SP-06: Recent Studies on the Geology of Kachchh

The early part of the Cenozoic succession of Kutch is still debated for the age of the formations overlying the Deccan basalt. Authors have differed on the question of the presence of marine Palaeocene, Lutetian and Priabonian sediments in Kutch. This paper integrates the larger foraminifer based shallow benthic zones (Serra Kiel et al., 1998) and the planktic foraminiferal zones (Berggren and Pearson, 2005) of the Eocene succession to present a revised chronostratigraphy. It is concluded that there is no evidence of Palaeocene marine sediments in on-land Kutch. The Eocene successions are referred to Ypressian and Bartonian stages. A small thickness of sediments below the Bartonian lacks age-diagnostic taxa and could questionably be of late Lutetian age. There is no evidence of Priabonian sediments in the area. Based on the present biostratigraphic study and the latest Geologic Time Scale (Gradestein et al., 2012), hiatuses of ~7 Myr between the Deccan Trap and Naredi Formation, ~7 Myr between Naredi Formation and Harudi Formation and ~4 Myr between Fulra Limestone and Maniyara Fort Formation are estimated.

Two depositional sequences are recognized in the Eocene succession of Kutch. The Sequence-I in the lower part is of Ypressian age and 6 Myr duration. A hiatus of about 7 Myr is estimated between the Deccan trap and Sequence-I and a hiatus of equal magnitude exists between sequences I and II. The Sequence-II is of Bartonian (and possibly extending to late Lutetian) age and a major part of it formed during a small interval of planktic foraminiferal zone E12 (0.5 Myr). A hiatus of ~7 Myr is estimated between the two sequences. Five depositional cycles of low amplitude, low frequency and confined to inner shelf are inferred in Sequence-I. In contrast, eight depositional cycles are recognized in Sequence-II and cycles in the lower part are of high amplitude, reaching to middle shelf. The average cyclicity is of > 1 Myr duration in Sequence-I and < 0.1 Myr duration in Sequence-II.

Passive source seismic imaging delineates fine crustal and lithospheric structures associated with the Kachchh rift zone (KRZ), Gujarat, India, which suggests a 4-7 km crustal thinning and 6-12 km asthenospheric thinning below the KRZ relative to surrounding regions. The study also depicts a constant lithospheric thickness (~76-78 km) below the median high towards the west of Kachchh rift zone while a marked decrease in lithospheric thickness as well as shear velocity (Vs) is noticed across west to east and south to north of the rift zone, which perhaps suggests an increase in partial melt contents below the central KRZ in comparison to surrounding regions. Further, we notice that the seismogenic zone extends up to 34 km depth below the central KRZ. The hypocenters of the Bhuj 2001 earthquake sequence are found to be mainly concentrated in the mafic-to-ultramafic lower curst (14-34 km depth) below the central KRZ. The coincidence of common area of crustal thinning, asthenospheric upwarping and confined aftershock activity suggests that there is a possible causal relation between the occurrences of continued aftershock activity and a 6-10% drop in Vs (or presence of carbonatite melts) at lithosphere-asthenosphere boundary (LAB) below the central KRZ. This 6-10% drop in Vs can be attributed to a ~1-2% carbonatite melt currently present at the LAB. Thus, we interpret that the presence of aqueous fluids (released during the prograde metamorphic reactions of lower crustal olivine-rich rocks) and volatile CO2 [emanating from the crystallization of carbonatite melts at shallow upper mantle depths (i.e. 50-70 km)] at the hypocentral depths, might be playing a key role in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.

