Kachchh Basin Research Articles

Upper Callovian-Middle Oxfordian belemnite assemblage from Jara Dome, Western Kachchh

Belemnites and ammonites were collected from the Dhosa Oolite beds of Jara Dome in Kachchh. The Dhosa Oolite is an important marker of the Mesozoic sediments exposed in the Kachchh Basin. Nine belemnite species are reported belonging to four genera Belemnopsis, Demubelus, Hibolithes and Conodicoelites. These include Belemnopsis cf. moluccana Boehm, B. calloviensis Oppel, Demubelus weberi Stolley, Hibolithes boloides Stolley, H. longiscissus Stolley, H. cf. compressus Stolley, H. jumarensis Waagen, H. budhaichus Stoliczka and Conodicoelites cf. abadi Challinor. Besides the belemnites, diagnostic ammonoid fauna are also recorded from the studied section enabling better controlled stratigraphic resolution and biostratigraphic correlation. Taxonomic studies on the different belemnite species are presented with illustration for reference.

Sedimentology, sequence stratigraphy and syn-rift model of younger part of Washtawa Formation and early part of Kanthkot Formation, Wagad, Kachchh basin, Gujarat

The 600 m thick prograding sedimentary succession of Wagad ranging in age from Callovian to Early Kimmeridgian has been divided into three formations namely, Washtawa, Kanthkot and Gamdau. Present study is confined to younger part of the Washtawa Formation and early part of the Kanthkot Formation exposed around Kanthkot, Washtawa, Chitrod and Rapar. The depositional architecture and sedimentation processes of these deposits have been studied applying sequence stratigraphic context. Facies studies have led to identification of five upward stacking facies associations (A, B, C, D, and E) which reflect that deposition was controlled by one single transgressive - regressive cycle. The transgressive deposit is characterized by fining and thinning upward succession of facies consisting of two facies associations: (1) Association A: medium - to coarse-grained calcareous sandstone - mudrocks alternations (2) Association B: fine-grained calcareous sandstone - mudrocks alternations. The top of this association marks maximum flooding surface as identified by bioturbational fabrics and abundance of deep marine fauna (ammonites). Association A is interpreted as high energy transgressive deposit deposited during relative sea level rise. Whereas, facies association B indicates its deposition in low energy marine environment deposited during stand-still period with low supply of sediments. Regressive sedimentary package has been divided into three facies associations consisting of: (1) Association C: gypsiferous mudstone-siltstone/fine sandstone (2) Association D: laminated, medium-grained sandstone - siltstone (3) Association E: well laminated (coarse and fine mode) sandstone interbedded with coarse grained sandstone with trough cross stratification. Regressive succession of facies association C, D and E is interpreted as wave dominated shoreface, foreshore to backshore and dune environment respectively. Sequence stratigraphic concepts have been applied to subdivide these deposits into two genetic sequences: (i) the lower carbonate dominated (25 m) transgressive deposits (TST) include facies association A and B and the upper thick (75m) regressive deposits (HST) include facies association C, D and E. The two sequences are separated by maximum flooding surface (MFS) identified by sudden shift in facies association from B to C. The transgressive facies association A and B represent the sediments deposited during the syn-rift climax followed by regressive sediments comprising association C, D and E deposited during late syn-rift stage.

Ground-motion Attenuation Relation from Strong-motion Records of the 2001 Mw 7.7 Bhuj Earthquake Sequence (2001–2006), Gujarat, India

