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Abstract

The Mohand anticline located south of Dehra Dun (U.P.) exposes the middle and upper Siwaliks (Sarmatian to Cromarian). The lower Siwalik was encountered in a deep well, a little south of Mohand.

The middle and upper Siwalik sediments are classified into two (A and B) and three (C, D and E) groups respectively on the basis of heavy mineral suites. The suite of group A consists of garnet, tourmaline, epidote, staurolite, and kyanite besides minor quantities of sphene, rutile, chloritoid, and monazite, and fairly abundant ore minerals (magnetite, hematite, limonite, pyrite and chromite?). Kyanite is marker for group A. Addition of hornblende to the above assemblage marks the suite of group B, and that of hornblende and sillimanite, the suite of group C. The suite of group D is characterized by flooding of ore minerals,. impoverishment of nonopaque minerals, very low garnet and very rare kyanite, and rounded zircon as well as tourmaline. The suite of group E is comparatively richer than that of group D. It mainly consists of very low garnet, rare but persistent sillimanite and abundant kyanite.

The absence of mineral markers in older rocks e.g., that of sillimanite in middle Siwalik and kyanite in lower Siwalik does not seem to be a function of intrastratal solution effects. Alternatively, it is concluded that their appearance was due to uplift and successive unroofing of more metamorphosed rocks constituting the provenance.

The heavy mineral suite of Siwaliks exposed along the Himalayan foothills vary from area to area. The suite of lower Siwalik extending from Jammu to eastern Uttar Pradesh is characterised by presence of staurolite marker and is similar throughout and consitutes a terrigenous mineral super-province. The suite of middle Siwalik sediments varies from area to area and the diversities are ascribed to splitting of the super-province into three provinces extending from Jammu to the Yamuna river, the Yamuna river to the Ganga river, and the Ganga river to Mithawali fault. The mineral province extending from Jammu to the Yamuna river gave rise to two sub-provinces extending from Jammu to the Markanda river and the Markanda river to the Yamuna river during the deposition of upper Siwalik sediments. The respective provinces are delimited by faults presently occupied by the Markanda, Yamuna and Ganga river courses. Tectonic activities along these faults in relation to post-Miocene, Himalayan orogeny vis-a-vis evolution of Siwalik lithotypes requires detailed analyses and, together with the recognition as well as definition of provinces, appears a necessary prerequisite for attempting any regional correlation on the basis of heavy minerals.

The heavy mineral suites of Siwalik rocks exposed in Mohand Anticline are not wholly comparable to those of Jammu, Punjab and Tanakpur (U.P.).

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The Pranhita-Godavari graben, located in the eastern Indian Peninsula, is aligned NW-SE and covers an area of nearly 40,000 sq. km. The graben exposes sediments ranging in age from Proterozoic to Lower Eocene. The oldest, the Pakhal Series, are disconformably overlain by Sullavai Series and together comprise at 1.5-2.0 km thick sequence. The Sullavai Series are unconformably overlain by approximately 6.5 km thick Gondwana sequence which is in turn, unconformably overlain by infra-trappean beds. Deccan Trap effusives contain inter-trappean beds and are unconformable with the underlying infra-trappean beds.

Genesis of hydrocarbons in non-marine sediments containing predominantly vegetal constituents is feasible. The Lower Gondwana sediments of the Pranhita-Godavari graben contain coal seams, and dispersed organic matter. Sediments of Talchir Stage are poor in organic matter content and their hydrocarbon-generating potential appears doubtful. The Barakars contain coal seams and are rich in dispersed organic matter. The carbon ratios, computed from proximate analyses of coals from various localities, indicate that the stage of maturity of organic matter varies within such limits which do not exclude the Barakar sediments from being considered as potential source rocks of hydrocarbons. The Kamthi sediments are dominantly arenaceous and contain small amounts of dispersed organic matter. Lack of relevant analytical data makes it difficult to assess their hydrocarbon-generating potential.

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Outcrops ranging in age from Late Jurassic to Recent are located nearer the western margin of the basin. The eastern parts of the basin are covered by alluvium and contain a relatively thicker and more complete sedimentary sequence. The exposed and subsurface sedimentary sequence are adequately fossiliferous and several faunal as well as palynological zones are established.

The basin was formed during Late Jurassic by sagging of a part of the Indian shield, mainly along the dominant NE-SW Eastern Ghat trend. The basin was well differentiated into depressions and ridges formed owing to taphrogenic fragmentation and block faulting along the dominant basement trends during the onset of Late Jurassic. Initial sedimentation was under non-marine environment. The first marine transgression occurred during the close of Late Jurassic. Marine environment of sedimentation continued till Cretaceous, although through a series of minor transgressions and regressions. The depocentres were mainly due west.

A major regression occurred during the close of Cretaceous. The basin underwent an easterly tilt and the depocentres shifted due east prior to marine transgression during the beginning of Tertiary. The differentiation of the basin into depressions and ridges was almost lost during Eocene. Sedimentation during the Tertiary period was marked by repeated transgressions and regressions as is evidenced by biofacies, isopachs, and several sedimentation breaks which are relatively more pronounced in the western parts of the basin.

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