Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

M. Raza ¹, S. M. Casshyap ¹, A. Khan ¹

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh - 202002, IN

Abstract

Well-preserved accretionary lapilli of different types are reported from the basal volcanic sequence (Khairmalia basalts) of Lower Vindhyan occurring to the south of Chittaurgarh. Four types of lapilli are identified on the basis of the nature of their core, surrounding rim zone and physical appearance. The first two types comprise cores of volcanic glass and welded tuff surrounded by rims similar to the coated accretionary lapilli or type-B of Reimer. The third type is a typical armoured lapilli, and the fourth type is a core type lapilli or type - A of Reimer. The internal structure and characteristics of the lapilli suggest their formation from ash-charged volcanic cloud produced by repeated volcanic explosions. Their excellent preservation, lack of fragmentation, presence of large size vesicles and absence of sedimentary structures in the matrix suggest little post-depositional transport. The initial explosive volcanic events and resultant basic flows and volcaniclastics, succeeded by deposition of interlayered fine clastics and conglomeratic coarse cross-bedded sandstone in the adjoining down warped (? half graben) basin (s) has heralded the onset of Vindhyan sedimentation in a fluctuating shore zone environment in southeastern Rajasthan.

Keywords

Volcanism, Mesoproterozoic pyroclastics, Accretionary Lapilli, Tectono-Magmatic Event, Vindhyan Basin, Chittaurgarh, Rajasthan.

Full Text

     

S. M. Casshyap ¹, B. D. Bhardwaj ¹, M. Raza ¹, A. Singh ¹, A. Khan ¹

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh - 202 002, IN

Abstract

Khardeola Formation (∼200 m), representing the lowermost clastic assemblage of Vindhyan Supergroup in southeast Rajasthan, consists of a fine clastic sequence in the lower part and a gritty, coarse to medium grained sandstone in the upper part. The formation crops out as discontinuous linear patches close to the western boundary of the basin. The Khardeola assemblage is investigated in this study for sedimentary facies, palaeocurrents, depositional environments and palaeogeography at the onset of Vindhyan sedimentation.

The lower fine clastic sequence (15-60 m thick) consists of interlaminated shale and siltstone, thinly bedded sandstone and mudstone,and red sandstone in a coarsening upward sequence, showing parallel to wavy lamination, ripple marks, desiccation cracks, bidirectional cross lamination, and gently inclined lamination. The facies assemblage, their characteristics, and occurrence alongside linear bodies of Khardeola sandstone, call for a protected depositional environment (back barrier lagoonal to tidal flat). The succeeding Khardeola sandstone, forming the upper part of the assemblage, represents 40-150 m thick sequence of moderately well sorted quartzarenite, subarkose and sublitharenite, and crops out as narrow linear ridges trending north-South. These sandstone units are divisible into four facies on the basis of fining upward texture, bedding types, and scale of sedimentary structures. Conglomeratic facies occurs in the basal part of sandstone with well defined scour base. The succeeding facies of lenticular sandstone with scour base comprises upward thinning sets of large, medium and small scale planar and trough cross bedding. The paleocurrent pattern reflects bipolar, bimodal to trimodal dispersal from the base of the sequence upward, directed broadly towards east and west, and occasionally towards north or south. Inasmuch as the source area providing quartzo feldspathic sediments was located mainly to the west of study area, the easterly paleocurrents were directed basinward (seaward). These deposits are interpreted as originating within laterally migrating tidal inlets (barrier inlets), in which easterly oriented ebb currents were dominant, with intermittent influx of flood oriented and north or south directed longshore currents. Plane bedded to gently inclined units of quartzarenite are interpreted as beach foreshore (spit) deposits that occur in places, capping the barrier inlet sequence. A combination of barrier inlet migration and shoreline transgression resulted in a truncated and modified barrier island sequence, dominated by inlet fill sandstone that merges with or oversteps westward (landward) the interlayered, fine grained clastic lagoonal/Tidal flat facies.

The Khardeola sedimentation was terminated with the decline in sediment supply, rise in sea level and landward migration of shoreline, followed by deposition of the overlying algae dominated Bhagwanpura Limestone in a quiet,open shelf of transgressive phase.

Keywords

Mesoproterozoic, Cratonic Basin, Sedimentary Facies, Paleocurrents, Beach, Barrier Island, Palaeogeography, Vindhyan, Rajasthan.

Full Text

     

M. Raza ¹, S. M. Casshyap ¹, A. Khan ¹

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh - 202 002, IN

Abstract

The western margin of the Vindhyan Basin of north Indian shield, is characterized by well developed Mesoproterozoic Lower Vindhyan succession. It contains thick units of shales and sandstonc with minor conglomcratc and limestone at different stratigraphic levels. Five shale units, namely Khardeola Shale (KHS), Palri Shale (PRS), Binota Shale (BNS), Bari Shale (BRS) and Suket Shale (SKS) occurring in chronological order, have been examined for their geochemistry to determine the composition of their source rocks, to understand the environments of weathering and sedimentation and the tectonic conditions at the time of their deposition.

The Lower Vindhyan shales are characterized by high SiO₂, low CaO and MgO, very low Na₂O and very high K₂O/Na₂O ratio relative to average Proterozoic shales. All the shale units are severely depleted in Ca, Na and Sr and slightly enriched in some trace elements such as Co and Nb, relative to the early Proterozoic upper continental crust. It appears that the high field strength and transition elements remained immobile throughout the sedimentary processes and represent the source rock composition. Geochemically, the individual shale units do not show any significant variation with age, excepting the Khardeola Shale that is relatively more enriched in ferromagnesian and large ion lithophile elements.

Geochemical data suggest that sediments of Lower Vindhyan shales had their source in stable continental areas and deposited in intracratonic basin away from active plate boundaries. The immobile trace element data suggest that the source material of these shales have been derived from felsic and mafic components of Banded Gneissic Complex occurring to the west of the Great Boundary Fault in the proportion of about 4: 1. Severe but non-steady-state weathering conditions in the source region having humid, and tropical climate appcar to have prevailed throughout the sedimentation of Lower Vindhyan sequence. The sequence of events suggcsts passive type rifting environment during the Vindhyan Basin formation.

Keywords

Geochemistry, Shales, Vindhyan, Mesoproterozoic, Chittaurgarh, Rajasthan.

Full Text