Predictive relations are developed for peak ground acceleration (PGA) from the engineering seismoscope (SRR) records of the 2001 Mw 7.7 Bhuj earthquake and 239 strong-motion records of 32 significant aftershocks of 3.1 ≤ Mw ≤ 5.6 at epicentral distances of 1 ≤ R ≤ 288 km. We have taken advantage of the recent increase in strong-motion data at close distances to derive new attenuation relation for peak horizontal acceleration in the Kachchh seismic zone, Gujarat. This new analysis uses the Joyner-Boore’s method for a magnitude-independent shape, based on geometrical spreading and anelastic attenuation, for the attenuation curve. The resulting attenuation equation is, $$ \eqalign{ \ln ({\hbox{Y}}) = -7.9527 + 1.4043\,{\hbox{M}}_{\rm{W}} - \ln \left( {{\hbox{r}}_{\rm{jb}}^{2} + 19.82^{2}} \right)^{1/2} - 0.0682\,{\hbox{S}} \hfill \cr {\hbox{for}}\, 3.1 {\hbox{ < \;M}}_{\rm{W}} \le 7.7\quad \quad {\hbox{std}}.\,{\hbox{dev}}.\,\left(\sigma \right): \pm 0.8243, \hfill \cr } $$ where, Y is peak horizontal acceleration in g, Mw is moment magnitude, rjb is the closest distance to the surface projection of the fault rupture in kilometers, and S is a variable taking the values of 0 and 1 according to the local site geology. S is 0 for a rock site, and, S is 1 for a soil site. The relation differs from previous work in the improved reliability of input parameters and large numbers of strong-motion PGA data recorded at short distances (0–50 km) from the source. The relation is in demonstrable agreement with the recorded strong-ground motion data from earthquakes of Mw 3.5, 4.1, 4.5, 5.6, and 7.7. There are insufficient data from the Kachchh region to adequately judge the relation for the magnitude range 5.7 ≤ Mw ≤ 7.7. But, our ground-motion prediction model shows a reasonable correlation with the PGA data of the 29 March, 1999 Chamoli main shock (Mw 6.5), validating our ground-motion attenuation model for an Mw6.5 event. However, our ground-motion prediction shows no correlation with the PGA data of the 10 December, 1967 Koyna main shock (Mw 6.3). Our ground-motion predictions show more scatter in estimated residual for the distance range (0–30 km), which could be due to the amplification/noise at near stations situated in the Kachchh sedimentary basin. We also noticed smaller residuals for the distance range (30–300 km), which could be due to less amplification/noise at sites distant from the Kachchh basin. However, the observed less residuals for the longer distance range (100–300 km) are less reliable due to the lack of available PGA values in the same distance range.

Estimation of Site Response in the Kachchh Seismic Zone, Gujarat, India

Abstract The horizontal components of S -wave data from 38 aftershocks ( M w 3.0–5.6) of the 2001 Bhuj earthquake ( M w 7.7) recorded at 18 strong-motion sites in the Kachchh seismic zone, Gujarat, India, were analyzed to estimate the site response, in the frequency range 0.2–10.0 Hz using the reference site generalized inversion technique. The site response values thus obtained at each site were averaged (logarithmically) over two frequency bands, 0.2–1.8 and 3.0–7.0 Hz, respectively, to study the spatial variation of site response in the Kachchh seismic zone. In the 0.2–1.8 Hz frequency band, the high site response values (1.0–1.7) were observed to be associated with the regions south of the Kachchh mainland fault that suffered extensive ground failure during the 2001 M w 7.7 Bhuj earthquake. In the 3–7 Hz frequency band, the average site response estimates in the area of Jurassic and Quaternary formations vary from 0.22 to 1.53, whereas those of Tertiary formation lie between 0.60 and 1.30. The maximum site response values at both 0.2–1.8 Hz (∼1.7) and 3–7 Hz (∼1.53) are estimated at the Sikara site, which lies in a zone of geological contact between Jurrassic and Tertiary formations. The deamplification observed at the AMS station in the 0.2–10 Hz frequency band could be attributed to a very localized geological variation of the soil type or geometry of the basin near the site. It is apparent that in both 0.2–1.8 and 3–7 Hz frequency bands, the presence of geological contacts and sediment thicknesses controls the site responses in the Kachchh seismic zone. It is also inferred that larger site response (>1.2) values in the 0.2–1.0 Hz frequency band could be indicating the probable presence of soil class C () and D (180 V s ≤360 m/sec) in the Kachchh basin (according to 1997 National Earthquake Hazards Reduction Program [NEHRP] provisions).

