Page images
PDF
EPUB

Mr. Lahiri's present season's work in sheets 44 M 9, M/13 and M/14 completes the geological map of the Hoshiarpur district. The

portion of the Siwalik range lying in these Hoshiarpur district, Punjab.

sheets consists, as in the area to the south

east previously noted, of Upper Siwalik beds, both the Pinjor and Boulder-conglomerate stages being present. The central portion of the range is composed of sand-rock and conglomerates belonging to the Pinjor stage, the Boulder-conglomerates being confined to its either edge. The uppermost 1,000 feet of the Pinjor stage in 44 M/13 is highly conglomeratic, the crestal regions of the range consisting entirely of these conglomerates.

The major structure of the Siwalik range is an asymmetric anticline, the strata of the north-east flank of which are characterised by low north-easterly dips, whilst the dips on the south-west limb, which are low in the crestal area of the fold, steepen progressively towards the plainward edge of the range. In sheet 44 M/13, the crestal beds of the main anticline are folded into a very shallow but broad syncline pitching north-west. The Hoshiarpur portion of the hilly ground east of the Sohan nadi in 44 M/13 consists of Pinjor beds which are thrust upon the Upper Siwalik boulder beds of the dun country on the west. This fault is the north-westerly continuation of the Satlitta fault1 mentioned in the Director's General Report for 1933.

Mr. Lahiri mapped portions of sheets 53 B/13 and B/14 lying to the east of the Ambala-Kalka railway line in continuation

his previous work in the area to the west, Ambala district and of the line. Patiala State, Punjab.

The 'geological formations

Subathus (Eocene), Nahans or Lower Siwaliks, Upper Siwaliks and sub-Recent gravels. The Subathus consist of greenish or brownish shales and occasionally limestone and carbonaceous shales. They are faulted on the south-west against the Nahans which form the low hills occurring to the south and southeast of Kalka. The Nahans, in their turn, are faulted against Upper Siwalik boulder-gravels on the south-west. This fault is of the reversed type and is the southerly continuation of that running along the eastern edge of the Pinjaur and Nalagarh dun noted in the Director's General Report for 1935.2

arc

1 Rec. Geol. Surv. Ind., LXVIII, p. 67, (1934). 2 Op. cit., 71, p. 78, (1936).

The portion of the Siwalik range on 53 B/14 examined during the season consists, as usual, of Upper Siwalik beds. Both the Pinjor and Boulder-conglomerate stages are represented, the latter being confined to the northern flank of the range and the dun country to its north. The structure is, as in the Hoshiarpur area, an asymmetric anticline. The axis of the fold passes quite close to the plainward margin of the range. While the strata of the north limb of the anticline have low northerly dips, those of the south flank are steeply inclined as a rule.

111. Mr. J. B. Auden spent five months in the Himalayan foot-hills between November, 1936, and April, 1937, mapping on two miles

to one inch map sheets 53 F/S. E., 53 JAN. W., Garhwal district, Tehri Garhwal State,

S. W., S. E., 53 K/N. W., N. E. He has Dehra Dun district, completed the survey of the outer Himalaya United Provinces, Siró between Salon and the Ganges river and has mur State, Punjab.

remapped a portion of the area in Garhwal district described by Middlemiss in 1887.1 An account of the salient tectonic features of this zone, including the results of a part of his survey during this season, has already been published and it is unnecessary to discuss here the structure of the area described.2

Mr. Auden has determined the following unconformities in the outer Himalaya of this region : (1) at the base of the Eocene, which lies on Simla slates, Krol

limestone and Tal beds : (2) at the base of the Upper Tal limestone, which in Garhwal

cuts out the Upper Tal quartzites towards the south, as

near Gadmola (30° 2' : 78° 33') : (3) at the base of the Lower Tal shales, an unconformity which

is not pronounced, and which over most of the area may be a disconformity ; phosphatic nodules were corded many years ago at this horizon near Mussoorie, 3 and have recently been found by Mr. Auden at Masrani

(30° 27' : 78° 9') : (4) at the base of the two boulder-bed horizons of the Blaini;

the Nagthat quartzites appear to be cut out completely by the Blaini both at Kimora (30° 29' : 78° 10') and at Bhitarli Kalan (30° 26' : 78° 3'):

re

[blocks in formation]
[ocr errors]

(5) at the base of the Nagthat series in maps 53 F/14, 15,

where there is a pronounced unconformity across pre

viously folded Chandpur phyllites. It is clear that there have been several periods of folding prior to the Tertiary era, but it is difficult to assess magnitudes of the diastrophism responsible for these unconformities.

Mr. Auden has traced the Tons thrust from Sarog (30° 42' : 77° 44') to Khand (30° 32' : 78° 21'), where it appears to terminate abruptly along the Bhagirathi river against another major tectonic line which divides the Barahat series from the overlying Chandpurs. The Barahat series is a newly recognised group of rocks, consisting predominantly of quartzites together with limestones and occasional lavas. The quartzites resemble those of both the Nagthat and Tal series, but from their general relationships, Mr. Auden favours their correlation with the Nagthat (Jaunsar) series. They occupy extensive areas to the north-east of the Bhagirathi river and, like the Nagthat quartzites, show complex secondary folds. Near Pirhi hill (30° 26' : 78° 32') and Partabnagar, the associated limestones are mostly siliceous dolomites containing bands of pale chert; but towards the north-west this type disappears, metamorphism in

