Page images
PDF
EPUB

s Chinji stage
Kamlial stage

Lower Siwaliks

transition.

Murrees.

The Murrees, which occupy the whole of the north-east corner in sheet 43 P1, are not a uniform or homogenous series, but are composed of at least two contrasted rock facies. One facies can easily be identified as the Lower Murrees, composed usually of hard, fine-grained, calcareous sandstones and purple splintery shales and nodular clays. The other facies is principally composed of comparatively soft fine to sugary textured sandstones of pale yellow, grey or grey-brown colour and dull red to reddish brown shales and clays. The rocks of the second facies show neither the high dips nor the crushing and crumpling which characterise the Lower Murrees; on the contrary, they have low dips (20°-40°) and form prominent dip-slopes and scarps. The sandstones show ripple-markings and, being well jointed, break into huge blocks sometimes measuring over 20 feet. The plant fossils obtained from these sandstones at Maneh (32° 59' : 75° 14') and Ka (32° 58' : 75° 11'), resemble very closely the plant fossils from the Murree and Kasauli beds described by 0. Feistmantel. The lithological characters of the second facies of Murree rocks are similar to those of the Kasaulis of Dharmsala (Kangra District) and so it is suggested that this second. facies might possibly be of Kasauli or Upper · Murree age. Rocks of typical Upper Murree age, comprised of Siwalik-looking sandstones and characteristic Lower Murree shales, were observed near Krimchi (32° 57' : 75° 7'). However, it was not possible conveniently to map the two rock facies separately in this area in the north-east corner of sheet 43 P/1, and they have accordingly been classed together tentatively as Lower Murree.

In the south-west corner of sheet 43 P/1, the “Mansar-Suruin Sar” anticline, definite Upper Murrees pass conformably into Kamlials. Although no particular bed can be taken as the exact boundary between the two stages, yet the group characters of the two are markedly distinct. The approach of the Kamlials is indicated by the occurrence of massive beds of dark hard sandstone. In this area, it was noticed that a prominent bed of ossiferous clay-conglomerate was invariably present at the base of the massive sandstones,

1 Rec. Geol. Surv, Ind., XV, p. 53, (1882).

an

and as this bed was easily identified in the field, it was taken as the base of the Kamlials. A somewhat similar bed of ossiferous clayconglomerate, found at the top of the Kamlials, served to demarcate the boundary between them and the succeeding, conformable Chinji stage.

The Chinjis are essentially argillaceous formation, being composed of characteristic brick-red to deep red clays, soft lightcoloured sandstones and pseudo-conglomerates. The proportion of clays to sands is 2 : 1 at the base but 1:1 at the top.

The Chinjis are gradually succeeded by a vast thickness -over 6,000 feet--of massive, coarse sandstones of greenish grey or pepperand-salt” colour and inconspicuous clays of grey or blue-grey colour belonging to the Nagri stage of the Middle Siwaliks. The coarse sandstone is locally calcified and, on weathering, gives rise to characteristic balls' of various sizes and shapes at the foot of the hills.

In the Jammu-Udhampur road section in sheet 43 L/13, the Nagris are conformably succeeded by the Dhok Pathan stage ; but in the Tawi section in Udhampur, sheet 43 P/1, the Nagris are directly succeeded by Upper Siwalik beds. The Dhok Pathan stage is most probably overlapped here.

The Dhok Pathan stage in the road section is succeeded by the Pinjor and Boulder Conglomerate stages of the Upper Siwaliks. The former stage is composed of coarse, crumbling, pebbly sand-rock and variegated silts and clays, and the latter stage of boulder conglomerates with inconspicuous sandy matrix.

The principal structural feature of the Murrees is the geniculate deflection at the crossing of the Biruin Nala where the strike of the rocks changes from N. W.-S. E. to N. E.-S. W. and later to E. by N.-W. by S. The cause of the bend is uncertain.

The chief tectonic features of the Siwaliks are the two northwesterly pitching anticlines and synclines. The axes of the folds are fairly straight, trending S. E.-N. W.; but in sheet 43 L/13, they swing to S. S. E.-N. N. W. The Mansar-Suruin Sar anticline in the south-west corner of sheet 43 P/1 is a highly compressed anticline with almost vertical limbs, the component beds dipping at 60°-85° N. E. and S. W. The anticline is followed to the northeast by a syncline, the northern limb of which, as well as the southern limb of the succeeding anticline, is entirely cut out by a low-hading reversed fault (Kishanpur fault). The Middle Siwaliks which occur in the syncline also dip at high angles. The high dips of the zones

to a

of strata composing the above folds suggest that they were subjected

severe compressional movement coming from the north. Beyond the Kishanpur fault, the principal structure is an asymmetrical anticline of the Lower Siwaliks which, in turn, passes into a shallow syncline containing the Middle and Upper Siwaliks. The Udhampur syncline is cut out to the north-east by the Murree boundary thrustfault. The boundary fault trends roughly N. W.-S. E., but makes a sweeping curve on crossing the Biruin Nala. The inclination of the thrust plane varies from about 30° to about 70°.

109. During March and April, 1937, Mr. W. D. West continued his survey of the Simla Hills, the area mapped falling within the one

inch sheets 53 A/16, E/3, E/4, E/7 and E/8, Simla Hills, Punjab.

in the States of Suket, Bhajji and Baghal. Mr. West first mapped the rocks occurring along the Sutlej valley between Chaba (31° 13' : 77° 10') and Talah (31° 15' : 77° 15'). In this area there occur bands of massive pink and white quartzites, with subordinate purple slates, in the midst of the Lower Shali limestone. These bands are seen to be sharp folds closing upwards, and they are clearly the crests of anticlinal folds. Their position in the centre of the great Shali anticlinorium supports this view, and indicates that they form the lowest horizon of the Shali series seen in this area.