Comparative Study of Distance Attenuation in the Central United States and Western India

Seismic waveform data, recorded by short-period instruments of the Cooperative New Madrid Seismic Network (CNMSN), and a temporary aftershock deployment for the 2001 Bhuj M W 7.6 earthquake are corrected to the theoretical Wood-Anderson response, and the horizontal peak amplitudes are used to determine local magnitude scales for the Mississippi embayment of the central United States and the Kachchh basin of western India, with a focus to understand the distance attenuation in these two regions. Results show that the distance-correction function for the Mississippi embayment of the central United States is

Determination of directions of horizontal principal stress and identification of active faults in Kachchh (India) by electromagnetic radiation (EMR)

Abstract The Kachchh basin in the western India is known for its recent high seismicity. This study presents an application of the geogenic Electromagnetic Radiation (EMR) technique for deciphering the directions of principal horizontal stress in the eastern Kachchh. The principal direction of horizontal stress obtained from EMR differs from those obtained from earthquake focal plane solutions. The major horizontal principal stress based on the EMR study shows an azimuth of N60°E ± 10°. The principal directions of EMR emissions are parallel to the acute bisector of conjugate microcracks. The azimuthal distribution of EMR signal and dimension of microcracks suggest that the EM emissions are transversely polarized. The study also deals with the first application of electromagnetic radiation emissions to identify active fracture planes in sandstones that could become potential active faults later, which might be seismogenic or nonseimogenic. This study is based on linear profiling at six different places across two major faults, the Kachchh Mainland Fault (KMF) and the South Wagad Fault (SWF) in the eastern Kachchh. Anomalously, high EMR emissions are observed in the eastern part of the KMF, indicating active surface deformation.

Shallow Seismicity in Stable Continental Regions

A worldwide compilation of well-constrained fault ruptures and focal depths of earthquakes reveals that the Earth9s crust in many stable continental regions (SCRs) is characterized by a bimodal depth distribution with a very shallow upper crustal component. The distributions can vary in a) depth of the modes and b) strength of bimodality, probably due to intracrustal boundaries, differences in frictional and rheological properties, heat-flow densities, strain-rates, and tectonic forces or forces stemming from the surface. Overall, SCR ruptures and SCR earthquakes are confined within the upper third (0–10 km) and/or the lower third of the crust (20–35 km), while the midcrust (10–20 km) tends to be aseismic. Historical data indicate that some SCRs show very well-developed bimodal distributions of focal depths ( e.g. , North Alpine foreland basin in Europe, Kachchh basin in India), while others show weak to no developed bimodal distributions ( e.g. , Charlevoix seismic zone, New Madrid seismic zone in the central United States). Moreover, many large SCR earthquakes ( M w 4.5–8.0) nucleate on reverse faults and close to the surface ( m b e.g. , ground motion).

Sediment Thicknesses and Qs vs. Qp Relations in the Kachchh Rift Basin, Gujarat, India Using Sp Converted Phases

Delineation of the top sedimentary structure and its Qs vs. Qp relationship using the travel-time difference of direct S and converted Sp phase is key to understanding the seismic hazard of any sedimentary basin area. We constructed filtered displacement waveforms from local ETNA Episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 Bhuj earthquake recorded by the Kachchh seismological network of the National Geophysical Research Institute (NGRI), Hyderabad, India during 2001–2004. Stations are within 15–70km of epicenters, and the resulting displacement waveforms are generally simple, displaying prominent P, Sp, and S wave pulses. Particle motion of P and S waves suggest near-vertical raypaths consistent with preliminary depth estimates. The direct S wave on the horizontal component is characterized by lower frequency content than the converted Sp phase on the vertical component. This difference in frequency content between S and Sp phases can be explained in terms of different attenuation effects for P and S waves in the unconsolidated sediments. The Sp phase is generated by S-to-P phase conversion at the base of Mesozoic sediments of the Kachchh basin. Travel-time inversion (VELEST) of 2565 P and 2380 S arrivals from 658 well located aftershocks recorded at 8–14 three-component local seismic stations led to 1 D velocity models indicated very slow sediments in the upper 0–2 km depth range (Vp: 2.92 km/s and Vs: 0.90 km/s) and an increasing trend of velocities with depth at 2–40 km depth. The estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel-time difference between S and converted Sp phases reaches a maximum of (1.534 ± 0.117) km beneath Bandri (near the location of 2001 Bhuj mainshock) and attains a minimum sediment thickness of (0.858 ± 0.104) km beneath Ramvav and Burudia. The spectral ratios between Sp and S from 159 three-component accelerograms have been used to study seismic wave attenuation in the Kachchh rift basin. The estimated Qs vs. Qp relations for 12 accelerograph sites vary from Qs = 0.184 Qp (at Chobari) to Qs = 0.505 Qp (at Dudhai). For stations Chobari, Chopdwa, Jahawarnagar, Vondh and Tapar, the spectral ratio slopes and hence the calculated Qs vs. Qp relations are effectively the same, and the correlation coefficients are quite high (0.91–0.93). Stations Adhoi, Manfara, New Dudhai, Dudhai and Sikara have similar Qs vs. Qp relationships to each other and also have high correlation coefficients (0.78–0.87). The spectral ratios for stations Anjar and Ramvav are small and poorly constrained, resulting in less reliable Qs vs. Qp relations. This could be due to noisy data, fewer available waveforms, or scattering due to velocity heterogeneities and/or interface irregularities.

