Modification in Textural and Geochemical Characteristics of Shallow Marine Black Shale in Proximity to Igneous Dykes in Kimmeridgian Jhuran Formation, India

Abstract

This study reports organic-rich shale in Kimmeridgian Jhuran Formation in western India and discusses effects of small-scale igneous dyke intrusions on textural and geochemical characteristics of the shale. Field, petrography, Rock-Eval pyrolysis and GC-MS data are presented to document changes in texture and geochemistry for the systematically collected samples in relation to the distance of dykes in outcrop. Unconformably resting over a carbonate succession, the overall coarsening-upward Jhuran Formation exhibits laterally extensive organic-rich shale (average thickness ~60 m) in the lower and middle segments, which are intervened by siltstone and sandstone interbeds of variable thicknesses. Inner to middle-shelf origin of the carbonaceous shale is inferred by abundant plant leaves in shales, hummocky cross-stratified and wave rippled sandstones, polymodal paleocurrent direction of sole marks and trace fossils Gyrochorte, Arenicolites, Skolithos, Planolites. TOC content of shales vary from 1.7% to 7.5%, with an average Tmax 430°C. HI and OI range from 46 to 15 and 19 to 71 mg HC/g Corg respectively, suggesting dominance of type-III kerogen. Phyllocladane, ent-kaurane, retene and simonellite dominates the biomarker assemblage indicating predominantly coniferal source of organic matter. Mafic igneous dykes (75–68 Ma old) intrude the shale at several localities, altering its textural and geochemical characteristics. The dykes form an intensely burnt zone within the shale at its contact to about 2 m, and a weakly burnt zone up to around 8 m distance. Average Tmax value of samples at the intensely burnt zone is around 595°C. Original wavy and lenticular micro-texture of shale is completely lost as quartz silts exhibits floating texture within organic-rich matrix. Biomarker is represented by poly-aromatic fractions such as fluroanthrene, pyrene, benzo-fluorene, methylated pyrenes, benzo-naptho-thiophenes, anthracenes. Tmax sharply drops to ~460°C and thereafter it decreases gradually within the weakly burnt zone. Average HI and OI values of the intensely burnt zone are 10 and 20 mg HC/g Corg respectively which increase gradually as the wavy and lenticular micro-texture of shale gradually reappears with increasing distance from dykes. This study documents a sharp rise of Tmax and sharp fall in HI and OI in the intensely burnt zone adjacent to dykes and gradual changes in these parameters within the weakly burnt zone to the unaffected zone.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015