The Geology of the Mergui District
This article is the continuation of the previous Episode 38, extracted from the Geology of Mergui District, reported by Sethu Rama Rau in 1930.
Geological Formation
The rocks of this series are phyllites, argillites, shales, conglomerates, sandstones, quartzites, tuffs, and agglomerates. The shales that predominate are indurated into phyllites and argillites and are dark grey. On weathering, they give rise to variously coloured clays, from buff yellow to brownish red, according to the proportion of iron and the degree of decomposition which they have undergone. The hills, rising to heights of from 2,000 to 2,500 feet, are situated west of the Tenasserim Valley, and their continuation southwards up to the source of the Klong Yung, Klong Banhuni, and Klong Ngan tributaries of the Pakchan River, consist of quartzite. The rivers flowing westwards, such as the Lenya, have a few waterfalls and rapids along their courses. The angular turn taken by the river and the islands in its bed are caused by the resistance of the quartzites to denuding agencies. The northerly continuation of the quartzites is visible in the Palauk, Palaw, Dathwe, and other rivers which have a westerly course.
Impure limestones, weathering into brownish-red soil, occur near the sources of easterly tributaries of the Pabyi Chaung and west of Sinlauk Taung.
Carboniferous shales have a graphitic appearance and occur in many localities. The best sections of these rocks are to be found near Tharabwin, Mawton, and Tagu. Similar shales occur in the Tavoy district as almost continuous beds at the 11th mile on the Tavoy-Myitta road and at Onbinkwin near Kanbauk.
The ‘pseudo-porphyritic grits and conglomerates described by Dr Oldham are represented in the district by volcanic tuffs and agglomerates in many of the islands of the Archipelago.
Sedimentary rocks, which are partly of volcanic origin, were observed by Dr A M Heron in some of the islands of the Archipelago. In his examination of Maingay and Elphinstone Islands, Dr Heron has mapped the foci of eruptions, which have distributed their debris widely through the rocks of the Mergui series on Maingay Island. A significant development of volcanic agglomerates, composed of quartz, felspar, and biotite fragments, with chips of glass and quartzite, evidently the product of paroxysmal eruptions of a magma corresponding in composition with granite and rhyolite occurs on Maingay Island, etc.
The author has observed similar rocks, resembling ash beds, weathering to a grey colour, on Singu Island, north of Kyaukpeik, which island forms the southerly continuation of the Elphinstone Island rocks. Another easterly band of hard, tough, indurated, volcanic tuffs, with angular fragments of felspar and subangular pebbles of slates of various sizes, occurs on the easterly slopes of Manoron taung at the source of the Pong chaung, one of the tributaries of the Manoron chaung, N N W of Thetke.
Hard, fine-grained, finely laminated argillites of a bluish-grey and black colour occur in many parts of the district and are abundant in the chain of islands extending from Tavoy Island, southwards through King, Maingay, and Lampi (Sullivan), to “Sir W. James Island. Similar rocks occur east of the Yanngwa, Karathuri, and Maliwun ranges near the source of the Lenya and Pakchan Rivers and extend north and south.
The rocks resemble micaceous slates and break easily along their bedding planes into splinters. Along the peripheries of the granite of the Tagu are many instances in which the argillite is altered to hornstone, as seen on the Sindin-Tagu Road. On the Island of Pulo Kanna (11° 0’ 30” and 98° 36’30”), the argillite is metamorphosed by the adjoining granite. The fresh rock has the black appearance of basalt but not its specific gravity. A microscopic examination shows brown biotite, magnetite, and garnet. In many localities, the slates under the microscope also display well-developed greenish chlorites due to metamorphism.
On the east coast of Lampi Island, north of Kayang-aw and south of Katyang, the argillites are interbedded with conglomerates. The pebbles consist of granite, quartz, and quartzite, ranging in diameter from one to ten inches. The granite is of the biotite-hornblende variety. From the nature and size of the boulders, it appears that they could not have come from a long distance. On the west coast of Pulo To-ton Islands, a conglomerate band consisting of pebbles of quartz, from a half to one inch in diameter, occurs above high tide level. Beds of conglomerate, passing gradually into slates, also occurred on the easterly slopes of the ridge dividing the drainage areas of the Tenasserim and Kyaukpya Rivers and were traced southwards up to the Sindin-Tagu Road.