and the limestones occur as finely banded marbles similar to those in the Mandhali series south of Chakrata. These marble are to be seen near Guinota (30° 45' : 78° 17') and just north of the Barahat Rest House (30° 45' : 78° 27'). Amygdaloidal lavas occur near hill 5838 (30° 30' : 78° 28') and may represent a local development of the Panjal traps. A volcanic suite, likewise associated with quartzites, occurs south of Chamoli (30° 24' : 79° 20').1 The Barahat series extends to the south-east into 53 J/S. E., cropping out on Maniknath hill (30° 22' : 78° 40'), and certainly joins up with the Chamoli group which Mr. Auden noticed along the Alaknanda river in 1932 in sheet 53 N/S. W. The south-west boundary of this series with the overlying Chandpurs is regarded as a thrustplane and has been traced from 30° 46' : 78° 17' to 30° 24' : 78° 32'.

A large intrusion of dioritic rock, grading into dolerite, occurs on the east side of the Khurmola Gad, just west of Guinota (30° 45' : 78° 17'). This diorite is similar to that found on the ridge west of hill 7027 (30° 42' : 78° 03').

creases

[ocr errors]

1 Rec. Geol. Surv. Ind., LXIX, p. 134, (1935).

Mr. Auden revisited a portion of sheet F/14 in the hopes of arriving at a better understanding of the Mandhali rocks. The typical banded phyllites of the Chandpur series (which have a striking resemblance to the Lower Cambrian slates north of Srinagar in Kashmir) only occur above the Bansa limestone, but some of the pelitic rocks below this limestone are very similar to those of the Chandpur series, and it is uncertain if the arbitrary boundary between the two series, previously drawn at the top of the Bansa limestone, has any tectonic significance. That the Mandhalis underlie the Chandpurs on both sides of the syncline running across sheet 53 F/14 seems indisputable, but the correlation of the Bhadraj quartzite horizon with the Nagthat series leads to the difficulty that the same quartzitic rocks both underlie and overlie the Chandpurs. Mr. Auden hopes to discuss this problem in a Memoir it is intended to write.

Southern Circle. 112. The Southern Circle consisted of Mr. H. Crookshank, Superintending Geologist, in charge, Dr. P. K. Ghosh, Mr. D. Bhattacharji, Mr. B. C. Gupta, and Dr. A. K. Dey.

113. Mr. Crookshank continued his mapping in Bastar State and completed parts of sheets 65 F/1, 3, and 7. He states that the

mapping of both sides of the Bailadila ridge Bastar State, Eastern is now complete for about two-thirds of its States Agency,

length. It is now definitely established that the banded hematite-quartzites which cap the ridge are not a deepseated formation, but lie more or less horizontally on the surface of the older sediments which form the flanks of the ridge. To the west of the ridge these ancient sediments are almost entirely sericitic quartzites. They differ greatly from the slates, shales, and phyllites so strongly represented on the eastern flank of the range. The horizontality of the base of the hematite-quartzites, and the way in which they appear to overlap the various beds of the older series, are believed to indicate the presence of an unconformity at their base.

Mr. Crookshank goes on to show that the banding of the hematite-quartzites in this area is probably not connected with the original stratification of the rocks. He considers that it is due to the deposition of iron-ore along the planes of cleavage or schistosity in ferruginous phyllites or slates.

as

In mapping sheets 65 F/3 and 7 Mr. Crookshank noted numerous small lenticular masses and two large patches of a rock which he regards as a diorite which has to a variable extent assimilated the surrounding hornblende-schists. This rock is commonly mineralised. Dr. Dunn examined the opaque minerals in it. He reported that the rock greatly resembles the vanadium-bearing ones of Bihar, The only ore minerals which he could find were ilmenite altered marginally to rutile, pyrite, and a little hematite. No concentration of ore-minerals likely to be of any value was seen.

Dr. P. K. Ghosh continued his mapping in Bastar State, Eastern Bastar State, and completed the geological surStates Agency.

vey of sheets 65 F/6 and 65 B/5. On sheet 65 F/6 he noted the same sequence as that given in the General Report for 1936 with some variations, which are follows:

A series of amphibolites is associated with the calc-gneisses and ferruginous schists. These are probably a mere variation of the calc-gneiss.

Some basalts seen to the north-west of sheet 65 F/6 connect up with similar rocks mapped by Mr. Crookshank further to the north. Their field relations are by no means clear, but they appear to be definitely older than the dolerite intrusions in this region.

The calc-gneisses and ferruginous schists are overlain unconformably by quartzites in the north-west corner of sheet 65 F/6. In the same area andalusite and cordierite schists, gneisses, and phyllites were observed.

Phenomena representing assimilation and hybridisation are common in the granitic areas. The effects of these processes are particularly well seen in a suite of rocks closely resembling charnockites. These are well developed in sheet 65 B/5, where in the same exposure variations from acid to basic and ultra-basic charnockites were seen.

In sheet 65 B/5 aplitic gneisses, charnockite-like rocks, calcgneisses, ferruginous schists, and quartzites of Archæan age occupy the castern and northern portions, while the south and west consist of Cuddapah rocks. These differ from the Cuddapahs seen in other parts of Bastar and the Central Provinces, as quartzites overlie shales and limestones instead of underlying them. The Cuddapah basin is here faulted into the Archæan complex. This may be the reason why the basal quartzites were not seen.

« PreviousContinue »