Ripple Marks in Alwar Quartzites East of Mera Ka Gurha Near Udaipur, Rajasthan

RIPPLE MARKS IN ALWAR QUARTZITES EAST OF MERA KA GURHA NEAR UDAIPUR, RAJASTHAN

BY V. K. VERMA (Department of Geology, University of Jodhpur) Received Deeember 7, 1963 (Communicated by Dr. R. S. Varma, r.A.SC.)

ABSTRACT The present note includes a brief account of the ripple marks in the Alwar quartzites of ah area East of the village Mera Ka Gurh~ near Udaipur. Both symmetrical and asymmetrical types of ripple marks have been noticed. In the case of asymmetrical ripples, the ripple index and the horizontal forro hadex have been dealt with in detail. An attempt is made to decipher the palaeogeographical, ecological and other conditions that were present at the time of formation of the riDples.

/NTRODUCTION TH~ present paper embodies the results of the study carried out on the ripple marks of the Alwar quartzites of the Delhi System. The material was coUected by the author from a quartzite hill, about 2 km. east of the village Mera Ka Gur~a and about 10.5 km, i'qE of Udaipur City (N. Lat. 24 ~ 35', E. Long. 73 ~ 41')in Rajasthan. Here the quartzites are thrusted over Aravallis. Large portions of Western and North-western Rajasthan are covered with sands. Among the prominent rock types constituting the geology of Rajasthan are those that belong to the Banded Gneissic Complex, the Aravallis and the Vindhyans. Isolated exposures of Jurassics are noticed between Jaiselmer and Barmer. Nummulitic limestones of Laki age are exposed near Khewansar. Kirthars have been reported from an area three miles WSW of Marh village near Kolayat, Bikaner.

PREVIOUS LITERATURE In the absence of any identifiable fossils in sedimentary formations, a study of fossil ripples is an excellent aid in deciphering the palaeogeographical and the environmental conditions, the depth of formation and in the elucida- tion of structures in highly disturbed tectonic area. 313 314 V.K. VERMA

One of the eafliest references available on ripple marks is perhaps that of Darwins's (1883). The effects of velocity, depth of water, viscosity, size and density of the grains have been dealt with by Bucher (1919). Gilbert (1899) observed that for moderate depths, size of the ripples is not very sensitive to depth. Among the other pioneer contributors to the knowledge of ripple marks are Bethune (1936), Evans (1940), Johnson (1916) Hjulstrom (1936), Hantzshell (1938), Lamont (1936), and Woodford (1935).

CHARACTERS OF THE RIPPLE MARKS Both symmetrical and asymmetrical types of ripple marks are met with in this arca. In one of the types, a section transverse to the ripple presents an asymmetrical profile, having a gentIe slope on one side, called the stoss anda steep slope on the other side, called the leeside. Each one of these ripple marks may be considered asa miniature dune, stoss being in place of topset and leeside in place of foreset. The crests are devoid of s•arp edges and are perfectly well rounded with smooth outline, constituting a part of a sphere with an average radius of about 7 mm. Kindle (1917) defines the distance between the adjacent crests or troughs of a ripple asa "wavelength" and the elevation of the crest above the adjacent trough as the "amplitude ". A few representative ripples were taken for the determination of the wavelength and the amplitude; and the ripple index-- the ratio between lhe wavelength and amplitude--calculated. The results are recorded in Table I. TABLE I

Wavelength Amplitude Ripple Index x--~ nlm.. raro. x

17"0 4.0 4.25 --0-72 22.0 5.0 4.40 --0-57 20"0 4"0 5"0o +0-03 16-0 2"5 6.40 +I" 43

17"0 3.0 5.60 +0.63 19-0 4.5 4.20 --0.77 Ripple Marks in Alwar Quartzites East of Mera Ka Gurha 315