Ichnological studies in sedimentary rock formations of Kachchh Basin commenced in the late 1970s, despite an allusion to a 'fossil burrow' in 1872. Numerous research articles have since been published. Most of them merely record trace fossils from sporadic sites, with an unwarranted plethora of new ichnotaxa, mainly due to lack of total comprehension of ichnotaxonomic norms. Though of late, a few articles concerning definite environmental/stratigraphic issues have become available, it is now time to introspect our work vis-?-vis advancements made by fellow ichnologists. Recognising the potential of trace fossils, novel applications with emphasis on organism behaviour and substrate nature are being discovered, pertaining to various problems prompting a reorientation in our perception. Here, we attempt such an exercise in our observations on Diplocraterion-rich horizons from three localities viz., Gamdau, Khari Nadi north of Bhuj City, and near Dahisara on Bhuj- Mandvi road. They occur in the Bhuj Formation, which otherwise is sparsely fossiliferous. The strata under consideration are thoroughly bioturbated with monogeneric ichnofauna, overlying and underlying beds being barren or sparse in traces. These beds represent colonization windows, in turn linked to changes in hydrodynamic regime. While the monogeneric ichnocoenoses suggest opportunistic taxa (r-selected) typical of inhospitable conditions to most life forms; such assemblages characteristically reflecting brackish conditions, where salinity fluctuations are inherent. Prevailing sedimentary structures, presence of colonization events by opportunistic ichnospecies, changing hydrodynamic conditions, preservation of petrified wood and foliages, and stark absence of animal remains advocate transitional ecological setting such as prevalent in a deltaic set-up.

A combined sedimentological, stratigraphical, mineralogical and geochemical investigation on Palaeogene glauconites of Kutch address some of the crucial issues related to origin of glauconite and its depositional and stratigraphic significance. Glauconite occurs within fossiliferous green shale units of Naredi, Harudi and Maniyara Fort Formation. It occurs primarily in two modes, either as an altered form of fecal pellets or as infillings within the bioclasts. The origin of glauconite may be best explained by direct precipitation of Fe-rich glauconitic smectite within the pores of bioclasts and fecal pellets followed by its maturation to glauconitic mica by the addition of K. Fecal pellet is a more favourable substrate than bioclasts for glauconitization because of easy percolation of pore water within and availability of relevant ions in pellets. Strong biostratigraphic control allows us to clearly establish that relatively high rate of sedimentation (>27 m/Ma) discourage glauconite formation in the Fulra limestone while low rate of sedimentation rate (<5m /Ma) allowed its formation. Occurrence of glauconite exclusively within the transgressive systems tracts and its absence within the highstand systems tracts corroborates the requirement of low rate of sedimentation. Formation of mature glauconite in shallow marine open as well as in protected lagoon suggests that glauconite may be a poor indicator of depositional environment, despite the fact that it forms in modern deep marine conditions. However, positive Ce-anomaly in all glauconites suggests that dys-oxic depositional setting encourages glauconite formation. Absence of glauconite within the unfossiliferous and emerged red shales, immediately overlying and underlying the glauconitic green shales suggest that oxic condition discourages glauconite formation.

The Deccan Traps are of global interest for their possible links to Cretaceous-Tertiary (K-T) mass extinction event and global climate change. Radiometric dating of Deccan Trap lavas and intrusions have shown that bulk of the magmatic activities occurred 65 (?1) Ma. Isotopic similarity between Deccan and Reunion lavas have been used as an evidence to suggest that Deccan magmas were supplied by the Deccan-Reunion plume-head. Kutch is located in northwest India where, based on the trajectory that Indian plate took while crossing over the Deccan-Reunion plume, the earliest Deccan lavas would be expected to erupt. Contrary to this expectation, published 40Ar/39Ar dates of igneous activities suggested that Kutch volcanics are as old as the rest of the Deccan province, even with a small "tail" eruption occurring as late as ca. 61 Ma.Kutch's geology is complicated by the fact that it is marked by old rifting events and significant neotectonic activities. Here we present four new 40Ar/39Ar ages of igneous intrusions from Kutch. Three such intrusions occur in the so-called 'Island Belt' that juts out of the great salt flats (Great Rann of Kutch) in northern Kutch. The fourth intrusion is a gabbroic body that occurs at Dhar Dongar in southern Kutch. Two of the Island Belt intrusions (Sadara Sill and Nir Wandh gabbro) give ages of ca. 75 Ma. A lamprophyre dike from the Island Belt gave an age of ca. 67 Ma. The fourth intrusion from Kutch Mainland yielded the youngest age found in Kutch of ca. 61 Ma. These ages, combined with those determined in previous studies (65-67?1 Ma), suggest the following: Earliest rift-related magmatism occurred 75 Ma, and is clearly of pre-Deccan age. The rest of the igneous activities occurred in two episodes - the most voluminous episode coincided with Deccan age (65-67 Ma) whereas a small volume igneous activity took place at ca. 61 Ma, following a 6 m.y. long hiatus. The 61 Ma episode appears to occur in many different locations within the Deccan. We suggest that the 75 Ma pre-Deccan rifting-magmatic event is a relict of magmatism that occurred during separation of Madagascar from India, which was caused by the Marion plume.

