The magnetotelluric (MT) techniques are used to study the electrical conductivity distribution in the earth’s interior by using the surface measurements of the naturally occurring electric and magnetic field variations to delineate deep-seated structures with precision. Gautam Gupta and S G Gokarn in their paper, “Geoelectric Structure over the Deccan Flood Basalts Using Magnetotelluric Studies”, have identified a layer comprising the deccan basalts and the pre-Trappean Bagh sediments in the Deccan Volcanic Province (DVP) in the region near Nasik, Maharashtra, India. The individual thickness of the formations, however, has not been determined due to similar resistivities of both the layers. The study delineated a deep-seated thrust zone along which the Bagh sediments are subducting eastwards. This, along with the strong undulations in the top layer observed in the western part of the study area, is indicative of the compressional tectonics that this region may have undergone prior to the Deccan volcanic event. The observed sporadic seismic swarm in the region indicates that the thrust zone is seismically active at present. The authors have suggested further studies towards the east and west of the study region, which might throw more light on the exact nature of the thrust zone and the underlying structure.

The Cartagena volcano field, South-East of Spain, comprised a series of small ash cones and lapillis, sometimes accompanied by short lava flows of basic composition. The rocks and minerals of three ash cones of the volcano have been analyzed by A Aparicio and R García, in their paper, “Basanites and Crustal Contamination in the Cartagena Volcano Field (South-East of Spain)” by various geochemical methods. The study indicates that most of the enclaves correspond to schists and gneisses having different degrees of assimilation by the magma. The basanitic-basaltic magma of the Cartagena volcano field is estimated to have formed at depths of 35-42 km under temperature and pressure conditions of 1,175-1,216ºC and 10-12 kb respectively. A genetic relationship is found to exist between the basanites and the pyroxenites. The percentage melting of the peridotites was estimated to be about 7-15%. Metamorphic enclaves, mainly with pyroxenites and dunites, were also indicated. The percentage assimilation of metamorphic rocks by the basanitic-basaltic magma is relatively high (13-14%). The chemistry of the basanitic-basaltic magma reflects an Oceanic Island Basalt (OIB) type composition and the occurrence of Within Plate Basalt (WPB) or Columbia River Basalts (CRB) intraplate continental drift.

Facies interprets every precise manner of depositional setting that indicates ancient environmental condition of a sequence. The depositional environments of the lithologic units are established on the basis of lithology, lithofacies, sedimentary structure, texture and fossil content. Mrinal Kanti Roy, Partha Jit Roy, Mst. Shamima Akter and Bikash Kumar Chowdhury in their paper, “Facies and Depositional Environment of the Sylhet Limestone Formation and Kopili Formation as Exposed in the Surma Basin, Northeastern Bangladesh”, have described various facies and tried to explain the depositional environment of the Sylhet formation and Kopili formation exposed in the Surma basin, south of Dauki fault, in the Sylhet district of Bangladesh. The study indicates that the Sylhet Limestone Formation consists of crystalline limestone facies and fossiliferous limestone facies. Kopili Formation is dominantly composed of black to dark green shale. Crystalline and fossiliferous limestones are ofepeiric sea and shallow marine shelf deposit respectively. Black to dark green shale and red shale are designated as deep marine shale of abyssal to bathyl basin plain. The study further suggests that the ripple laminated very fine sandstone-siltstone and parallel laminated very fine sandstone-siltstone are transported and deposited distally to the anoxic reducing environment of deep marine black to dark green shale.

The stratigraphic sequence, lithologic interpretation and analysis of major and minor facies cycle provide information regarding environments of deposition of the sedimentary sequence in a well. Sedimentary electrofacies always bear the evidence of depositional condition. Electrofacies analysis is mainly done on the basis of different log signatures. Debashish Mondal, Md. Sultan-Ul-Islam and Aminul Islam in their paper “Electrofacies Analysis of Neogene Sequence in the Well Shahbazpur-1, Bhola, Bengal Basin”, have analyzed different wireline logs, such as, GR, SP of the well Shahbazpur-1 of the Shahbazpur structure, and reconstructed the paleoenvironments of deposition. Four well-developed sequences—alluvium of Miocene to recent ages—were identified in the structure. The sequences/cycles of coarsening upward and fining upward, and linear were observed. Based on the log response, the lower and upper parts of the Shahbazpur sequence-4 (Depth range: 3,400-2,750 m) were deposited under the marginal estuarine influence and marshy environments respectively. The sequences 2 and 3 might have been deposited under marine transgressional environments. Shahbazpur sequence-1 (Depth range: 1,480-480 m) was deposited under a phase of slow rate of deltaic progradation and then marine transgressional phase and upper part over bank environments. The overall nature of the log suggests a phase of slow rate of deltaic progradation and then marine transgressional phase during the deposition of the sequence-1. Although several cycles marine transgressions and regressional phases have been interpreted, the overall sequence forms an upward coarsening progradational deltaic phase of deposition.

The basement rocks of the Obudu Plateau, Southeastern Nigeria, represent a high grade metamorphic terrain with polyphase deformation. Ukaegbu V U and Beka F T in their paper, “LILE and HFSE Fingerprints on the Geotectonic Settings of Some Pan-African Granitoids from Obudu Plateau, Southeastern Nigeria”, have derived new geochemical data set, using the trace elements as geotectonic markers. The granitoids are characterized by high and variable concentrations of the Large Ion Lithophile Elements (LILE) and High Field-Strength Elements (HFSE). Trace element contents of the granitoids indicated the important role in the partial melting of continental crust played on their petrogenesis, hence their orogenic affinity. The trace element geochemistry of the representative samples indicates that granitoids display S-type affinity, and mixture of crustal and mantle sources. Further, the analysis suggests a syn-continent-continent collision setting between the West African craton and the westward drifting and subducted Eastern Sahara plate during the Pan-African orogeny. The observed paucity of syn-collision granitoids in Obudu Plateau and other Pan-African orogenies suggests that the crustal thickening is low, but uniform throughout these orogenies.

-- S V Srirama Rao
Consulting Editor