Accretionary Lapilli from the Basal Vindhyan Volcanic Sequence, South of Chittaurgarh, Rajasthan and their Implication

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.