Ismail Hossain

Geochemical characteristics of Holocene sediments from Chuadanga district, Bangladesh: Implications for weathering, climate, redox conditions, provenance and tectonic setting

The present research deals with the geochemical characteristics of the Holocene sediments from Alamdanga area, Chuadanga district, Bangladesh. Main goals of the study are to delineate source rock characteristics, degree of chemical weathering and sorting processes and behavior of redox conditions during deposition of the sediments. Geochemical characteristics of the sediments show comparatively a wide variation in accordance with stratigraphy in their major element contents (e.g. SiO2 69.46–82.13, Al2O3 2.28–8.88 in wt%), reflecting the distinctive provenance and in part an unstable period in terms of tectonic activity. Geochemical classification of the sediments shows mostly sub-arkose with few sub-litharenites. Some major and trace elements display comprehensible correlation with Al2O3 confirming their possible hydraulic fractionation. The chemical index of alteration (CIA*), W* index, index of compositional variability (ICV), plagioclase index of alteration (PIA*) values and the ratio of SiO2/Al2O3, suggest low degrees of chemical weathering in the source areas as well as immature to moderately mature the sediments. The sediments suggest semi-arid climatic trends within oxic deltaic depositional conditions during the Holocene, at 3–12 ka. Whole rock geochemistry and discrimination diagrams demonstrate the continental signature derivatives, which might have been derived from the felsic to intermediate igneous rocks (granitic plutonic rocks) as well as from quartzose sedimentary/metamorphic provenance. These typical sources are present in a vast region of the Himalayan belt and catchment areas of Ganges. The tectonic setting of the sediments demarcates typically passive margin with slightly continental arc system.

Analysis of sedimentary facies and depositional environments of the Permian Gondwana sequence in borehole GDH-45, Khalaspir Basin, Bangladesh

Lithofacies analysis of the Permian Gondwana sequence in borehole GDH-45 of the Khalaspir Basin was performed with a view to deduce the nature of depositional environments. On the basis of dominant lithofacies association, the sequence is divided into six lithostratigraphic units (units A to F). Five lithofacies (conglomerate, sandstone, siltstone, mudstone/shale and coal) are identified within these units. Several sub-lithofacies, such as massive, crudely stratified, cross-stratified, ripple and parallel laminated sandstones are also identified within these lithofacies. The sequence forms a fining-upward trend with a rare coarsening-upward unit. The generalised Gondwana sequence is characterised mainly by channel lags, pebbly massive to crudely cross-stratified sandstone, trough and planar cross-stratified sandstone, ripple laminated sandstone/siltstone, massive to parallel laminated siltstone, mudstone/shale and coal in ascending order. The facies associations represent several repeated fining-upward units and cycles, indicating various sub-environments (channel, floodplain, flood basin/backswamp) in fluvial regime. The conglomerates might have been deposited as debris flow or channel lag deposits. The sandstones were deposited mainly as multistoried channel and lateral bars in moderately braided and sinuous streams. The siltstone and mudstone lithofacies indicate bar top, natural levee or floodplain to flood basin environments. The coal lithofacies suggests deposition in low-lying, short to long persistent, moderately to well drained and sparse to densely vegetated backswamps in fluvial channel-flood-plain complex. The overall succession of the Gondwana borehole sediments suggests that the depositional basin became, with time, gentler in slope gradient, resulting in a more sinuous stream setting.

Crystallization conditions and petrogenesis of the paleoproterozoic basement rocks in Bangladesh: An evaluation of biotite and coexisting amphibole mineral chemistry

The Paleoproterozoic (∼1.73 Ga) basement rocks from Maddhapara, Bangladesh show a large range of chemical variations including diorite, quartz diorite, monzodiorite, quartz monzonite and granite. These are composed of varying proportions of quartz+plagioclase+K-feldspar+biotite+ hornblende±epidote+titanite+magnetite+apatite and zircon. Amphibole and biotite, dominant ferromagnesian minerals, have been analyzed with an electron microprobe. The biotite, Mg-dominant trioctahedral micas, is classified as phlogopitic nature. Relatively high Mg (1.33–1.53 pfu), Mg# (0.52–0.59) and low AlVI (0.13–0.25 pfu) contents in the biotite reflect slightly fractionated magma, which might be a relative indicator for the origin of the parental magma. Biotite is also a very good sensor of oxidation state of the parental magma. Oxygen fugacity of the studied biotites estimate within the QFM and HM buffers and equilibrate at about −12.35 and −12.46, which exhibit the source materials were relatively higher oxidation state during crystallization and related to arc magmatism. Whereas, calcic amphiboles, a parental member of arc-related igneous suite, display consistent oxygen fugacity values (−11.7 to −12.3), low Al# (0.16–0.21) with H2Omelt (5.6 wt.%–9.5 wt.%) suggest their reliability with the typical values of calc-alkaline magma crystallization. The oxygen fugacity of magma is related to its source material, which in turn depends on tectonic setting. Discrimination diagrams and chemical indices of both biotite and amphibole of dioritic rocks reveal calc-alkaline orogenic complexes; mostly I-type suite formed within subduction-related environments. Moreover, igneous micas are used as metallogenic indicator. The biotites with coexisting amphibole compositions show an apparent calc-alkaline trend of differentiation. The study suggests that the trend of oxidized magmas is commonly associated with compressive tectonic and convergent plate boundaries.

Endangered Geoheritage in Bangladesh: A Case Study of Eocene Sylhet Limestone and Adjoining Areas, Jaflong, Sylhet

The Eocene Sylhet Limestone is highly endangered outcrops exposed only in the eastern bank of the Dauki River, Jaflong, Bangladesh. These outcrops and associated stratigraphic units have enormous geo-scientific and educational contents. So, the present study deals with the characterization, identification and evaluation of Jaflong Eocene Sylhet Limestone along with surrounding areas as a geoheritage. Characteristics of the calcitic-rich limestone indicate low energy shallow marine depositional condition. This limestone (48–50 Ma) also includes an abundant and diverse larger foraminifer assemblage and genera, a sparse diverse ostracode assemblage and a very rich, diversified dasycladalean algal assemblage. The foraminifer assemblage has zoogeographic affinities with Eocene Tethys and Indopacific assemblages. Overall evaluations provide score as Rank III of an open system of qualification of the geological heritage content as of its regional importance and the indicial, symbolic, documental and scenic types of abstract perception. Results and level of rank from different quantitative and qualitative assessment also provide excellent geoheritage category. However, the main entity, Eocene Sylhet Limestone of the area is in extremely vulnerable conditions, which recommend strong justification to grant protection at the level of geoheritage as well as geopark and it is so much important to make legal base for conservation.