Coda Q in the Kachchh Basin, Western India Using Aftershocks of the Bhuj Earthquake of January 26, 2001

Q C -estimates of Kachchh Basin in western India have been obtained in a high frequency range from 1.5 to 24.0 Hz using the aftershock data of Bhuj earthquake of January 26, 2001 recorded within an epicentral distance of 80 km. The decay of coda waves of 30 sec window from 186 seismograms has been analysed in four lapse time windows, adopting the single backscattering model. The study shows that Q c is a function of frequency and increases as frequency increases. The frequency dependent Q c relations obtained for four lapse-time windows are: Q c =82 f1.17 (20–50 sec), Q c =106 f1.11 (30–60 sec), Q c =126f1.03 (40–70 sec) and Q c =122f1.02 (50–80 sec). These empirical relations represent the average attenuation properties of a zone covering the surface area of about 11,000, 20,000, 28,000 and 38,000 square km and a depth extent of about 60, 80, 95, 110 km, respectively. With increasing window length, the degree of frequency dependence, n, decreases marginally from 1.17 to 1.02, whereas Q0 increases significantly from 82 to 122. At lower frequencies up to 6 Hz, Q c −1 of Kachchh Basin is in agreement with other regions of the world, whereas at higher frequencies from 12 to 24 Hz it is found to be low.

Unbiased Moment-Rate Spectra and Absolute Site Effects in the Kachchh Basin, India, from the Analysis of the Aftershocks of the 2001 Mw 7.6 Bhuj Earthquake

What can be learned about absolute site effects on ground motions, with no geotechnical information available, in a very poorly instrumented region? In addition, can reliable source spectra be computed at a temporary deployment? These challenges motivated our current study of aftershocks of the 2001 M w 7.6 Bhuj earthquake, in western India, where we decouple the ambiguity between absolute source radiation and site effects by first computing robust estimates of coda-derived moment-rate spectra of about 200 aftershocks in each of two depth ranges. Crustal attenuation and spreading relationships, based on the same data used here, were determined in an an earlier study. Using our new estimates of source spectra, and our understanding of regional wave propagation, for direct S waves we isolate the absolute site terms for the stations of the temporary deployment. Absolute site terms for each station were determined in an average sense for the three components of the ground motion via an L1-norm minimization. Results for each site were averaged over wide ranges of azimuths and incidence angles. The Bhuj deployment is characterized by a variable shallow geology, mostly of soft sedimentary units. Vertical site terms in the region were observed to be almost featureless (i.e., flat), with amplifications slightly <1.0 within wide frequency ranges. As a result, the horizontal-to-vertical (H/V) spectral ratios observed at the deployment mimic the behavior of the corresponding absolute horizontal site terms, and they generally overpredict them. This differs significantly from results for sedimentary rock sites (limestone, dolomite) obtained by Malagnini et al. (2004) in northeastern Italy, where the H/V spectral ratios had little in common with the absolute horizontal site terms. Spectral ratios between the vector sum of the computed horizontal site terms for the temporary deployment with respect to the same quantity computed at the hardest rock station available, BAC1, are seriously biased by its nonflat, nonunitary site response. This indicates that, occasionally, the actual behavior of a rock outcrop may be far from that of an ideal, reference site (Steidl et al. , 1996).

