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Geoelectric Structure over the Deccan Flood
Basalts Using Magnetotelluric Studies
-- Gautam Gupta and S G Gokarn
Magnetotelluric (MT) studies over the northern part of the Deccan flood basalts have delineated a
conductive layer comprising the Deccan basalts and the pre-Trappean Bagh sediments. This layer is about 2.5 km
thick on the western part of the survey profile, whereas on the east its thickness is about 1.5 km. The
individual thicknesses of these formations could not be determined presumably because both these layers have
similar resistivities. A deep-seated thrust zone is delineated 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 Bagh sediments are known to have deposited during the Cretaceous and thus the
compressional forces leading to the thrust zone observed here may have been active during the period between the
Cretaceous and the Deccan volcanism.
© 2009 IUP. All Rights Reserved.
Basanites and Crustal Contamination in the
Cartagena Volcano Field (South-East of Spain)
-- A Aparicio and R García
The volcanic province of southeastern Spain, which is of Miocene-Pliocene age, is characterized in its
first phase by the emission of calcalkaline materials (andesites-dacites-riolites). In the last phase, which is
Pliocene in age, basic rocks were emitted, the composition of which varies from basanite to trachybasalt. The
ash cones of the Cartagena volcano field rest on a 22-km thick continental crust containing Palaeozoic
metamorphic rocks that appear as enclaves among vulcanites. Most of these enclaves correspond to schists and
gneisses showing different degrees of assimilation by the magma. Metamorphic enclaves can also be found,
mainly with pyroxenites and dunites. The presence of metamorphic enclaves lies at the root of the contamination
of the volcanic rocks, with large amounts becoming incorporated into the magma (13% assimilation).
The origin of the magma source of the basic rocks in the Spanish southeast was the melting of ultramafic rocks
of the mantle similar to those of the pyroxenite enclaves in the basanites.
© 2009 IUP. All Rights Reserved.
Facies and Depositional Environment
of the Sylhet Limestone Formation and Kopili
Formation as Exposed in the Surma Basin, Northeastern Bangladesh
-- Mrinal Kanti Roy, Partha Jit Roy,
Mst. Shamima Akter
and Bikash Kumar Chowdhury
The Sylhet Limestone Formation and Kopili Formation are exposed in the Gwainghat area, Sylhet Trough
of the Bengal Basin. The Sylhet Limestone Formation consists of crystalline limestone facies and
fossiliferous limestone facies. Fossiliferous limestone overlies the crystalline limestone, which looks like an assemblage
of foraminifera and other invertebrates. The Kopili Formation overlies the Sylhet Limestone Formation
with drowning unconformal contact that was later faulted. Black laminated shale, red shale (clay), ripple
laminated very fine sandstone-siltstone and parallel laminated very fine sandstone-siltstone are the sedimentary facies
of the Kopili Formation. The facies associations are: the limestone facies association of epeiric sea to
shallow marine environment and the shale facies association of deep marine environment. Crystalline and
fossiliferous limestones are of epeiric 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 drowning unconformal
contact indicates the shutdown of the carbonate factory of the Sylhet Limestone Formation following a rise in
relative sea level that onsets the deep abyssal plain environment of the Kopili Formation in the study area
within the short temporal and spatial sense.
©2009 IUP. All Rights Reserved.
Electrofacies Analysis of Neogene Sequence in the
Well Shahbazpur-1, Bhola, Bengal Basin
-- Debashish Mondal, Md. Sultan-Ul-Islam and Aminul Islam
The present research work presents a detailed analysis of electrofacies, cycles, sequences, associations
and limited core of the Neogene sedimentary sequence in the well Shahbazpur-1 of the Shahbazpur structure,
for reconstruction of paleoenvironments of deposition. The Neogene sequence of the well in the
Shahbazpur structure is characterized by four sequences (Sequence 1-4) that consist of 15 para-sequence sets and 27
para-sequences. The identified electrofacies in GR logs are bell, serrated bell, funnel, serrated funnel,
cylindrical, egg or bow and linear-shaped. The Shahbazpur Sequence-4 (SBS4) consists of two para-sequence sets
(one fining upward and one coarsening upward) and 16 para-sequences (nine fining upward, six coarsening
upward, and one coarsening upward then fining upward). The lower part of the sequence was deposited in
marginal estuarine influence and the upper part in marshy environments including shallow marine to
transitional estuary tidal channel, mud flat, etc. The Shahbazpur Sequence-3 (SBS3) consists of four para-sequence sets
(one coarsening upward then fining upward, one coarsening upward, one linear and one fining upward) and
five second-order (one coarsening upward and four fining upward). The sequence (SBS3) might have been
deposited under shallow marine (lower part) to deep marine (upper part) environments. The Shahbazpur
Sequence-2 (SBS2) consists of five para-sequence sets (three coarsening upward and two fining upward) and two
para-sequences (coarsening upward). It might also have been deposited under progradational delta or
marine regression and marine transgressional environments. The Shahbazpur Sequence-1 (SBS1) consists of four
para-sequence sets (two coarsening upward and two fining upward), and four para-sequences (two coarsening
upward and two fining upward). Overall, the nature of the log suggests a phase of slow rate of deltaic progradation
and then marine transgressional during the deposition of the sequence (SBS1). Although several cycles,
marine transgressions and regressional phase have been interpreted, yet the overall sequence
forms an upward coarsening progradational deltaic phase of deposition.
© 2009 IUP. All Rights Reserved.
LILE and Hfse Fingerprints
on the Geotectonic Settings
of Some Pan-African Granitoids
from Obudu Plateau, Southeastern Nigeria
-- Ukaegbu V U and Beka F T
Structurally-controlled and spatially-restricted Late Neoproterozoic granitoids intrude a
multi-deformed amphibolite-granulite facies terrain in Obudu plateau in a regional N-S to NE-SW pattern. The
mineral contents of the granitoids are mainly potassium feldspar, quartz, plagioclase and biotite, with
subordinate muscovite, hornblende, garnet, iron ores and zircon. The granitoids show a continuous
compositional variation from largely granites (sensu
stricto) to a few granodiorites, with
SiO2 range of 65-75 wt%. They are strongly potassic
(K2O/Na2O>1) and show a strong peraluminous composition (A/CNK>1), S-type
feature, mostly corundum normative composition and low iron enrichment. They 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 indicate an important role in partial melting of continental
crust played on their petrogenesis, hence their orogenic affinity. In particular, Y+Nb versus Rb and Y versus
Nb discrimination diagrams suggest 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 suggest that
crustal thickening was low but uniform throughout these orogenies.
© 2009 IUP. All Rights Reserved.
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