The general strike of the sedimentary rocks of the Mergui series is from 10° to 15° west of north, with steep angles of dip. The vast extent to which these rocks are exposed is due to their folding. In the islands of the Archipelago, marine denudation has exposed the structure of the rocks along the coastline to show the different variations in the dip. On Tavoy and King Islands, in the north, Dr A M Heron has marked dips at several places, which proves the folding of the Merguie. The rocks of Tavoy Island dip 70° N E near Malizayatseik; on Rich Island east of Tavoy Island, the dip is from 10° to 20° E. Further east, the angle of dip increases to 30° on Edward Island. On the north coast of King Island, Dr A M Heron has observed strata, which are vertical in some places, sometimes rolling, and at other locations with gentle angles of dip, in a nearly eastern direction. The sedimentaries of Julia Island, situated northeast of King Island, are inclined at angles of 50°to 60° in a roughly northeast direction. But the direction of the dip on Polly Island is from west to west-south-west. This change in the direction of the dip may be explained by an anticlinal fold, whose axis forms the line of weakness. As the longer axis of the granitic boss of this island coincides with the anticlinal axis, it may be inferred that the granite has been intruded along this line of weakness, etc.
On Lampi and adjoining Islands in the south, which lie on a line coinciding with the general strike of the sedimentary rocks of the district, the author observed several opposing dips that prove folding. Near the landing place of Mr Ahmed’s mine, on the east coast, a small island in the bay of Kayaung-aw exposes strata at low tide, which dip inwards at a low angle and mark the position of a syncline, followed by an anticline. Other islands that dip in opposing directions are well exposed, such as Pulo Kamat and Pulo Pyindaung, Sir Charles Forbes Islands, etc.
Mr J J A Page examined many of the islands of the Archipelago and has adduced additional evidence of folding. He observed the crossing of two anticlines in Hayes Island (12( 30’; 98( 10’)) and noticed steeply inclined folds on an island near the southwest corner of Elphinstone Island. All these folds are more or less parallel to one another. The overlying limestones also show evidence of folding in Kyauktaung Hill. The crustal movements just have been in part after the deposition of the Moulmein series, i.e. post-Permo-Carboniferous, but earlier folding also antedated, in all probability, the Moulmein series. In their memoir on the Tavoy district, Drs J Coggin Brown and A M Heron have observed that the Mergui series is repeated several times in an east-to-west section, in closely pressed folds.
Age of Mergui Series
A careful search for fossils in the rocks which underlie the Moulmein series was unsuccessful. The age of the Moulmein series has been fixed as Carboniferous from an examination of the fossils collected by Mr P N Bose. All that can be stated, therefore, is that the Merguis are older than the Carboniferous. From their lithology characters, they may be tentatively correlated with the Chaung Magyi series of Upper Burma – Pre-Cambrian and, possibly in part, older Palaeozoic.
Mr W R Jones ascribes a Carboniferous age to the limestones and a Permo-Carboniferous age to a part of the Mergui series. The limestones referred to by Mr Jones as forming the bed of the Kanbauk Valley are not the characteristic limestones of the Moulmein series. In the rocks of the Mergui series, there are many grades of calcareous shales and sandstones with varying quantities of lime in them; the limestones of the Kanbauk valley are possibly such limestones interbedded with the quartzites and shales. Limestones of this kind were mapped on the hill ranges situated west of the Tenasserim River at the foot of Sinlauk Taung in the Mergui district and near the source of the Ban Chaung in the Tavoy district. Similar limestones of a blackish or red colour, sometimes crystalline, with field relationships to the Merguis leaving little doubt that they form part of them, were examined by Dr A M Heron between Yapu and Migyaung Laung on the Tavoy River.
Older intrusives in the Merguis
Near Talobusa, there are many bands of acid and basic rocks with gently folded ridges that intrude into the slates and quartzites of the Mergui series. The junction is obscure and it is difficult to state whether it is conformable or not. The acid bands are composed of quartz approaching a quartzite with little biotite, while the bare rocks are rich in hornblende, pyrite, and biotite with very little quartz. The rocks are undulating and gently folded, and the axis of the folds corresponds with the strike of the sedimentary rocks of the district, i.e. 20° W of N to 20° E of S. Pegmatitic veins traverse them and branch irregularly in various directions. Quartz veins of a later age, having an N E – S W direction, have cut across the pegmatites.