Sedimentary and crustal structure beneath Kachchh and Saurashtra regions, Gujarat, India

Strong and distinct Ps conversions from Moho and top sediments characterize the radial receiver functions (RF) estimated for stations in Kachchh basin, while, these for stations in Saurashtra are characterized by a strong Ps conversion from Moho and a weak or absence of Ps conversion from top sediments. The jumping inversion of stacked radial RFs suggest that 1–3 km thick top low velocity sediments ( V p: 1.8–2.5 km/s; σ: 0.28–0.34) and a 39–48 km thick crust ( σ: 0.25) (thicker than average Precambrian crust) characterize the Kachchh basin area. While, the crust beneath Saurashtra is characterized by a 0–1 km thick moderate velocity sediments ( V p: 2.0–3.56 km/s; σ: 0.25–0.28) and a north dipping shallow Moho (35–42 km; σ: 0.248). The estimated average crustal σ values from the arrival times of PpPms and PpSms + PsPms phases suggest a relatively stronger crust in Kachchh ( σ: 0.22–0.27) in comparison to the crust in Saurashtra ( σ: 0.18–0.19). In the epicentral zone of 2001 Bhuj earthquake, the estimated Moho thickness reveals a Moho upwarping (3–5 km change in the E–W direction and a 6–9 km change in the N–S direction) beneath the epicentral zone of 2001 Bhuj earthquake, which might have induced large intraplate stresses to cause the M w 7.7 earthquake at 23 km depth.

Palaeoclimate reconstructions of the Middle Jurassic of Kachchh (western India): an integrated approach based on palaeoecological, oxygen isotopic, and clay mineralogical data

The Middle Jurassic sedimentary succession of the Kachchh Basin shows significant changes in lithology, faunal content and diversity across the Bathonian–Callovian boundary. The change in lithology from predominantly carbonates in the Middle and Late Bathonian to siliciclastics in the Callovian is accompanied by a drop in species diversity from high to moderate. Characteristic faunal elements of the Bathonian (corals, sponges, the bivalves Eligmus and members of the Opinae) are absent or rare in the Callovian. In contrast, Callovian sediments contain comparatively high proportions of nuculid bivalves. The clay mineral assemblage reveals prominent variations in the smectite vs. kaolinite abundance, especially during the Bathonian to Callovian transition. From the Bajocian to the Middle Bathonian, smectite becomes the dominant clay mineral and is nearly exclusively present in the Late Bathonian. Towards the Middle Callovian, its content gradually decreases and kaolinite becomes the dominant clay mineral. Illite generally is only a minor component throughout the Middle Jurassic. Based on these results, the Bajocian to Middle Bathonian time interval is interpreted to represent subtropical climatic conditions with seasonal droughts and a moderate supply of terrigenous clastics to the basin. The Late Bathonian was a period with a semi-arid climate, hot seasonal droughts and a minor input of terrigenous clastics. The increase in kaolinite contents in the Callovian suggests a subtropical humid climate with less prominent seasonal droughts and a higher input of siliclastics into the basin accompanied by a higher nutrient influx. Oxygen isotope ratios measured on Bathonian to early Callovian brachiopod shells give palaeotemperatures of 19–24 °C. Callovian to Oxfordian paleotemperatures calculated from oxygen isotopes measured on belemnite rostra range from 11 to 21 °C. The lower palaeotemperatures estimated from the oxygen isotopic composition of belemnite calcite are explained by assuming that the belemnites spent most of their life span in colder regions of the Malagassy Gulf and migrated into the warmer subtropical waters of the Kachchh Basin. Combining these various lines of evidence, the changes in lithology and faunal composition taking place in the Kachchh Basin during the Middle Jurassic appear to reflect largely a change towards increasing humidity rather than a distinct decrease in water temperature.

