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calcareous series. These make up the ridge of Pyingyitaung south of Kokku and continue to the southern edge of the sheet; northwards they are found again in the valley of Kyunbyingyi chaung. West of the latter, the outer line of intrusion, which in this sheet consists predominantly of fine to medium-grained granite of the Kabaing type, makes up the hill mass of Ngawuntaung (1,217) in the south, emerges from the alluvium some NgawuntaungKaduda Taung and three miles to the north to form Kaduda Taunggyun Taung. Taung and then from the calcareous series just over a mile further north to join the granite hills of which Taunggyun Taung forms the highest point. The latter hills continue to the north of the sheet but in the north, crystalline limestones, calciphyres and calcareous gneisses are very clearly seen caught up in the medium-grained granite on Chaunggyi chaung between Chaunggyi and Zibyugon.
West of this line of intrusion in the south, Recent alluvium. flanks the granites but to the north rocks of the calcareous series occur. The latter differ little from those to Hornblendic possibly altered Creta- the south on the east side of the outer line of ceous series. granites, but on the west, in the extreme north of the sheet, crystalline limestones occur in association with a hornblende series, the rocks of which Mr. Clegg regards as more metamorphosed types of the altered limestones, lavas, tuffs, and ashes found south of Male and in the Cretaceous series in sheet 93 A/4 north of Thabeitkyin.
On the west side of the Nweyon and Ywathit chaungs olivine basalts of late Tertiary age protrude through the soil cap; they encroach across the river at the bend one and a half miles south-west of Konaing. Further west similar lavas occur in sheet 84 N/14 where they were mapped by Mr. V. P. Sondhi as forming the line of hills between the rightangled bend of the Irrawaddy below Thabeitkyin, to Singu.
94. In sheets 93 A/4 and 93 A/8 Mr. Clegg continued northwards Dr. Iyer's mapping of sheet 93 B/1 and 5. Last year Mr. Clegg collected some specimens of massive limestone from an area about ten miles south-east of Tagaung from which Dr. M. R. Sahni isolated an Orbitolina of Cretaceous age. As it was obvious from the topographical map that this massive limestone series continued for some distance to the south, Mr. Clegg's endeavour was to see if it linked up with the Mogok series.
This work was only partially successful, as in the south of sheet 93 A/4 there is a pronounced change in the strike of the rocks. Nevertheless the strike of the less metamor
Disturbed area bounds
Mogok series on the phosed series in the north is found to conform, within the limits which might be expected in a very disturbed and intruded area, with that of the Mogok series to the south, and it appears fairly certain that Cretaceous rocks are included in part in the Mogok series, rocks which in the past have been classed as of Dharwar age. A gap of about three miles exists between definitely shelly limestone rocks and totally re-crystallised and homogeneous crystalline limestones of the Mogok series.
The rocks of the area are metamorphosed to varying extents and they can be divided into three zones:
(1) A northern, little altered zone consisting of quartz and felspar porphyries, calcareous tuffs, ashes, agglomerates, and limestone in
Varying degree of which traces of fossils can still be seen. metamorphism of the quartz-porphyries form quite a considerable mass in the ridge and hills west of Baladokhta Taung and continue to two and a half miles W. S. W. of Nansein Taung. The shelly limestones occur mostly on the eastern side of Nansein chaung where they form a series of steep hillocks and small cliff sections and are very much intermixed with lavas and ashes.
(2) An intermediate zone composed of metamorphosed ashes, blotchy limestone, in sections of which no trace of organic structure can be seen but which, megascopically, looks fossiliferous, phyllitic rocks of ashy origin, serpentinised peridotite and crushed granite. The acid intrusions occur near the contact with the Mogok series, whilst the basic intrusions occur to the north and are more remote.
(3) A southern zone composed of completely metamorphosed rocks of the Mogok series in which granites of the Kabaing type, crystalline limestones, calciphyres and calcareous gneisses predominate. This series occupies the high mountainous tracts of the Pinkan, Kyaukyi and Shwe-u-daung forests.
No definite boundaries exist between the various zones but the limestone rocks can be taken as an indicator. In the least metamorphosed rocks to the north unmistakeable shells can be seen both with the naked eye and as traces in a section; in the more metamorphosed zone the limestones are blotchy and sometimes look almost brecciated owing to coarser fragments of calcite
in a finer grained groundmass; in the Mogok series the calcareous rocks of individual exposures preserve a more homogeneous texture and no blotchiness at all is seen.
In the Paungsho chaung, where a good section can be seen of the middle series, the rocks consist of fine-grained banded tuffs, in places very tough and slaty and at others schistose and gneissic. The more schistose and gneissic consist of hornblende, sometimes much chloritised, in an aluminous ground mass.
The same series of rocks passes eastwards into sheet 93 A/8, where a very brecciated zone exists and the change from blotchy crystalline limestones with intruded acid granites, as seen south of the Twinnge-Mongmit P. W. D. road, to lavas, tuffs, and ashes north of the road, is very marked and it certainly seems as though major faulting occurs along this line. Were it not for the blotchy limestones which occur on the south side of this line in sheet 93 A/8 and the blotchy limestones which occur with granite-gneisses and altered tuffs in sheet 93 A/4, Mr. Clegg would have no hesitation in inserting a fault along the line of the Twinnge-Mongmit road. Possibly the situation will become clear when further work is accomplished. What has eventuated from the survey is that the autochthonous rocks of the Mogok series may include metamorphosed derivatives of any stratigraphical horizon from the Chaung Magyis to the Cretaceous.