It is difficult to fix the age of these acids and basic rocks. The original minerals have been altered by metamorphism, and they have been subjected to severe crustal movements, including the folding of the Merguis. Their general appearance resembles that of gneiss. They were either intruded into the Merguis before the latter were folded or are older than the Merguis.
The Moulmein limestones
The Moulmein limestones are coarse-grained and crystalline, some of the crystals being ¼ to ½ inch in width. The rocks form ridges and bills with precipitous sides, rugged and bare of vegetation, standing like fortresses, and are surrounded by alluvium. They are easily removed by solution, producing caves of various dimensions, ranging from 50 to 150 feet in breadth and from 60 to 250 feet in length, resembling enormous halls. The infiltration of calcium carbonate from surface drainage has produced many stalactitic growths. Thick deposits of the excrement of bats occur on the floors of the caves; this was at one time collected and used for the manufacture of saltpetre. The limestone islands of the Mergui Archipelago are the habitation of small swifts (little rock martin), the mo-swe-angst and ziwaso of the Burmans (Hirundo sp. and Hirundo collacalia), which make their nests out of gelatinous saliva and form the delicacy known as ‘edible birds’ nests’. At present, the right to collect nests is leased out by the Government. The Chinese value them highly and purchase them at about their weight in silver.
One of the chief outcrops of the limestone of the Moulmein series is the Thampra hill (Khao Thampra, 11° 14’; 99° 11’), and a range of precipitous hills to the south situated near the source of the eastern tributary Klong Thampra, of the Lenya River. This outcrop forms, with others, a limestone chain from the source of the Ngawun Chaung, the southern tributary of the Little Tenasserim River. This chain crosses the Great Tenasserim and continues west of the river to the well-known limestone hills of Kyauktaung and Tharabwin. The north and south of these hills are covered in some places with thick alluvium, and other areas are covered by lakes or lowlands that are submerged during the rains. The bottoms of some of the lakes, when dried up, show limestone in situ. Further northwards, this chain is represented by another outcrop of coarsely crystalline limestone near the source of the Peinchaung River, which continues to Kyauktaung Hill, west of Pawut. This central chain is the longest and best exposed. The westerly line of the limestones is exposed in isolated islands situated some distance apart. The ‘High Black Rocks’ (12° 58’; 98° 15’) and ‘High Rocks’ (12° 52’; 98° 15’) islands are situated west of Tavoy Island and to the east of Middle Moscos, which are visible from Maungmagan, is the northernmost outcrop of this line. These groups and another, east of Domel Island, named Elephant Island or ‘Marble Rocks’ were examined by Dr A M Heron. Mr J J A Page examined the southernmost group forming St. Matthew’s Island. The author mapped the central group of the Turret Islands east of Sir William James Island. The island of Pulo Ampat (Marble Rocks) 11° 10’; 98° 37’, situated along this line, is made up of white quartzites and not of limestones.
The limestone collected by the author from the Turret Islands was analyzed in 1926 in the laboratory, and the following results were obtained. Rock Register No 34-729. Sp Gr 2.764: –
Insoluble residue. . . . . . . . . .. .. . . . . . .0.10%
Oxide of iron and alumina. . . . . . . . . . .0.27%
Calcium oxide. . . . . . . . . . . . . . . . . . . .55.51%
Magnesium oxide. . . . . . . . . . . . . . . . . 1.00%
Loss on ignition (CO2 and H2O) . . . . . 43.72%
The above-quoted analyses show that limestones, with their low percentage of magnesia, would be suitable material for the manufacture of Portland cement. From fossil evidence, Dr Noetling thinks the age of the limestones of the Tenasserim Valley tends to be an Upper Carboniferous period.
To be continued.
References:
Sethu Rama Rau, Rao Bahadur 1930: “The Geology of the Mergui District”, Memoirs of the Geological Survey of India, Vol LV, Part 1.