Tectonics and crustal structures related to Bhuj earthquake of January 26, 2001: based on gravity and magnetic surveys constrained from seismic and seismological studies

Gravity and magnetic data of the Kachchh basin and surrounding regions have delineated major E–W and NW–SE oriented lineaments and faults, which are even extending up to plate boundaries in the north Arabian Sea and western boundary of the Indian plate, respectively. The epicentral zone of Bhuj earthquake and its aftershocks is located over the junction of Rann of Kachchh and median uplifts viz. Kachchh mainland and Wagad uplifts, which are separated by thrust faults. Gravity data with constraints from the results of the seismic studies along a profile suggest that the basement is uplifted towards the north along thrust faults dipping 40–60° south. Similarly gravity and magnetic modeling along a profile across Wagad uplift suggest south dipping (50–60°) basement contacts separating rocks of high susceptibility and density towards the north. One of these contacts coincides with the fault plane of the Bhuj earthquake as inferred from seismological studies and its projection on the surface coincides with the E–W oriented north Wagad thrust fault. A circular gravity high in contact with the fault in northern part of the Wagad uplift along with high amplitude magnetic anomaly suggests plug type mafic intrusive in this region. Several such gravity anomalies are observed over the island belt in the Rann of Kachchh indicating their association with mafic intrusions. The contact of these intrusives with the country rock demarcates shallow crustal inhomogeneities, which provides excellent sites for the accumulation of regional stress. A regional gravity anomaly map based on the concept of isostasy presents two centers of gravity lows of −11 to −13 mGal (10 −5 m/s 2) representing mass deficiency in the epicentral region. Their best-fit model constrained from the receiver function analysis and seismic refraction studies suggest crustal root of 7–8 km (deep crustal inhomogeneity) under them for a standard density contrast of −400 kg/m 3. It is, therefore, suggested that significant amount of stress get concentrated in this region due to (a) buoyant crustal root, (b) regional stress due to plate tectonic forces, and (c) mafic intrusives as stress concentrators and the same might be responsible for the frequent and large magnitude earthquakes in this region including the Bhuj earthquake of January 26, 2001.

Palaeoecology of Middle to lower Upper Jurassic macrofaunas of the Kachchh Basin, Western India - an overview

Palaeoecology of Middle to lower Upper Jurassic macrofaunas of the Kachchh Basin, Western India - an overview

Ground-Motion Scaling in the Kachchh Basin, India, Deduced from Aftershocks of the 2001 Mw 7.6 Bhuj Earthquake

We studied the excitation, propagation, and site effects in the Kachchh basin of India by using ground-motion recordings from a temporary seismograph network deployed to study aftershocks of the M w 7.6 Bhuj earthquake of 26 January 2001. The Kachchh basin has been proposed as a useful analog region for studying hazard in other earthquake-prone but slowly deforming regions, such as the central United States. The earthquakes we studied ranged in size from about M 2 to M 5.2, and travel paths ranged from a few kilometers to about a hundred kilometers. There was a broad range of focal depths among the aftershocks, so the data were divided into two overlapping subsets to test the sensitivity of the derived propagation and source parameters to focal depth. Parameters we constrained include the source excitation terms (related to stress drop), a frequency-dependent attenuation operator, a geometric spreading function, and an operator to account for site effects. Our results indicate that seismic-wave attenuation in Kachchh crust is very low, similar to other continental intraplate areas such as central and eastern North America. We also estimated seismic moments and stress drops for the earthquakes by fitting single-corner-frequency source-model spectra to the observed spectra, corrected for propagation by using our derived parameters. Stress drops were found to scale with seismic moment and to be rather high overall. By using a stochastic point-source model to estimate mainshock ground motions, we found that the distance decay of expected peak ground motions, assuming a stress drop of 15-20 MPa, compare well with the scant observations for the Bhuj earthquake. Ground-motion predictions for Kachchh, based on Bhuj aftershock data, support the idea that the region may have similar hazard to proposed analog areas in North America.

