Research Notes

RESEARCH NOTES

SHEAR PLANE FRACTURES IN A GARNET PORPHYROBLAST by K. C. Sahu and S. N. Kulkarni, (Indian Institute of Technology, Bombay)

In the course of exmination of thin sections of a garnetiferous biotite schist, peculiar sets of conjugate fractures have been noticed in some of the garnet porphyro-blasts (Fig. 1). The fractures, oriented in a direction subparallel to the direction of schistosity of the rock, are confined to the garnet crystals only and are easily distin-guished from the usual irregular conchoidal fractures characteristic of garnet.

The development of irregular and conchoidal fractures in garnet is essentially due to tensional force produced during cooling and shrinkage, a phenomenon some-what equivalent to the development of mudcracks, because of uniform tension in all directions on a structurally homogenous material devoid of strong cleavage or weak plane. Conversely a hydrostatic compressive force will produce similar fractures in the mineral.

STRESS

!

SHEAR PLANES

Figure I. Shear plane fractures in garnet porphyroblast.

While under a directional stress the ductile and flaky minerals like biotite yield easily or continue to grow in the direction of strain producing planar feature called schistosity, brittle minerals like porphyroblasts of garnet may develop shear planes by failure if the elastic limit of the mineral is exceeded. In the present case it is interesting that the set of conjugate fractures in garnet are almost parallel to the shear planes of the strain ellipsoid produced by the metamorphic compressive stress at right angle to the plane of schistosity.

Theoretically the shear planes make an angle of45° with the principal stress axis in complete absence of frictional force; but this angle is likely to be smaller(300

_ 35°)

in actual practice because of the existence of the frictional force.

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ofsimilar worm tracks in the rocks of late Precambrian age not only in India but also in other parts of the world. These worm casts, considered as fossils, are further significant as they indicate the existence of animal life in the late Precambrian period. Acknowledgement: The junior author thanks the Council of Scientific and Industrial Research for the award of a Fellowship.

REFERENCES

ALF, R. M., (1959) Possible fossils from the early proterozoic Bass formation, Grand Canyon, Arizona. Plateau,v. 31, pp. 60-63*.

PAUL, H., (1948) Pre-Cambrian fossil tracks from Ajibik quartzites of Michigan (Abstract).

Geol. Soc. Am. Bull.,v,59,p. 1320.

FRAREY, M. J. and MCLAREN, D. J., (1963) Possible metazoons from the early Proterozoic of the Canadian Shield. Nature, v,200,pp, 461-62.

GLAESSNER, M. F., (1961) Pre-Cambrian animals. Scientific American, v,204, pp. 72-78. GLAESSNER, M. F. and WADE, M., (1966) The Late Pre-Cambrian fossils from Edicora, South

Australia. Palaeontology,v. 4, pt. 4, pp. 599-628.

MISRA, R. C. and AWASTHI, N., (1962) Sedimentary markings and other structures in the rocks of the Vindhyan formations of the Sone valley and Maihar-Rewa area, India. Jour. Sed.

Petrol, v. 12, no. 4,pp. 764-75.

YISWANATHIAH. M. N., (1968) Badami Series. A new Post-Kaladgi fermat ions of Mysore Stale.

.Bull. Geol. Soc. India,v.5, no. 3.

VISWANATHIAH, M. N. and VENUGOPAL, K., (1970) Badami series of Gokak, Belgaum district, Mysore State. Abstract. 57th Session of the Indian Science Congress-Geology and Geo-graphy Section.

WALCOTT, C. D, (1899) Pre-Cambrian fossiliferous formations. Geol. Soc. Am. Bull.,v. 10,pp, 199-244*.

*Original papers not referred.

SURFACE INDENTATIONS OF PEBBLES AND THEIR SIGNIFICANCE by P. S. N. Murty, (National Mineral Development Corporation Limited)

During the course of an investigation on the sphericity trends of pebbles of three different rock types, the author observed that many of the pebbles are characterised by depressions of various sizes and shapes.

