Page images
PDF
EPUB

association with local gravel and sand. Conglomeratic pebbles are often cemented by lateritic material.

96. Mr. A. B. Dutt, before commencing systematic survey work on the Shan scarps, spent about a week familiarising himself with the Palæozoic formations in the Sedaw-Zebingvi-Maymyo area and the metamorphosed rocks and granites east of Mandalay, Belin and Kyaukso.

In the map accompanying La Touche's memoirl the hill five miles east of Mandalay is shown as consisting of Mogok gneiss. According to Mr. Dutt, however, this hill is composed of alternating bands of crystalline limestones, diopside-granulites, calcareous carbonaceous sericite-schists and calcareous sericite-quartzite at the base and crystalline limestones on the flanks and summit.

The geology of the hills east of Belin and Kvaukse has been dealt with in a recent paper2 by Mr. Clegg.

In the General Report for 1936,3 in giving Mr. Clegg's ideas about the systematic position of the metamorphosed sediments of Kyaukse, it is stated that “the metamorphosed sediments mentioned, which consist of diopside-granulites, marbles and calc-sericite-schists will eventually be found to link up with the Coal Measures which overlie the Plateau Limestones in the Kalaw area to the south”. The views expressed by Mr. Clegg have now been supported by Mr. Dutt's mapping in the

south of Kyaukse. The metamorphosed calcareous sediments of the Kyaukse area occupy the same stratigraphical position as the Coal Measures further south, inasmuch as both sets of bells overlie the Plateau Limestones and have the same general strike, N. N. W.-S. S. E., although locally, at Kyaukse, the strike is roughly E.--W. It should be noted, however, that the beds of this horizon at Kyaukse show evidence of having undergone a considerably higher degree of metamorphism than those of the southern area.

Mr. A. B. Dutt's main work consisted in the mapping of parts of sheets 93 C/3 and 7, C/4 and 8 and D/1 and 5. The country Yeng à 11

State, mapped is situated on the edge of the Shan Kyaukse and Meiktila Plateau in the Yengan State, and the districts.

Kyaukse and Meiktila districts. The western

area

[ocr errors]

I Mem. Geol. Surv. Ind., XXXIX, Pt. 2, pp. 1-379, (1913).

- E. L. G. Clegg, Note on Rocks in the vicinity of Kyaukse, Burma ", Rec. Geol. Surv. Ind., 71, Pt. 4, pp. 376-379, (1938).

Op. cit., 72, Pt. 1, p. 61, (1937).

3

portion of this area was previously mapped by Mr. P. N. Datta and the eastern portion by Mr. V. P. Sondhi whilst Dr. Cotter surveyed a small area along the Thazi-Kalaw road in sheet 93 D!1 and 5.

The geological formations mapped were as follows :(6) Alluvium. (5) Granite intrusions and associated rocks. (1) Red Beds. (3) Metamorphosed sediments consisting of mica-schists and lime

stone. (2) Coal Measures. (1) Plateau Limestone with limestone conglomerate and breccia.

The oldest formation of the area is the Plateau Limestone which consists of grey massive limestones, much brecciated and traversed

by numerous veins of calcite. Occasionally a Plateau Limestone.

limestone conglomerate or breccia is met with. The Plateau Limestone maintains its typical characteristies throughout the area examined, except near its contact with intrusive rocks where it has been transformed into metamorphic calcareous rocks such as calciphyre, an outcrop of which occurs south-east of Taungdaw (21° 16' : 96° 18').

Overlying the Plateau Limestone are the Coal Measures consisting mainly of shales, sandstones, conglomerates and quartzites,

though occasional bands of shaly sandstones, Coal Measures.

coarse felspathic sandstones and limestones are also noticeable. The shales are greenish to brownish in colour and occasionally contain nodules of hardened clay, as for instance on the river bank N. N. W. of Kunaw (20° 58': 96° 27'). Conglomerate outcrops are generally confined to sheet 93 D/1 and 5, although an outcrop of metamorphosed conglomerate is seen in sheet 93 C3 and 7. At the junction of the granite and the Coal Measures the sandstones are metamorphosed to quartzites. The junction between the Coal Measures and the Plateau Limestone is invariably a zone of high calcification and is often obscured by calcareous tufa. The general strike of the beds is N. N. W.-S. S. E. and the dip is either in an E. N. E. or W. S. W. direction, the amount of dip varying between 60% and 70°.

The metamorphosed sediments consist of mica-schists and greyish limestones and occupy a triangular area at the south end of sheet

ments.

93 D/1 and 5. They are largely intruded by Metamorphosed sedi.

igneous rocks and were previously mapped by

Dr. Cotter as Chaung Magyis. Mr. Dutt is, however, inclined to think that they are really members of the Coal Measures, subsequently metamorphosed by the intrusion of a granitic magma, but that their exact correlation will only be possible when the unexplored area to the south has been examined ; he has, therefore, refrained from assigning any age to them at present. Only a small outcrop of the Red Beds occurs in the area. They

consist of brick-red to purplish soft sandstones, Red Beds.

sometimes having a mottled appearance. By far the most predominant rocks are granites and porphyries. The granites are generally of the biotitic type and are often gneissic

in the fine and the medium varieties. Locally Granite intrusions and associated rocks.

they are porphyritic and with an increase of

hornblende they grade into hornblende-granites. They are traversed by quartz veins and pegmatites; dark basic segregations are occasionally seen in them, while basic doleritic rocks showing ophitic texture were noted in them in three places. Another interesting rock type of the area is volcanic tuff which occurs in small outcrops and consists of angular and rounded fragments of quartz and felspar in a groundmass of quartz and sericite. The porphyries include both quartz and felspar-bearing varieties. They are found in lenses and rounded patches intrusive into the Coal Measures.

