Petrogenesis of igneous intrusions reveals the geotectonics of orogenic and folded belts. The granitoids are one such intrusion, and the petrologic and structural features of the basement units throw light on the regional and local changes in rock types and environmental variables of pressure, temperature and fluid activity— since the heat supplied from these intrusions result in remobilization, redistribution and recrystallization. Granitoids are the ubiquitous igneous intrusives in Bansara area of the Bamenda Massif, which is wedged between two major tectonic plates: West African Craton (WAC) to the west and Gabon-Congo Craton (GCC) to the east. The basement rocks appear to have undergone high grade metamorphism and consist dominantly of granite gneisses, migmatitic gneisses and migmatitic schists. In the first paper, “Petrology and Major Element Geochemistry of Late to Post Neoproterozoic Peraluminous Granitoids in Parts of Bansara Southeastern Nigeria”, the authors, N Egesi and V U Ukaegbu, have analyzed the geochemical data of a set of representative granitoid samples from parts of Bansara region, Bemenda Massif of Southeastern Nigeria, and integrated with their petrogenetic and geotectonic features, in an attempt to use the intrusions as petrogenetic markers of the rocks of the Bansara area of the Bamenda Massif. The authors infer that the geodynamic disturbances may have been produced by differentiation, fractional crystallization and concurrent contamination of charnockitic magma, while the uniformity is indicative of the petrogenetic link of the bulk of the parent magma composed of granitoids (enderbite, granodiorite and granites) with major element abundances closely approximating tholeiites emplaced in continental margin environment. Fractional crystallization, partial and contamination processes may have acted sequentially to account for the various granitoid suites in the area. The uniform behavior of some oxides present in the three rock suites probably suggests a common tectonic environment and association.

Foraminifera are found abundantly in many types of marine sediments and are known to be successful inhabitants of aquatic environment from deep oceans to brackish water lagoons, estuaries and even rarely in freshwater streams, lakes, etc. Foraminifera have long been considered valuable tools to reconstruct paleoenvironments, and recent studies have shown that the most important environment factors regulating foraminiferal occurrences are oxygen and food. The distribution and abundance of living foraminifera are controlled by a number of natural factors, which include temperature, salinity, water depth, nutrients, sediments substrate, dissolved oxygen content, calcium carbonate, organic matter, etc. Meager attention has been paid so far to the foraminiferal occurrences and the associated environmental mineralogy. With a view to knowing the distribution of foraminifera in different habitats in Mandapam and Tuticorin, Gulf of Mannar and the Tamiraparani estuary sediments along the southeast coast of India, M Suresh Gandhi and A Solai, in their paper, “Distribution and Ecology of Foraminifera Studies Between Mandapam and Tuticorin, Southeast Coast of India”, have carried out a detailed analysis of the occurrence of foraminifera. A total of 117 foraminiferal species belonging to 61 genera, 30 families, 14 superfamilies and five suborders were identified. Among these, 49 species were identified out of six beach sample analysis. It is inferred that the sand was deposited all along the coast with silt and silty sands. Totally, 82 species were identified in the estuary region. The sand-silt-clay ratios were estimated and silty sand, and sandy substrates have been predominantly found in the study area. The distribution of foraminifera in the depositional basin is mainly controlled by the species enrichment, grain size, wave motion, beach morphodynamics, etc. From the faunal distribution, it is observed that the total distribution of foraminifera is higher at Mandapam and Tuticorin sector, whereas in Kallar and Vallinokam, it is different with lesser amount. The arcuate nature of bay-like appearance and wave energy conditions are the major controlling factors for the distribution of foraminifera. The coastal configuration of the beach may be controlled by the distribution of foraminifera species from offshore to beach.

About 80% of the landmass in Bangladesh is constituted by floodplains, and flooding is the common phenomenon in the region. In 1988 and 1998, the floods caused severe damage in living memory when over 60% of the area suffered flooding, affecting half of the population. Chapai Nawabganj District of Bangladesh is frequently inundated by the Padma-Mahananda river system. The mighty river Padma (the main flow of the Ganges) mainly controls the natural drainage pattern of Chapai Nawabganj District. The Padma-Mahananda confluence makes the area more susceptible to flooding. In the last paper, “Back Flow of the River Water Causing Floods in Bangladesh: A Mineralogical Study”, the authors, Shabiha Yeasmin and Sohail Kabir, have systematically examined the mineral assemblages in the sediments of the Padma and Mahananda, near the Padma-Mahananda confluence, in order to observe the back flow of water from the river Padma to the Mahananda. The heavy mineral assemblage of the Mahananda river at a distance of up to 10 km upstream from the confluence indicates the mixed heavy minerals of the Padma and the Mahananda, which support the back flow of water from the river Padma to the Mahananda. The rising of the river bed of the Padma due to siltation, along with the neotectonic upliftment, hinders the flow of the water during monsoon causing back flow of the water, resulting in the flooding of the floodplains and lowland along the Mahananda.

-- S V Srirama Rao
Consulting Editor