PHOTONIRVACHAK Jour. Ind. Soc. Photo-Int., Vol. V, No. 2, 1977

NOTE A NOTE ON PHOTOGEOLOGICAL MAPPING OF THE CRETACEOUS

TERTIARY FORMATIONS IN SOUTHERN FRINGES OF MEGHALAYA

SURI3SH CHOPRA

ABSTRACT

Mapping of the Cretaceous-Tertiary sedimentary formation in the southern fringes of the Khasi Hills and the adjoining districts of the Shillong Plateau through Photo-interpretation techniques has been found to be fairly accurate and reliable. Their unique photo characters and other recognition

elements are briefly described in this paper.

INTRODUCTION

Rocks of the Cretaceous-Tertiary age belonging to shelf facies occur in the southern part of the Khasi Hills and the

adjoining districts of the Shillong Plateau. These sedimentary rocks were deposited in shallow basins, are horizontlly disposed and occur as cappings over the Archeans and the younger Shillong Series and Sylhet Traps unconformably (Murthy et al. 1971). They increase in thickness

towards the South. Their typical photo characters greatly facilitate their mapping through photo interpretation techniques unlike Tertiaries of Arunachal Pradesh, Assam and adjacent areas because of their different lithology and structural disposition. This also helps in delinea- ting the surface of unconformity to a large extent.

PHOTO CHARACTERS OF THE OLDER UNDERLYING ROCKS

The underlying rocks mostly consist of gneisses, granites, quatrzites, phyllites

and traps. These are exposed in the rivers and nala cuttings, show dark tones, thick vegetation, and moderate to steep dips. The texture is medium to coarse and cultivation is almost absent. Drain- age pattern is sub-parallel to sub-dendri- tic with moderate density.

PHOTO CHARACTERS OF CREtA- CEOUS-TERTIARY FORMATIONS

These have almost flat topography with very low rounded hillocks forming typical plateau and show light to very light tone, fine, smooth texture and are almost devoid of vegetation. This mode of occurrence coupled with tone is a typical recognition criteria. They show numerous fractures and lineaments and a well developed pattern of two to three sets of joints can be easily deciphered. Faults, sympathetic and parrallel to the Dauki fault with clear difference in eleva-

tion on the two sides are easily picked up on the photographs whereas these

82 SURESH CHOPRA

are likely to be missed in the field (Fig. are no other structural 1). Except some fau'ts and joints there observed.

PHOTO-INTERPRETED MAP OF A PART OF SOUTHERN

features to be

FRINGE OF

MEGHALAYA DISI'RICT KHASI HILLS SHOWING DIFFERENCE IN THE RECOGNITION ELEMENIS OF TERTIARY/CRETACEOUS

FORMATIONS AND OLDER ROCKS

L E G E N D

TERTIARY AND. CRETACEOUS FORIIATIOtll

ARCHA£NS /INO OLDER METAMOI~ISHICS

UNCONFORMITy

I - ~ OIP HORIZ0tiTal,.

STREAM

Drainage is mostly structure controll- ed and the pattern is either rectangular or parallel to sub-parallel. At places internal drainage is observed indicating the previous nature of the material, drainage density being low to medium. Limestones show typical karst topography with innumerable sinkholes. Land use and cuhivation is limited and vegetation is observed mostly along the water

channels. No significant gullying is seen

but the area appears to be undergoing sheet wash type of erosion and soil for- mation is insignificant. The crocodile skin type of appreance which is due to the formation of low, small rounded hillocks with some vegetation at the tops is a typical photo character.

Sometimes these rocks occur as 'Mesa' with fiat tops and steepslopes. Their geomorphic expresion and the outcrop pattern itself suggests low dips.

A NOTE ON PHOTOGEOLOGICAL MAPPING OF THE CRETACEOUS-TERIIARY 83

Bedding is often seen in the uppermost reaches of the nala cuttings and the distinct break in slope marks the un- conformable contact. The vertical scarp

faces of the Cretaceous-Tertiary forma- tions in the gorge sections are responsi- ble for the presence of numerous water falls in the area. In fact if all the falls are plotted the line joining these will approximately mark the suface of un- conformity.

CONCLUSIONS

with similar formations of adjoining areas, the Tertiaries in the southern- most part of the plateau that is in the Garo Hills show different photo charac- ters because of different structural dis- position. Their mapping thus becomes much accurate, speedy and economical by photo interpretation methods and increases the scope of photo-geological method for rapid mapping of the virgin areas of north-east India.

ACKNOWLEDGEMENTS

It is evident that Cretaceous-Tertiary The author is thankful to Shri K. formations of this area exhibit typical Gogoi for his guidance and encourage- photo characters specially when compared ment.

REFERENCE

Murthy, M.V.N., Chakravarty, G. and Talukdar, S.C., 1971 Stratigraphic Revision of the Cretaceous-Tertiary sediments of North Eastern India. Unpublished paper read in the seminar on Geology of North East India, held at

Shillong in 1976.

Address of the author: Suresh Chopra, S. C. O., 98/'100, Sector 17-C, Chandigarh.