95. Dr. L. A. N. Iyer continued the geological survey of the Amherst district begun by him in the 1935-36 field season, northwards in sheets 95 E/11 and 15, 95 E/14 and 95 I/1. 2, 6 and 7. Many of the observations recorded in the General Report for 1936, naturally apply with equal force to the area mapped during this field season. The main geological formations mapped were as follows:
Traverses made during the season in the Ye and Kya-In townships have given some idea of the geological structure of the area. Apart from the granitic intrusions, the sedimentary rocks of the Mergui and Taungnyo series have a general strike of N. W.-S. E. The main ridge of the Taungnyo series in sheet 95 E/14 appears
Geological structure of the area.
to be the axis of a synclinal fold, whilst in the eastern portion of sheet 95 I/2, the Zami valley occupies a synclinal basin, the rocks to the west of it dipping east and east-north-east, and the rocks east of it dipping west and west-south-west. In this latter area the rocks are shallow-water sandstones and coarse gravels, and are significant of a gradually silting up shallow basin.
Evidence of faulting is seen in the Moulmein limestone exposures, but owing to their isolation in laterite, these effects are not visible. on the underlying rocks. Near Lutshan1 (15°
48: 97° 54′), Mr. Leicester has described the effects of strike-slip faults on the limestones and the Taungnyo series below. A similar phenomenon occurs in the south-east corner of sheet 95 E/13 on the western spur of Winpauk Taung. The dip of the rocks is east; near Kyauktaga (15° 47′: 97° 59′), the dip becomes steeper and further south the rocks appear to have been affected by a pivotal fault with translatory movement, which has reversed the dip to the west. The south face of Allantaya Taung shows a smooth slickensided surface, and Karesaw Taung seems to have been faulted off from it. Minor faults and slips are a common feature in the limestone exposures.
It has not been possible to divide the Taungnyo series from the Mergui series in sheets 95 E/15 and E/14. Mr. Leicester has suggested that the Taungnyo series is younger than the Merguis or that it must be considered to form the upper portion of the Merguis; no conclusive evidence for the complete separation of the two series has, however, yet been obtained.
Taungnyo series and Mergui series.
The rocks of the Mergui series consist of sandstones, sandy shales and shales, and when affected by contact metamorphism are changed to quartzites, phyllitic mica-schists and phyllites. Where intense granitisation has occurred, the product is a biotite-granulite. The rocks of the Taungnyo series generally consist of sandstone and shales at times passing to quartzite and phyllite. The sandstones become coarse, gritty, and conglomeratic in the Zami valley.
The granite intrusion mapped in sheet 95 E/11 and 15 continues in a northerly direction into sheet 95 E/14 and thence sweeps round to the coast. Numerous veins and apophyses
of granite also occur in the rocks of the Taungnyo and the Mergui series. A dyke of granite, four miles long, runs
1 Rec. Geol. Surv. Ind., LXIII, pp. 93-97, (1930).
south-south-east from Kawlawa, and with other minor intrusives is responsible for the impregnation of tin in this area in rocks consisting of coarse biotite-granite or gneiss, medium- and fine-grained aplititic rocks, and pegmatite. In some of the banded gneissic varities, felspar and quartz form definite bands whilst bands rich in biotite also occur. Crushed bands in the series contain biotite, quartz, felspar, iron-ore and apatite. The felspar is mostly microcline with some microperthite. The pegmatite near Kawlawa (15° 40′ 97° 57') is composed of microcline, quartz, muscovite, tourmaline and occasionally epidote. Aplite veins were seen in the Myettaw stream.
The Moulmein limestone occurs as towering ridges rising precipitously out of the alluvium and laterite in the Kya-In township, and also overlying rocks of the Taungnyo series. The general relationship of the limestone to the underlying sedimentary rocks has been dealt with by Mr. Leicester1. From his study of the contact in the Beke Taung he concluded that the junction is undulating, and that both the series have been folded together. The limestone passes to a calcareous shale at the base and in the absence of faulting at the contact, there appears to be little if any unconformity. Mr. Leicester also mentions that no definite conclusion could be reached regarding the relation of the Taungnyo series and the Moulmein limestone until further contacts had been examined.
A number of contacts examined by Dr. Iyer during the last season, led him to conclude that the limestone has been highly folded in with the underlying rocks.
The limestone is white, grey or darkish in colour, saccharoidal, fine-grained, massive, partly crystalline or amorphous. The results of solution weathering are seen in giant stalactites on scarp faces and swallow-holes. At the northern end of Winpauk Taung and near Kyauktaga (15° 47': 97° 57′), the limetone is brecciated and veins of calcite cement the brecciated pieces. This is a characteristic feature of the Lower Plateau Limestone of the Southern Shan States. Laterite usually occupies the low ground and foot-hills near the coast, forming a cap over rocks of the Mergui series, and is in course of formation at the present day. It is also found in the flat country east of the Taungnyo range in sheets 95 I/1, 2 and 95 E/13 in
Laterite and laterite with gravel.
1Op. cit. pp. 94-97.