Pebbles and cobbles present in an unit area of one square metre were collected at six points located along the course of the Bhadra river in Mysore State, and separat-ed into three rock types viz, vein quartz, magnetite quartizite and amphibolite. The number of pebbles with indentations was counted for each rock type and converted into percentages; these are recorded in Table I.

TABLE I

Percentage of pebbles with pits at Average of wear and

Rock types different places , percentage tear deter- Specific - - - at all the mined by gravity A B C D E F places _experimentslaboratory

Amphibolites 13.92 24.08 27.14 17.54 30.95 12.80 20.43 20.6 2.84

8.M.Q. 46.98 44.05 50.00 17.86 61.58 31.50 39.65 26.2 3.553

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(iv) If the pebbles and cobbles of the river are to be used for concrete mixing purposes, it would be best to use amphibolites, magnetite quartzite and vein quartz in that order of importance, subject to other properties being equally favourable. Selection can be made by visual inspection of relative size of pits and percentage of pebbles with pits, from the same area.

Acknowledgements: The author is deeply indebted to Shri O. P. Vasudeva, Project Manager, Kudremukh Iron are Project for his kind encouragement and giving facilities. He is highly thankful to Dr. B. P. Radhakrishna, Director, Department of Mines and Geology, Bangalore for his kind advice from time to time. He is also thankful to Drs. R. Vaidyanathan and M. Subba Rao, Department of Geology, Andhra University, Waltair, B. K. Sahu, Rourkela and Prof. B. S. Basavarajaiah, Suratkal, for going through the manuscript and offering helpful criticism.

REFERENCE

PETTIJOHN, F. J., (1957) Sedimentary Rocks, Oxford Book Company,pp,68-72.

A CONGLOMERATE ABOVE THE LOWER VINDHYAN LIMESTONES OF CHITOR-GARH, RAJASTHANby Sagar Roy, (Department of Geology, G.C. College, Silchar-4 (Assam)

The author reports for the first time the occurrence of a conglomerate horizon sandwiched between the lower and the upper Vindhyan rocks of Chitorgarh, Raja-sthan. Upon the lower Vindhyan Chitor limestone (Nimbahera limestone of Heron, 1936), occurs an impersistent conglomerate horizon with a slight disconformity. The rock units of the upper Vindhyans of the area are shown below:

Thickness in feet

Chitor Fort quartzites

Scarp sandstones

Upper shales

l

ortho-quartzite 30 Massive

ortho-quartzite 140 Fine sandstone

with thin shaley

layers 100

Micaceous shale with thin sand-stone layers 300± Quartz-pebble

conglomerate (?)

d i s c o n f o r m i t y -Chitor limestones Lower Vindhyan

SHORTER COMMUNICATIONS 427 the south of the Sainti village has an approximate outcrop width of 100 yards and can be traced for a distance of about1000 feet. The conglomerate here dips easterly at low angles (exact dip could not be reliably measured).

Description of the conglomerate: This is a pebble conglomerate with subanglular to subrounded pebbles, cobbles and coarse sands of quartz set in a brown ferruginous cement. A small amount of calcareous cement is also present. The rock resembles in many respects the orthoquartzitic conglomerate of Pettijohn (1957, p. 256).

Under the microscope, the sandy matrix of the conglomerate shows a poorly sorted framework of granules and coarse to fine angular to subangular grains of quartz. There are a few metaquartzite fragments along with some grains of staurolite. Abundant cement is present (about 35%), mostly Fe oxides with a small amount of calcite (5%).

A metamorphic source for the quartz grains is indicated by the presence of meta-quartzite fragments, quartz with strain shadows and a few staurolite grains. High rounding of the larger grains forming the framework, again, indicates a reworked sedimentary source.

It is interesting to note that the conglomerate horizons separating the upper Vindhyan formations viz. Kaimur, Rewa and Bhander, are known to contain 'nuggets' of diamond. In view of this, the present conglomerate also on detailed

study may prove to be diamondiferous.

REFERENCES HERON, A. M., (1936) Mem. Geol. Surv. India,v.68, PartI.