. 9 Dense mollusk shell assemblage containing two taxa

of shell, Bellamya and Bithynia.

According to the shell bed was partially disturbed by mining activities, the real dimension of the shell bed is, thus, impossible to reconstruct. However, the shell bed is between two layers of coal in vertical relationship and thinned out to be claystone layer between the two layers of coal. The shell bed is described to be a linear terrigenous shoreline where lake currents were strong enough to redistribute land-derived detritus of mollusk shell like a shell beach running parallel to the coast. Upward-coarsening regressive sequences are a characteristic of the linear terrigenous shoreline. The shells were deposited in sub-aqueous condition under wave base affects generating differential transportation forming upward-coarsening sequences. The upper part of the shell bed on the western rim shifting to the north shows evidence of oxidation producing yellowish orange color indicating that the shell bed was sometimes sub-aerial condition when water level in the lake regressed.

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CONCLUSION Sedimentary facies of the unusual thick shell bed consisted of these sedimentary characteristics of which is interpretable its depositional environments:

1. The shell bed was formed by two genera of mollusks including Bellamya and Bithynia with turtle plates in common

2. The shell bed extended from north to south about 1,000 meters in distance with 12 meters as the thickest part at about the middle of the shell bed shifting to the north and thinning out toward north and south looking like a lens in N-S cross section.

3. The east-west cross section showed fault boundary on the west and slightly thinning out in the eastward direction.

4. The shell bed possessed over 30 interformational layers of shell that each layer was upward-coarsening grade bedding with some upward-fining grade bedding in some places.

5. The shell bed is entirely contained by detritus of shell of both fragmented and non-fragmented. The shell bed continuously increases in clay content with finer shell debris toward south indicating paleocurrents were from north to south.

6. The shells were carried by a river to deposit at the river mount and were redistributed as a linear terrigenous shoreline as a shell beach by wind generated currents.

7. Huge volume of the shell bed had been removed to pile up somewhere else by mining activities leaving just a small volume as a conservational site. Many important sedimentary features were destroyed. Many pictures of the shell bed in this paper cannot be seen in the field of which

many important real scientific references of the shell bed had been unreturnably vanished.

ACKNOWLEDGEMENTS I thank my bosses from the Department of Mineral Resources including Director General, Mr. Somsak Potisat, Director of Bureau of Geological Survey, Mrs. Benjawan Charukalas, and Paleontological Specialist, Mrs. Benja Sektheera for many helps in both field and office works. This is also my colleagues including Mr. Somkiat Maranate, Dr. Assanee Meesook, Mr. Wattana Tansathien, Mr. Suvapak Imsamut, and Mr. Wirote Saengsrichan. Many helpful staffs of the Electricity Generating Authority of Thailand are specially thanked including Mr. Pairote Anupandhanant, Mr. Prajuab Donkhammoon, Mr. Visut Bunthai and Mr. Aroon Hoonkao who provided many things in both academy and facilities including logistical supports. REFERENCES Benammi, M., Urrutia-Fucugauchi, J., Chaimanee, Y., and

Jaegaer, J.J., 2002. Magnetostratigraphy of the Middle Miocene continental sedimentary sequences of the Mae Moh Basin in northern Thailand: evidence for counterclockwise block rotation. Earth and Planetary Science Letters, 204: 373-383.

Silaratana, T., (2003). Effects of environmental factors on accumulation of fossil fuel deposits in northern Thailand. Unpublished Ph.D. Thesis, Graduate School, Chiang Mai University: 340 p.

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