SEDIMENTOLOGICAL STUDIES OF THE f^fii JODHPUR GROUP IN THE DISTRICTS OF JODHPUR & NAGAUR, RAJASTHAN, INDIA

Abstract

"Rocks of the Jodhpur Group which forms the lowermost stratigraphic unit of the Marwar Supergroup (Proterozoic to Lower Cambrian?), occur as flat-topped hills on the eastern fringes of the Thar desert in Western Rajasthan. These unfossiliferous rocks are horizontally bedded and are dominantly sandstones with subordinate shales, deposited in an intracratonic basin. On the basis of lithological variation, the rocks of the Jodhpur Group have been classified into five Formations - A,B,C,D, and E. The sandstone bearing Formation A is the oldest among these formations. It is overlain by the Formation B which consists mainly of shale with minor sandstones. The Formation C is mainly sandstone bearing. It overlies the Formation B and underlies the Formation D which consists dominantly of shale. The youngest Formation E which overlies the Formation D is almost entirely sandstone bearing. Within the Jodhpur Group, six lithofacies have been recognised on the basis of lithological characters, sedimentary structures, paleocurrents and grain size of sediments. These lithofacies have been interpreted in terms of environmental facies as braided fluvial facies, low energy beach facies with tidal effects during depositional regression, storm influenced beach facies during depositional regression, low energy beach facies during depositional transgression, lagoonal facies and shelf facies. Paleocurrent directions obtained from the foreset azimuths of cross-beds in the sandstones of the braided fluvial facies, have unimodal distribution with small variance values (1000 to 4000). The dispersal pattern of sediments appears to have remained more or less same during the deposition of the FormationsC and E. The paleocurrent directions were mainly northerly in the eastern part and north-westerly to westerly in the western part of the area of study. The streams during the deposition of sediments of the Jodhpur Group were of very small stature with low discharge. The sinuosity of the streams was of the order of 1.07. The mean channel depth, bankful depth and width to depth ratio have been estimated to be of the order of 1.22 m, 2.92m and 81 respectively. The mean annual discharge and the mean annual flood discharge were 60 cu.m/sec and 490-1860 cu. m/sec respectively. The cyclic nature of sedimentation in the braided fluvial facies has been studied by facies-state transition probability matrices. A typical cycle starts with an erosional surface at the bottom of relatively coarse grained facies_states such as crudely bedded or planar cross-bedded conglomerate or trough cross-bedded sandstone or large scale planar cross-bedded sandstone. These facies-states pass upward to finer facies-states— cross-laminated sandstones and siltstone, either directly or through horizontally bedded and small scale planar cross-bedded sandstone. Within this facies, the channel floor lag deposits are probably represented by conglomerate facies-state. It appears that the migration of bars and development of plane beds in the channel resulted in the formation of trough cross-bedded. large scale planar cross-bedded and coarse grained horizontally bedded sandstones. The fine grained horizontally bedded and small scale cross-bedded sandstones were probably associated with the bar tops and nearly filled channels. Cross-laminated sandstones and siltstones might have been formed by vertical accretion. In the beach facies, to the west of Jodhpur city, asymmetric ripple marks show bipolar nature of paleocurrents with two modes 180° apart. These modes give north-westerly and south-easterly paleocurrent directions which may be due to waves travelling towards and away from the northeast-southwest trending coast. In the beach areas, foreshore,shoreface and offshore facies have been identified. Petrographically, the sandstones of various formations of the Jodhpur Group, are mainly quartz arenite with minor amounts of sub-feldspathic lithic and feldspathic arenites and arkose.Quartz is the dominant constituent of these sandstones. Among the non-opaque, non-micaceous heavy minerals, zircon, tourmaline and rutile are the most common minerals. However, garnet, kyanite, sphene, anatase, actinolite, hornblende', monazite and dumortierite also occur in minor quantities. Amongst the opaque minerals, magnetite, ilmenite, hematite, limonite and leucoxene are frequently encountered. The skeletal grains of sandstones are bonded together mainly by siliceous and ferrugenous cement. Whne the sedimentary and metasedimentary rocks of the Aravallis supplied the bulk of the sediments to the Jodhpur Group as indicated by high maturity of these sandstones, the igneous >

and metamorphic source rocks such as the Malani Rhyolite^the jalor-Siwana Granite and the schist, gneiss, phyllite and slate of Archeans, also acted as minor provenances. Criteria to distinguish sandstones of different formations have been evolved using discriminant functions based on the modal constituents and heavy minerals of the sandstones. This technique has also helped in identifying zircon, tourmaline and rutile as those heavy minerals which made significant contributions in discri minating the sandstones from different formations. Clay minerals of shales from the Formations B and D are mainly kaolinite and illite with minor amounts of montmorillonite and chlorite. On the basis of illite intensity ratio and crystallinity index, shales from these two formations can be differentiated. Lateral variation in clay minerals indicate 'marine-like' environments in the northern and western part of the area of study. Variation in the illite index of shales indicates existence of a closed basin in the Central part of the area in which sediments of the Formation D were deposited. The sandstones are fine to medium and coarse grained with moderate to moderately well sorting. On the whole, the Jodhpur Group has 'coarsening up' aspect which was probably controlled by gradual increase in the tectonic activity of the area. Different sedimentational environments have played little role in modifying textural characteristics of sandstones. Amongst the bivariate plots, however, the scatter diagrams of mean sphericity versus sphericity

variance of quartz grains and mean grain size versus size standard deviation can be used to some extent to decipher and distinguish one environment of deposition from the other.Discriminant function analysis using moments of size, sphericity and roundness of quartz grains has helped in differentiating beach from shelf and shelf from braided fluvial facies of the Formations C and E. A series of transgressions and regressions of sea took place during the formation of the Jodhpur Group in this area. The lower part of the Formation A was formed during a regressive phase. A transgressive phase then started and continued till the sediments of the Formation B were deposited. The Formations C and E with the intervening Formation D which was formed under stable sea conditions, were deposited during regressive phases. Petrographic and textural studies of sandstone and, conglomerate reveal that the sediments of the Jodhpur Group were deposited in warm and humid climate. Paleolatitude of about 35° to 36° H of the Jodhpur area probably also supports this view."