A final examination of the structure at Chaba confirmed the view previously held that there occurs here a subsidiary thrustfault which brings a thick slice of the Upper Shali limestone and Shali quartzite to rest on the Madhan slates (normally above the Shali series). The Power Station is situated on this thrust mass.

Having thus completed the mapping of what may be termed the Shali area, Mr. West proceeded to map the Shali thrust west and south-west from Tatapani (31° 15' : 77° 5'). Between Chaba and Tatapani both sides of the Sutlej valley are occupied by rocks of the Chail series. But at Tatapani the Shali series re-appear once more, coming up in the core of a pitching anticline of the Chail series, from which they are separated by the Shali thrust. From Tatapani the line of the Shali thrust on the south side of the Sutlej valley runs nearly due west for about eight miles, and then rapidly swings round through a right angle to run south towards Arki (31° 8' : 77° 58'). In the same manner the strike of the rocks above and below the thrust also swings round,

[ocr errors]

At Tatapani the Chail series is thrust over the Lower Shali limestone, though a few feet of chlorite-schist intervene in places along the line of the thrust. To the west this bed of green schist gradually thickens, until by Kaslog (31° 15' : 77° 58') its outcrop is nearly a mile wide. As it thickens it becomes less schistose, and can be recognised as a definite volcanic rock, a common variety being full of amygdales of quartz and chlorite. South-west from Kaslog its thickness decreases, until by hill 5998 (31° 13' : 76° 57') it is finally cut out by the Shali thrust, which brings the Chail series to rest once more upon the Shali limestone.

Followed to the north-west from Tatapani this volcanic bed continues along the north side of the Sutlej valley, and it seems likely from Medlicott's observations (Mem. Geol. Surv. Ind., III, 1864) that it joins up with the traps of Mandi State. These were described by McMahon, and are almost certainly Panjal trap. It thus seems possible that the Panjal trap is represented in the Simla area.

Southwards from A 5998 the trace of the Shali thrust follows a very straight course, running almost due south towards Hat Kot (31° 5' : 76° 58'). From A 5998 to as far as the Bilaspur-Arki road the Chail series is thrust over the Shali limestone, which to the west is overlaid normally by the Tertiary rocks. But south of this road the Shali limestone is cut out, and the Chail series rests directly upon the Tertiary rocks, the Chail series forming an escarpment overlooking the low-lying Tertiary country to the west.

The Shali thrust has thus been followed to within about a mile and a half of Hat Kot. Further south it seems likely, from what is known of the strike of this area, that it will run first S, S. E. and then south-east. Should this prove to be correct, then it is likely that it will join up with the Giri thrust by Kandaghat (30° 57' : 77° 7').

As regards the age of the Shali limestone, its correlation with the Krol limestone cannot yet be regarded as certain. Between the last exposure of the Shali limestone west of Arki and the Krol limestone of the Krol mountain there is a gap of about 15 miles. Moreover it cannot be overlooked that the lithological sequence within the two series is not identical. The probability is that the two series are the same age, but the question must be left unsolved for the time being.

110. During the field-season 1936-37, Mr. H. M. Lahiri mapped sheet 52 D/4 and a portion of 43 P/16, lying in the Kangra district and the Chamba State, portions of 44 M/9, M/13 and M/14 situated entirely in the Hoshiarpur district and portions of 53 B/13 and B/14, lying partly in the Ambala district and partly in the Patiala State.

The geological formations met with in 52 D/4 and 43 P/16 are the Nahan or Lower Siwalik, the Middle Siwalik, the Upper Siwalik

and sub-Recent deposits. The lithology of the Kangra district and

various formations is in general the same as Chamba State, Punjab.

that given in previous reports on the areas to the south-west. It may be noted, however, that the Middle Siwalik sandstone, which contains only scattered pebbles in the southern and western parts of 52 D/4, becomes highly conglomeratic in the exposures in its

extreme north-east corner. The Pinjor (Upper Siwalik) strata also become increasingly coarse to the north-east.

The central structure in this area is an anticline of Nahan beds which form the ridge with on it the villages Masrur (32° 4' : 76° 8') in 52 D/4 and Mastgarh (32° 13' : 76° 0') in 43 P/16. South-west from this ridge, there is a regular succession from the Nahans upwards to the Upper Siwaliks, all the beds being characterised by high south-westerly dips. The structure of the Upper Siwalik beds beneath the sub-Recent deposits of the Bubl-Dehar-Gáj valley, which occupy a large area in the eastern part of 43 P/16 and the south-western quarter of 52 D/4, is an asymmetric syncline, the strata of the south-west flank of which have low north-easterly dips whilst those of the north-east limb are steeply inclined as The Upper Siwalik beds of the north-east flank of the Masrur anticline are faulted against the Nahans of the Bohar Kawalu (32° 5' : 76° 12') ridge. This is the Gumber fault of Medlicott.1 North-east of this fault there is again a regular sequence up to the Upper Siwaliks until the Shahpur valley in the north-east quarter of 52 D/4, where they are overlain unconformably by valley deposits of probable sub-Recent age. The Middle and Upper Siwaliks reappear north of this valley, their junction with the sub-Recent beds being a low-angle thrust well seen in some sections. The structure of the Siwalik beds north of this fault is an anticline pitching east.

a rule.

1 Mem. Geol. Surv. Ind., III, p. 134, (1864).

н

« PreviousContinue »