ENVIRONMENTS AND FAUNAL PATTERNS IN THE KACHCHH RIFT BASIN, WESTERN INDIA, DURING THE JURASSIC

Marine Jurassic sediments (Bajocian-Tithonian) of the Kachchh Basin were deposited in a ramp setting. Except during the Middle and Late Bathonian, when a carbonate regime became established, the fill of the basin consists predominantly of siliciclastics. The sediments represent environments that range from coastal plains (rivers and associated flood plains with caliche nodules), deltas, brackish water lagoons, nearshore sand and iron-oolite bars of the inner ramp, generally situated above fair-weather wave-base, to the middle ramp influenced by storm-waves and by storm-generated currents, and finally to the outer ramp which is characterised by low energy, fine-grained sediments. Changes in relative sea level produced a cyclic sedimentation pattern. The rich benthic fauna of macroinvertebrates is dominated by bivalves, followed by brachiopods, gastropods, corals, serpulids, and sponges. The analysis of 370 statistical samples and more than 27, 000 specimens produced more than 40 benthic associations and assemblages. They show a relationship to several environmental parameters, two of which, salinity and climate, are briefly discussed. The spatial distribution of the facies and biota is outlined for two time slices, the Bathonian and the Callovian-Oxfordian, respectively.

Studying earthquake recurrence in the Kachchh region, India

The occurrence of two large earthquakes (M&gt;7.5) in the Kachchh basin within a short span of ∼200 years is quite unusual for an intracratonic region (Figure l). What factors could possibly make this region different from other, mid‐plate settings? Could some of these factors be responsible for the relatively shorter recurrence times? Does the tectonic history provide us with any clues about the uniqueness of this intraplate region?

Sequence stratigraphic significance of sedimentary cycles and shell concentrations in the Upper Jurassic–Lower Cretaceous of Kachchh, western India

Upper Jurassic and Lower Cretaceous siliciclastic shallow water sediments of the Kachchh Basin, western India, form strongly asymmetric coarsening-upward cycles, which are interpreted as recording changes in relative sea level (deepening–shallowing cycles). These cycles correspond to depositional sequences, in which deposits of the lowstand systems tract are not present, the sequence boundary coinciding with the transgressive surface. Shell concentrations are found in the transgressive lags at the base of the transgressive systems tract (TST), in the maximum flooding zone (MFZ), and at or close to the top of the highstand systems tract. They belong to six assemblages, five of them dominated by large bivalves such as Seebachia, Herzogina, Gryphaea, Gervillella, Megacucullaea, Pisotrigonia and Indotrigonia, the sixth by the coral Amphiastraea. Three types of shell concentrations can be distinguished that differ from each other in a number of ecological and taphonomic features, such as species diversity, preservation quality, orientation in cross-section, percentage of disarticulation, and degree of biogenic alteration. Characteristic features of concentrations at the base of the TSTs are moderate time-averaging, sorting, a preferred convex-up orientation, and nearly total disarticulation of shells. They are suggestive of an environment in which reworking and local transport were frequent events. Similar features are shown by concentrations near the tops of the HSTs, except that there shells were largely concentrated in lenses and in pavements rather than in beds as in the transgressive lags. Associated sedimentary structures indicate deposition above fair weather wave base in a high-energy environment. Concentrations occurring in the MFZ, in contrast, are autochthonous and highly time-averaged, having accumulated during times of low rates of sedimentation below storm wave base. This is supported by their high preservation quality (comparatively high percentage of articulated shells, shells of infaunal organisms commonly preserved in life position), biogenic alteration being the most important taphonomic agent. The dominant elements of these shell concentrations, i.e. Seebachia, Megacuccullaea, and Indotrigonia in the Upper Jurassic, and Pisotrigonia in the Lower Cretaceous, are endemic to the Ethiopean faunal province and belong to lineages that rapidly evolved during this time period.

The oldest Jurassic cyathophorid coral (Scleractinia) from siliciclastic environments of the Kachchh Basin, western India

CyathophoraMichelin, 1843, hitherto well known from the Upper Jurassic and Cretaceous, has been found in the Middle Jurassic (Bajocian) of the Kachchh Basin, western India. Eleven specimens ofCyathophora bourgueti (Defrance, 1826) from the Babia Cliff Sandstone member of the Kaladongar Formation, exposed along the northern scarp of the Kala Dongar, Pachchham Island, Kachchh, are described and illustrated as the earliest Jurassic record of the family CyathophoridaeVaughan & Wells, 1943. It is suggested that the monospecific occurrence ofCyathophora bourgueti was controlled by salinity.

Marker Beds in the Jurassic of the Kachchh Basin, western India : Their depositional environment and sequence-stratigraphic significance

Marker Beds in the Jurassic of the Kachchh Basin, western India : Their depositional environment and sequence-stratigraphic significance