97. Mr. V. P. Sondhi's field season was spent with the Sino-British Boundary Commission and his field work was restricted to the area

traversed by the Commission or by topograSino-British Boun: dary Commission.

phical survey parties, ration parties and patrols

working in conjunction with it. La Touche's published map of the Northern Shan States1 extends to about two miles east of Tangyan (22° 29' : 98° 26'30"), the roadhead of the Commission, and east of this point Mr. Sondhi's work may be regarded as reconnaissance traverses, the formations alluded to being those described by La Touche in the memoir quoted.

Mr. Sondhi reports that for a few miles to the east of Tangyan, the plateau-like rolling country is occupied by Plateau Limestones, but as the foot of the great hill range, with its highest peak shown as 6,758', is approached, different members of the Lower Palæozoic rock formations, namely Namhsim sandstones, Hwe-Maung beds and Naungkangyi series, are exposed. From these a few fossils were collected. With the exception of a relatively narrow basin of Red Beds with interbedded fossiliferous limestone in the vicinity of Man Nagang (22° 16' 30" : 98° 33') this Lower Paleozoic sequence lies in a north-south belt about two miles wide and is overlapped on the east by Plateau Limestones which form the Loi-se hill-range. Plateau Limestones then cover .practically the whole of the country to within one mile of the Salween River; their general strike is N. N. W.-S. S. E. and they have strong easterly dips.

1 Mem. Geol. Surv. Ind., XXXIX, Pt. 2, (1913).

The gorge of the Salween in sheet 93 J/12 is walled on the west by an enormous well-defined scarp of Plateau Limestone, along the eastern base of which the river flows some thousands of feet below. The limestones rest directly upon shales and sandstones belonging to the older rock formation of the Chaung-Magyis.

The valley of the Salween forms not only an important physical feature, being almost 5,000 feet deep, but also an equally remarkable geological one, as it separates two distinct types of country. On the west, although Plateau Limestones predominate, there are widespread deposits representing different ages in the geological sequence, from the Chaung-Magyi series at the base to Red Beds and lacustrine deposits at the top. As soon as the Salween is crossed, however, the greater part of the country visited by Mr. Sondhi was found to be covered by older deposits of shales, phyllites, slates, and sandstones, considered to be equivalents of the various members of the Chaung-Magyi series. Plateau Limestone, Red Beds and lacustrine beds are also present in lesser quantities but nowhere have Lower Palæozoic rocks and Rhætic beds been identified.

Near Ta Manhsom (22° 13': 98° 36'), where the Salween was crossed, several good sections occur on the river banks. The rock consist of tabular beds of quartzitic sandstones intercalated with thinner layers of dark sheared shale, which dip E. N. E. at from 60-70 degrees. These continue eastwards through Kawn-ye (22° 10': 98° 41') to Panghsang, and show a gradual change in lithology and degree of metamorphism. Eastwards the relative proportion of shale and degree of metamorphism increase, until near longitude 99° sandstones disappear altogether and bluish green, lustrous phyllites are all that one sees.

3

At Panghsang (H 17)1 the Nam Hka makes a sharp double bend, apparently due to an east-west fault passing through the area. South of the town, younger rocks are seen for the first time since leaving Ta Manhsom : the low hills immediately to the south of Panghsang are composed of hard grit and arkose and represent the base of the Red Beds which are well developed in the Mengma (L 16)1 valley to the east. South of this occurrence, across the Nam Hka stream, Plateau Limestone appears as a line of crags and cliffs, and an extensive sheet of the same rock formation caps the phyllites north-west of Panghsang.

The Red Bed deposits mentioned above extend to the head of the Mengma valley and form steep hills to the north and south. On the track several good sections of their unconformable junction with the phyllites are exposed.

West of Monglem (N 15)1 a belt of Plateau Limestone runs first to the northeast towards Ching Maw, (Q 13)1 and then turning north runs through the line of the well-known peaks of Pola She (P 11) and Hko Di Shan (Q 9)? In the south the belt is over four miles wide but it narrows down considerably towards the north, where presents a fault-scarp on the west, overlooking the Monghsaw (9 11) valley. The limestones lie unconformably upon older phyllites, which here contain beds of tuff.

The phyllites and associated tuffs occur as an elongated inlier in the limestones, since on the east they are again wrapped round by limestones which join the western Pola She belt, on the north and south.

The phyllitie series underlying the Pola She-Hko Di Shan scarp is exposed in a limited area only, along the foot of the scarp. Imme

diately beyond it on the west, the country is

of the covered by a large basin of Red Beds which Monghsaw area.

overlap phyllite and in places come in direct contact with the limestone. On the east their boundary with the older rock formation runs practically due north and south, along the crest of the ridge that bounds the Monghsaw valley on the east. The rocks consist of deep red sandy shales and sandstones deposited on a base of coarse grits and arkose which form the crest of the ridge, and which dip to the west at high angles. East of Monghsaw, however, the dips change to the east, thus demonstrating that the

Red Beds

'Numbers in brackets are those of squares on the Sino-British Boundary Commission mar.

« PreviousContinue »