The Kutch basin is a western margin pericratonic Mesozoic rift basin. The rift basin originated during break up of Eastern Gondwanaland. The rift basin is bound by Nagar Parkar uplift on the north and Kathiawar uplift (Saurashtra horst) on the south respectively along Nagar Parkar and North Kathiawar faults (NPF&NKF). The rift is styled by three main uplifts along three master faults, (from north to south) Island Belt, Kutch Mainland and Wagad uplifts, with intervening grabens and half-grabens. The uplifts are upthrust basement blocks tilted along sub-vertical faults with initial normal separation. The NKF is the principal master fault along which the rift subsided most. The structure is thus styled by tilted blocks and half-grabens within a south tilted asymmetric rift basin. Blanketing carapace of sediment over the basement drape folded over the tilted edges of the basement blocks as marginal flexures. The flexures are knee-bend folds with narrow deformation zones along the crest. The deformation zone encloses complicated folds, locally much faulted and intruded by igneous rocks. In the western part the uplifts are tilted to the south with flexures draped over the faulted up northern edges. In the eastern part a large uplift, Wagad Uplift, occurs between the Mainland and Island Belt uplifts. It is tilted towards the north along the South Wagad Fault (SWF), opposite to other uplifts, with a narrow deformation zone along the faulted up southern edge. The back slope ends up against Bela horst of the Island Belt uplift along Gedi Fault.A subsurface basement ridge - Median High, crosses the basin in the middle at right angle to its axis. Acting as a hinge it divides the basin into a deeper western part and a shallower and more tectonised eastern part. The rift is terminated in the east against a transverse subsurface basement ridge, Radhanpur-Barmer Ridge, which is the western shoulder of the adjacent N-S oriented Cambay rift. To the west, the rift merges with offshore shelf.The Kutch basin is the earliest pericratonic rift basin to form in the western volcanic passive margin of the Indian craton during the Late Triassic break up of Gondwanaland. The rift evolution with syn-rift sedimentation continued through Jurassic till Early Cretaceous as Indian plate separated from Africa and drifted northward. The rift expanded from north to south by successive reactivation of primordial faults of Mid-Proterozoic Delhi fold belt. The faults strike E-W but eastward the strike swings to NE-SW merging with the Delhi- Aravalli strike.The rifting was aborted by the trailing edge uplift during Late Cretaceous pre-collision stage of the Indian plate. The uplift caused structural inversion during rift-drift transition stage when most of the uplifts with drape folding over the edges came into existence by upthrusting along the master faults. The motion during the drift stage of the plate induced horizontal stress and the near vertical normal faults, which were reactivated as reverse faults during initiation of inversion cycle, became strike-slip faults involving divergent oblique-slip movements.The present structural style evolved by right lateral slip, which shifted the uplifts progressively eastward relative to each other from south to north. This resulted in the present en echelon positioning of the uplifts with respect to Kutch Mainland uplift. The strike slip related structuring modified the linear flexures breaking them into individual folds at the restraining and releasing bends. Narrow deformation zones are complicated by conjugate reidel faults formed along the master faults modifying the initial drape folds. Upward migration of basement faults through ductile sediment drape and the accompanying fault propagated folds further complicated flexural deformation zone. Syntectonic intrusions modified the shape of the structures. The structure of the SWF deformation zone demonstrate a typical strike-slip fault related structural style with two intertwining faults and related folds.Igneous rocks extensively intruded the Mesozoic sediments. Synrift plutonic activity and post rift hotspot related Deccan volcanism are two major magmatic events. Studies on the intrusive bodies and seismological data suggest the presence of an ultramafic magmatic body close to the crust-mantle boundary. This suggests mantle stretching, thinning and rupturing during rifting which led to deep crustal melting and formation of magma that intruded during inversion stage.