It is observed that the average wavelength and the amplitude come out to be 18.5 mm. and 3.8 mm. respectively. The moan ripple index ~ is obtained by the formula

x=¡ lE x where, n is the number of observation. Substituting the values, ~ is found to be 4.97. However, it has been felt that for the same mean value the distri- burlon of the ripple index may vary considerably. Thus, the mean deviation, i.e., the mean of the deviations of the observations from the mean, is calculated which is given by the formula,

d= ¡1)-~, (x-Z) where d = mean deviation, x -- ~ = deviation of the observation from the mean. Substituting the values from the Table I in the above formula, the mean deviation, 1 d= • 0.03

= .005. Such a small value of the mean deviation is indicative of the fact that the ripple index obtained above is highly representative of the quartzites. To investigate the horizontal form index--the ratio of the length of the stoss to that of leeside--a few representative ripples were selected and the observations recorded us given in Table II. TABLE II

Length I..,ength Horizontal of stoss of leeside form h--ti in mm. in mm. Index, h

10"0 6.0 1 "7 +0"4 10"0 5-0 2.0 --0-1 10.0 6.0 1 "7 --0.4 9"0 5.0 1"8 --0.3 10'0 4"0 2"5 +0"4 10"0 3"5 2"8 +0"7 316 V.K. V]~.MA

Ir is interesting to note that out of six, in five cases the measure of stoss is unchanged, whereas the leeside is different in every case. It is further observed that the stoss makes an angle between 15 ~ and 20 ~ with the horizontal and the leeside forros the angle between 30 ~ and 35 o. From Table II, the average length of the stoss is about 10 mm. and that of the leeside about 5 mm. The mean horizontal form index is 2" 1. The mean deviation in this case is 0.016, a figure which is rather small, and hence the mean horizontal form index may be taken as most representative. These are asymmetrical ripples. The other type of ripples is symmetrical in which case the horizontal form index is obviously 1. The order of the ripple index is of the same range as above because of similar values of wavelength and amplitude. Here both the stoss and leeside ate inclined to the horizontal at ah angle of 30 ~ of less. DISCUSSION A close examination of a fresh surface of a transverse sectiort of the ripple, with a hand lens, reveals a fine texture of the quartzites. A low value of ripple index eliminates the possibility of wind ripples, which are difficult to be preserved in the fossil state and hitherto perhaps not reported. It must be an aqueous ripple. In contrast, to the sharp erests of oscillation ripples formed due to wave motion, these ripples have rounded crests and, therefore, are eurrent rip?les. The wavelengths and amplitudes of the ripp'.es do not indicate the p;esence of high velocity current. A low value of the horizontal form index in the case of asymmetrical ripp'.es and the presence of symmetrical ripples suggest that the velocity of the current during the course of ripple formation was moderate. In some tases, it is noticed that the crest of one ripple is just in front of the trough of the other. This suggests the presence of more than one set of ripp!es. A clean linear depression corresponding to zero displacement is observed. The presente of sucia interference ripptes suggests the cross- current activity, and this is further supported by the occurrence of current bedding. These ripples "ivere developed in exceedingly shallow water which might have been exposed intermittently and not at a depth greater than about 300 m. Immediately after the formation, these structures must have been buried without disturbance to escape erosion and facilitate preservation. Since aqueous ripple marks are formed only on the granular sediments, their presence in the Alwar quartzites indicates the granular character of the composing material at that time. Later on, they underwent metamorphism Ripple Marks in Alwar Quartzites East of Mera ka Gurha 317 but the intensity of the metamorphism was not such as to obliterate the original sedimentary structures. The Aravallis and Delhis are geosynclinal deposits. The occurrence of ripples and current beddings are characteristics of the platform facies of sedimentafion, which in fact is taken to contrast with the graded bedded arenites of geosynclines. The Alwar quartzitos with these mechanical inorganic structures (Pettijohn, 1949) must be shallow water deposits, or wero formed at the margin of the geosyncline. They may also represent isolatod masses in the geosyncline.

REFERENCES

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