Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

A. H. M. Ahmad ¹, G. M. Bhat ², S. Rais ¹, C. Saikia ¹, A. E. Khan ¹

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh - 202 002, IN
2 Postgraduate Department of Geology, University of Jammu, Jammu - 180 006, IN

Abstract

This paper describes 10 microfacies types in the Jumara Coral Limestone and Echinoderm Packstone Members (early Late Bathonian age) of Jhurio Formation of the Jumara Dome. The temporal distribution of these microfacies associations reflects an ideal shallowing upward sequence, representing a system of bioclastic bars separating the basin from lagoon environment. Evidence suggests evolving of this bioclastic bar system upward into an oolitic Bar-to-Bank system. Three types of cements (bladed to fibrous, blocky and syntaxial overgrowth) occur in these carbonates. The study also reveals that chemical compaction followed the two phases of early mechanical compaction that largely governed porosity of these limestones. Diagenetic signatures in these carbonates suggest that marine phreatic, under saturated fresh water phreatic and mixing marine fresh water phreatic environments dominated but deep burial diagenesis also played its role in shaping these rocks through time. Evidence suggests that both primary and secondary porosity exist in these carbonates, which determine the permeability in these rocks.

Keywords

Carbonates, Depositional Environments, Diagenesis, Jurassic, Jumara Dome, Kachchh, Gujarat.

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A. H. M. Ahmad ¹, G. M. Bhat ², A. F. Khan ¹, C. Saikia ¹

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh - 202 002, IN
2 PG Department of Geology, University of Jammu, Jammu - 180 006, IN

Abstract

The sandstones of the Upper Katrol Formation were studied with a view to understand the nature of Kachchh basin in relation to provenance and tectonic setting in the vicinity of Nakhtarana area. These Upper Katrol sandstones are fine to coarse grained, moderately to poorly sorted, sub-Angular to sub-Rounded and of moderate sphericity. These sandstones are composed of several varieties of quartz, feldspar, rock fragments, micas and a suite of heavy minerals derived from the Aravalli Range and Nagarparkar massif. Our study reveals that these sediments were derived from the continental block orogen provenance characterized by collision suture and fold thrust belts. Three types of cement encountered in this study include iron oxide, calcite and silica. Evidence of diagenetic modifications suggests an early stage of burial to a depth of 748 to 932 m for these sandstones. Porosity reduction is mainly due to early stage mechanical compaction and subsequent pervasive calcite and Fe-Calcite cementation, which was followed by the late stage authigenic kaolinization in the secondary pore space.

Keywords

Katrol Formation, Kimmeridgian, Sandstones, Petrography, Diagenesis, Tectonic Provenance, Nakhtarana, Gujarat.

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A. H. M. Ahmad ¹, A. F. Khan ¹, C. Saikia ¹

Affiliations
1 Department of Geology, AJigarh Muslim University, Aligarh-202002, IN

Abstract

The Middle Jurassic Athleta Sandstone of Jhurio Dome is mainly medium grained, moderately sorted to moderately well sorted, subarkose and is composed of four major lithofacies representing near shore tidal environments. Iron oxide, silica and glauconite cements mark the sandstones. Compaction is mainly mechanical and sandstones show good amount of porosity. Porosity reduction is mainly due to mechanical compaction in the early stage of diagenesis and subsequent pervasive iron oxide cementation.

Keywords

Depositional environment, Athleta Sandstone, Middle Jurassic, Jhurio Dome, Kachchh, Gujarat.

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C. Saikia ¹, A. H. M. Ahmad ¹, S. M. Wasim ¹

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh - 202 002, IN

Abstract

The Bayana Basin forms the eastern most limit of north Delhi fold belt covering parts of northeastern Rajasthan. The deposition of sediments took place during middle Proterozoic in fluvial and shallow marine environments. The rocks are mainly clastics and include conglomerate, sandstone and shale. During mechanical compaction rearrangement of grains took place and point and long contacts were formed. The sandstones are cemented by iron oxide, silica, carbonate and barite. The porosity has developed due to dissolution of iron, carbonate cement and feldspar grains. Dissolution of quartz along grain boundaries and silica rich compaction pore water seems to be the main source of silica. These observations suggest progressive compaction which initiated at the sediment water interface and continued till deep burial diagenesis in a rapidly subsiding basin.

Keywords

Facies Controlled Diagenetic Evolution, Delhi Group, Bayana Basin, Rajasthan.

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